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radiant heat back to the char bed for the reduction chemistry to take place. The product chemicals are molten, drained from the char bed to collectors, and then poured into water to produce green liquor.

Thus, the pulp and paper industry offers unique opportunities for the production of synthesis gas insofar as an important part of many pulp and paper plants is the chemicals recovery cycle where black liquor is combusted in boilers. Substituting the boiler by a gasification plant with additional biofuel and electricity production is very attractive, especially when the old boiler has to be replaced. The equivalent spent cooking liquor in the sulfite process is usually called brown liquor, but the terms red liquor, thick liquor, and sulfite liquor are also used. Approximately seven units of black liquor are produced in the manufacture of one unit of pulp (Biermann, 1993).

      2.3.8 Mixed Feedstocks

      Any carbonaceous feedstock may be co-gasified with waste or biomass for environmental, technical or commercial reasons (Chapter 5, Chapter 6, Chapter 7). It allows larger, more efficient plants than those sized for grown biomass or arising waste within a reasonable transport distance; specific operating costs are likely to be lower and fuel supply security is assured.

      Although the gasification processes used for individual feedstocks are relatively straightforward, process efficiency depends for the most part on the unique characteristics of each feedstock. In addition, the non-uniformity of the feedstocks and the variability of the specific compositions over time require flexible and robust gasifiers. For example, consideration should be given to the individual constituents of each feedstock to determine if there are any reactions between the constituents that can have an adverse effect on the process. An example is the composition of any mineral matter that may cause the constituents to interact to form a troublesome slag that may cause harm to the gasifier.

      However caution is advised when using mixed feedstocks for gasification or, for that matter, for any conversion process. It has been the method in the past (and even continued to be the method in some cases) to assess the properties of the mixed feedstock by calculating an average value for one or more of the properties of the mixed feedstock. This is a dangerous practice because it ignores the potential for interaction of the contents of each feedstock that can cause changes in the chemistry of the process as well as a loss of process efficiency.

      One approach to avoid a significant decrease in conversion efficiency is to develop a formulation of mixed feedstocks in order to produce a more consistent material. Formulation combines various preprocessed resources and/or additives to produce a feedstock that provides process consistency.

      Feedstock formulation is not a new concept and has been used for decades by the coal industry. For example, different grades of coal are blended for power generation to reduce the sulfur content and the nitrogen content of the feedstock. In the current context, biomass blending feedstocks refers to the combination of multiple sources of the same biomass resource to average out compositional and moisture variations, whereas aggregation refers to the combination of different raw or preprocessed biomass resources to produce a single, consistent feedstock with desirable properties. Examples include mixing blended corn stover with blended switchgrass; mixing blended wheat straw with blended softwood residuals; and mixing blended Miscanthus with blended rice hulls. This strategy allows the desirable characteristics of many types of feedstocks to be combined to achieve a better feedstock than any of the feedstocks alone (Shi et al., 2013; Lu and Berge, 2014).

Examples to emphasize the above considerations are presented below and relate to (i) the gasification of biomass and coal, (ii) the gasification of biomass and municipal solid waste.

       2.3.8.1 Biomass and Coal

      The gasification of biomass and coal (Chapter 4, Chapter 6) blends is of considerable current interest because of the reduction of the usual high yield of tar products that result from biomass gasification. Various operations involved in the biomass-coal-gasification process such as the typically high moisture-content biomass is usually not just dried, but also subject to torrefaction which involves heating to temperatures typically ranging between 200 and 320^oC 390 to 610^oF) in the absence of oxygen, at which point the biomass undergoes a mild form of pyrolysis) and possibly compacted – this improves the quality as a feedstock for the process. Also, size reduction of both the biomass and the coal to uniformly sized particles is required for optimum gasification.

      The product gas compositions are influenced by both the type of biomass co-gasified, as well as its proportion in the feed mixture. Generally, higher H₂ content results from greater biomass inclusion; in particular, lignin in woody biomass seems to boost H₂ yield in syngas. A wide range of proportions of coal and biomass may be possible for given applications, but the optimum is a complex function of the type of coal used, type(s) of biomass, gasifier type and operating conditions, desired syngas composition, etc., not to mention the available quantities of the biomass which may be considerably less than the coal available.

      Furthermore, the cleanup of synthesis gas derived from mixed feedstocks may be more complicated than for gasification or of the individual feedstocks because the species present in each raw feedstock as well as those (environmentally unfriendly) species that are present in elevated amounts from, say, biomass gasification (such as tars and alkalis) may need to be addressed.

       2.3.8.2 Biomass and Municipal Solid Waste

      The gasification of biomass and municipal solid waste differ in many ways from the gasification of crude oil coke or conversion of natural gas to synthesis gas. While the gasification technologies used with biomass (Chapter 6) or municipal or municipal solid waste (Chapter 7) are relatively straightforward, performance depends greatly on the unique characteristics of the feedstock. These feedstocks have much higher moisture content and less heating value by volume than coal. In addition, the non-uniformity of the feedstocks and the variability of the specific compositions over time require flexible and robust gasifiers.

      Co-gasification technology varies, being usually site specific and feedstock dependent. In fact, biomass and municipal solid waste feedstocks are highly variable feedstocks that present issues for feed systems as these feedstocks are largely heterogeneous in their delivered state. Some biomass, such as sawdust from lumber mills, can be in a condition suitable for many existing feed systems most municipal solid wastes require extensive preparation or feed system customization. Biomass and municipal solid waste may also have characteristics such as higher moisture content which may necessitate pre-gasification drying. The mineral matter content of each feedstock can also vary widely and the gasifier must be able to handle variable (even high) levels of mineral matter and the ensuing ash.

At the largest scale, the plant may include the well-proven fixed-bed and entrained-flow gasification processes. At smaller scales, emphasis is placed on technologies which appear closest to commercial operation. Pyrolysis and other advanced thermal conversion processes are included where power generation is practical using the on-site feedstock produced. However, the needs to be addressed are (i) core fuel handling and gasification/ pyrolysis technologies, (ii) fuel gas clean-up, and (iii) conversion of fuel gas to electric power (Ricketts et al., 2002).

2.4 Influence of Feedstock Quality

      The influence of physical process parameters and the effect of feedstock type on gasification include, for example, the fact that the reactivity of coal generally decreases with increase in rank (from lignite to subbituminous coal to bituminous coal anthracite). Furthermore,

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