1. INTRODUCTION:
Hydrogen (H2) is widely produced and used fuel, considered as an energy carrier for stationary power and transportation market. According to EIA 2008, yearly approximate 10-11 million metric tonnes of hydrogen is produced in US. There are various well known technologies for hydrogen production which namely includes steam reforming of natural gas, partial oxidation or gasification of coal or biomass, advanced thermochemical techniques, biomass fermentation with microorganisms and electrolysis. Some of the important uses of hydrogen in current state scenario involve oil refining, hydrogenation, ammonia synthesis, fuel and energy.1
Energy recovery from waste is one of the hot topics in current science. It involves conversion of waste to fuel (e.g. hydrocarbon, hydrogen, etc). In last few decades this topic has got much attention because of the serious environmental problems caused by the plastic and other polymeric waste. Every year there is a great enhancement is observed in discarding plastic waste. These materials are non biodegradable causing serious disposal problems. There are some methods for the disposal which are practiced since decades include incineration, landfilling and plastic recycling. Dumping plastics neither help in recovery of material nor energy.
With land fill space is getting reduced with time. For the energy recovery plastics are incinerated using municipal waste or steel plants. However during this process original material is lost but includes emission of carcinogens like polychlorinated dioxins and polychlorinated furans and also poisonous gas like phosgene. Then the third method is plastic recycling based on palletizing and moulding into low grade plastics as chemical recycling technologies have attracted many researchers. But the recycled plastic possesses poor mechanical strength and color qualities hence having low market price.2-6 By considering these environmental issues, there is a great demand for the conversion of plastic waste to fuel/energy. Out of all these techniques mentioned above this article is emphasized on gasification of waste and biomass and recent trends in catalyst materials for hydrogen production.
1.1 Gasification:
Gasification is a simple and commercially well established technology practiced more than 50 years. Typical gasification process involves conversion of carbon and hydrogen containing materials to syngas (mixture of H2 and CO). Syngas is an energy source for combustion process used to generate electricity. The ratio of carbon to hydrogen determines the application of syngas e.g. syngas containing mainly hydrogen is preferred for refineries, chemical industries mainly used syngas containing roughly equal proportion of hydrogen and carbon monoxide. The United State Environmental Protection Agency (EPA) defined gasification as “a process that converts carbonaceous materials through a process involving partial oxidation of the feedstock in a reducing atmosphere in the presence of steam at temperatures sufficient to convert the feedstock to synthesis gas, to convert inorganic matter in the feedstock (when the feedstock is a solid or semi-solid) to a glassy solid material known as vitreous frit or slag, and to convert halogens into the corresponding acid halides7”
1.1.1 Need of gasification:
There are great concerns regarding the climate change and effect of green house gases. In this regard, the demand of finding low carbon energy technologies for future world will increase exponentially. It has been now well estimated that, every year approximately 50 million tons of hydrogen are produced.8 To fulfill the increasing demand of hydrogen; effective and efficient technologies for hydrogen production is must. Many developing countries with their gradual development, they are facing problem of collection of huge municipal waste. Many researchers have extensively worked on conversion of waste to energy using pyrolysis or gasification techniques. Among these two techniques gasification is favored because of the reason mentioned in section 1.1.2. 9-11
1.1.2 Difference between gasification and incineration:
Gasification is not incineration; they are markedly different from each other. Technically in gasification, carbon feedstock is converted into carbon monoxide (CO) and hydrogen (H2) while incineration converts the feedstock to Carbon dioxide (CO2) and water (H2O).
|
CO+H2 Syngas |
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Incineration |
CO2+H2O Combustion product |
Fig. 1 Difference between gasification and incineration
2. GASIFICATION TO FUEL:
There are various attempts made for conversion of waste to fuel or energy. Various gasification techniques include thermal, catalytic, plasma, etc. Toshiro and Yoshika (2000) reported two-stage thermal gasification of plastics. For this polyethylene (PE), polypropylene (PP) and polystyrene (PS) were used for two-stage thermal decomposition. In a typical two-stage gasification technique, plastic is first converted into liquid and then to gas in next gasification step. However maximum conversion to gases in case of chosen polymers followed the order of PE(max)>PP>PS. The main components of a gases formed, found to have methane and gaseous olefins (ethylene, propylene).12
In pyrolysis process, plastics produced high calorie gas with aromatic liquids like benzene, toluene and xylene, generally designated as BTX. But there are no reports yielding exclusively hydrogen gas during gasification technique. But a well known key factor to maximize the hydrogen production is the use of catalyst.
2.1 Catalytic Materials for energy:
The catalysts employed for the waste to energy conversion include oxides, mixed oxides, supported metal oxides. In1986, Ueno et al, studied deactivation of catalyst for steam gasification of wood. During this study, 20 wt% Ni/Al2O3 catalyst was prepared by conventional impregnation method. Aqueous solutions of nickel nitrate and aluminium sphere (100-200A0) were used and catalyst was further reduced at 7000C in hydrogen steam for 4 h. The catalyst was well characterized by SEM, XRD, EPMA and BET surface area. When reaction temperature was elevated to 7500C the yield of gas (CO, H2) reached approximately to 50 %. 12
For utilization of renewable sources, conversion of biomasses to hydrogen and syngas is becoming important now days. 13Conversion of biomass to hydrogen or syngas can be done by both non catalytic and catalytic ways. But use of metal catalyst reduces the tar content formed during gasification.14-16 Tomishige et al., studied extensively on gasification of cellulose and biomass with air over Rh/CeO2/SiO2 .17-25 Ni and dolomite catalysts used in steam reforming suffer from coke deposition and eventually deactivate the catalyst while Rh/CeO2/SiO2 was found comparatively more effective and resistant to coking.
