3.1 Introduction

The term "opportunity fuels" is typically defined by listing combustible resources that are outside the mainstream of commercial fuels, but that can be used productively in the generation of electricity or the raising of process steam and space heat in industrial and commercial applications. Under certain circumstances, materials once defined as opportunity fuels can emerge as mainstream and highly popular energy resources. Letheby ^{[1] [2]}, for example, contends that Powder River Basin coals originally were opportunity fuels that emerged as a dominant commercial energy source when industry learned how to handle and use these materials, and when regulatory and economic pressures provided a highly receptive marketplace.

Because opportunity fuels are outside the mainstream of fuels of commerce, the most common types are residues or low-value products from other processes. These can include, for example, petroleum coke, sawdust, hogged wood waste (mixtures of sawdust, hogged bark, planer shavings, and other solid wood products' residues), spent pulping liquor, rice hulls, oat hulls, animal fats and proteins from meatpacking and rendering plants, wheat straws and other straws from agricultural activities, unusable hays, and a host of other solid and liquid materials ^{[3] [4]}. In addition, blast furnace gas, coke oven gas, refinery off-gases, and like products also can be considered as opportunity fuels and used primarily on-site in processindustries. Because this book focuses on solid fuels, these liquid and gaseous fuels will not be discussed here, but several are...