The advantages and disadvantages of the co-gasification of coal and biomass mixture are briefly illustrated in Table 7.3. [12-14]. As shown in this table, co-gasification provides many advantages to the production of the syngas as well as generation of power. The pure biomass gasification process is limited to the small scale, has high capital (fixed) cost, has lower thermal efficiency, and carries shutdown risk. All of these are alleviated by the use of coal. A mixture of coal and biomass provides a stable and reliable feed supply for large-scale operations. Coal can be considered as the "fly wheel" that allows

TABLE 7.3

Advantages and Disadvantages of Use of Coal and Biomass Mixture Feed for Gasification

Disadvantages

Source: Prins, Ptasinski, and Janssen, 2006. Torrefaction of wood. Part 1. Weight loss kinetics, J. Anal. Appl. Pyrolysis, 77: 28-34; Shafizadeh, 1985. Pyrolytic reactions and products of biomass. In R. P. Overend, T. A. Mime, and L. K. Mudge (Eds.), Fundamentals of Biomass Thermochemical Conversion, London: Elsevier, pp. 183-217; and Shafizadeh, 1983. Thermal conversion of cellulosic materials to fuels and chemicals. In Wood and Agricultural Residues, New York: Academic Press, pp. 183-217.

a continuous plant operation when biomass fuel is not easily available. This concept in principle can be applied to any mixed feedstock. Co-gasification reduces the cost associated with fossil fuel consumption although some types of biomass can add significant cost to fuel production.

Co-gasification also reduces CO2 discharge in the atmosphere. When emis­sions related to harvesting, transportation, and other elements of the biomass supply chain are not included, biomass is considered to be a CO2-neutral fuel. A life-cycle assessment study shows that in comparison with coal-based systems, the use of biofuels for gasification results in environmental benefits. Unlike coal, the use of biomass fuel sources result in the generation of sig­nificantly lower quantities of anthropogenic CO2 emissions during power or fuel productions. A 70/30 mixture of coal and biomass generally produces a carbon-neutral process [15]. These advantages provide more security and less risk for the project financiers than the use of pure biomass, and are likely to engender more positive public attitudes toward the use of a fossil fuel sup­ply as a part of the mixed feedstock.

The mixed feedstock of biomass and coal also carries some disadvantages. As shown in Table 7.3, feed preparation and complex feed systems for mixed feedstock can be expensive. Two separate feed injectors, versus a single feed injector, may affect the gasifier performance. The gas cleaning system has the additional complications due to impurities in biomass. Although SOx emissions during gasification with mixed feedstock generally decreases,

NOx emission can increase, decrease, or remain the same depending on fuel type, firing, and operating conditions. Also, depending on the gasifica­tion technology, the presence of more tar or oil in the product gas may be problematic [1, 6]. Co-firing mixed feedstock with high chlorine content can increase corrosion in the system.

The ash coming from coal gasification processes is different from that coming from biomass gasification processes. When coal and biomass are gasified separately and ashes are kept separate, coal ash is generally used for the construction industry and biomass ash is recycled to the biomass origin or used as fertilizer, a building material, or as a fuel for power and heat gen­eration. The last option is possible only for biomass ash with a high energy content such as that from a fluidized bed gasification process. Biomass ash is often sent to a landfill for disposal. When coal and biomass are gasified together, the combined ash may not be useful for concrete and other con­struction industry applications. The slagging behavior of the combined ash of coal and biomass in the gasifier may also have a negative impact [1, 6]. The hydrophilic character of biomass and alkaline metals in biomass ash can also create fouling of heat transfer surfaces within gasifiers.

The effects of various physical properties and chemical constituents of biomass on the various aspects of gasification process are summarized in Table 7.4. These and other constraints of mixed feedstock processing as well as possible solutions are further discussed in subsequent sections.

TABLE 7.4

Effects of Various Physical and Chemical Properties of Biomass on Gasification Process

Relevant Physical or Chemical Properties

Source: Modified from Maciejewska et al. 2006. Co-Firing of Biomass with Coal: Constraints and Role of Biomass Pre-Treatment, DG JRC Institute for Energy Report, EUR 22461 EN.