Another large source of renewable carbon supplies is waste biomass. It consists of a wide range of materials and includes municipal solid wastes (MSW), municipal biosolids (sewage), industrial wastes, animal manures, agricultural crop and forestry residues, landscaping and tree clippings and trash, and dead biomass that results from nature’s life cycles. Several of these wastes can cause serious health or environmental problems if they are not disposed of properly. Some wastes such as MSW can be considered to be a source of recyclables such as metals and glass in addition to energy. Thus, waste biomass is a potential energy resource in the same manner as virgin biomass.

To assess the potential impact of energy from waste biomass on supplying energy demand, it is necessary to consider the amounts of the different types of wastes generated, their energy contents, and their availabilities. Every person in the United States, for example, discards about 2.3 kg (5 lb) of MSW per day. From an energy standpoint, one short ton of MSW has an as-received energy content of about 9.5 GJ (9.0 X 106 million Btu), so about 2.2 EJ/year (2.1 quad/year) of energy potential resides in the MSW generated in the United States.

As for the amount of energy that can actually be recovered from a given waste and utilized, much depends on the waste type. The amount of available MSW, for example, is larger than the total amounts of available agricultural wastes even though much larger quantities of agricultural wastes are generated. This is caused by the fact that a larger fraction of MSW is collected for centralized disposal than the corresponding amounts of agricultural wastes, most of which are left in the fields where generated. The collection costs are prohibitive for most of these wastes. Note that municipal biosolids on a dry solids basis is generated in the smallest quantity of all wastes. Its disposal, however, is among the most costly and difficult of all waste treatment opera­tions.

Many studies have been carried out to estimate the potential of available virgin and waste biomass as energy resources. One is presented in Table 2.7 for the United States for the year 2000 (Klass, 1990). The estimated energy potential of the recoverable materials is about 25% of the theoretical maximum. Wood and wood wastes are about 70% of the total recoverable energy potential and 50% of the estimated maximum energy potential. These estimates of virgin and waste biomass energy potential are based on existing, sustainable biomass production and do not include new, dedicated biomass energy plantations that might be developed and placed in commercial operation.

An assessment of the energy potential of waste biomass that is more localized can often provide better leads for the development of biomass energy supplies. The results of one such preliminary study performed for the state of Indiana are summarized in Table 2.8 (Klass, 1981). Indiana is a farm state. More than 60% of the state area was devoted to cropland at the time of the study and about 52% of state area was under active cultivation. The major agricultural crop and farm animal wastes as well as forestry and municipal wastes were therefore selected for the assessment of waste biomass energy potential. The waste biomass generated in the state each year was first inventoried, and each waste was then converted to gross energy content using generic conversion factors as a first approximation of energy potential. Comparison of the results with annual commercial energy utilization in the form of petroleum motor

TABLE 2.7 Potential Biomass Energy Available in United States in 2000“

“Klass (1990). The energy values are the higher heating values of the indicated biomass or derived methane. The conversion of biomass or methane to another biofuel or to steam, heat, or electric power requires that the process efficiency be used to reduce the potential energy available. These figures do not include additional biomass that could be grown as a dedicated energy crop.

fuels indicated that grain crop residues, particularly corn and soybean residues, and cattle manures have the largest potential as feedstocks for conversion to substitute motor fuels. Most of the other wastes are generated in insufficient quantities to make a large contribution. This simple assessment provided direction to the initiation of programs to develop systems using waste biomass feedstocks generated in the state of Indiana.

An example of a different type of assessment of waste and virgin biomass energy potential is one performed for the state of Wisconsin, another farm state in the Corn Belt of the United States. This assessment evaluated the economic impacts of shifting a portion of Wisconsin’s future energy investment from imported fossil fuels toward renewable energy resources. It assumed a 75% increase in the state’s renewable energy use by 2010—775 MW of new electric generating capacity to supply electricity to 500,000 Wisconsin homes, and 100 million gallons per year of new ethanol production to supply gasohol (blends of 10 vol % ethanol and 90 vol % gasoline) to 45% of Wisconsin’s automobiles (Clemmer and Wichert, 1994). This scenario generated about three times more jobs, earnings, and output (sales) in Wisconsin than the same level of imported fossil fuel usage and investment, and was equivalent to 63,234 more job-years of net employment, $1.2 billion in higher wages, and $4.6 billion in additional output. Over the operating life of the technologies

analyzed, about $2 billion in avoided payments for imported fossil fuels would remain in Wisconsin to pay for the state-supplied renewable resources, labor, and technologies. Wood, corn, and waste biomass contributed 47% of the increase in net employment.

This review of the concept of utilizing biomass energy shows that when sufficient supplies of renewable carbon are available, virgin and waste biomass have the potential of becoming basic energy resources. Presuming that suitable conversion processes are available, and that the demand for energy and estab­

lished organic fuels and intermediates continues, an industry based on renew­able biomass fuels and feedstocks that can supply a significant portion of this demand is, at the very least, a technically feasible concept.