Gasification

Gasification of biomass is to convert it into useful gases such as carbon monoxide, hydrogen and light hydrocarbons (Brown, 2003). Since the mid-1980s, inter­est has grown on the subject of catalysis for biomass gasification. The advances in this area have been driven by the need for producing tar-free gases from biomass. The avoidance of tars and the yield of hydorgen are deciding factors the economic viability of the biomass

gasification process. Major reactions in gasification are as follows (Brown, 2003).

1

C + — O2 4 CO

2 2

C + CO2 4 2CO C + H2O 4 H2 + CO C + 2H2 4 CH4 CO + H2O 4 H2 + CO2 CO + 3H2 4 CH4 + H2O

The desired product from gasification of biomass is hydrogen or syngas. Syngas can be burned directly in gas engines, be used to produce methanol, or be con­verted into synthetic fuels via the Fischer—Tropsch process. Though gases are target products, gasification of biomass leaves behind solid residuals such carbon and inorganic compounds (ash).

Gasification of biomass is normally performed in the presence of steam and the process depends on the occur­rence of the steam-reforming reactions. Water, in the form of steam, is often added to promote additional produc­tion of hydrogen via the water—gas shift reaction. As the biomass is heated, moisture contained in the biomass is converted to steam, which can react with biomass. However, in practice, proper drying of biomass before feeding it into gasification equipment is still needed in view of energy-input.

Small amounts of oxygen can also be added to the gas feed. The heat from exothermic oxidation reactions can then be used by the endothermic steam-reforming reac­tion. In addition, oxygen has a function to delay the cata­lyst deactivation by helping burn off some of the coke formed.

Catalytic Gasification

Table 15.2 lists some typical results from the gasifica­tion of lignocellulosic biomass in the presence of catalysts.

Biomass gasification is inevitably accompanied with tar formation. Nevertheless, tar can be effectively minimized by catalytic cracking. Naturally occurring dolomite (CaMg(CO3)2), for example, has been used as a catalyst for gasification of biomass in a fluid bed reactor to reduce the tar content by transforming it to gases (Delgado and Aznar, 1997). The mineral-based catalyst generally con­tains CaO, MgO, CO2 and trace minerals such as SiO2, Fe2O3 and Al2O3. The tar cracking efficiency over the dolomites depends on their chemical composition. In general, dolomites with the lowest content of CaO and MgO show the lowest tar cracking efficiency. Yu et al. gasified birch on the four types of dolomites (deposites in Zhenjiang, Nanjing, Shanxi, and Anhui, China) and a Swedish dolomite (Sala) (Yu et al., 2009). The result was that Anhui dolomite showed a low catalytic capacity to crack tar at 973 and 1073K due to its lowest content of CaO and MgO among the tested dolomites. An alterna­tive can be naturally occurring particles of olivine, which are a mineral containing magnesium oxide, iron oxide and silica. Regarding their attrition resistance, Olivine is advantageous over dolomite (Devi et al., 2005).

Alkali salts are often added to biomass by dry mixing or wet impregnation and used as catalysts for the elim­ination of tar and upgrading of the product gas (Li et al., 1996; Encinar et al., 1998). But it has considerable difficulty in catalyst recovery and disposal of ash. Carbonates, oxides and hydroxides of alkali metals can effectively cata­lyze the decomposition of tar during catalytic gasification (McKee, 1983). Earlier, for example, Mudge et al. investi­gated the catalytic steam gasification of wood using alkali carbonates and naturally occurring minerals (trona, borax), which were either impregnated or mixed with the biomass (Mudge and Baker, 1985). The order of activity reported was potassium > carbonate > sodium carbonate > trona > borax.

The Ni-based catalysts for biomass gasification in a fluid bed reactor are typically Ni-Al based one (Garcia et al., 2002; Arauzo et al., 1997) and Ni/olivine one (Courson et al., 2002, 2000). Ni catalysts help to remove tars and methane and to adjust the composition of syn­thesis gas. Sinag et al. studied the effect of nano-sized and bulky ZnO and SnO2 at 573 K on the water-gas shift reaction in gasification of cellulose. The results showed that the water-gas shift reaction proceeded faster over ZnO catalysts than that over SnO2 catalysts. Therefore, a higher yield of hydrogen was obtained in the presence of ZnO (Sinag et al., 2011).

However, catalysts often suffer from deactivation by sintering and/or coke deposition. The use of supercriti­cal water can prevent catalyst from deactivation by means of extracting the coke precursor from the catalyst surface (Baiker, 1999). In addition, it can improve solubi­lity of cellulosic materials and thus reduce mass-transfer limitation. It is also worth noting that, in addition to the active component in a catalyst, usually the acidity and basicity of a support is also an influential factor on product distribution and coke formation. Tasaka and coworkers disclosed that steam reforming of tar derived from cellulose gasification was efficiently catalyzed by 12 wt% Co/MgO catalyst at 873 K in a fluidized bed reactor (Tasaka et al., 2007).

Supported Ru, Pt or Pd catalysts also appear prom­ising in the catalytic gasification of lignocellulosic biomass. They were able to overcome the shortcomings of Ni-based catalysts and dolomite catalysts, although they are relatively costly. Usui et al. gasified cellulose in hot-compressed water at 623 K in the presence of a series of supported catalysts such as Zr(OH)4, (CH3COCH=C(O-)CH3)3Fe, ferrocene, Ru3(CO)12,

(CH3COCH=C(O-)CH3)2Co, NiC2O4, NiO, Ni(OH)2, PdI2 and Cu(OH)2. After reaction for 3 h, 5 wt% Pd sup­ported on Al2O3 showed the highest catalytic activity, leading to a 42.3 vol% yield of H2 and a 7.7 vol% yield of CH4 (Usui et al., 2000). Tomishige et al. found that the order of M/CeO2/SiO2 catalyst activity in the cedar wood gasification at 823 K was the following: Rh > Pd > Pt > Ni=Ru (Tomishige et al., 2004). For Rh/ CeO2/M-type (M=SiO2, Al2O3, and ZrO2) catalysts for cellulose gasification in a continuous-feeding fluidized — bed reactor, Asadullah et al. found that Rh/CeO2/SiO2 exhibited the best performance in terms of generating syngas or hydrogen (Asadullah et al., 2001, 2003).