Rev Chem Eng 2017; aop
Tumelo Seadira, Gullapelli Sadanandam, Thabang Abraham Ntho, Xiaojun Lu, Cornelius M. Masuku and Mike Scurrell*
Hydrogen production from glycerol reforming: conventional and green production https://doi.org/10.1515/revce-2016-0064 Received December 13, 2016; accepted July 13, 2017
Abstract: The use of biomass to produce transportation and related fuels is of increasing interest. In the traditional approach of converting oils and fats to fuels, transesterification processes yield a very large coproduction of glycerol. Initially, this coproduct was largely ignored and then considered as a useful feedstock for conversion to various chemicals. However, because of the intrinsic large production, any chemical feedstock role would consume only a fraction of the glycerol produced, so other options had to be considered. The reforming of glycerol was examined for syngas production, but more recently the use of photocatalytic decomposition to hydrogen (H2) is of major concern and several approaches have been proposed. The subject of this review is this greener photocatalytic route, especially involving the use of solar energy and visible light. Several different catalyst designs are considered, together with a very wide range of secured rates of H2 production spanning several orders of magnitude, depending on the catalytic system and the process conditions employed. H2 production is especially high when used in glycerol-water mixtures. Keywords: evolution; glycerol; green; hydrogen; production; reforming.
1 Introduction Energy is a vital part of human civilization, which enables the development of different technologies that we use in *Corresponding author: Mike Scurrell, Department of Civil and Chemical Engineering, University of South Africa, Florida Campus, 1710 Florida, South Africa, e-mail:
[email protected] Tumelo Seadira, Gullapelli Sadanandam and Cornelius M. Masuku: Department of Civil and Chemical Engineering, University of South Africa, Florida Campus, 1710 Florida, South Africa Thabang Abraham Ntho: Advanced Materials Division, Mintek South Africa, 2125 Randburg, South Africa Xiaojun Lu: Material and Process Synthesis Research Group, University of South Africa, Florida Campus, 1710 Florida, South Africa
our daily lives. Most of the world’s energy is produced from fossil fuels, such as petroleum, coal, and natural gas, because of their low cost and availability (Solomon et al. 2009). However, the growing demand for energy in today’s world has put a strain on the limited supply of these fossil fuels, which will be depleted someday. Furthermore, the combustion of these fossil fuels produces greenhouse gases, such as carbon oxides and nitrogen oxides, which are responsible for global climate change and severe health problems for both humans and animals due to environmental pollution associated with toxic byproducts resulting from the combustion of these fossil fuels (Primio et al. 2000). Therefore, the need to replace fossil fuels with alternative renewable, clean, and lower carbon emission energy sources is urgent. The alternative energy sources include wind, hydropower, geothermal, and solar. However, the major drawbacks of wind, hydropower, and geothermal energy is the high cost involved in the construction of these energy plants as well as the difficulties and limitations in operating the plants (Barbier 2002, Yuksel 2010). Among several sources of renewable energy available, solar energy is the most available one by far. Solar energy can be converted to power or heat using solar cells (Parida et al. 2011) or concentrators (Xie et al. 2011). However, the intermittent nature of the sun, such as the intensity of the solar irradiation, which depends on the geographical location and weather, is the major drawback of solar energy. Hydrogen (H2) as an energy carrier is one of the most promising ways of storing solar energy in the form of the chemical bond between two H2 atoms. The H2 molecule can be combusted with oxygen (O2) in air to produce heat and by-product water, which has no carbon footprint to contaminate the environment. Some estimates of the availability of bioenergy may be made by considering the carbon tonnage production via biomass, which is of the order of 1 × 109 tC/km2/year, much of it in the equatorial regions of the planet (McKendry 2002). The total carbon production via biomass production for these regions alone accounts for more than 1000 times our present carbon consumption in the form of crude oil. Biomass is, in this sense, very available but, of course, demands further chemical processing to obtain useful fuels, which
