Commercialization of Adsorbed Natural Gas Storage ...

NEW JERSEY INSTITUTE OF TECHNOLOGY MSEMBC PROGRAM

Commercialization of Adsorbed Natural Gas Storage Technology for Use in Stationary Emergency Power Systems ECE 700B Michael Hainzl, CBCP

e-mail: [email protected]

4/21/2017

Abstract: This paper examines the engineering, operational, regulatory, and financial challenges that must be overcome to successfully commercialize adsorbed natural gas storage for use in stationary Emergency Power Systems. Natural gas cannot yet economically meet on-site fuel storage requirements. The on-site fuel storage requirement continues to drive the use of diesel fueled engines in stationary emergency power systems, even when an owner might otherwise prefer to use natural gas as a fuel source. The costs and regulatory challenges of using adsorbed natural gas as an on-site fuel source for emergency power generation are identified through a practical design example.

Keywords: Adsorbed Natural Gas, ANG, Emergency Power, Emergency Power Systems, EPS, Emergency Generators, Standby Generators.

Acknowledgements: Cenergy Solutions, CP Industries, FIBA Technologies, Generac Power Systems

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Table of Contents: 1

Introduction ......................................................................................................................... 4

2

Background and Benefits .................................................................................................... 4

3

4

5

2.1

Benefits to Society and the Environment...................................................................... 4

2.2

Operational Benefits .................................................................................................... 7

2.3

Relevant Codes and Standards – System Performance .............................................. 7

2.4

Relevant Codes and Standards – Use and Storage of Natural Gas ............................. 9

Engineering........................................................................................................................12 3.1

Compressed Natural Gas (CNG) – Technology Background ......................................12

3.2

Adsorbed Natural Gas (ANG) – Technology Background............................................15

3.3

ANG – Adsorption and Desorption Cycle ....................................................................15

3.4

Technical Obstacles to Commercialization ..................................................................18

3.5

Design Example ..........................................................................................................18

3.6

Compressors and Regulators .....................................................................................19

3.7

Fuel Storage – Cylinder Volume Requirements ..........................................................19

3.8

Packaging and Construction .......................................................................................22

3.9

Size and Weight ..........................................................................................................22

3.10

Material Cost...............................................................................................................24

3.11

System Design Schematic ..........................................................................................27

3.12

Sequence of Operation ...............................................................................................28

Operational Considerations ................................................................................................29 4.1

Safety .........................................................................................................................29

4.2

Scalability ...................................................................................................................31

4.3

Life-Cycle Costs ..........................................................................................................32

4.4

End of Life – Recycling and Disposal ..........................................................................33

Summary & Recommendations..........................................................................................33 5.1

Regulatory Changes to Support ANG Technology ......................................................33

5.2

Next Steps for Industry and the Engineering Community ............................................34

6

Appendix 1 – Full Size Functional Block Diagram ..............................................................35

7

Glossary ............................................................................................................................37

8

Endnotes............................................................................................................................38

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1 Introduction This paper examines the engineering, operational, regulatory, and financial challenges that must be overcome to successfully commercialize adsorbed natural gas (ANG) storage for use in stationary Emergency Power Systems (EPS). Today, natural gas continues to gain market share against diesel fuel for EPS applications. Natural gas has several practical benefits including lower maintenance costs and reduced emissions compared to diesel, which can bring a societal benefit through improved ambient air quality. However, natural gas cannot yet economically meet the on-site fuel storage requirements identified in NFPA 110 §5.1. Many authorities having jurisdiction (AHJs) require on-site fuel storage regardless of the natural gas supply’s historic reliability. Similarly, some EPS owners prefer an on-site fuel supply even when it is not specifically required by the AHJ. The on-site fuel storage requirement continues to drive the use of diesel fueled engines in stationary EPS, even when an owner might otherwise prefer to use natural gas as a fuel source. The objective of this paper is to demonstrate how existing Adsorbed Natural Gas (ANG) technology can be leveraged to develop a commercially viable on-site natural gas storage system that is:    

Safe, meeting applicable national safety standards. More economical than traditional high-pressure compressed natural gas storage. Reliable with minimal maintenance requirements over a 25 year projected life-cycle. Space efficient and scalable.

2 Background and Benefits 2.1

Benefits to Society and the Environment

According to a 2007 study by the US Department of Energy, there are approximately 12 million emergency generators in the United States with an installed capacity of approximately 170 Gigawatts (GW).1 Historically, units larger than about 100 kW have been powered by diesel engines and still account for the majority of installed emergency power generation capacity. The historic reliance on diesel fuel was rooted mainly in economics; widely available fuel that was easy to manage, engines that were reliable, compact and cost effective. Diesel also met the “on-site fuel supply” requirement. Even if a specific fuel supplier was unable to make a delivery, the perception was, until recently, that delivery arrangements could be made quickly in the event of an emergency. Two things happened to change the perceived reliability of diesel fuel in EPS applications:  Ultra-low sulfur diesel (ULSD) was introduced nationwide in 2010 for non-road use in response to new EPA emissions requirements.2

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

Hurricanes Irene (2011) and Sandy (2012) impacted diesel fuel supplies all along the East Coast for weeks, while natural gas supplies suffered only localized short-term outages.3

Ultra-low sulfur diesel (ULSD, less than 15 ppm sulfur) was a prerequisite for the exhaust aftertreatment systems introduced after 2006, enabling engines to achieve exhaust emissions that are more than 99% cleaner than engines manufactured prior to 1996. Diesel engine emissions from stationary sources are part of a larger group of EPA air-quality regulations known as RICENESHAP.4 Implementing cleaner diesel engines is estimated to reduce annual emissions from stationary engines by:    

1,000 tons per year of air toxics, 2,800 tons per year of fine particulate matter, 14,000 tons per year of carbon monoxide, and 27,000 tons per year of volatile organic compounds 5

However, ULSD has a substantially shorter shelf-life compared to its older high-sulfur predecessor; 6-12 months at most before fuel polishing6 is required. This is driven by changes in the fuel and the engines:  





The catalytic cracking processes used to increase per-barrel yield and remove sulfur results in a less stable finished fuel compared to product from a distillation process. Sulfur in diesel fuel acts as a biocide, slowing the growth of algae and other bacteria that would otherwise begin to break down the fuel. While high-sulfur diesel fuel was not completely immune to fuel degradation, it could be stored reliably for 1-2 years in most environments before fuel polishing may have been required. High pressure fuel systems (>30,000 lb/in^2 on the latest generation of engines) to meet today’s strict emissions requirements increase fuel temperature and the rate of fuel oxidation. Fuel oxidation leads to the formation of gums that can clog filters and fuel injectors. 7, 8 Fuel systems on older diesel engines were more tolerant of degraded fuel.

With ULSD requiring more rigorous maintenance of the fuel itself and the EPA’s mandate for cleaner burning diesel engines requiring a higher quality fuel; diesel fuel was no longer the lowcost, low-maintenance emergency power source it had been for decades past. The environmental risk related to diesel fuel leaks, spills and the associated costs of cleanup also weigh heavily on the continuing use diesel fuel for emergency power. EPA regulations require reporting of oil spills, which include diesel, even in very small quantities. The “sheen rule” states that oil spill reporting does not depend on the specific amount of oil spilled, but on the presence of a visible sheen (on the water) created by the spilled oil.9 As if the environmental risk, maintenance challenges posed by ULSD and more complex albeit cleaner burning engines weren’t enough; Hurricanes Irene and Sandy destroyed the longstanding conventional wisdom that diesel fuel delivery was assured even during the most severe emergencies. For owners and operators of emergency power systems, the changing landscape had them looking for alternatives. Natural gas appeared to address all their concerns: cleaner exhaust emissions, equal or better fuel availability, reduced fuel maintenance and environmental liability.

