Jacob J. Jacobson and Kara G. Cafferty

Idaho National Laboratory, U. S.A.

2.2 Introduction

In 1978, the United States enacted the Public Utilities Regulatory Policy Act (PURPA) giving small electricity producers (less than 80 MW) a natural monopoly by requiring electric utilities to purchase the small companies’ surplus electricity at a price equal to the cost the utility would have incurred by producing the electricity themselves. As a result, biopower experienced a threefold increase in grid-connected capacity, created 66 000 jobs, and had an industrial investment of $15 billion dollars during the next decade. Despite these historic advancements, biopower has not experienced further substantial growth. Currently, avoidance costs from electric utilities remain low due to the vast supply of natural gas and innovations in natural gas turbines. As a result, it is difficult for renewable fuels to compete and developments in renewable energy technology have slowed (Figure 3.1). However, interest in environmental sustainability has caused some state governments to implement renewable portfolio standards (RPSs) requiring that a minimum amount of renewable energy (wind, solar, biomass, or geothermal) be included in the electricity generation portfolio of each state. As of February 2012,30 states and the District of Columbia have enforceable RPS programs [2]. Despite these regulations, biomass makes up a small portion of the current power industry because of high biomass feedstock costs and low overall efficiencies.

Biopower produces less than 2% of the total electricity in the United States [1], while coal and natural gas supply over 65%; hydropower and nuclear make up the remainder. The primary reasons biopower contributes such a small percentage of the overall electricity production are the size and efficiency of its plants. The average size of biopower plants is

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

20 MW (maximum 75 MW); while a typical coal plant ranges from 100 to 250 MW. The small plant sizes (which lead to higher capital cost per kilowatt-hour of power produced) and low efficiencies (which increase sensitivity to fluctuation in feedstock price) have led to electricity costs of 8-12 c/kWh [3]. Therefore, for biopower to increase its contribution to the U. S. energy supply, plant size and efficiency must increase to be competitive with current fossil fuel technologies. Additionally, as biopower increases the demand for biomass supply will tend to increase the price of biomass. For the biopower industry to continue to expand the biomass must remain cost competitive.

Biomass is a desirable source of energy because it is renewable, sustainable, widely available throughout the world, and amenable to conversion. Biomass is composed of cel­lulose, hemicellulose, and lignin components. Cellulose is generally the dominant fraction, representing about 40-50% of the material by weight, with hemicellulose representing 20-50% of the material, and lignin making up the remaining portion [4-6]. Although the outward appearance of the various forms of cellulosic biomass, such as wood, grass, munic­ipal solid waste (MSW), or agricultural residues, is different, all of these materials have a similar cellulosic composition. Elementally, however, biomass varies considerably, thereby presenting technical challenges at virtually every phase of its conversion to useful energy forms and products.

Despite the variances among cellulosic sources, there are a variety of technologies for converting biomass into energy. These technologies are generally divided into two groups: biochemical (biological-based) and thermochemical (heat-based) conversion pro­cesses. Although there are specific technologies within each of these general categories, biochemical conversion technologies (i. e., enzymatic hydrolysis), generally operate on wet feedstocks with a high carbohydrate content at the time of conversion [7]. In contrast, ther­mochemical conversion processes (e. g., combustion, gasification, and pyrolysis), generally require a dry feedstock, low in ash content, and having a small, consistent particle size [8,9]. As a result of these generalizations, herbaceous feedstocks that are naturally higher in ash and carbohydrates are generally allotted to biochemical conversion, while woody

feedstocks with their lower ash content are directed to thermochemical conversion. Ther­mochemical conversion is more aptly used to create heat and electricity due to destruction of chemical bonds while biochemical processes are more suited to develop liquid fuels. With the exception of anaerobic digestion, biochemical conversion is not discussed in this chapter. The main thermochemical processes under which biomass can be converted into energy include:

• Combustion

• Gasification

• Pyrolysis

• Hydrothermal Liquefaction.

In general, the specific thermochemical process being used is determined by the operating air supply and temperature conditions. Combustion occurs in the presence of excess oxygen, gasification takes place when the quantity of oxygen is insufficient for stoichiometric requirements, and pyrolysis happens in the complete absence of air. As a result, gasification can actually be characterized as an intermediate between combustion and pyrolysis; an alternative between having an over-sufficient oxygen supply to biomass and its absolute absence from the process. The operating conditions required for the main thermochemical conversion processes are summarized in Table 3.1.

2.3 Combustion

Combustion or burning is the most common means of converting biomass to usable energy and heat. Historically, woody biomass, from timber harvesting, sawmills, and pulp and paper production, has been used to generate electricity and heat at co-located, direct-fired boilers. Agricultural residue, primarily from wheat and corn harvests, has also contributed to biopower production. These practices have grown the biopower industry into the third largest generator of renewable electricity in the nation, providing 12% of the United States’ renewable generation capacity in 2010 [11]. There are two mainstream methods of com­bustion, direct combustion and co-fired combustion.