The goal of any gasification technology processing a mixed feedstock for specific applications such as IGCC, syngas generation, and the like is threefold. First, the gasifier operation and performance meet the desired

objectives. Second, the products must meet the requirements of all the down­stream operations such as gas turbine or FT catalysts, and so on, and third the gasifier must operate within its design limits. This goal is affected by the characteristics and properties as well as any synergistic effects of the mixed feedstock, composition of the total feed, the approach and equipment used to co-feed and the designs of the gasifier and the syngas clean-up equipment.

When possible, it is more economical to process a mixed feedstock in the process already being used for a single feedstock. For coal and biomass mixtures, it is more likely that coal processing plants will be retrofitted to accommodate the use of biomass in the feed. The pretreatment should allow this to happen as inexpensively as possible. Compared to coal, biomass has higher H/C and O/C contents (see Figure 7.1) which make biomass more active and easily degradable (more susceptible to fungal attack) when subject to the natural environment. Biomass also contains some inorganic elements (such as K, Na, Cl, Br, P, S, Ca, Mg, Si, Al, Fe, etc.) that if not removed can accumulate in the remaining solids of the various thermochemical processes causing slagging, reactor blockage, and other problems or end up with prod­uct gas requiring downstream cleaning operations. The ash from biomass gasification is different from that of coal gasification. Water removal from the biomass is also important for energy efficiency of the gasifier operation.

Pretreatment of biomass for its use in mixed feedstock is done for various reasons. A wide variety of biomass streams often does not match with nar­row fuel specifications of feeding systems and the desired conversion pro­cesses. Other reasons are to reduce the plant’s investment, maintenance, and personnel costs by using homogeneous fuel that is suitable for automatic fuel feeding and to reduce the need to invest in complex and novel gasification

systems. Pretreatment will also reduce the cost of transportation, storage, and handling of biomass. When coal and biomass are fed together as a mix­ture into a gasifier, the process of feeding must be uniform, consistent, and one that allows for easy particle fluidization in any type of gasifier. With popular entrained bed gasifiers, dry feeding is preferred because it allows maximum flexibility in allowable operating conditions and composition of the coal-biomass mixture feedstock. In addition to dry feeding a coal-bio­mass mixture successfully, the biomass needs to be prepared such that it forms a homogeneous mixture with coal.

The choice of pretreatment method used often depends on the downstream use of biomass as well as the local conditions and the need. As shown later, the choice of pretreatment method will also depend on the choice of the pro­cess configuration. The three most important parameters in feeding mixed feedstock are particle size and uniformity, level of inorganic impurities in the biomass, and moisture content of the biomass. The particle size affects the feeding and fluidization process within the reactor. A level of inorganics and other elements affects the quality of syngas and ash (and slag) formed within the reactor and the moisture content affects the energy efficiency of the gas­ifier. Various methods used to pretreat biomass for these purposes and their advantages and disadvantages are described in Table 7.6 [17]. The inorganic impurities can be partially removed by the leaching process mentioned in Table 7.6 [17]. Washing biomass with hot (around 60-80°C) water will largely remove ions of K, Na, Cl, and Br and somewhat remove the elements P, S, Ca, and Mg. Most other elements including Si, Fe, and Al will remain in the bio­mass. Process configuration and the downstream treatments are important for handling impurities in gas and solid product streams. The best method to handle volatile matter and particle size is torrefaction with or without pelletization. For an entrained bed reactor, a particle size of 1 mm. or less is required and this can be achieved through torrefaction. Sizing and milling of herbaceous biomass such as straw and switchgrass is very difficult and can be achieved via use of torrefaction. Pelletization of compressed biomass is possible, but it does not provide the additional benefits associated with the prevention of water absorption and fungal attack during storage and trans­portation of biomass that are provided by torrefaction. Furthermore, bio­mass pelletization alone does not easily allow the generation of fine particles on the order of several microns. In contrast, pyrolysis is a chemical process that significantly breaks down the chemical bonds within the biomass. It, however, requires biomass feeding in a slurry form.