A fossil-fuel based recipe for clean energy

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A fossil-fuel based recipe for clean energy Surendra K Saxena, Vadym Drozd, Andriy Durygin CeSMEC (Center for the Study of Matter at Extreme Conditions), College of Engineering and Computing, Florida International University, Miami, FL 33199, USA

art i cle info

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Article history:

A zero-emission process of hydrogen production from fossil fuel through a system of

Received 15 December 2007

reactions involving hydroxide, carbon, CO, CO2 and water is described here. It provides for a

Received in revised form

complete sequestration of carbon (CO2 and CO) from coal/natural-gas burning plants. The

27 April 2008

CO and or CO2 produced in coal or natural gas burning power plants and the heat may be

Accepted 28 April 2008

used for producing hydrogen. Economically hydrogen production cost is less than the

Available online 17 June 2008

current price of fossil-fuel produced hydrogen with the added benefit of carbon

Keywords: Hydrogen Carbon-sequestration

sequestration. The reduced cost of the hydrogen may aid in making a hydrogen fueled automobile economically viable. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Coal-burning power plant

1.

Introduction

Steam methane reforming is the most common and the least expensive method to produce hydrogen at present [1]. Coal can also be reformed to produce hydrogen, through gasification. Hydrogen production by CO2-emitting-free methods are either more expensive compared to those using fossil fuel or are in the very early stages of development. Examples are the methods proposed by Gupta et al. [2], the zero-emission coal technology (ZEC) by Zlock et al. [3,4], GE’s fuel-flexible technology [6] and several others [7–9]. Since United States has vast proven coal reserves, coal based technology of hydrogen production is very attractive. However, effective and low cost carbon sequestration technology has yet to be developed. Hydrogen is regarded as the energy for future but to produce and use hydrogen either by direct combustion or in a fuel cell, we need to use other sources of energy. Thus hydrogen or use of any material in producing energy cannot be an environmentally clean and economically viable solution unless we sequester carbon. We may eventually have the hydrogen solution for our transportation and other energy

reserved.

uses. However, such energy will continue to be dependent on the use of fossil fuel for long time and may not be economic. To turn things around, we have to use alternate methods of using coal, producing hydrogen and hydrides. Many hydrides are currently under consideration for use in on-board generation of hydrogen and the cost of producing the hydride is an important consideration. Coal is used extensively in producing synthetic fuels [1]. Use of coal in gasifiers is well established and hydrogen may be produced by the reaction: C+2H2O ¼ CO2+2H2. Gasifiers are operated between 500 and 1200 1C, and use steam, oxygen and/or air and produce a mixture of CO2, CO, H2, CH4 and water. The CO produced can be further processed by the shiftgas reaction to produce H2 with production of CO2: CO+H2O ¼ CO2+H2. The following is an extract from a report by National Academy of Engineering, Board on Energy and Environmental Systems [5] and shows the importance of the present study: ‘‘At the present time, global crude hydrogen production relies almost exclusively on processes that extract hydrogen from fossil fuel feedstock. It is not current practice to capture and store the by-product CO2 that results from the production of hydrogen from these feed stocks. Consequently,

Corresponding author.

E-mail address: [email protected] (S.K. Saxena). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.050

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more than 100 Mt C/yr are vented to the atmosphere as part of the global production of roughly 38 Mt of hydrogen per year.’’ It would then appear that when coal is used in gasifiers or in direct burning in power- and other manufacturing-plants, CO2 and CO are prominent among other gases released to

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atmosphere. Their emission is not only harming the environment but as considered here is also a waste of resources. For industry this has been an economic issue. This study will provide a clear economic incentive to sequester carbon (CO2 and CO) without significantly affecting our current modes of operations.

Fig. 1 – (a) Phase equilibrium in the system Ca(OH)2+C+H2O; (b) equilibrium in the system 2NaOH+C+H2O; (c) equilibrium composition in the system 2NaOH+CO; (d) equilibrium in the system 4NaOH+C+CO2; and (e) the gas-shift reaction produces a more complex gas composition and at higher temperature than reactions (2)–(4).