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Except one: on-site fuel storage. NFPA 110 §5.1 specifies energy sources for emergency power systems. Natural gas is permitted as a fuel source, but carries with it the following exception: For Level 1 installations in locations where the probability of interruption of offsite fuel supplies is high, on-site storage of an alternate energy source sufficient to allow full output of the EPSS to be delivered for the class specified shall be required, with the provision for automatic transfer from the primary energy source to the alternate energy source. Unfortunately, NFPA 110 does not specify a level of availability that is acceptable. What is a high probability of interruption? What is reliable? NFPA forces this decision to the local AHJ. Needless to say, there are few consulting engineers, building inspectors or gas companies willing to assume liability and declare a natural gas fuel source as “reliable” without a quantifiable and legally rigorous definition from NFPA or other applicable codes. Even in rare cases where the AHJ has approved natural gas for life-safety EPS, there are owners and operators who are not comfortable being fully reliant on pipeline natural gas delivery to power their life-safety and mission critical systems. Developing a safe and cost-effective means to provide on-site natural gas storage addresses all three challenges; the cautious consulting engineer, the AHJ unwilling to accept natural gas as a reliable fuel source, and the owner who is reluctant to be without on-site fuel. With the exception of seismically active areas, natural gas distribution systems operate at a reliability exceeding 99.999% according to a 2013 report by MIT’s Lincoln Laboratory.10 Incidentally, that makes the natural gas distribution system approximately three orders of magnitude more reliable than a single engine generator set. The same MIT paper notes that many of the compressors on the transmission network are powered by natural gas; using approximately 3% of the natural gas produced. The distributed nature of the natural gas production and transmission networks result in an exceedingly low probability of cascading failure. Each individual location has its own unique risk profile, but an illustrative example is helpful. Assume 99% availability on the electric utility (87.6 hours of outage time per year). Further, let’s say a particular city has older natural gas infrastructure and availability is only 99.9% (two orders of magnitude less reliable than cited by the MIT paper). 𝑃(𝑢𝑡𝑖𝑙𝑖𝑡𝑦 𝑝𝑜𝑤𝑒𝑟 𝑓𝑎𝑖𝑙) = 0.01 𝑃(𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠 𝑠𝑢𝑝𝑝𝑙𝑦 𝑓𝑎𝑖𝑙) = 0.001 For the purpose of maintaining electrical power to a facility using a natural gas generator, a natural gas supply failure is a problem only if the utility power is in a failed state simultaneously: 𝑃(𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑔𝑎𝑠 𝑠𝑢𝑝𝑝𝑙𝑦 𝑓𝑎𝑖𝑙 | 𝑢𝑡𝑖𝑙𝑖𝑡𝑦 𝑝𝑜𝑤𝑒𝑟 𝑓𝑎𝑖𝑙) = 0.0001 × 0.01 = 0.00001 Expressed in terms of availability, there is a 99.999% probability that utility power or the natural gas supply will be available at any given time. “Five nines” availability equates to an average of five minutes and fifteen seconds a year where both electric and gas service are simultaneously unavailable. However, the even more remote possibility of a ruinous lawsuit in civil court drives consulting engineers towards on-site fuel storage.

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Adsorbed natural gas (ANG) storage technology can provide the safe, cost-effective, on-site storage solution that enables wider use of clean burning natural gas fueled EPS; ensuring reliable emergency power even in the rare instance of a concurrent electric and natural gas utility failure.

2.2

Operational Benefits

An ANG storage system is not intended to replace the utility gas provider. Rather it serves as a backup to the pipeline natural gas supply; a “standby fuel”. The EPS will be able to operate for several hours independent of the utility gas supply using the stored natural gas. An example design in Section 3.5 illustrates the expected runtime, space requirements, and cost associated with a practical system. In the past several years, natural gas generator sets in the range of 500 kW to 1 MW have become available from the major American manufacturers; Caterpillar, Cummins, Generac, and Kohler.11 Units are routinely operated in parallel to create multi-megawatt emergency power systems. It is with these larger systems in particular that a shift to natural gas generation coupled with ANG storage can see significant operational, financial, and environmental risk benefits; eliminating diesel storage tanks on the order of 10,000 – 20,000 gallons.

Relevant Codes and Standards – System Performance

2.3

To provide context, it is useful to begin with an introduction to the relevant national codes and standards. First, we’ll introduce the four main standards which drive the design and performance requirements of an EPS. In the following section we’ll look at the standards impacting the design of an ANG storage system and the regulatory gaps that may confound the deployment of ANG storage. It is not practical to assess the innumerable building and electrical code variations at the State and City level in this paper. However, the national codes and standards of the National Fire Protection Agency (NFPA)12 form the foundation on which many of the State and City codes are based. The reader must understand that our high-level review of the NFPA standards will not address all local requirements, which may be more stringent or have additional requirements. Direct quotes from the NFPA standards cited below are in italics. NFPA 70 – National Electric Code13 The National Electric Code (NEC) has broad scope, covering many aspects of low-voltage (150 V/V at 500 lb/in^2 is considered to be a minimum threshold to enable development of low-cost tanks constructed specifically for ANG. Entergris

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Inc. recently released an engineered carbon called BrightBlack capable of storing 180 Vd/V at 500 lb/in^2 that also claims to have excellent thermal conductivity.34 For our example design we will presume a nominal adsorbent efficiency of 160 Vd/V at 500 lb/in^2 (35 bar). Figure 4: Performance of various carbon based adsorbents and one MOF (HKUST-1). The green vertical bar is centered around 3.5 MPa (500 lb/in^2). From work by Nie, Lin, & Jin.

The most common and economical adsorbent materials are carbon based, made from wood, coal, coconut shells, and other organic base materials. Because adsorbents are used in many industrial applications to remove undesired components from liquids and gasses, a vast array of physical and chemical compositions are commercially available. For ANG storage, there are two competing design objectives: high effective density and high thermal conductivity. The effective density of an adsorbent material is a physical characteristic. Powders can be packed tightly and have a high effective density. Minimizing the amount of gas stored in the vapor phase is desirable to maximize the adsorbed gas storage capacity of the cylinder. However, densely packed adsorbent in a fine granular or powder state has very low thermal conductivity.35 Recall from the earlier discussion that desorption (delivering gas to the system) is an endothermic process. If sufficient heat cannot be transported into the adsorbent, internal cylinder pressure may drop due to auto-refrigeration to the point that sufficient gas volume would not be delivered to the load. This would result in an unexpected shutdown of the engine. Vapor phase gas is more effective at transporting heat from the cylinder walls into the core of the adsorbent material. Pelletized or larger granules of adsorbent are available, but there is a penalty in terms of total cylinder storage capacity. Gas stored in the vapor phase between adsorbent particles to facilitate heat transport is volume not used efficiently at the relatively low working pressures within the adsorbent material. Total storage capacity of the system suffers. As noted earlier (Chang and Talu paper), cylinder construction techniques such as a central dip tube through the cylinder length spread the desorption process across the entire cylinder,

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reducing the localized cooling and improving gas delivery when high effective density adsorbents are used. Given these design challenges, it is easy to see that even if the physical space is available for a single large diameter ANG cylinder, that may not be desirable because the thermal effects of desorption would be more difficult to manage.

3.4

Technical Obstacles to Commercialization

Today, there are no technical obstacles prohibiting the construction of an ANG storage system. All the components are already in use for CNG applications.  





High pressure ASME tanks for CNG filling stations are becoming more common to support the growing fleet of CNG vehicles. DOT cylinders are also readily available. FMVSS 304 CNG cylinders, while lighter and less expensive than ASME tanks, still require periodic testing that typically cannot be done while the tank is in service. One of the potential cost advantages of an ANG storage system is the ability to use cylinders that have a service-life of at least 20 years without the need for periodic requalification and/or a limited service life. Compressors have become reliable and durable. However, 3,600 lb/in^2 compressors for CNG remain very expensive. One of the cost advantages to an ANG storage system is expected to come from compressors operating at much lower pressures, on the order of 500 lb/in^2. As a “standby fuel”, another option is to exclude a permanently installed compressor altogether. A service technician could provide a portable compressor, connected with quick connect fittings similar to those used on CNG vehicles. This would eliminate the cost of a permanent compressor and reduce the maintenance risk on a piece of equipment with a very low anticipated duty cycle. The ANG storage would only need to be replenished if the natural gas supply failed while the generator was running.

The main risks at this time come from the uncertain regulatory and financial aspects. Earlier we identified the regulatory risks. In the upcoming section we will attempt to quantify the financial risks and benefits necessary for commercialization.

3.5

Design Example

A practical design example is useful to develop an understanding of the operational benefits and limitations associated with existing natural gas adsorbent materials. We will design the ANG system around a Generac SG50036 natural gas fueled generator, capable of producing 500 kilowatts (kW) at 100% load. The generator set is powered by a 12cylinder 25.8 liter engine producing 809 horsepower at full load. It burns 6,000 standard cubic feet (SCF) of natural gas at full load per hour of operation. Recall from the earlier discussion on the relevant standards, NFPA 70 requires on-site fuel storage sufficient to run the generator for a minimum of two hours at full load. For the SG500, two hours of runtime at full load requires a storage system capable of delivering 12,000 SCF in two hours. Delivery time is relevant because desorption in an ANG system is endothermic. If the tanks cannot absorb sufficient heat from the ambient environment, the tank temperature and pressure may drop so low that gas will not be desorbed at a sufficient rate to support the load on the engine. The generator will shut down due to low gas pressure.

