More recently, animal biomass has also been considered as a possible feedstock for smaller, on-the-farm combustion systems designed to properly dispose of animal solids and wastewater. Using commercially available equipment like solid separators, augers, and dryers, DB can be pre­pared for smaller combustion processes (Carlin etal., 2007). A study by (Rodriguez etal., 1998) investigated the effect of drying on biomass heating values. Moreover, additional information on biomass fuel properties and heating values can be found in Annamalai and Puri (2007) and Annamalai et al. (1987). In the following sections of this chapter, the facilities and experimental work being conducted at the TAMU Department of Mechanical Engineering will be discussed.

It has been proposed that cattle biomass (CB) and litter biomass (LB) can be used as a fuel source for power generation. The CB includes both FB and DB. Previous attempts to use CB as a sole fuel source in gasifiers or direct combustion have resulted in limited technical suc­cess due to the high moisture content, high ash content, and/or low heating value of manure (Sweeten et al., 2003). These properties which are found in most animal manure-based biofuels cause flame stability problems, and the high ash/soil content can clog conventional combustion devices and accelerate boiler tube erosion and corrosion. The more the volatile matter and lower the temperature at which volatiles are released, the better the flame stability in the boiler burner. Thus, co-firing biomass with coal may improve flame stability. FB, DB or LB could be used as a fuel by mixing it with coal and firing it in an existing coal suspension fired combustion system. This technique is known as co-firing. The high temperatures produced by the coal will allow the biomass to be completely combusted. If only 10% by mass LB is used, the fuel proper­ties will not change radically, and few adjustments will have to be made to existing combustion system equipment. Previous boiler co-firing experiments involving biomass with pulverized coal have included: wood waste (Gold etal., 1996), switch-grass (Aerts et al., 1997), straw (Hansen et al., 1998), sewage sludge, tire-derived refuse (Abbas et al., 1994), or grass (Spliethoff et al.,

1998) . Chapter 5 of this book deals with combustion of biomass fuels and various combus­tors used for energy conversion: pile, grate, fluidized beds, suspension burners (Desidiri and Fantozi, 2013).

The use of FB as a co-firing fuel was previously investigated by Frazzita et al. (1999) using a small-scale boiler burner to co-fire FB and coal under transient conditions. A lack of adequate insulation and steel combustor walls allowed the experiments to obtain only transient results at low temperatures. Additional co-firing experiments are summarized by Sami etal. (2001).

Figure 3.19 presents an overview of a number of co-firing plants in Europe. All together, there have been around 100 co-firing units in Europe. Co-firing plants in the Netherlands, Denmark, Finland and Sweden are mostly operating on a commercial basis while many of the plants in the UK are in trials or demonstrations. Positive experience in co-firing has been made mainly with woody biomass (EUBIA). There are different biomass employment methods for co-combustion and three main co-firing combustion methods (direct, indirect, parallel). Co-firing can be accomplished by different technologies like atmospheric or pressurized flu­idized bed combustors, pulverized or grate combustors (EUBIA). The majority of the European co-fired power plants operate in direct co-combustion with circulated fluid bed boilers (EUBIA).

It is apparent from a literature review that there are no prior data on the effect of co-firing low quality and high nitrogen DB on the combustion and emission characteristics.