Abstract
In the next quarter-century, global demand for energy is expected to increase more than 25%, while some analysts are predicting that output of petroleum will soon peak. This reality of increasing demand in the face of diminishing fossil supplies is spurring interest in renewable energy sources. An array of biomass-for-bioenergy resources has been proposed, with perennial, lignocellulosic feedstocks showing the greatest potential. Assessment of potential biomass energy resources is difficult, however, as uncertainties over available land and crop yields swing reported estimates from 35 to 1135 EJ/year. In the USA, it has been suggested that more than 1 billion tonnes (910 million Mg) of biomass could be sustainably harvested, but these estimates are dependent on continued gains in plant productivity, nutrient use efficiency and soil and water conservation. Variables of population growth and increased standards of living will also affect the availability of land for these energy-producing endeavours. Several biofuel sources have been identified to include waste streams, microalgae and woody biomass plantations. With herbaceous-based systems, much effort is currently being given to corn and other starch or grain crops that can be readily converted to ethanol. While these crops may serve to jumpstart the biofuel industry, they have much less potential to meet demand and much greater potential for negative environmental impacts. Long-term, high-yielding perennial species will be better suited to meet the requirements for sustainability, but they are currently plagued by limitations in pretreatment and processing technologies. Further upstream of the conversion facility loom the questions of logistics, e.g. how does one handle and move a bulky, distributed resource in a cost-effective manner? In addition, there are important interactions between each component of the supply chain - agronomy, logistics and processing - that are best not studied in isolation. Important social issues also stand to influence farmer decisions regarding market entry which will affect in turn the function and profitability of a biorefining industry. Once at the facility, several possible biomass conversion technologies may be employed, including (but not limited to) pyrolysis to syn-gas and bio-oils or sugar hydrolysis and fermentation to ethanol. Both have drawbacks, and the limitation for cellulosic feedstocks remains an economical method of conversion. It is widely believed that this challenge will be met, but the question of 'when?' remains. Other, larger questions about the development of bioenergy resources reside outside the system, with policy being one of the biggest. For example, bioenergy systems may be more cost-competitive if policy allows them to benefit from the potential ecosystem services they provide, e.g. sequestering carbon. However, such policies may also encourage farmers to convert food crop land to energy crop land, with concomitant ripple effects throughout the economy. These and other policy-related risks increase uncertainty; this uncertainty will limit investments, especially large-scale investments, in such industries. Bioenergy systems may also face social hurdles from those who view them as inimical to the interests of local and global producer.