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Available space to install an emergency generator is often limited on the building property. Typically, a diesel generator set is seen sitting above a base tank which holds hundreds to several thousand gallons of diesel fuel. A similar footprint may be desirable for an ANG system. The challenge is designing a structure that will support the nearly 13,000 pounds of generator set sitting on top of it, provide protection to the ANG system components while still meeting the criteria of NFPA 55 for “weather protection” and other regulatory requirements.

3.6

Figure 5: Generator set on a diesel base tank.

Compressors and Regulators

Compressors can represent a substantial cost of a natural gas storage system. A 3 kW CNG compressor capable of delivering ~250 cubic feet an hour (CFH) at 3,600 lb/in^2 is approximately $16,000 USD.37 The compressor would have to run approximately 48 hours to store enough gas to meet the minimum two-hour runtime in our design example. However, it would be well matched for replenishing the system and could be permanently installed or temporarily connected by a service technician. Because the market for stationary ANG storage is very small today, comparative data on low pressure natural gas compressors designed specifically for ANG is not readily available. One manufacturer, Cenergy Solutions, is expected to release 60 CFH and 180 CFH compressors designed specifically for ANG storage sometime in 2017. List price is estimated to be $3,000 and $7,000 USD, respectively.38 Compressor utilization is expected to be very low. To make ANG storage economically viable, the cost of a permanently installed compressor option must be minimized. Alternatively, equipping the system with quick-connect fittings and deploying a portable compressor to the site only when required can reduce the capital and maintenance costs. Multi-stage regulators for CNG vehicle systems are readily available from manufacturers. The same basic design principles can be used to scale up to the 25.8 liter engine in our design example. One commercially available heavy-truck conversion package that includes DOT cylinders (1,230 liters total volume, 11.7 MMBTU of storage), pressure relief devices, regulators, and valves is priced at $22,125.39 Excluding the cylinder cost of approximately $15,000, the rest of the system components are $7,125. This is an important data point because it represents a commercially available CNG system with all the basic components needed to store and deliver the gas required for two hours of runtime on a 500 kW generator set.

3.7

Fuel Storage – Cylinder Volume Requirements

An ANG storage system that closely resembles a diesel base tank is one practical form factor to consider. It uses real estate efficiently by storing the fuel underneath the generator set.

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Constructing the system to resemble a diesel base tank provides a familiar reference point for consulting engineers and end users to help illustrate the potential of ANG storage. The first step to determining the storage volume requirements is calculating the amount of fuel necessary to achieve a desired runtime. Earlier, it was noted that NFPA 70 requires a minimum of two-hours of on-site fuel storage where emergency and/or life-safety loads exist. Some cities across the United States require on-site fuel storage to sustain runtimes longer than two hours.40 Fuel storage to support the 500kW machine in terms of standard cubic (SCF), conventional CNG tank volume, and ANG storage using a nominal carbon based adsorbent with a working capacity of 160 Vd/V at 500 lb/in^2 are listed in the table below: Table1: Natural gas storage requirements for specified runtime:

Runtime Hours

1

2

3

4

5

6

Total SCF Required

6000

12000

18000

24000

30000

36000

Conventional CNG Cylinder volume, CF at 3,600 lb/in^2 (248 bar.) 296 Vd/v

20.3

40.5

60.8

81.1

101.4

121.6

ANG Cylinder volume, CF, at 500 lb/in^2 (35 bar), 160 Vd/v

37.5

75.0

112.5

150.0

187.5

225.0

Based on the table and taking into account the cushion gas that will remain in the adsorbent after discharge, it can be seen that a minimum of 75 cubic feet of ANG storage volume is required to meet the minimum on-site fuel storage requirement in NFPA 70. It is also instructive to note that 225 cubic feet provides six-hours of runtime; the requirement for the City of New York. With a basic idea of the fuel storage volume required, we turn attention to location of the cylinders. The frame dimensions and the nominal height of base tanks designed to fit below a diesel generator afford a significant volume to consider various ANG cylinder array configurations. Table 2: Typical generator base tank dimensions for a 500 kW diesel: Unobstructed dimensions within base tank volume: Base Tank

L (in.)

W (in.)

H (in.)

Style 1

123.8

60

14

Style 2

193.8

60

36

Style 3

255.8

60

36

Other dimensions are achievable, but we have decided to determine how much ANG can be stored in a form factor that is similar to what engineers and end users are accustomed to seeing today with a diesel generator set. Take away the size constraints and the problem simplifies to cost and code acceptance.

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Using a Style 2 base tank (193.8 inches long), constraining the cylinder length to 120 inches leaves up to 74 inches for the compressor, valves, regulators, cylinder manifolds, and other system accessories. That gives an available volume for ANG cylinders of 120” L x 60” W x 36” H. Finally with a constrained volume, various cylinder configurations can be considered. The diameter of each cylinder, number of rows and columns, will impact the heat transfer in and out of the cylinders. For example, a 4x3 cylinder array would result in two “core” cylinders, completely surrounded by other cylinders. During a desorption cycle, the inner cylinders may have difficulty absorbing sufficient heat from the surrounding environment to deliver the stored gas at the required rate. Similarly, as tank diameter increases, it becomes increasingly difficult to transport ambient heat to the core of the adsorbent material. Table 3 lists a range of cylinder configurations given the volume constraints noted previously. Configurations where the cylinder array dimensions exceed the size constraints noted earlier are shaded grey. The 36” height constraint is not arbitrary; rather it is connected to another requirement in the National Electric Code that limits the maximum height of the main circuit breaker located inside the generator set. Cylinder array heights greater than 36” can be used, but will typically require a platform around the generator for service access. Aesthetically, the increased height is undesirable in many applications and is another practical reason to limit the height of the cylinder array. Table 3: Possible cylinder array configurations within available volume: Cylinder diameter, inches

10

12

14

16

18

20

Max number of cylinders across:

6

5

4

3

3

3

2 rows high, height in inches

20

24

28

32

36

42

3 rows high, height in inches Volume at 120" cylinder length, CF, 1 row Volume at 120" cylinder length, CF, 2 rows

30

36

42

48

54

63

32.72

39.27

42.76

41.89

53.01

65.45

65.45

78.54

85.52

83.78

106.03

130.90

98.17

117.81

128.28

125.66

159.04

196.35

Volume at 120" cylinder length, CF, 3 rows

Referring back to the minimum required ANG storage volume from Table 1, at least two rows of cylinders at least 12” in diameter will be required to achieve the minimum on-site fuel storage requirement of two-hours specified by NFPA 70. Table 4 lists the runtimes achievable using the various cylinder array configurations listed in Table 3 at 100% load. Table 4: Estimated System Runtime, based on ANG Volume Delivered (Vd/V) Cylinder diameter, inches Runtime hours, 1 row of cylinders Runtime hours, 2 rows of cylinders Runtime hours, 3 rows of cylinders

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0.9

1.0

1.1

1.1

1.4

1.7

1.7

2.1

2.3

2.2

2.8

3.5

2.6

3.1

3.4

3.4

4.2

5.2

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Storing sufficient fuel in a volume associated with a typical diesel base tank is achievable. Our attention now turns to economics, dynamic thermal performance, and the required runtime for the ANG system.

3.8

Packaging and Construction

Thanks in large part to continuing development in the natural gas fueled vehicle industry, there exists a wide variety of components and materials available to implement CNG in a stationary application. However, few components specifically designed for ANG storage are on the market at this time. The significantly lower storage pressures afforded by ANG allow for variations from the traditional form factor of a gas cylinder. Conformable tanks made with fiberglass or carbon fiber reinforced plastic can more effectively use the irregular spaces available in a modern automobile. Conformable composite tanks are also much more resistant to cyclic fatigue. Research continues to identify the optimal balance of storage pressure and tank construction cost. Current research is focused on 500 lb/in^2 (35 bar), although higher pressures offer increased working capacity. There is ongoing research in adsorbents as well, ranging from highly engineered carbon nanotubes to metal oxide frameworks (MOFs). These advancements strive to improve the amount of gas that can be adsorbed into a given volume of adsorbent, preferably at lower pressures, and to improve thermal conductivity. The literature review for this project revealed dozens carbon based adsorbents available in physical configurations ranging from loose powder, granular, toroidial pellets to small bricks. The shapes and exact chemical composition of each are designed to optimize a desired performance characteristic. The adoption of technical advances in the natural gas vehicle sector will largely come down to economics. For a stationary emergency power application it is not necessary to get the best performing adsorbent, the best cyclic adsorption/desorption, lightest weight, or the most space efficient conformable composite tank available. As noted earlier, the probability that the ANG storage system will ever be called on to perform in its lifetime is relatively small. The primary design objectives for stationary emergency power include:     

3.9

Safety. Low capital cost; no need for the lightest cylinders or the most efficient adsorbent. Minimum 25 year design life on cylinders, without periodic inspection or requalification. Stable adsorbent with minimum 25 year design life. Minimal maintenance requirements.

