Let us make critical system engineering analysis of the paper : http://www.ika.rwth-aachen.de/r2h/index.php/Thermochemical_conversion_of_biomass#Introduction

1.Thermo Conversion Route for Fuel cell hydrogen :

Introduction Biomass gasification has attracted the highest interest amongst the thermochemical conversion technologies as it offers higher efficiencies in relation to combustion while flash pyrolysis is still in the development stage. Gasification is an energy process producing a gas that can substitute fossil fuels in high efficiency power electric generation, heat and/or CHP applications, and can be used for the production of liquid fuels, chemicals and hydrogen via synthesis gas (typical composition of a synthesis gas from an FICFB gasifier[1]). (Figure) Gasification technology consists of several unit operations, the most critical of which is gas cleaning and conditioning for end-user hydrogen appliances. Numerous types of gasifiers have been developed (Girard et al., 2003; Knoef, 2000), originally for coal feedstocks, and tested in many industrial applications. Significant progresses have been achieved over the last five years and some applications are on the threshold of becoming commercial or are already on the market. However, for most of the applications (in particular for fluidized beds) the efficient and economic removal of tar and inorganic compounds still presents the main technical barrier to be overcome. However, although biomass gasification technologies have recently been successfully demonstrated at large scale and several demonstration projects are under implementation (Costello, 1999; Maniatis, 1999), they are still relative expensive in comparison to fossil based energy and, therefore, face economic and other non-technical barriers when trying to penetrate the energy markets (Haas, 2000; McDonald et al., 1998). Their penetration into the energy markets can only be achieved at present via economic development through biomass systems integration (Maniatis & Millich, 1998). An extensive review of gasifier manufacturers in Europe, USA and Canada (Knoef, 2000) identified 50 manufacturers offering "commercial" gasification plants from which: a large majority of the designs were downdraft type, but an significant increasing proportion of fluidized bed systems. On the large scale applications, the successful operation of the Värnamo plant (6 MWe + 9 MWth, Foster Wheeler) in Sweden for over 3,800 hours on Integrated Gasification Combined Cycle provided credibility and the expected operation of the ARBRE plant (8MWe, TPS) in the UK during the first months of 2001 will be the first step towards commercialisation of the IGCC technology. Serious delays have been reported with the Brazilian project (32 MWe, TPS), however, it is expected that this project will also been implemented soon. The Battelle technology has also been successfully demonstrated at the Burlington, Vermont, project in the US. One of the most interesting demonstration operation is the FICFB (Fast Internal circulating Fluidized Bed) plant in Güssing (Austria), which was launched in 2005 and is in operation for over 6500 hours for power and heat generation with a global energy yield close to 50%.One of the most interesting demonstration operation starts in 2005 and is in operation for over 6500 hours for power and heat generation with a global yield close to 50%.

1. ↑ Composition dry gas: HYDROGEN: 42 %, CO: 23 %, CO2: 23 %, CH4: 10 %, N2: 3 %, Tars: 0,5-1,5 g/Nm3, Dust: 5-10 g/Nm3.

Hydrogen from Biomass pyrolysis

Pyrolysis is the heating of biomass at a temperature of 650-800 K at 0.1-0.5 MPa in the absence of air to convert biomass into liquid oils, solid charcoal and gaseous compounds. Pyrolysis can be further classified into slow pyrolysis and fast pyrolysis. As the products are mainly charcoal, slow pyrolysis is normally not considered for hydrogen production. Fast pyrolysis is a high temperature process, in which the biomass feedstock is heated rapidly in the absence of air, to form vapour and subsequently condensed to a dark brown mobile bio-liquid. The products of fast pyrolysis can be found in all gas, liquid and solid phases (Jalan & Srivastava, 1999):

• Gaseous products include H2, CH4, CO, CO2 and other gases depending on the organic nature of the biomass for pyrolysis.
• Liquid products include tar and oils that remain in liquid form at room temperature like acetone, acetic acid, etc.
• Solid products are mainly composed of char and almost pure carbon plus other inert materials. Although most pyrolysis processes are designed for biofuel production, hydrogen can be produced directly through fast or "flash" pyrolysis if high temperature and sufficient volatile phase residence time are allowed as follows:

