Use of catalysts in the thermochemical conversion of biomass may not be essential, but it can help under certain circumstances. Two main motivations for catalysts are:

* Removal of tar from the product gas, especially if the downstream applica­tion or the installed equipment cannot tolerate it (see Chapter 4 for more details).

* Reduction in methane content of the product gas, particularly when it is to be used as syngas (CO, H2 mixture).

The development of catalytic gasification is driven by the need for tar reforming. When the product gas passes over the catalyst particles, the tar or condensable hydrocarbon can be reformed on the catalyst surface with either steam or carbon dioxide, thus producing additional hydrogen and carbon mon­oxide. The reactions may be written in simple form as

Steam reforming reaction:

CnHm + nH20 catalyst >(n + m/2) H2 + nCO (5.20)

Carbon dioxide (or dry) reforming reaction:

CnHm + nC02 catalyst > 2nCO + (m/2)H2 (5.21)

As we can see, instead of undesirable tar or soot, we get additional fuel gases through the catalytic tar-reforming reactions (Eq. 5.20). Both gas yield and the heating value of the product gas improve.

The other option for tar removal is thermal cracking, but it requires a high (>1100 °C) temperature and produces soot; thus, it cannot harness the lost energy in tar hydrocarbon.

The second motivation for catalytic gasification is removal of methane from the product gas. For this we can use either catalytic steam reforming or catalytic

carbon dioxide reforming of methane. Reforming is very important for the production of syngas, which cannot tolerate methane and requires a precise ratio of CO and H2 in the product gas. In steam reforming, methane reacts with steam in the temperature range of 700 to 1100 °C in the presence of a metal — based catalyst, and thus it is reformed into CO and H2 (Li et al., 2007):

CH4 + H2O catalyst > CO + 3H2 + 206 kJ/mol (_

— steam reforming of methane

This reaction is widely used in hydrogen production from methane, for which nickel-based catalysts are very effective.

The carbon dioxide reforming of methane is not as widely used commer­cially as steam reforming, but it has the special attraction of reducing two greenhouse gases (CO2 and CH4) in one reaction, and it can be a good option for removal of carbon dioxide from the product gas. The reaction is highly endothermic (Wang and Lu, 1996):

CH4 + CO2 catalyst > 2CO + 2H2 + 247 kJ/mol (_

— dry reforming of methane

Nickel-based catalysts are also effective for the dry-reforming reaction (Liu et al., 2008).

Catalyst Selection

Catalysts for reforming reactions are to be chosen keeping in view their objec­tive and practical use. Some important catalyst selection criteria for the removal of tar are as follows:

• Effective

• Resistant to deactivation by carbon fouling and sintering

• Easily regenerated

• Strong and resistant to attrition

• Inexpensive

For methane removal, the following criteria are to be met in addition to those in the previous list:

• Capable of reforming methane

• Must provide the required CO/H2 ratio for the syngas process

Catalysts can work in in-situ and post-gasification reactions. The former may involve impregnating the catalyst in the biomass prior to gasification. It can be added directly in the reactor, as in a fluidized bed. Such application is effective in reducing the tar, but it is not effective in reducing methane (Sutton et al., 2001). In post-gasification, catalysts are placed in a secondary reactor downstream of the gasifier to convert the tar and methane formed. This has the additional advantage of being independent of the gasifier operating condition.

The second reactor can be operated at temperatures optimum for the reforming reaction.

The catalysts in biomass gasification are divided into three groups: earth metal, alkali metal, and nickel based.

Earth metal catalysts. Dolomite (CaCO3.MgCO3) is very effective for disposal of tar, and it is inexpensive and widely available, obviating the need for catalyst regeneration. It can be used as a primary catalyst by mixing with the biomass or as a secondary catalyst in a reformer downstream, which is also called a guard bed. Calcined dolomite is significantly more effective than raw dolomite (Sutton et al., 2001). Neither, however, is very useful for methane conversion. The rate of the reforming reaction is higher with carbon dioxide than with steam.

Alkali metal catalysts. Potassium carbonate and sodium carbonate are important in biomass gasification as primary catalysts. K2CO3 is more effec­tive than Na2CO3. Unlike dolomite, they can reduce methane in the product gas through a reforming reaction. Many biomass types have inherent potas­sium in their ash, so they can benefit from the catalytic action of the potas­sium with reduced tar production. However, potassium is notorious for agglomerating in fluidized beds, which offsets its catalytic benefit. Ni-based catalyst. Nickel is highly effective as a reforming catalyst for reduction of tar as well as for adjustment of the CO/H2 ratio through methane conversion. It performs best when used downstream of the gasifier in a secondary bed, typically at 780 °C (Sutton et al., 2001). Deactivation of the catalyst with carbon deposits is an issue. Nickel is relatively inex­pensive and commercially available though not as cheap as dolomite. Appropriate catalyst support is important for optimum performance.