2.

Reactions

We may divide the hydrogen producing reactions in two categories. The first category reactions are for the gasification process unrelated to coal-burning power plants. These are: CaðOHÞ2 ðsÞ þ CðsÞ þ H2 OðgÞ ! CaCO3 ðsÞ þ 2H2 ðgÞ;

DH ¼ 606 kJð627 CÞ

2NaOHðsÞ þ CðsÞ þ H2 OðgÞ ! Na2 CO3 ðsÞ þ 2H2 ðgÞ; DH ¼ 645:8 kJð600 CÞ

(1)

(2)

The use of the following reactions may be considered in relation to coal-burning power and other industrial plants: 2NaOHðsÞ þ COðgÞ ! Na2 CO3 ðsÞ þ H2 ðgÞ; DH ¼ 119 kJð600 CÞ

(3)

4NaOHðsÞ þ CðsÞ þ CO2 ðgÞ ! 2Na2 CO3 ðsÞ þ 2H2 ðgÞ;

DH ¼ 662 kJð600 CÞ

(4)

Sodium in the above reactions may be replaced by potassium. We performed both thermodynamic equilibrium calculations using the software FACTSAGE and the databases therein and conducted several experiments for verification. Reaction (1) was considered by Saxena [10] and Xu et al. [11]. In the reaction, the gas is multicomponent and hydrogen yield is only partial (see Fig. 1a) (Moles H2:H2O:CH4:CO ¼ 1.5:.27:. 113:.044). Reaction (2) was proposed by Saxena [6] and although endothermic, it produces a much cleaner hydrogen yield than reaction (1) (Fig. 1b) and over a wider temperature range. Reactions (3) and (4) are exothermic. Reaction (4) can be considered as a combination of the Boudouard reaction: C þ CO2 ! 2CO

(5)

and reaction (3). Reaction (4) may also be considered as a combination of 2NaOH þ CO2 ! Na2 CO3 þ H2 O

(6)

and 2NaOH þ C þ H2 O ! Na2 CO3 þ 2H2

(2)

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An equilibrium calculation (Fig. 1b) shows that Na2CO3 also known as soda ash and hydrogen are produced over a wide temperature range starting from 100 to 800 1C. Similar compositions result by the use of reactions (3) and (4) (Fig. 1c and d). We may compare reactions (2)–(4) with the gas-shift reaction (CO+H2O ¼ CO2+H2), which differs only in the form of introduction of water. It is quite clear that there is a significant advantage in using reactions (2)–(4) over the gasshift reaction (Fig. 1e). Reaction between sodium hydroxide and carbon monoxide yielding sodium formate was described by Berthelot in 1856. When heated above 250 1C, sodium formate transforms into oxalate with release of hydrogen: 2HCOONa ! Na2 C2 O4 þ H2

(7)

In 1918 Boswell and Dickson [12] demonstrated that when carbon monoxide is heated with excess of sodium hydroxide at temperatures at which formate is transformed into oxalate, oxidation almost quantitatively to carbon dioxide occurs with the evolution an equivalent amount of hydrogen: CO þ 2NaOH ! Na2 CO3 þ H2

3.

(8)

Experimental results

Experiments were conducted to verify the theoretical predictions for reactions (2) and (3) using an in-house method involving measurement of evolving hydrogen (Fig. 2). Anhydrous sodium hydroxide, supplied by Alfa Aesar (97%), was allowed to react with a mixture of carbon and water for reaction (2) and with CO and N2 (carrier gas) for reaction (3). The reaction between carbon, sodium hydroxide and water was carried out in a gas-flow system (Fig. 2). Sodium hydroxide was dissolved using a minimal amount of distilled water in an alumina boat and then activated carbon was immersed into this solution. The alumina crucible was put in a tubular furnace with a quartz tube. Nitrogen gas with a flow rate of 50 ml/min was used as a carrier to deliver steam to the reactor.