Biomass + heat → H2 + CO + CH4 + other products

(1)

Methane and other hydrocarbon vapours produced can be steam reformed for more hydrogen production:

CH4 + H2 → CO + 3H2

(2)

In order to increase the hydrogen production, water–gas shift reaction can be applied as follows:

CO + H2 → CO2 + H2

(3)

Besides the gaseous products, the oily products can also be processed for hydrogen production (Evans et al., 2003). The pyrolysis oil can be separated into two fractions based on water solubility. The water-soluble fraction can be used for hydrogen production while the water-insoluble fraction for adhesive formulation. The material flow is summarized in figure below. Experimental study have shown that when Ni-based catalyst is used, the maximum yield of hydrogen can reach 90%. With additional steam reforming and water–gas shift reaction, the hydrogen yield can be increased significantly. Temperature, heating rate, residence time and type of catalyst used are important pyrolysis process control parameters. In favour of gaseous products especially in hydrogen production, high temperature, high heating rate and long volatile phase residence time are required (Demirbas, 2002b). These parameters can be regulated by selection among different reactor types and eat transfer modes, such as gas–solid convective heat transfer and solid–solid conductive heat transfer. Fluidized bed reactor type exhibits higher heating rates and thus it appears to be the promising reactor type for hydrogen production from biomass pyrolysis. Some inorganic salts, such as chlorides, carbonates and chromates, exhibit beneficial effect on pyrolysis reaction rate (Rabah & Eldighidy, 1989). As tar is difficult to be gasified, extensive studies on the catalytic effect of inexpensive dolomite, olivine and CaO on the decomposition of hydrocarbon compounds in tar have been conducted (Simell et al., 1997; Simell et al., 1999). The catalytic effects of other catalysts (Nibased catalysts (Narvaez et al., 1997), Y-type zeolite, K2CO3, Na2CO3 and CaCO3 (Chen et al., 2003)) and various metal oxides (Al2O3, SiO2, ZrO2, TiO2 (Sutton et al., 2002) and Cr2O3 (Chen et al., 2003)) have also been investigated. Among the different metal oxides, Al2O3 and Cr2O3 exhibit better catalytic effect than the others. Among the catalysts, Na2CO3 is better than K2CO3 and CaCO3. Although noble metals Ru and Rh are more effective than Ni catalyst and less susceptible to carbon formation, they are not commonly used due to their high costs (Garcia et al., 2000). In order to evaluate hydrogen production through pyrolysis of various types of biomass, extensive experimental investigations have been conducted in recent years. Agricultural residues, peanut shell, post-consumer wastes such as plastics, trap grease, mixed biomass and synthetic polymers (S. Czernik et al., 2003), have been widely tested for pyrolysis for hydrogen production. In order to solve the problem of decreasing reforming performance caused by char and coke deposition on the catalyst surface and in the bed itself, fluidized catalyst beds are usually used to improve hydrogen production from biomass-pyrolysis-derived bio-fuel (K.A.M. Bair et al., NREL, 2002 and 2003). Yeboah et al. (Yeboah et al., 2002) constructed a demonstration plant for hydrogen production from peanut shells pyrolysis and steam reforming in a fluidized bed reactor and the production rates of 250 kg H2/day was achieved. Padro and Putsche (Padró et al., 1999) estimated the hydrogen production cost of biomass pyrolysis to be in the range of 6.07 €/GJ to 10.6 €/GJ depending on the facility size and biomass type. For comparison, the costs of hydrogen production by wind-electrolysis systems and PV-electrolysis systems are 18.8 €/GJ and 28.6 €/GJ, respectively. It can be seen that biomass pyrolysis is a competitive method for renewable hydrogen production.