Size and Weight

There are no ASME or DOT cylinders on the market today constructed specifically for the lower pressure associated with ANG storage. Cylinders designed for the higher working pressure of CNG are heavier and more costly than necessary for ANG service. Although manufacturers’ technical and pricing data are readily available for CNG cylinders at operating pressures of 3,600 – 5,000 lb/in^2 41; there are no 500 lb/in^2 class cylinders that can serve as a direct cost comparison for ANG service. Currently, we can only interpolate the cost of low pressure liquidpropane cylinders, some 250-500 lb/in^2 compressed air receivers, and high pressure CNG

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cylinders to estimate the cost of an ANG cylinder. Compared to 3,600 lb/in^2 CNG, cylinder weight and cost could be reduced by 50% - 75% for 500 lb/in^2 ANG service. Liquid propane (LP) tanks have been converted and represent the lowest cost tank option for ANG storage, but can only operate up to a maximum working pressure of 250 lb/in^2. The low pressure does not make very efficient use of the adsorbent’s capacity and so we will not consider them further in this project. Table 5 lists typical Type IV cylinder weights and the total weight of the cylinder array prior to filling with adsorbent material. Even though Type IV cylinders are the most expensive today for CNG use, we expect they will be more economical to manufacture for 500 lb/in^2 ANG service. However, since the potential weight reduction is not known at this time, we retain the average Type IV cylinder weight for 3,600 lb/in^2 CNG service in Table 5. Keeping with the physical constraints identified earlier, the cylinder length in all configurations is 120” (10 feet); a commercially available size. Table 5: Estimated cylinder weight, without adsorbent material, 120" overall length: Cylinder diameter, inches Type IV composite, avg. weight per CF of volume (pounds)

10

12

14

16

18

20

15

15

15

15

15

15

Cylinders Across: Estimated cylinder array weight, 1 row (pounds) Estimated cylinder array weight, 2 rows (pounds)

6

5

4

3

3

3

491

589

641

628

795

982

982

1,178

1,283

1,257

1,590

1,963

1,473

1,767

1,924

1,885

2,386

2,945

Estimated cylinder array weight, 3 rows (pounds)

The effective density of carbon based adsorbents ranges from 29-32 lb/ft^3.42 The example system design presumes a carbon based adsorbent with an effective density of 31 pounds/cubic foot. Table 6 lists the amount of adsorbent material required to fill the respective cylinder array volumes. The adsorbent is placed inside the cylinder by removing the valve neck and compacted until the required density is reached. A perforated dip tube that is not filled with adsorbent may be installed. The valve assembly is reinstalled and the cylinder is evacuated to desorb as much air as possible. The cylinder is then allowed to fill to atmospheric pressure with natural gas and the evacuation/natural gas refill process is repeated until the oxygen and nitrogen levels are reduced at least 90% from the baseline. Table 6: Adsorbent material required for each cylinder array configuration, pounds: Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across: Pounds of adsorbent material, 1 row Pounds of adsorbent material, 2 rows

6

5

4

3

3

3

1,014

1,217

1,326

1,299

1,643

2,029

2,029

2,435

2,651

2,597

3,287

4,058

3,043

3,652

3,977

3,896

4,930

6,087

Pounds of adsorbent material, 3 rows

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The addition of adsorbent increases the weight of the cylinder array, but is still very manageable given the typical weights associated with stationary power generation equipment. Safety considerations will also restrict the installation of ANG building rooftops, so total weight is not a significant concern save for having the appropriate equipment on site during installation. An additional 500-1000 pounds is estimated for the remaining supporting structure. Table 7 lists the total weight of the adsorbent filled cylinders. Table 7: Total ANG cylinder array weight with adsorbent added, pounds: Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across: Total ANG cylinder array weight, 1 row (pounds) Total ANG cylinder array weight, 2 row (pounds)

6

5

4

3

3

3

1,505

1,806

1,967

1,927

2,439

3,011

3,011

3,613

3,934

3,854

4,877

6,021

4,516

5,419

5,901

5,781

7,316

9,032

Total ANG cylinder array weight, 2 row (pounds)

In comparison, a 500 kW diesel generator burns approximately 30 gallons per hour at full load. A base tank sized to carry 100% load for 24 hours, plus an additional margin to allow for generator exercise, must hold about 1,000 gallons. The diesel fuel alone weighs 7,100 pounds.

3.10 Material Cost Type I CNG cylinders are the lowest cost CNG cylinder. Constructed entirely of steel, they can be manufactured cost effectively up to 16 inches in diameter. Manufacturing costs increase beyond 16 inches for steel cylinders. Type IV composite cylinders become more cost effective for diameters greater than 16 inches in diameter. Today, Type IV composite CNG cylinders average about $360/cubic-foot of cylinder volume. The construction methods and materials for Type IV are similar to those required for conformable tanks. By comparison, LP tanks rated for 250 lb/in^2 are on the order of $30/cubicfoot of cylinder volume. If we assume a linear relationship between design working pressure and price, a cylinder designed specifically for ANG storage at 500 lb/in^2 working pressure would cost about $55/cubic foot of storage. If ANG gains acceptance in the vehicle sector, $55/CF may be attainable, but that would be some time in the future. For our example design, we’ll assume a more modest price reduction and use $150/CF for ANG cylinder cost in Table 8 (following page), about 60% less than Type IV CNG average.

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Table 8: Estimated cylinder array cost, based on cubic foot of storage volume: Cylinder diameter, inches 500 lb/in^2 ANG Cylinder Estimated Price per CF storage Cylinders Across: Estimated Cylinder Array Cost at 120" length, 1 row Estimated Cylinder Array Cost at 120" length, 2 rows Estimated Cylinder Array Cost at 120" length, 3 rows

10

12

14

16

18

20

$150

$150

$150

$150

$150

$150

6

5

4

3

3

3

$4,909

$5,890

$6,414

$6,283

$7,952

$9,817

$9,817

$11,781

$12,828

$12,566

$15,904

$19,635

$14,726

$17,671

$19,242

$18,850

$23,856

$29,452

Adsorbent is sold by weight and ranges from $1.00 - $2.00/pound for carbon based materials. Pelletized and other engineered shapes (toroids for example) are closer to $2.00/pound while granules and powders tend to be less than $1.50/pound. The example system presumes use of a nominally priced adsorbent at $1.50/pound. Table 9 lists the adsorbent cost for the respective cylinder array configurations. Table 9: Adsorbent material cost (USD): $ 1.50 per pound Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across: Cost of adsorbent material, 1 row Cost of adsorbent material, 2 rows

6

5

4

3

3

3

$1,522

$1,826

$1,988

$1,948

$2,465

$3,043

$3,043

$3,652

$3,977

$3,896

$4,930

$6,087

$4,565

$5,478

$5,965

$5,843

$7,395

$9,130

Cost of adsorbent material, 3 rows

In addition to the cylinders and adsorbent, which have relatively good pricing data available, the regulators, valves, pressure relief devices, monitoring, and structural components to support the cylinders and generator set are more difficult to estimate. The only data point for regulators, PRDs, and valves capable of delivering the flow required for an 809 horsepower engine is the American CNG package noted earlier, approximately $7,125. Add an estimated $3,000 for structural components to construct the cylinder and generator supporting structure, for a total of $10,000 in system integration cost in addition to the cylinders and adsorbent. The total system cost in Table 10a reflects the cylinders, adsorbent, and $10,000 for system integration components. It excludes the optional on-board compressor which has significant price uncertainty today. An on-board compressor designed specifically for ANG is estimated to cost an additional $3,000 to $7,000, expected availability is sometime in 2017. By comparison, a 3,600 lb/in^2 CNG compressor delivering 250 CFH can be purchased today for approximately $16,000.