Catalyst preparation methods also affect profoundly on catalytic activity. Ni/CeO2/Al2O3 prepared by co-impregnation method and sequential impregnation method both showed different activity in steam gasification of biomass (Cedar wood) in terms of yield of coke and tar. Catalyst prepared by co-impregnation method was found to have high catalytic performance than catalyst prepared by sequential impregnation and Ni/Al2O3 catalyst.26 The steam gasification of biomass was carried out in a laboratory scale continuous feeding dual-bed reactor for testing catalytic activity. Among all the catalyst Ni/CeO2/Al2O3 showed highest rate of formation of hydrogen. Steam gasification of carbonaceous materials mainly includes two reactions namely reaction between water and carbon and water gas shift reaction between hydrogen and carbon dioxide. The reaction between water and carbon is highly endothermic and requires very high temperature (700-1200°C). These high energy demanding processes/ steps make these technologies expensive and unsustainable. Hydrothermal gasification involves low temperature treatment of organic waste for production of hydrogen.27-28 Alkali like potassium carbonate increased the gas yield of glucose solution (Kruse et al) 29 while Schmeider et al., found that KOH and potassium carbonate as effective for gasification of carbohydrates, glycine and aromatics at 6000C and 250 bar to produce hydrogen rich gas.30 Lin et al., reported calcium based catalysts like calcium oxide and calcium hydroxide for biomass gasification under hydrothermal condition.31
By considering environmental concerns, Williams et al., reported hydrothermal catalytic gasification of municipal solid waste. During experimental work, in order to improve sample homogeneity, municipal solid waste was used in the form of refused derived fuel (RDF). In his study, he studied the effect of water density, reaction temperature on hydrothermal condition using sodium hydroxide.32 Guilhaume et al reported hydrogen production from catalytic pyrolysis of oil with sequential catalytic reactions. For the study the catalyst Ni-K/La2O3-Al2O3 (purchased from Johnson Matthey) and Ni/Alumina prepared by impregnation method. Both the catalysts exhibited good catalytic performances and 45-50% H2 was recovered from product stream.33
There are few reports on production of hydrogen from reforming of bio-oil over noble metal catalysts. The type of support used, fine tunes not only catalytic activity but also selectivity of the product. There are many reports on Ni based supported catalysts like Ni/MgO, Ni/Al2O3, Ni/zeolite, etc 34-37 Recently Wu et al, reported effect of Ni particle location within MCM-41 support for hydrogen production in catalytic biomass gasification. Ni/MCM-41 catalyst was prepared by impregnation method and well characterized by SEM, TEM, XRD, TPR and TGA techniques. Catalytic performance of NiO particles inside and outside of MCM-41 pores was tested for hydrogen production and coke deposition on catalyst during biomass gasification. It was observed that NiO particle inside the MCM-41 mesopore generated lowest carbon deposition and enhanced the hydrogen production.38
The catalytic gasification of biomass is explained in details in a review reported by Lasa et al., 2011.39 Municipal solid waste (MSW) is currently a main problem of many developing as well as developed countries. Incineration burns MSW directly to thermal energy and subsequently electrical energy in same manner gasification process is useful for converting MSW to syngas and subsequently energy. The reactors in current state in gasification technologies operate in between temperature range of 400-8500C. Their mechanism based on the air-breathing combustion of a portion of a feedstock for production of heat to facilitate gasification. In real sense they are partial combustors and partially gasifiers. Very recently Chung et al., published review article on steam gasification of MSW.40
In upgrading and recovery of oil sands because of asphatenes removal, it is further used in gasification for hydrogen generation. In this concern, gasification of asphaltenes on nano sized catalyst materials was reported by Nassar et al., (2011). In this study, nanoparticles of Fe3O4, Co3O4, and NiO were used. The catalytic steam gasification of adsorbed asphaltenes over metal oxide nano particles was carried out using simultaneous DSC-TGA analysis. Percent conversion at the onset temperature for NiO, Co3O4, and Fe3O4 nanoparticles were found to be 37, 32, and 21%, respectively. Thus among all these oxides, NiO nanoparticles showed better catalytic activity.41 Thus there is a great scope of improvement in catalytic material to get hydrogen production selectively. Very recently our mini review on HDPE valorisation to fuel in accepted in green chemistry which deals with the role of catalytic material for conversion of plastic waste to fuel with details.42
3. FUTURE PROSPECTS:
“Waste to gas (energy)” transformation processes are a great challenge because of requirement of high reaction temperatures, and product selectivity. Therefore lowering the reaction temperature is always being a long-term goal in catalytic processes which can be lead by the appropriate designing of advanced catalytic materials. Though there are various routes using catalytic materials for energy production; the role of catalyst is not fully understood for selectivity of product (s). Therefore there is a wide scope for understanding the role of various materials and their surface science.
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Received on 13.01.2015 Modified on 05.02.2015
Accepted on 10.02.2015 © AJRC All right reserved
Asian J. Research Chem 8(3): March 2015; Page 201-204
DOI: 10.5958/0974-4150.2015.00036.X