Hydrogen from Biomass gasification

Generally, conventional gasification of biomass and wastes is employed with the goal of maximizing hydrogen production. Some experimental efforts are currently described to demonstrate that the pilot-scale research on hydrogen-production by catalytic coal gasification can be extended to wood. According to the current experiences (Demirbas, 2001; Pinto et al., 2003) the coal technology seems to be fully transferable to wood, subject to minor substitution in feeding and solids-handling components Biomass can be gasified at high temperatures (above 850°C). The process performs partial oxidation to convert carbonaceous feed stock into gaseous energy carrier consisting of permanent, non-condensable gas mixture (CO, CO2, CH4, H2 and H2O). In an ideal gasification process biomass is converted completely to CO and H2 although in practice some CO2, water and other hydrocarbons including methane are formed. Most simple biomass gasifiers produce approximately equal proportions of CO and hydrogen. This conversion process can be expressed as:

Biomass + heat + steam → H2 + CO + CO2 + CH4 + Tars + Char

(1)

In a gasification processes, the solid fuels are completely converted (except the ashes in the feed) to gaseous products having different compositions. Because of the production of cleaner gaseous fuel as well as almost complete conversion of biomass, the gasification process for converting biomass into energy is becoming an attractive option. The char produced from the fast pyrolysis of biomass is highly reactive and can be gasified with gasifying agents such as steam, CO2, oxygen and hydrogen to gaseous fuels. Unlike pyrolysis, gasification of solid biomass is carried out in the presence of low concentration of oxygen. Besides, gasification aims to produce gaseous products while pyrolysis aims to produce bio-oils and charcoal. The gases produced can be steam reformed to produce hydrogen and this process can be further improved by water–gas shift reactions as discussed in the previous section. The gasification process is applicable to biomass having moisture content less than 35% (Demirbas, 2002a). One of the major issues in biomass gasification is to deal with the tar formation that occurs during the process. The unwanted tar may cause the formation of tar aerosols and polymerization to a more complex structure, which are not favourable for hydrogen production through steam reforming. Currently, three methods are available to minimize tar formation:

• proper design of gasifier,
• proper control and operation
• additives/catalysts.

There are three technologies available for biomass gasification:
1. Air gasification: Air gasification is most widely used technology as single product is formed at high efficiency and with out requiring oxygen. A low heating value gas is produced containing up to 60% N2 having a typical heating value of 4–6 MJ/Nm3 with by-products such as water, CO2, hydrocarbons, tar, and nitrogen gas. The reactor temperature of 650–800 °C was achieved.
2. Oxygen gasification: Yields a better quality gas of heating value of 10–15MJ/Nm3. In this process relatively a temperature of 750–1100°C is achieved. But it requires an O2 supply with simultaneous problem of cost and safety. 3. Steam gasification: Biomass steam gasification results in the conversion of carbonaceous material to permanent gases (H2, CO, CO2, CH4 and light hydrocarbons), char and tar. To avoid corrosion problems, poisoning of catalysts and to improve the overall efficiency of the gasification process, tar components needs to be minimized. Andersson and Harvey (Sweden, 2006) described two alternative options for the production of hydrogen as follows:

• Pulp-mill-integrated hydrogen production from gasified back liquor.
• Stand-alone production of hydrogen from gasified biomass. The comparison assumes that the same amount of biomass that is imported in option A, is supplied to a stand-alone hydrogen production plant and that is gasified black liquor in option B, is used in a black liquor gasification combined cycle (BLGCC) CHP unit. The comparison is based upon equal amounts of black liquor fed to the gasifier, and identical steam and power requirements for the pulp mill. The two systems are compared on the basis of total CO2 'emission consequences, based upon different assumptions for the reference energy system that reflects different societal CO2 'emissions reduction target levels. Authors have reported that the hydrogen production from gasified black liquor (option A) is best from a CO2 'emissions’ perspective. Whereas, with high CO2 'emissions associated with electricity production, hydrogen from gasified biomass and electricity from gasified black liquor (option 2) is preferable.

The operation parameters, such as temperature, gasifying agent and residence time, play an important role in formation and decomposition of tar. It has been reported that tar could be thermally cracked at temperature above 1000°C (Milne et al., 1998). The use of some additives (dolomite, olivine and char) inside the gasifier also helps tar reduction (Gil et al., 1999). When dolomite is used, 100% elimination of tar can be achieved (Sutton et al., 2001). Catalysts not only reduce the tar content, but also improve the gas product quality and conversion efficiency. Dolomite, Ni-based catalysts and alkaline metal oxides are widely used as gasification catalysts. Process modifications by two-stage gasification and secondary air injection in the gasifier are also useful for tar reduction (Sutton et al., 2001).