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Table 10a: Total system cost, including fuel system delivery components ($150/CF ANG cylinder cost): Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across:

6

5

4

3

3

3

Total System Cost, 1 row

$16,430

$17,717

$18,402

$18,231

$20,417

$22,861

Total System Cost, 2 rows

$22,861

$25,433

$26,805

$26,462

$30,835

$35,722

Total System Cost, 3 rows

$29,291

$33,150

$35,207

$34,693

$41,252

$48,583

Looking in terms of the cost per cubic foot of storage, we can see that a 2x5 array of 12” diameter cylinders provides 2.1 hours of runtime and represents the lowest cost alternative that meets the NFPA on-site fuel requirement. Referring back to Table 3, the 2x5 array of 12” cylinders will be approximately 24” high. A walkway around the generator set will not be required. If the ANG cylinder cost did indeed drop all the way to $55/CF, the system cost is reduced significantly, as shown in Figure 10b. Table 10b: Total system cost, including fuel system delivery components ($55/CF ANG cylinder cost): Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across:

6

5

4

3

3

3

Total System Cost, 1 row

$13,322

$13,986

$14,340

$14,252

$15,381

$16,643

Total System Cost, 2 rows

$16,643

$17,972

$18,680

$18,503

$20,762

$23,286

Total System Cost, 3 rows

$19,965

$21,958

$23,021

$22,755

$26,143

$29,929

In general, the cost per cubic foot of storage decreases as the volume increases because the cost of the regulator and valve package is fairly stable regardless of the total volume stored. Table 11: Cost per cubic foot of storage ($150/CF cylinder cost): Cylinder diameter, inches

10

12

14

16

18

20

Cylinders Across:

6

5

4

3

3

3

Cost per CF stored, 1 row

$502

$451

$430

$435

$385

$349

Cost per CF stored, 2 rows

$349

$324

$313

$316

$291

$273

Cost per CF stored, 3 rows

$298

$281

$274

$276

$259

$247

Assessing ANG’s value relative to conventional CNG:    

For comparison, a 3,600 lb/in^2 CNG system with two-hours of storage capacity using ASME cylinders is about $33,000. This does not include a permanently installed compressor.43 A 1000 gallon (24 hour) diesel tank costs about $21,500. The greatest uncertainty is the cost for ANG cylinders or conformable tanks. There simply is no sizable manufacturing capacity in existence today for comparison. With ANG cylinder cost at ~$150/CF, the value case is marginal. The value proposition improves dramatically if the ANG cylinder manufacturing cost trends closer to $55/CF.

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3.11 System Design Schematic Figure 6 depicts the natural gas supply to the generator with the ANG storage operating as a standby fuel (according to NFPA 54 definition). A lockup type regulator with integral or external check valve prevents gas from the ANG storage from flowing back into the gas company’s mains. Dual fuel configurations on smaller generator sets using natural gas as their primary fuel and propane as a standby fuel are implemented with controls similar to those in Figure 6. Using ANG as a backup fuel allows the generator to continue to produce the maximum rated power.44 The drawing shows the compressor permanently installed, but as noted previously, it could be implemented with quick connect fittings and the compressor deployed by the service technician only when the ANG system required refilling. The pressure relief system shows remote temperature sensors at both ends of each cylinder. The importance will be covered in section 4.1, Safety. An ANG system similar to the one below could be deployed with any natural gas fueled generator and operates completely independently from the generator set. In our design example, the physical configuration is constructed to resemble a diesel base tank simply to serve as a familiar point of reference. The ANG storage could be located remotely from the generator set, and in some designs that may be desired or necessary. Figure 6: System Design Schematic (Full size view in Appendix): Radiator and Exhaust Airflow

Normal gas supply secondary regulator, 11" w.c Lockup Type

Utility Gas Supply, 28-55" w.c. Main shutoff

Compressor #1, Permanently installed or portable S1

S2

ANG Supply, Primary Regulator 5 psi into heat exchanger

Pressure Transducer #1 Pressure Transducer #2

Overpressure Relief Valve, Vented to atmosphere

ANG Supply, Secondary Regulator 11" w.c. to engine

ANG Storage Cylinder 400-700 psi ~15-35 CF Heat Exchanger ANG Storage Cylinder 400-700 psi ~15-35 CF

ANG Storage Cylinder 400-700 psi ~15-35 CF

Pressure Relief System. Valves at top end of each cylinder, remote temperature sensors at far end of each cylinder.

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3.12 Sequence of Operation At initial commissioning of the ANG storage system, the compressor (permanently installed or portable) fills the cylinders. Depending on the amount of storage and the size of the compressor, this may take 24-36 hours. Once the initial fill is completed, the control system will monitor the pressure in the ANG cylinders and when equipped, start the on-board compressor to “top off” the system as necessary. The compressor may be locked out when the gas supply is being used to run the generator. Alternatively, fuel pressure in the ANG storage system can be monitored remotely. A portable compressor can be deployed to the location to top off the ANG system when required. The ANG storage is designed to maintain the engine’s fuel supply should the pipeline gas supply fail entirely or drop below the minimum pressure required. The switchover from pipeline gas to the ANG storage must be seamless, sustaining engine speed while under load. Pressure Transducer #1 monitors the gas company’s pipeline supply pressure. When the normal pipeline gas supply is available at an acceptable pressure ahead of the first stage regulator (>15” w.c. for example) the solenoid valve from the ANG supply (S2 in Figure 6) remains closed. Pressure sensing of the commercial natural gas supply is done ahead of the second stage regulator to provide more reliable detection of pressure drop. Figure 7: Sequence of Operation (Full size view in Appendix): Close solenoid valve S2

Alarm cleared: External gas supply restored. Engine connected utility gas supply.

Open solenoid valve S2

Alarm condition: Engine connected to ANG system.

YES PT #1 at or above min. pres?

ANG Storage Ready

Alarm condition: External gas supply failure.

NO

PT #1 at or above min pres, >1 min?

YES

Wait until generator stops to attempt ANG refill.*

YES

NO Generator ready to run or running on ANG supply.

Is generator running?* NO

PT #2 below max fill pres?

Close solenoid valve S2, open solenoid valve S1.

YES

NO

PT #2 below min YES operating pres?

Engine Stop. ANG Storage Empty. External gas supply failure.

NO Start & Run compressor to fill ANG storage cylinders.

PT #1 at Or above min. Pres.?

YES

PT #2 at max fill pres?

Compressor stop. Gas supply fail.

YES

NO Stop compressor. Close solenoid valves S1 and S2.

NO

Notes: NO

Generator start detected?* YES

* Default setting: ANG system will not attempt to refill while the generator is running. This is to ensure sufficient gas supply to the generator set. If sufficient gas supply volume is available to refill while the generator is running, this check can be disabled.

On the low pressure side of the regulator, it becomes impractical to differentiate between a transient pressure drop caused by a large load step applied to the generator and a genuine commercial gas supply outage. Should the pressure from the commercial gas supply drop below the acceptable range (anywhere from 15” w.c. up to 5 lb/in^2 depending on the gas

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supply system design), solenoid valve S2 will open. The ANG storage will begin supplying gas until such time as the stored gas is exhausted, the commercial gas supply returns to an acceptable pressure, or commercial power returns and the generator shuts down. The compressor will start to refill the ANG storage after the generator has stopped (commercial power restored) and acceptable gas pressure is detected at Pressure Transducer #1. Note that disabling the gas compressor while on generator power is a decision that could be driven by the available gas supply volume. There may be conditions where the utility is unable to supply sufficient volume to run the generator set and refill the storage system simultaneously. Given that situation, the gas compressor would be disabled while operating on generator power. Similarly, depending on the size of the generator and the gas compressor, the compressor load may represent a substantial portion of the generator capacity. The end user may not want to size the generator large enough to run the building load and cover the remote possibility of a simultaneous need to run the gas compressor. The decision is one of design constraints at a specific location, not a limitation of the gas storage system.

4 Operational Considerations The use of ANG storage as a backup on-site fuel source for emergency power systems will be perceived as “new” in the industry. Even though the natural gas fueled vehicle industry has used nearly all of the same technology for more than a decade, overcoming the perceived safety risks and unfamiliarity with the technology is a prerequisite to market acceptance.

4.1

Safety

In early conversations about using ANG storage for emergency power, one engineer commented to the author, “A bomb underneath a generator set? Yes, that will go over well with the building inspector.” It will take time, testing, and demonstrated successes through pilot projects to change attitudes. The following discussion uses CNG vehicles as a proxy for quantifying risk because industry and the engineering community have substantial experience and historical data within the CNG vehicle sector. CNG fueling stations could be considered, but still the numbers worldwide are small. In addition, the regulations typically view fuel dispensing and fuel use differently. No perfect comparison exists today. It is instructive to note that stationary uses of CNG avoid the greatest risk of fire and compromised cylinder integrity – being out on the road in traffic. All stored fuel presents some safety risk; unknown or unfamiliar risks are often perceived as “more dangerous” than risks we have dealt with for long periods.  