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2 T. Seadira et al.: Glycerol to hydrogen satisfy the combustion requirements of today’s engines. In essence, the challenge of biomass-derived fuel conversion is that of O2 removal, although this does not have to be exhaustive. The loss of O2 may be contemplated via decarbonylation, decarboxylation, and dehydration or a combination of all three, usually accomplished via thermal treatment (Mattos et al. 2012). Catalytic deoxygenation could also be considered but would normally demand the availability of H2 (Melero et al. 2012). Alternatively, the use of photocatalysis to decompose biomass-derived oil and materials is an attractive option and one leading to H2 production, for example. H2 can be produced from biomass (such as glycerol) via many processes such as gasification, pyrolysis and steam reforming, O2-blown gasification, and anaerobic fermentation (Abd-Alla et al. 2011, Sarma et al. 2012). Biomass can undergo thermochemical conversion for the production of H2 and other gases; however, a separate stage is required to separate H2 from other gases. Also, biomass can be subjected to thermal treatment for the production of bio-oil, which can be reformed to generate H2 (Sarkar and Kumar 2010). Most of the H2 in the world is currently produced by steam reforming (Hou and Hughes 2001) and coal gasification (Nowotny et al. 2005) processes. However, the drawback of these two processes is that they both produce carbon dioxide (CO2) as a by-product; also, high energies are required for H2 production by these processes. Therefore, solar energy as an alternative energy source for H2 production by the water-splitting reaction is a promising clean/green technique. Water splitting for H2 production can be achieved thermochemically, photobiologically, and photocatalytically by the reforming of renewable feedstock. The major drawback of the thermochemical watersplitting process is the heat control and management as well as the lack of appropriate heat-resisting materials. For the photobiological water-splitting process, CO2 is generated as a by-product and also the low H2 yield (Steinfeld 2002, Guan et al. 2004). Therefore, the photocatalytic water-splitting reaction and photocatalytic reforming are promising clean technologies to produce H2 because of the following advantages: (1) efficiency of solar irradiation to H2, (2) low-cost process, (3) potential to achieve the separation of H2 and O2 produced during reaction, and (4) bench-scale reactors that can be used for household applications, therefore having a huge market potential. However, biomass photocatalytic reforming is thermodynamically more feasible than water splitting, and its application in biomass-derived substrates processing can simultaneously produce H2 and process wastes from the wine, food, or biodiesel industry.
Biomass oxygenated hydrocarbons such as ethanol, methanol, glucose, and glycerol have been employed as sacrificial agents, which can easily undergo oxidation than water and thus improve the efficiency of water splitting during H2 production. However, the demand for ethanol, methanol, and glucose for industrial processing poses a drawback for photocatalysis because of high cost of these products. On the contrary, glycerol is relatively low cost because it is produced in large volumes as a by-product during the biodiesel production and therefore is a promising sacrificial agent for H2 production via photocatalytic reforming (Daskalaki et al. 2010). This process uses semiconductor materials such as titanium dioxide (TiO2), zinc oxide (ZnO), and ferric oxide (Fe2O3). Among these semiconductor materials, TiO2 has been given most attention due to its low cost, nontoxicity, great stability to light irradiation, and activity properties as a photocatalyst (Watts et al. 1995, Kanki et al. 2004). In this article, we review the production of H2 by glycerol reforming using conventional processes and explore the potential application of photocatalysis or photocatalytic reforming process for H2 production, focusing mainly on the development and activities of catalysts. Recent reviews dealing with various aspects of photoreforming of biomass-derived oxygenates, including glycerol, are available (Bowker 2012, Rossetti 2012, Montini et al. 2016).