Another problem of biomass gasification is the formation of ash that may cause deposition, sintering, slugging, fouling and agglomeration (Hurt et al., 1995). To resolve the ash-associated problems, fractionation and leaching have been employed to reduce ash formation inside the reactor (Arvelakis & Koukios, 2002). Though fractionation is effective for ash removal, it may deteriorate the quality of the remaining ash. On the other hand, leaching can remove biomass’ inorganic fraction, as well as improve the quality of the remaining ash. More recently, gasification of leached olive oil waste in a circulating fluidized bed reactor was reported for gas production that demonstrated the feasibility of leaching as a pre-treatment technique for gas production (Arvelakis et al., 2001). Hydrogen can be produced from the gasification gaseous products through the same procedure of steam reforming and water–gas shift reaction as discussed in the pyrolysis section. As the products of gasification are mainly gases, this process is more favourable for hydrogen production than pyrolysis. In order to optimize the process for hydrogen production, a number of efforts have been made by researchers to test hydrogen production from biomass gasification with various biomass types and at various operating conditions, as listed in the following table below.

Table: Investigations on biomass gasification for hydrogen production

Feedstock	Reactor Type	Catalyst	Hydrogen production (%vol)	References
Sawdust	unknown	Na2CO3	48,3 % at 700°C	(SuPing et al., 2005)
		</font>	55,4 % at 800°C
			59,8 % at 900°C
Sawdust	Circulating Fluidized bed	not used	10.5 % at 810°C	(Turn et al., 1998)
Wood Epicea	Fixed Bed	not used	7.7 % at 550°C	(Y.Xia, et al., 2000)
Sawdust	Fluidized bed	unknown	57.4 % at 800°C	(Turn et al., 1998)
Sawdust	Fluidized bed	K2CO3	11.3 % at 964°C	(J.Jian-chun et al., 2001)
	Fluidized bed	Na2CO3	14.7 % at 1012°C
	Fluidized bed	CaO	13.3 % at 1008°C
Pine sawdust	Fluidized bed	unknown	26 - 42 % at 700-800°C	(W.Zhiwei, et al., 2002)
Bagasse	Fluidized bed	unknown	29 - 38 % at 700-800°C
Cotton stem	Fluidized bed	unknown	27 - 38 % at 700-800°C
Wood Eucaplyptus	Fluidized bed	unknown	35 - 37 % at 700-800°C
Wood Pinus	Fluidized bed	unknown	27 - 35 % at 700-800°C
Sewage sludge	Downdraft	unknown	10 - 11 % at 750 °C	(Midilli et al., 2002)
Almond shell	Fluidized bed	La-Ni-Fe	62.8 % at 800°C	(Rapagna et al., 1998)
	Fluidized bed	Perovskite	63.7 % at 900°C
Switchgrass	Moving bed	Cu-Zn-Al	27.1 % at 750°C	(Brown, 2003)

Using a fluidized bed gasifier along with suitable catalysts, it is possible to achieve hydrogen production about 60 % vol. Such high conversion efficiency makes biomass gasification an attractive hydrogen production alternative. In addition, the costs of hydrogen production by biomass gasification are competitive with natural gas reforming as illustrated in figure.

Taking into account the environmental benefit as well, hydrogen production from biomass gasification should be a promising option based on both economic and environmental considerations. In recent years, a novel gasification method, namely, Hydrogen Production by Reaction Integrated Novel Gasification (HyPr-RING) was proposed by Lin et al. (Lin et al., 2002). HyPr-RING method is an integration of the water-hydrocarbon reaction, water–gas shift reaction and absorption of CO2 and other pollutants in a single reactor under both sub-critical and supercritical water conditions. The main reaction for this new method can be expressed as:

C + 2H2O + CaO → CaCO3 + 2H2

DH 0 298 = - 88kJ/mol

This reaction is exothermic and high yield of hydrogen can be achieved at relatively low temperature (650 – 700°C). As compared with conventional gasification, the HyPr-RING process, as illustrated in figure below, can be conducted in a much simpler manner as the reaction for hydrogen production and gas separation are carried out in one single reactor at a lower temperature. This novel gasification process has been analyzed theoretically and demonstrated experimentally to be a very efficient technique for hydrogen production from biomass. When biomass has high moisture content above 35%, it is likely to gasify biomass in a supercritical water condition. Under the severe conditions obtained by heating water to a temperature above its critical temperature (374°C) and compressing it to a pressure above its critical pressure (22 MPa), biomass is rapidly decomposed into small molecules or gases in a few minutes at a high efficiency (Matsumura & Minowa, 2004). Supercritical water gasification is a promising process to gasify biomass with high moisture contents due to the high gasification ratio (100% achievable) and high hydrogen volumetric ratio (50% achievable).

In recent years, extensive research has been carried out to evaluate the suitability of various wet biomass gasification in supercritical water conditions. However, the works have been mostly on a laboratory scale and in an early development stage; hence, the principles and basic mechanisms are not well understood yet. Supercritical water gasification is still at its early development stage, the technology has already shown its economic competitiveness with other hydrogen production methods. Spritzer and Hong (Spritzer & Hong, 2003) have estimated the cost of hydrogen produced by supercritical water gasification to be about 2.1 €/GJ (0.24 €/kg). Hydrogen production from biomass thermochemical processes has already been shown to be attractive economically and demonstrated to be a feasible option. However, it should be noted that hydrogen gas is normally produced together with other gas constituents. Thus, separation and purification of hydrogen gas are required. Nowadays, several methods, such as CO2 absorption (Selexol® process for example), drying/chilling and membrane separation (Reij et al., 1998), have been successfully developed for hydrogen gas purification. It is expected that biomass thermochemical conversion processes, especially the newly developed gasification types, will be available for large-scale hydrogen production in the near future.

References

• Girard, P., Van, D., Koch, T., Antonini, G. & Bensakhria, A.
  Biomass gasification process, and apparatus, and their applications: EP Patent 1,312,662
  

• Knoef, H. A. M.
  Inventory of Biomass Gasifier Manufacturers & Installations
  Final Report to European Commission. Contract DIS/1734/98-NL, Biomass Technology Group BV, University of Twente, Enschede,[see http://btgs1.ct.utwente.nl/]

• Costello, R. (1999).
  An overview of the US Department of Energy’s biomass power program. Power Production from Biomass III, Gasification & Pyrolysis R&D&D for Industry,[Ed by K Sipila & M Korhonen]
  VTT Symposium 192

• Maniatis, K.
  Overview of EU THERMIE gasification projects. Power production from biomass III gasification and pyrolysis R&D for industry
  VTT Symposium 192, 1999

• Maniatis, K. & Millich, E.
  Energy from biomass and waste: The contribution of utility scale biomass gasification plants
  Biomass and Bioenergy 15, 195-200, 1998

• Haas, R.
  Promotion strategies for electricity from renewable energy sources in EU countries
  Institute for Energy Economics, Austria, December 2000

• McDonald, N., Harrisson, G. & Limbrick, A. J.
  Implementation Studies in Four Regions of the EU for Power-From-Biomass at the Scale of 1 to 5MW~e
  ETSU B/T1/00544/REP. ETSU REPORT ETSU B T1 REP, 1998

• Milne, T. A., Evans, R. J. & Abatzaglou, N.
  Biomass GasifierTars: Their Nature, Formation, and Conversion
  NREL/TP-570-25357; ON: DE00003726, National Renewable Energy Laboratory, Golden, CO (US), 1998

• Demirbas, A.
  Gaseous products from biomass by pyrolysis and gasification: effects of catalyst on hydrogen yield
  Energy Conversion and Management 43, 897-909

• Rabah, M. A. & Eldighidy, S. M.
  Low cost hydrogen production from waste
  International journal of hydrogen energy 14, 221-227. 1989

• Simell, P. A., Hakala, N. A. K. & Haario, H. E.
  Catalytic decomposition of gasification gas tar with benzene as the model compound
  Ind Eng Chem Res 36, 42-51, 1997

