7.1 Introduction

As the world supply and demand picture for fossil energy changes and the environmental regulations for greenhouse gas (GHG) emissions become more stringent, more efforts are being made to find alternate sources of energy. One source is renewable bioenergy which addresses environmen­tal regulations. Although bioenergy currently contributes to only a small percentage of worldwide energy production (it is about 5% of European Union energy supply and smaller in the United States), its worldwide usage is rapidly increasing. Furthermore, recent advances in sustainable waste management provide an additional opportunity for converting various cellulosic and polymer waste, rubber tires, MSW, and the like, to energy and products. Types of biomass feedstock used for energy purposes are described in Table 7.1 [1-3].

Although the use of biomass for power and fuel brings environmental benefits, its use involves high investment costs. Furthermore, the use of biomass raises concerns about the security of feedstock supply particularly for large power and fuel plants. The feedstock supply issue with biomass is caused by (a) the seasonal nature of biomass, (b) biomass resources are dispersed in many countries and an infrastructure for the biomass supply is not established, and (c) transportation of biomass can be very expensive because of its low mass and energy densities. Lower heating values and lower bulk densities compared to coal result in a much greater volume of biomass to be transported, handled, and stored and as a consequence, large biomass units (>300 MWe) are economically unattractive [4]. The argument of an inconsistent and unreliable feed supply to large plants also applies to the waste industry.

The biomass demand in stationary applications (heat and power) is the main driving force behind early expansion of bioenergy and this will remain the major demand source for bioenergy up to 2030. However, the need for more improved and efficient generation of stationary bioenergy, as well as the new efforts to use biomass and waste to generate biofuels and other products, will stimulate more and more use of biomass. These expansions

Types of Biomass Feedstock Used for Energy Purposes [1-3]

Source: 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; Loo and Koppejan (Eds.) 2004. Handbook of Biomass Combustion and Co-Firing, Prepared by Task 32 of the implementing agreement on bioenergy under an auspices of the international energy agency, Twente University Press; VIEWLS. 2005. Biofuel and Bio-Energy Implementation Scenarios, Final report of VIEWLS WP5, modeling studies.

will also stimulate the establishment of a new and improved supply infra­structure for biomass in the long term.

At the present time, a mixed feedstock of coal and biomass offers a possible solution to the above-described problem. Such a mixture can help mitigate environmental concerns of plants running on coal alone. On the other hand, coal can mitigate the effects of variations in biomass feedstock quality and buffer the system when there is a lack of sufficient required biomass quantity [3,5]. A mixed feedstock can be used in large units that have better thermal and economical efficiencies compared to small-scale systems. Furthermore, it is possible to adapt existing coal power plants or coal-based fuel refineries for mixed feedstock at a relatively lower cost compared to building new and dedicated systems [4].

The use of mixed feedstock started in co-firing because its major purpose was to generate heat and electricity and the compositions of gas and sol­ids were not important toward the end-use. However, now in addition to combustion, other thermochemical technologies such as gasification, plasma technology, pyrolysis, liquefaction, and supercritical technology have been further developed to generate heat, electricity, transportation fuel, chemi­cals, and materials. The use of mixed feedstock in these technologies is more complex because of the effects of mixed feedstock on the gas, liquid, and solid compositions and their subsequent use. As shown in Tables 7.2a to 7.2c [6, 7], the chemical compositions of various raw materials that can be used within a mixed feedstock vary substantially and these can significantly affect the gas, liquid, and solid product compositions.

Up till now at larger scales, co-utilization of waste and coal has received considerably more attention than co-utilization of coal and biomass. This is true for all thermochemical processes such as combustion, high severity pyrolysis, gasification (including supercritical), and plasma technology that are generating heat, electricity, or gaseous fuels and products. Investigated waste has been municipal solid waste (MSW) that has had minimal presort­ing or refuse-derived fuel (RDF) that has had significant pretreatment such as mechanical shredding and screening as well as shredded rubber tires, paper and pulp waste, and plastic waste.

In recent years, co-utilization of coal and biomass in combustion, gasifica­tion, pyrolysis, and plasma technology has been gaining significant accep­tance. Recent reviews of cofiring literature identify over 100 successful field demonstrations in 16 countries using many types of biomass in combination

Source: Modified from Sami, Annamalai, and Wooldridge, 2001. Co-firing of coal and biomass fuel blends, Prog. Energy Combust. Sci., 27: 171-214.

TABLE 7.2C

Source: Modified from Sami, Annamalai, and Wooldridge, 2001. Co-firing of coal and biomass fuel blends, Prog. Energy Combust. Sci., 27: 171-214.

with various types of coals in boilers [8]. Significant new efforts at the labo­ratory level as well as at semi-commercial or commercial scales to gener­ate energy or useful products have been carried out and these are reviewed in this chapter. A recent position paper indicates that co-firing represents among the lowest risk, least expensive, most efficient, and shortest term option for renewable-based electrical power generation [8]. Although in gen­eral co-firing is more expensive than the power plants based strictly on coal, CO2 emission reduction, global climate change mitigation, and sustainable resource management for waste on a large scale are the main motivations behind the development of mixed feedstock strategy.

In this chapter, our discussions on various thermochemical technologies processing a mixed feedstock are divided in two parts: those that are largely generating gases such as combustion, gasification (including high tempera­ture supercritical gasification and reforming technology), high severity (i. e., high temperature and high residence time) pyrolysis, and plasma technology;

and those that are focused in generating liquids such as low severity (i. e., low contact time or low temperature) pyrolysis, liquefaction (either by water or an organic solvent), and supercritical fluid extraction (either by water or other chemical substances). Because biochemical technologies are not extensively used for fossil fuels such as coal, tar sand, shale oil, and the like, their appli­cations for a mixed feedstock are not addressed here. Gasification technolo­gies are used for a mixed feedstock on a larger scale, and the development of liquefaction technologies for mixed feedstock is still at the laboratory and small scale stages.

Combustion, gasification, plasma technology, and high-severity pyroly­sis are commercially proven technologies for a single feedstock, however, improvements are constantly being made to adapt these commercial opera­tions for mixed feedstock. Major challenges in mixed feedstock processing are biomass fuel preparation, storage, and handling [8-10]. Other problems are associated with poor or incompatible fuel quality and these include fuel feeding, co-milling, deposit formation, increased corrosion and ero­sion, and need for new fly ash (and in general slag) utilization schemes [8, 10, 11]. These challenges are not significant in a mixed feedstock with low concentration of biomass but become more important as the concentration of biomass increases, particularly when low-quality biomass is used. In these cases an economy of the overall plant may be significantly affected. As for example, herbaceous biomass and coal are not as good a mixed feedstock as wood chips and coal. Similarly, stringlike biomass (e. g., straw or switchgrass) is not as good as RDF in the form of well-mixed pellets. As shown later, these drawbacks can to a certain extent be avoided by application of appro­priate biomass pretreatments. The present chapter addresses these topics. Also, as shown later, several gasification technologies using mixed feedstock have been demonstrated commercially and these are briefly described. The development of supercritical gasification for a mixed feedstock is only at the laboratory and demonstration levels. Unlike gasification, direct liquefaction technologies are still at a laboratory or a small-scale developmental stage.