2 H2 production from glycerol 2.1 Glycerol production Glycerol is produced as a by-product during biodiesel fuel production from a process known as the transesterification of animal fats with methanol or vegetable oil in the presence of acid or base catalysts. Maris and Davies (2007) described the overall transesterification reaction by the following reactions: Triglycerides + CH3OH ↔ Diglycerides + R 1COOCH3 (1) Diglycerides + CH3OH ↔ Monoglycerides + R 2 COOCH3 (2) Monoglycerides + CH3OH ↔ Glycerol + R 3COOCH3 (3) In general, for every 100 kg biodiesel, about 10 kg crude glycerol is generated as a by-product during the base-catalyzed transesterification process. Therefore, the produced crude glycerol contains 50–60% glycerol, 12–16% alkalis (especially in the form of alkali soaps and hydroxides), 15–18% methyl esters, 8–12% methanol, Brought to you by | University of Gothenburg Authenticated Download Date | 11/22/17 8:51 AM
T. Seadira et al.: Glycerol to hydrogen 3
Vacuum flash
Biodiesel
Alkyl-esters
Methyl esters
Acid Methanol Oil
Reactor Base
Settling tank
Strip.
Glycerol (50 wt%)
CSTR or plug flow 60°C r.t. 1 h
Acid
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Distillation of MeOH & Me-esters
Methanol Catalyst Soap Neutr. Sep.
FFA
Water washing
Water
Wash water
Methanol Methanol
Crude glycerol (85%)
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Vacuum flash.
Water
Figure 1: Basics of biodiesel process (Thanh et al. 2010). Reproduced with permission from Elsevier Publ. Co.
and 2–3% water; furthermore, various elements such as calcium, magnesium, phosphorus, and/or sulfur and other components are found in crude glycerol (Kocsisova and Cvengos 2006, Thompson and He 2006). The general industrial process of biodiesel production is shown in Figure 1. Wang et al. (2006) reported that the estimation of the global biodiesel market will reach 37 billion gallons in 2016 with an average growth of 42% annually, which means that at least 4 billion gallons of crude will be generated. This increasing abundance of glycerol has resulted in a dramatic 10-fold decrease in crude glycerol pricing in recent years and has raised environmental concerns related to the disposal of contaminated glycerol (Yazdani and Gonzalez 2007). Glycerol is currently used as an additive in different industries; for instance, the soap and cosmetics industry uses 28% of glycerol and the polyglycerol, ester, and food and beverage industry uses up to 47% of glycerol. However, the excess of glycerol is too much to use in such applications, which means that the industrial biodiesel production on a full scale would break down due to the balance between the supply and demand of glycerol. Therefore, to improve the biodiesel industry’s economic feasibility, research has been focused on identifying alternative technologies of using crude glycerol.
Glycerol has recently attracted a lot of attention as a renewable biomass source for H2 via various processes such as steam reforming (Sanchez et al. 2010), partial oxidation (PO; Dauenhauer et al. 2006), autothermal reforming (Authayanun et al. 2010), aqueous-phase reforming (APR; Wen et al. 2008), and photocatalytic reforming (Kondarides et al. 2008).
2.2 Steam reforming Steam reforming is a process that takes place at high temperatures (>673°C) and atmospheric pressure over a catalyst such as cobalt (Co), nickel (Ni), and noble metals; however, noble metals are usually avoided for commercial steam reforming as the processes are operated at elevated temperatures. The advantages of steam reforming over other reforming process include operations at moderate pressures and production of higher H2 concentrations with high fuel conversion. During steam reforming in the presence of a suitable catalyst, at elevated temperatures (627°C) and low pressures (1 bar), glycerol is reformed into H2 and other gaseous byproducts:
C 3H8O3( g ) + 3H2O( g ) → 7H2 ( g ) + 3CO2 ( g ) (4) Brought to you by | University of Gothenburg Authenticated Download Date | 11/22/17 8:51 AM
4 T. Seadira et al.: Glycerol to hydrogen Adhikari et al. (2009) investigated the effects of process parameters of glycerol steam reforming, with the aim of improving its efficiency during H2 production. In their studies, they investigated the activities of catalysts for the enhancement of H2 production from glycerol by employing Ni supported on CeO2, Ni supported on MgO, and Ni supported on TiO2. Their studies revealed that Ni/CeO2 (which had the highest surface area of 67.0 m2/g) was the most active catalyst for H2 production when tested under same experimental conditions of water/glycerol molar ratio of 12:1, at 550°C, and flow rate of 30 ml/h. Furthermore, it was found that Ni/CeO2 gave a maximum of 74.7% of H2 selectivity compared to NiOx-MgO and NiOx-TiO2, which achieved a maximum of 38.6% and 28.3%, respectively. Although the steam reforming process has great advantages, the major drawbacks of steam reforming of glycerol for H2 production are the large consumption of energy and catalyst deactivation due to coke formation.