• Simell, P. A., Hirvensalo, E. K., Smolander, V. T. & Krause, A. O. I.
  Steam reforming of gasification gas tar over dolomite with benzene as a model compound
  Industrial and Engineering Chemistry Research 38, 1999

• Narvaez, I., Corella, J. & Orio, A.
  Fresh tar (from a biomass gasifier) elimination over a commercial steam-reforming catalyst. Kinetics and effect of different variables of operation
  Industrial and Engineering Chemistry Research 36, 1997

• Chen, G., Andries, J. & Spliethoff, H.
  Catalytic pyrolysis of biomass for hydrogen rich fuel gas production
  Energy Conversion and Management 44, 2289-2296, 2003

• Sutton, D., Kelleher, B. & Ross, J. R. H.
  Catalytic conditioning of organic volatile products produced by peat pyrolysis
  Biomass and Bioenergy 23, 209-216, 2002

• Garcia, L., French, R., Czernik, S. & Chornet, E.
  Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition
  Applied Catalysis A: General 201, 225-239, 2000

• Yeboah, Y., Bota, K. B. & Wang, Z.
  Hydrogen from Biomass for Urban Transportation
  Proceedings of the US DOE Hydrogen Program Review, 2002

• Padró, C. E. G., Putsche, V. & National Renewable Energy, L.
  Survey of the Economics of Hydrogen Technologies
  National Renewable Energy Laboratory, 1999

• Demirbas, A. (2001).
  Biomass resource facilities and biomass conversion processing for fuels and chemicals
  Energy Conversion and Management 42, 1357-1378
• Pinto, F., Franco, C., Andre, R. N., Tavares, C., Dias, M., Gulyurtlu, I. & Cabrita, I.
  Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system
  Fuel 82, 1967-1976, 2003

• Milne, T. A., Evans, R. J. & Abatzaglou, N.
  Biomass GasifierTars: Their Nature, Formation, and Conversion
  NREL/TP-570-25357; ON: DE00003726, National Renewable Energy Laboratory, Golden, CO (US), 1998

• Gil, J., Corella, J., Aznar, M. P. & Caballero, M. A.
  Biomass gasification in atmospheric and bubbling fluidized bed: Effect of the type of gasifying agent on the product distribution
  Biomass and Bioenergy 17, 389-403, 1999

• Sutton, D., Kelleher, B. & Ross, J. R. H.
  Review of literature on catalysts for biomass gasification
  Fuel Processing Technology 73, 155-173, 2001

• Hurt, R. H., Davis, K. A., Yang, N. Y. C., Headley, T. J. & Mitchell, G. D.
  Residual Carbon from Pulverized-coal-fired Boilers
  Fuel 74, 1297-1306, 1995

• Arvelakis, S. & Koukios, E. G.
  Physicochemical upgrading of agroresidues as feedstocks for energy production via thermochemical conversion methods
  Biomass and Bioenergy 22, 331-348, 2002

• Arvelakis, S., Vourliotis, P., Kakaras, E. & Koukios, E. G.
  Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion
  Biomass and Bioenergy 20, 459-470, 2001

• Arvelakis, S., Vourliotis, P., Kakaras, E. & Koukios, E. G.
  Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion
  Biomass and Bioenergy 20, 459-470; 2001

• Lin, S. Y., Suzuki, Y., Hatano, H. & Harada, M.
  Developing an innovative method, HyPr-RING, to produce hydrogen from hydrocarbons
  Energy Conversion and Management 43, 1283-1290, 2002

• Matsumura, Y. & Minowa, T.
  Fundamental design of a continuous biomass gasification process using a supercritical water fluidized bed
  International journal of hydrogen energy 29, 701-707, 2004

• Spritzer, M. H. & Hong, G. T.
  Supercritical Water Partial Oxidation
  Poster paper at the Annual Peer Review Meeting, US DOE Hydrogen Program, 2003

• Reij, M. W., Keurentjes, J. T. F. & Hartmans, S.
  Membrane bioreactors for waste gas treatment
  Journal of Biotechnology 59, 155-167, 199