Diesel fuel is perceived as “safe” because it does not form an explosive mixture in the open air, spilled fuel is difficult to ignite at room temperature. Spilled gasoline easily vaporizes at room temperature into a flammable and potentially explosive mixture at vapor concentrations of 1.0% – 7.6%. However, we deal with it daily and think little about its presence even in enclosed areas like underground parking garages. Propane is stored under pressure and the vapor is heavier than air; a concentration between 2.1% – 9.5% is explosive. Leaking propane gas will tend to accumulate in the

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

lowest spot, even outdoors, sometimes with disastrous results. Still, millions of backyard LP grills operate without incident.45 Compressed natural gas has a wider explosive range, 5% - 15%, but the gas is lighter than air and will tend to dissipate naturally before the lower explosive limit is reached.

Stationary emergency applications using CNG or ANG are so small in number that it is impractical to make statistically meaningful measurements on safety. However, there are approximately 22 million CNG and LP fueled vehicles on the roads worldwide. Aside from a small “CNG” or “LP” decal on the fuel door, the vehicles are often indistinguishable from their gasoline or diesel fueled counterparts. In 2014, Berghmans and Vanierschot conducted a study on the fire risk associated with gasoline, LP, and CNG fueled vehicles.46 The study examines the relative fire risk posed by each fuel type. There are two main risks associated with a CNG fueled fire:  

Cylinder or fuel delivery system leak where the gas attains a concentration within its explosive limits and finds an external ignition source. The result is a gas explosion. Cylinder relief valve operation due to flame impingement (vehicle catches fire due to some other causal factor). The result is a jet flame that can extend up to 25 feet from the vehicle. The flame decays in approximately one minute as the gas is vented from the tank.

Data showed that the probability of an event occurrence on any single CNG vehicle was 1.86x10-6 / year; or about 41 vehicle fires per year out of the population of 22 million. While the risk mechanisms are different, the probability of a CNG fuel related fire event was found to be nearly the same as a gasoline fueled vehicle. In relative terms, the risk of an LPG related incident was 72 times greater compared to CNG, due to the possibility of overfilling LPG cylinders. ANG storage uses similar safety devices and cylinder designs. In addition, the adsorbent material itself represents a large heat sink that will slow the temperature and corresponding pressure rise in the cylinder during flame impingement. ANG cylinder pressures are on the order of 500 lb/in^2 compared to 3,600 lb/in^2 for CNG. A jet fire from a venting ANG cylinder would last longer as gas desorbs and the cylinder pressure drops rapidly due to auto-refrigeration; but the flame would extend a much shorter distance from the source. CNG vehicle fires that result in cylinder failure are rare. In one incident, the cylinder failure in an Indianapolis truck fire in 2015 was due to firefighting operations. Hose streams cooled the emergency relief valves so they did not operate while the opposite end of the cylinder, unreachable with a hose stream, weakened and failed due to flame impingement.47 A US Department of Transportation study48 completed in March 2013 examined existing Federal Motor Carrier Safety Regulations in comparison to NFPA 52 (discussed earlier) and SAE J2406, Recommended Practices for CNG Powered Medium and Heavy Duty Trucks. The study also reviewed National Fire Incident Reporting System (NFIRS) records from 1999 to 2009 and Clean Vehicle Education Foundation (CVEF) data from 1976 to 2010 for incidents involving natural gas fueled vehicles. NFIRS is a U.S. based standardized reporting system, CVEF is international. From a regulatory perspective, the study concurs that NFPA 52 is a valid starting point to ensure consistent safety, but gaps remain as technology continues to evolve. NFPA 52 is a Michael Hainzl

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consensus standard, and so it poses challenges for manufacturers to standardize equipment when the methods to implement a particular safety objective are open to interpretation. Even within the natural gas vehicle sector, industry recognizes that regulatory gaps must be addressed to enable more widespread adoption. Extending CNG or ANG to stationary applications using NFPA 52 as a foundation has similar challenges not with the technology, but implementing the requirements in a consistent manner acceptable to AHJs nationwide. Incidents involving CNG totaled 163 from the NFIRS data and 138 from the CVEF data covering more than a decade.49 Within the dataset, 14 incidents resulted in cylinder rupture as a result of faulty pressure relief valve (PRV) operation. Analysis of those specific incidents showed that larger cylinders had a portion of the cylinder weakened by flame impingement, while the PRV itself was not heated enough to cause the gas to release. The USDOT study recommends PRV systems with sensors at multiple exterior cylinder locations to detect localized flame impingement that may not heat the opposite end of the cylinder enough to cause PRV operation and depressurization. Distributed heat detection along the length of each cylinder, with the ability to trigger PRVs, should be implemented in stationary ANG storage systems. A study completed by the National Highway Transportation Safety Administration, Advanced Fuels Crash Safety50, identified mechanical damage, chemical exposure, and improper installation as contributing factors leading to damage or failure of sample cylinders subjected to non-destructive and destructive testing. The data further supports that proper cylinder manufacturing techniques, installation, operation, and maintenance practices are critical for safe operation of CNG containers. Many of the operational and environmental risk factors that can contribute to fire, cylinder damage or failure, in the previously cited sources can be more effectively controlled in stationary applications. For example, the risk of impact damage from road debris, traffic accidents, and the corrosive effects of road deicing compounds are significantly reduced.

4.2

Scalability

The tables in Section 3 identified a number of configurations that can meet the two-hour runtime requirement at 100% load on a 500 kW natural gas generator set. ANG storage can be easily scaled to meet the desired runtime for a given engine size. The working pressure of the ANG system is a factor in the economy and scalability. For the alternative fuel vehicle industry, 500 lb/in^2 working pressure is driven by economics; the ability to build low-cost compressors for home-fueling and the desire to use conformable tanks. For stationary power applications, higher working pressure on the order of 1000 lb/in^2 with current carbon based adsorbents, offers ANG working capacity on the order of 200-250 Vd/V. That value approaches 3,600 lb/in^2 CNG while still offering some cost advantage on cylinder construction. We discussed the implications of ASME storage tubes compared to DOT FMVSS 304 cylinders; cost and service life will be an important decision point. ASME storage tubes offer an unlimited service life, but at a much higher cost. DOT cylinders are lighter, but composites (Type II-IV) are currently limited to a 15 or 20 year service life. It would not be economical to have a DOT cylinder reach the end of its service life with five or more years on the generator set yet to go. A practical option would be to revisit the DOT cylinder requalification and service life limitations while operating at a lower pressure associated with ANG, between 500-1000 lb/in^2. A change

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in the regulations extending the service life of DOT cylinders to 30 years, for example, would provide immediate access to mature manufacturing resources and offer end users an ANG storage option with a life-cycle longer than the generator set. Consideration should be given to a portable compressor for filling smaller systems in order to reduce cost. The permanently installed, automatic compressor becomes less of a factor at lower ANG operating pressures of 500 lb/in^2. Use of higher operating pressure to better utilize the adsorbent’s storage capacity will drive compressor cost. At some pressure threshold, a permanently installed compressor will not be economically feasible.

4.3

Life-Cycle Costs

A valid comparison of ANG system life-cycle cost must consider a stationary emergency power application where on-site fuel is required by code. The ANG storage is a standby fuel, used only if the pipeline natural gas supply fails. When an emergency power system requires on-site fuel, the traditional solution has been to use diesel fuel. A diesel fueled emergency power system is sized to run for at least 24 hours. In some instances, 72-96 hours of runtime are desired or required by local codes. For larger emergency power systems, on-site diesel storage can exceed 10,000 gallons. In almost all cases, only a small portion of the total fuel load is burned in a given year for engine exercise and actual commercial power failures. If diesel fuel is not used within 6-12 months of manufacture, it must be maintained to remain usable. Diesel fuel storage comes at a cost as well. Using a diesel variation on the 500 kW design example, a 1000 gallon diesel tank capable of supporting 24 hour runtime will cost approximately $21,500. Once the on-site fuel requirement is established, there is a cost. Some will argue that 24 hours of diesel storage is not the same as 2 hours of ANG storage and therefore not a valid comparison. However, the diesel generator will burn diesel whenever it runs. The ANG storage on a natural gas fueled generator is only used given the remote probability of a pipeline natural gas supply failure concurrent with a commercial power outage. In terms of statistical availability, natural gas with on-site ANG storage wins over diesel. Section 2.1 briefly describes diesel fuel maintenance; polishing the fuel once per year and fuel quality testing at least twice per year. The polishing process circulates diesel fuel from the tank through a series of filters to remove contaminants and desiccant media to remove accumulated moisture. Preservatives and algaecide are added to the fuel before it is returned to the main storage tank. The cost is approximately $2/gallon, or $2,000/year to maintain a 1,000 gallon diesel storage tank. ANG storage eliminates the need for annual diesel fuel maintenance. Assuming a 5% cost of capital, the net present value of diesel fuel maintenance alone over a 20 year life-cycle is approximately $25,000. Recall the lowest cost 2.1 hour ANG storage option is estimated to cost about the same. ANG storage system maintenance consists primarily of periodic inspections. At the time of writing, the nationwide average retail diesel fuel price is $2.53/gallon and natural gas for commercial use is $7.58/million BTU.51 A 500kw diesel generator running at full load, consuming 30 gallons per hour will cost $75.90/hr. for fuel. A 500kw natural gas generator running at full load, consuming 6,000 cubic feet per hour will cost $45.48/hr. for fuel; about 40% less than diesel. ANG storage enables an end user to take advantage of the lower natural gas fuel cost, even when there is an on-site fuel requirement.