2.3 PO The PO process involves the reaction of glycerol with O2 at substoichiometric ratios to create partial combustion, whereby only a portion of the fuel is oxidized by the insufficient stoichiometric amount of O2 added. As this reaction is exothermic, the energy produced is used to heat up the mixture and activate the reaction; therefore, no addition external energy is required. The PO process is advantageous over the steam reforming process because it has a start-up time that is low due to the quick exothermic reaction and the experimental set-up is being moderated closely as it uses the energy produced by the exothermic reaction to vaporize the fuel. The process can be shown as follows:
C 3H8O3( g ) + air → CO/CO2 + H2 + N2 ; ∆H < 0 . (5)
H2 production by PO of glycerol over noble metal catalysts was studied by Rennard et al. (2009). In their study, they revealed that Rh catalysts exhibited equilibrium selectivity toward syngas, whereas platinum (Pt) catalysts generated nonequilibrium products. Furthermore, it was found that the selectivity of H2 production was enhanced when glycerol was diluted with water. The highest percentage of H2 produced was achieved when the ratio of steam to carbon was 2:3 in the presence of an Rh/Ce catalyst. The drawbacks of PO are the difficulty of controlling the catalyst temperature due to the energy produced during the process, the intermediates produced decreasing the purity of the desired product, and the formation of CO in the reformate stream (Salge 2006, Tang 2009).
2.4 Autothermal reforming Autothermal reforming is an integrated process of PO and steam reforming, whereby fuel, water, and gas are all charged at the same time into the catalytic reactor. The advantages of autothermal reforming are that, during the autothermal reforming process, the steam reforming process absorbs the energy produced by the PO process and a fast start-up period compared to steam reforming due to the exothermic reaction is taking place on the surface of the reactor. The overall autothermal reforming reaction can be described as follows:
Substrate (glycerol) + Air + Steam → Carbon oxides + H2 + Nitrogen. (6)
Swami and Abraham (2006) carried out glycerol autothermal reforming over alumina-supported palladium/ Ni/copper (Cu)/potassium catalysts under the process parameters of 823–1123 K, steam/carbon ratio of 3, and O2/carbon ratio of 3. Their study revealed that autothermal reforming produced greater amounts of H2 when compared to the conventional steam reforming processes. However, the drawbacks of this process are the creation of hotspots by the heat produced during PO, which can result in absorption by steam reforming and has a negative effect on the activity of the catalyst, as well as the coke formation that leads to the termination of the reforming process.
2.5 APR APR is a liquid-phase reaction that involves the catalytic reaction of glycerol and water under reaction conditions of 30–80 bar and 473–523 K than the conventional steam reforming process. During APR, oxygenated hydrocarbons are converted into H2, CO2, H2O, and CnH2n (Shabaker et al. 2004). The overall reaction of APR of glycerol for H2 production can be depicted as follows:
C 3H8O3 + xH2O → (2 x + 1)H2 + xCO2 . (7)
Wen et al. (2008) investigated the catalytic activity and stability of Ni, Cu, Co, and Pt catalysts and the effects of catalyst supports used for H2 production in a fixedbed flow reactor. They reported that the catalyst activity increased in the order of Co