Fig. 2 – Equipment for the study of hydrogen generation using laser break down spectroscopy.

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Fig. 3 – (a) Experimental data for reaction (2) (2NaOH(s)+C(s)+H2O (g) ¼ Na2CO3(s)+2H2). Temperature was increased at a rate of 4 1C/min; and (b) experimental data for reaction (3) (2NaOH(s)+CO(g) ¼ Na2CO3(s)+H2(g)). Temperature was increased at rate of 4 1C/min. Two different rates of flow of CO were used. Lower hydrogen yield for higher CO flow could be explained if one takes into account CO disproportionation reaction 2CO-CO2+C, the rate of which depends on the CO partial pressure. Released CO2 will react with sodium hydroxide decreasing amount of the latter available for the reaction with carbon monoxide.

The reaction between NaOH and CO was studied using the same experimental setup. Hydrogen concentration in the effluent gases from the reactor was determined by laser break-down spectroscopy. Before analysis the gases were passed through liquid nitrogen (NaOH/C/H2O reaction) or acetone/dry-ice (NaOH/CO reaction) cooled condenser to remove all hydrogen containing species except for H2 gas. The reactions were first explored with temperature increasing at a fixed rate, reaction (2) between 100 and 700 1C (Fig. 3a) and reaction (3) between 110 and 400 1C (Fig. 3b). Reaction (3) was studied at two different flow rates of CO (10 and 20 ml/min) (Fig. 3b). Both reactions were complete in less than 200 min. Results of isothermal kinetic experiments at

several temperatures are shown in Fig. 4a (reaction 2) and b (reaction 3).

4.

Discussion

For existing power stations, where CO2 is produced, we may choose reaction (4): 4NaOH þ C þ CO2 ! 2Na2 CO3 þ 2H2

(4)

One may compare this reaction with the combination of the gasifier reaction C+2H2O ¼ CO2 +2H2 and the CO2 absorbing reaction 2NaOH+CO2 ¼ Na2CO3+H2O to accomplish similar result. It is shown in Fig. 1b and c that the reaction (4) has


definite advantage being the carbon-sequester and hydrogen producing reaction. A comparison of the figures shows that much higher temperature is required to obtain a significant amount of hydrogen mixed with CO in Fig. 1e than is required when using reaction (4) (Fig. 1d). Catalysis of the reactions, where coal is involved may be needed and has been discussed

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(8)

This reaction is endothermic with DH of 1160 kJ/mol and is largely complete around 1127 1C. Since we rely on coal to provide the heat, the energy cost is not an issue. If we use this reaction to reduce the amount of sodium carbonate produced in reactions (2)–(4), we will further decrease the cost of hydrogen. US tops in CO2-emissions per capita; in 2003, 121.3 metric tons of CO2 were released in the atmosphere. In 2004 the total carbon release in North America was 1.82 billion tons. Worldwide industrial nations were responsible for 3790 million metric tons of CO2 (Kyoto-related fossil-fuel totals). It is clearly not practical to consider that we can sequester all this carbon with reactions (2–4) which would require production of NaOH on a massive scale which would cause further