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While it is beyond the scope of this work, additional factors in the natural gas vs. diesel life-cycle cost include:   

Economic benefit from participating in utility based demand response programs. Given today’s emissions regulations, it is typically uneconomical to participate using on-site diesel generation, but very economical for natural gas generation. Environmental insurance related to on-site diesel fuel storage. Community health benefits associated with the reduced emissions from natural gas engines.

Natural gas and diesel fueled generator sets have approximately equal initial capital costs of $260/kw of output up to about 150kw. With larger engine sizes however, the cost per kilowatt increases on natural gas to $410/kw compared to $190/kw for diesel at the 500kw node. A 500kw natural gas generator set will cost approximately $110,000 more than a comparably sized diesel unit. In spite of a natural gas engine’s higher capital cost, the ability to leverage ANG storage to meet an on-site fuel requirement enables end users to take advantage of reduced maintenance and operational costs, the opportunity to participate in demand response, and reduce emissions using natural gas a fuel source.

4.4

End of Life – Recycling and Disposal

The components used in ANG storage are relatively benign to the environment and recyclable:  Steel and/or aluminum cylinder material are non-hazardous and recyclable  Carbon based adsorbents start as natural elements and can be regenerated or landfilled once all the methane has been burned off or desorbed.

5 Summary & Recommendations An objective of this paper was to demonstrate through a practical design example, the potential of ANG storage applied to stationary emergency power generation. There is a vast amount of ongoing research on adsorbents and lightweight conformable tanks. The references cited in this work barely scratch the surface of what is available. The technology has already been applied on vehicles and there is no technological reason prohibiting its application to stationary emergency power generation.

5.1

Regulatory Changes to Support ANG Technology

As noted in the USDOT paper, NFPA 52 is a logical foundation for regulation, but it remains a consensus standard. It is also targeted to CNG fueling stations and vehicles. A 2014 study by the National Renewable Energy Laboratory also identified the lack of consistent regulations and standards as an impediment to more widespread adoption of natural gas in transportation and other industrial sectors.52 There are gaps that need to be closed to include stationary use of CNG or ANG storage systems. Standardized regulations at the Federal level will assist industry in development of components and systems that can be widely deployed and drive economies of scale. On the enforcement side, it will assist inspectors at AHJs who will be largely unfamiliar with the technology.

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5.2

Next Steps for Industry and the Engineering Community

The best way to address the regulatory gaps and start the long road to market acceptance is to begin building demonstration systems. The large North American manufacturers of stationary power systems, Caterpillar, Cummins, Generac, and Kohler; have engineering resources and test facilities capable of doing the work. Costs and market acceptance must be addressed simultaneously, and unfortunately that presents a “chicken and egg” problem. Manufacturers will be unwilling to invest in developing a technology that has unproven market acceptance or demand. The engineering community will be reluctant to base designs on an uncertain regulatory environment on a technology that is perceived as “new”. Stationary power system manufacturers should partner with the research community, companies developing engineered carbons, and regulatory advisory boards to drive awareness in the engineering community and the industry.

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6 Appendix 1 – Full Size Functional Block Diagram Full size view of Figure 6.

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Full size view of Figure 7.

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7 Glossary AHJ – Authority Having Jurisdiction ANG – Adsorbed Natural Gas ANSI – American National Standards Institute ASME - American Society of Mechanical Engineers BTU – British Thermal Unit CF and CFH – Cubic Foot (per hour), common usage in natural gas supply context CNG – Compressed Natural Gas CVEF – Clean Vehicle Education Foundation EPA – Environmental Protection Agency EPS – Emergency Power System FMVSS - Federal Motor Vehicle Safety Standards GW – Gigawatt HDPE – High Density Polyethylene kW – Kilowatt LP – Liquid Propane, used even when delivered in gaseous state MAQ - Maximum Allowable Quantity per Control Area MMBTU – Million British Thermal Units MOF – Metal Oxide Framework MW – Megawatt NEC – National Electric Code NFIRS – National Fire Incident Reporting System NFPA – National Fire Protection Agency PRV – Pressure Relief Valve RICE NESHAP – Reciprocating Internal Combustion Engines, National Emissions Standard for Hazardous Air Pollutants SCF – Standard Cubic Feet (atmospheric pressure) ULSD – Ultra Low Sulfur Diesel USD – United States Dollar, used throughout USDOT – United States Department of Transportation V/V – Volume at standard pressure to volume of storage vessel. Absolute uptake in the context of ANG Vd/V – Volume delivered at specified working pressure to volume of storage vessel. The amount of gas desorbed during a discharge cycle to a specified low pressure threshold. w.c. – Water Column (inches of)

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8 Endnotes 1

https://www.ferc.gov/legal/fed-sta/exp-study.pdf US Department of Energy, 2007. The Potential Benefits of Distributed Generation and Rate-Related Issues That May Impede Their Expansion, pp.34-35 2

https://www.epa.gov/diesel-fuel-standards Diesel Fuel Standards, EPA web page.

3

https://energy.gov/sites/prod/files/2013/04/f0/Northeast%20Storm%20Comparison_FINAL_041513b.pdf, US Department of Energy, Office of Electricity Delivery and Energy Reliability. Comparing the Impacts of Northeast Hurricanes on Energy Infrastructure. Specifically, Table 3 on page 11 notes the number of natural gas compressor stations impacted during Hurricane Sandy (1) in contrast to the number of petroleum terminals (62). 4

https://www.epa.gov/stationary-engines RICE-NESHAP: Reciprocating Internal Combustion Engines, National Emissions Standard for Hazardous Air Pollutants. 5

https://www.epa.gov/stationary-engines/fact-sheet-final-air-toxics-standards-neshap, Fact Sheet, Final Air Toxics Standards. In addition to the air-toxics, the EPA estimates public health benefits by avoiding:  110 to 270 premature deaths,  75 cases of chronic bronchitis,  170 nonfatal heart attacks,  160 hospital and emergency room visits,  180 cases of acute bronchitis,  15,000 days when people miss work,  1,900 cases of aggravated asthma, and  87,000 acute respiratory symptoms.  6 http://www.dieselfueldoctor.com/blog/?p=87 A brief description of fuel polishing and fuel filtration. 7

Christensen, E., McCormick, R., Sigelko, J., Johnson, S. et al., "Impact of a Diesel High Pressure Common Rail Fuel System and Onboard Vehicle Storage on B20 Biodiesel Blend Stability," SAE Int. J. Fuels Lubr. 9(1):2016, doi:10.4271/2016-01-0885. Available at: http://www.nrel.gov/docs/fy16osti/65397.pdf 8

Diesl Fuel Storage and Handling Guide. Coordinating Research Council, Inc. 2014. Available at: https://crcao.org/reports/recentstudies2014/CRC%20667/CRC%20667.pdf 9

https://www.epa.gov/emergency-response/when-are-you-required-report-oil-spill-and-hazardous-substance-release, Spill reporting requirements are covered under RCRA and the CWA. 10

Judson, N. "Interdependence of the Electricity Generation System and the Natural Gas System and Implications for Energy Security." Lincoln Laboratory, Massachusetts Institute of Technology (May 2013): n. pag. Web. 5 Jan. 2016. https://www.ll.mit.edu/mission/engineering/Publications/TR-1173.pdf 11

Industrial natural gas generators from major US based manufacturers:  Caterpillar: http://www.cat.com/en_US/products/new/power-systems/electric-power-generation/gasgenerator-sets.html  Cummins: http://power.cummins.com/commercial-industrial/generators  Generac: http://www.generac.com/resources-and-tools/engineer-resources/product-downloads  Kohler: https://power.kohler.com/na-en/generators/industrial/products/Gaseous+Generators

12

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards. This is the entire list of codes and standards published by the NFPA. 13

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=70 . NFPA 70 is more commonly known as the National Electric Code, or simply “NEC”. 14

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=99. NFPA 99 governs a wide range of design and operational aspects of health care facilities. Health care is typically an application where there are no exceptions to the on-site fuel storage requirement.

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http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=101 NFPA 101 addresses requirements for egress, fire protection, sprinkler systems, alarms, emergency lighting, smoke barriers, and special hazard protection. 16

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=110 NFPA 110 is the foundation for emergency power system design. 17

“Class” is the minimum runtime required, in hours, the system must be capable of operating without refueling. EPS Types and Classes are defined in NFPA 110 Chapter 4. 18

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=37 NFPA 37 address system installation, fuel supplies, lubricating systems, engine exhaust systems, control and instrumentation, and fire protection features. 19

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=52 NFPA 52 covers the design, installation, operation, and maintenance of CNG and LNG fuel systems on all vehicle types--plus their respective compression, storage, and dispensing systems. 20

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=54 NFPA 54 covers the installation and operation of fuel gas piping systems, appliances, equipment, and related accessories, with rules for piping systems materials and components, piping system testing and purging, combustion and ventilation air supply, and venting of gas-fired appliances and equipment. 21

http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-andstandards?mode=code&code=55 NFPA 55 covers fundamental safeguards for the installation, storage, use, and handling of compressed gases and cryogenic fluids in portable and stationary cylinders, containers, and tanks in all occupancy types. 22

http://www.afdc.energy.gov/fuels/natural_gas_locations.html

23

http://www.compositesworld.com/articles/the-outlook-for-composite-pressure-vessels

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FMVSS 304 and NGV2 set the 15-year service life and no hydrostat testing option allowed to extend life.

25

http://blog.fibatech.com/cng/cng-storage/ The information and photos in this section are from the FIBA Technologies website, a manufacturer of CNG cylinders and other system components. The cited page provides an extensive overview of the decision factors which go into cylinder type selection. 26

http://www.compositesworld.com/articles/the-outlook-for-composite-pressure-vessels In terms of relative weight/cost benefits for the different cylinder types:  Type I vessels are the least expensive, with estimated production costs of roughly $5 per liter of volume. The metalworking skills and equipment needed to produce them are widely available internationally. To their detriment, Type I vessels also are the heaviest, weighing approximately 3.0 lb/l (1.4 kg/l).  Type II vessels cost about 50 percent more to manufacture but can reduce the weight of the storage containers by 30 to 40 percent.  Type III and Type IV vessels take the weight savings even further, weighing between 0.75 and 1.0 lb/l (0.3 to 0.45 kg/l). The cost of Type III and Type IV vessels, however, is roughly 2 times greater than Type II vessels and 3.5 times greater than the all-metal Type I tanks. 27

http://www.bauercomp.com/en/products-solutions/cng/micro-series This is one of the smallest commercial grade compressors available on the market today. 28

Nie, Zhengwei, Yuyi Lin, and Xiaoyi Jin. "Research on the theory and application of adsorbed natural gas used in new energy vehicles: A review." Frontiers of Mechanical Engineering (May 2016): n. pag. Web. https://www.researchgate.net/publication/303091601_Research_on_the_theory_and_application_of_adsorbed_natur al_gas_used_in_new_energy_vehicles_A_review This is an excellent paper that is easily read by a broad audience on the theory and application of ANG. 29

R. A. Munson and R. A. Clifton, Jr., Natural Gas Storage with Zeolites, U.S. Dept. of the Interior, 1971.

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http://pubs.rsc.org/en/content/articlehtml/2014/sc/c3sc52633j#cit10a Evaluating metal–organic frameworks for natural gas storage. 31

Chang, K. J., , & Talu, O. (1996). Behavior and performance of adsorptive natural gas storage cylinders during discharge. Applied Thermal Engineering, 16(5), 359-374. http://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1057&context=encbe_facpub 32

Nie, Zhengwei, Yuyi Lin, and Xiaoyi Jin. "Research on the theory and application of adsorbed natural gas used in new energy vehicles: A review." 33

Li, Bin, Hui-Min Wen, Jeff Xu, and Banglin Chen. "Porous Metal-Organic Frameworks: Promising Materials for Methane Storage." Chem (October 13, 2016): 557-80. Web. 12 Nov. 2106. http://www.cell.com/chem/references/S2451-9294(16)30154-1 34

"BrightBlack." Brightblack. N.p., n.d. Web. 06 Apr. 2017. http://www.entegris-brightblack.com/applications/adsorbed-natural-gas-storage

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Kumar, K. Vasanth, Kathrin Preuss, Maria-Magdalena Titirici, and Francisco Rodríguez-Reinoso. "Nanoporous Materials for the Onboard Storage of Natural Gas." Chemical Reviews 117.3 (2017): 1796-825. Web. http://pubs.acs.org.libdb.njit.edu:8888/doi/pdf/10.1021/acs.chemrev.6b00505 36

http://gens.lccdn.com/generaccorporate/media/library/content/all-products/generators/industrialgenerators/gaseous/500kw/generac-product-500kw-gaseous-industrial-generator-model-sg500_0l5796.pdf 37

NGV Solutions MCH-5 CNG compressor. http://www.ngvsolutions.com/products/compressors

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Phone and e-mail conversation with Gary Fanger, Cenergy Solutions, February 2017.

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American CNG Class 8 Package, 103 GGE, http://www.americancng.com/packages/truck-highpressurepackage103-gge.html. 40

The City of New York requires six hours of on-site fuel storage for emergency systems. Local Law 111, Subsection 700.12(B)(2), https://www1.nyc.gov/assets/buildings/local_laws/ll111of2013.pdf 41

The weight and example pricing were gathered from the following online sources: http://www.cngschool.com/cng-tank-price-list http://www.americancng.com/cng-cylinders/type-1-category.html http://hansontank.us/lpg-tanks.html

Approximations were made where an exact match for the application was not available. 42

One example of the range of adsorbent materials available and their performance characteristics: http://www.tigg.com/vapor-phase-media.html

43

Phone conversation with Chris Fink, FIBA Technologies, April 6, 2017. A 20’ ASME tube containing just over 12,000 SCF at 3,600 lb/in^2 will cost approximately $23,000. Include $10,000 additional for regulators and other accessories (same as the ANG option), for a total of $33,000. 44

On smaller natural gas fueled generator sets, typically on the order of 150kw or less, propane gas can be configured as a standby fuel using controls similar to those shown in Figure 4. With a loss of pipeline natural gas, the engine automatically switches over to propane from an onsite LP tank. Unfortunately, because of the lower ignition temperature of propane, engines must be derated when running on propane. For small generators, this may be 10% or less and is usually tolerable. For larger engines using higher turbo boost pressures, the power derate when operating on propane as a backup fuel can be 25% or more and is often unacceptable to the end user 45

NFPA repots there are about 7,000 gas grill fires every year, but most are due to operator error. Very few are due to equipment failure. In the extremely rare case when a cylinder does vent, it almost never results in an explosion. 46

Berghmans, J., and M. Vanierschot. "Safety Aspects of CNG Cars." Procedia Engineering (2014): 33-46. Web. https://doi.org/10.1016/j.proeng.2014.10.407

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http://www.fireengineering.com/articles/print/volume-169/issue-7/features/commercial-cng-vehicles-a-whole-newdanger-for-responders.html 48

Natural Gas Systems: Suggested Changes to Truck and Motorcoach Regulations and Inspection Procedures." U.S. Department of Transportation, Federal Motor Carrier Safety Administration, n.d. Web. 20 Mar. 2017. https://ntl.bts.gov/lib/51000/51300/51333/Natural-Gas-Systems-Report-508.pdf 49

From the USDOT Study, page 17; root cause of fire incidents that resulted in cylinder rupture:  Manufacturing defect, 8  PRD altered or missing, 3  PRD did not release in a fire, 14  Impact damage to cylinder, 4  Cylinder corrosion or stress cracks, 11  Other cylinder damage, 3  Overpressure at fuel station, 3  Other, 4

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Natural Gas Systems: Suggested Changes to Truck and Motorcoach Regulations and Inspection Procedures." U.S. Department of Transportation, Federal Motor Carrier Safety Administration, n.d. Web. 20 Mar. 2017. https://ntl.bts.gov/lib/51000/51300/51333/Natural-Gas-Systems-Report-508.pdf 51

Price data from U.S. Energy Information Administration. Retail diesel fuel price: https://www.eia.gov/petroleum/weekly/ Natural gas price data: https://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm

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Zigler, Brad. "Possible Pathways for Increasing Natural Gas Use for Transportation." National Renewable Energy Laboratory (October 15, 2014): n. pag. Web. 7 Sept. 2016. http://www.nrel.gov/docs/fy15osti/63322.pdf

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