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in detail in literature [1]. A high production rate would result if the hydrogen is formed by continuous flow processes. As envisaged here, the equilibrium calculations are for a closed system with a complete conversion of fixed ratio of reactants and production of the carbonate and hydrogen. Catalysis and partial conversion of the reactants will affect the costs. Fig. 5 shows the cost analysis. Through reaction (4), we will sequester 11 kg of CO2 for every 40 kg of sodium hydroxide producing 1 kg of hydrogen and 53 kg of sodium carbonate. If we accept the following per kg prices: Sodium hydroxide $0.42 and sodium carbonate 0.36, the material cost of hydrogen production is $ 2.28 per kg giving us an advantage in offsetting the energy costs. The new hydrogen DOE cost goal of $2.00–3.00/gge (delivered, untaxed, 2005$, by 2015) is independent of the pathway used to produce and deliver hydrogen. Better cost calculations are needed to insure the economic viability of the process. Note that less energy is required to electrolyze sodium chloride to produce sodium hydroxide than to produce sodium. It will be necessary to integrate the production of NaOH at the power plants instead of purchasing it from an outside manufacturer. In-house sodium hydroxide manufacture will provide significant shipping cost savings, efficient process integration, and safety. There are many uses of Na2CO3 and as long as the use does not release the CO2 to the atmosphere, the carbon sequestration remains effective. We may also consider the following reaction to use sodium carbonate gainfully: Na2 CO3 ðsÞ þ 2CðgraphiteÞ ! 2NaðgÞ þ 3COðgÞ

300°C

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Fig. 4 – (a) Hydrogen generation in 2NaOH+C+H2O-Na2CO3 + 2H2 reaction studied at different temperatures. N2 carrier gas flow rate 50 mL/min; (b) hydrogen generation in 2NaOH (s)+CO(g) ¼ Na2CO3 (s)+H2(g) reaction studied at different temperatures. N2 carrier gas flow rate 50 mL/min; and (c) hydrogen flow rates in the CO+2NaOH reaction measured at different temperatures and CO flow rate of 20 mL/min and N2 flow rate of 50 mL/min. Hydrogen flow rate vs. time dependence at 300 1C is characterized by quite long (about 3 h) initialization period. However, after 3 h the reaction accelerated. It may be due to the formation on the initial stages of the reaction of some intermediates, which themselves or together with sodium hydroxide melt below 300 1C. The presence of a liquid phase promotes the reaction.

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Fig. 5 – Hydrogen production and carbon sequestration. The analysis depends on the current price structure of sodium products. We may also use the other two reactions (2) and (3).

emission of CO2 if fossil fuel is used in the production. However, in all situations where industry is producing carbon gases and heat anyway, the production of hydrogen according to the reactions presented here, would lead to reduction of carbon in the atmosphere. Most benefit will be obtained if non-fossil sources of energy (hydro-electricity, nuclear-energy, solar and wind) are used for NaOH production. More than 100 Mt C/yr are vented to the atmosphere as part of the global production of roughly 38 Mt of hydrogen per year. Through reaction (4), we will sequester 3 Mt carbon (11 Mt of CO2) for every 40 Mt of sodium hydroxide producing 1 Mt of hydrogen and 53 Mt of sodium carbonate. The US production of NaOH is currently 16 Mt per year. NaOH of 1300 Mt will be needed to sequester all the carbon which is currently emitted in hydrogen production. In this process 33 Mt of H2 will result. Sodium hydroxide is produced (along with chlorine and hydrogen) via the chloralkali process. This involves the electrolysis of an aqueous solution of sodium chloride. The sodium hydroxide builds up at the cathode, where water is reduced to hydrogen gas and hydroxide ion. The total H2 produced in these reactions (reactions 2–4 and electrolysis) if used in automobiles and other energy devices will have a very large effect on CO2-emission. The present work provides a system of reactions to produce hydrogen from sodium hydroxide and CO or CO2 and carbon. The carbon gases are produced in industrial plants burning coal and thus are available at no cost. These gases can also be obtained at relatively high temperature; the reaction of CO or CO2 with sodium hydroxide is exothermic and hence no additional heating would be required. The CO or CO2 would react to form sodium carbonate and hydrogen and thus

carbon will be sequestered. The hydrogen produced cheaply with no carbon release in the atmosphere may be used to synthesize hydrides at low cost.

Acknowledgments The authors’ work is supported through a grant from National Science Foundation (DMR-0231291 to K. Rajan, Iowa State University) and a grant from Air Force (212600548) US Patent files (PCT/US08/55586). R E F E R E N C E S

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