Second-generation technologies under development are expected to produce economically competitive liquid biofuels that can be used for transport from cellulosic feedstocks, including both agricultural residues and wood. It is anticipated that the technology for commercially competitive conversion of cellulose to liquid biofuels will be available within ten to fifteen years (Worldwatch Institute, 2007). Demonstration scale production is already under way (see www.iogen.ca), with bioethanol being the cellulosic liquid biofuel closest to commercialization. The United States Government is currently investing in small-scale cellulosic biorefineries (US Department of Energy, 2008).
Agricultural residues are likely to be among the lowest-cost liquid biofuel feedstocks. Bagasse and residues from the production of cereals, including maize, wheat, barley, rice and rye, are among the feedstocks that can be used to generate bioethanol. However, only about 15 percent of total residue production would be available for energy generation after accounting for needs related to soil conservation, livestock feed and factors such as seasonal variation (Bowyer and Stockmann, 2001). As bioenergy production increases, agricultural residues may become more important biofuel feedstocks, and their availability could increase through improved management practices.
Residues from the forest products industry and wood from forest plantations provide other potential sources of feedstock for commercial cellulosic biofuel production. Today, only a small proportion of liquid biofuels are forest-based, but the development of an economically viable process for producing cellulosic liquid biofuels could lead to the widespread use of forest biomass in the transport sector.
Two basic technologies are being developed to convert wood to liquid fuels and chemicals: biochemical conversions and thermochemical conversion (gasification or pyrolysis). In biochemical conversion, wood is treated using enzymes to release hemicellulose and cellulose as sugars. These sugars can then be further converted to ethanol or other products. The lignin residue is also converted to other products, or used to provide heat and power for the plant’s operation or for sale.
In gasification, wood and bark are heated in the minimum presence of oxygen to produce a mixture of carbon monoxide and hydrogen, which, after clean- up, is referred to as synthesis gas (syngas). Syngas may be further converted to liquid transportation fuels. Pyrolysis is the process of treating wood at a lower temperature, in the absence or minimum presence of oxygen to convert wood to char, non-condensable gases and pyrolysis oils. Pyrolysis oil may be used directly for fuel or refined into fuel and chemicals.
Currently, biochemical conversion technologies require clean wood chips (without bark), which could draw on the same wood resources as pulp mills. Thermochemical conversion, however, can use a mix of wood and bark.
An interesting prospect is that of biorefineries, which are expected to produce not only heat and power, but also transportation fuels and industrial products. Modern pulp mills, which in some cases are net producers of heat and power, can be described as prototypes of biorefineries. The vision is that pulp mills will go from being large energy consumers and producers of only pulp and paper, to being producers of pulp and paper, as well as heat, electricity, transportation fuels and speciality chemicals. There is potential for adjusting the product mix to market situations, thus optimizing the profit made from a given amount of wood (UNECE/FAO, 2007).
It is probable that second-generation processes will be more profitable when integrated into existing manufacturing facilities, such as paper mills, that produce or have access to low-cost or by-product biomass (Global Insight, 2007). Cellulosic ethanol production is likely to be limited outside the United States, Europe, and Brazil due to the limited size of the expected markets and the availability of imports.
At present, the United States is among the most advanced countries in terms of cellulosic conversion. Support there is being given for the development of integrated forest biorefineries that would be added to existing pulp mills and produce renewable bioenergy and bio-products from forest and agricultural materials (UNECE/FAO, 2007). Current efforts are in three focal areas:
- seeking cost-effective processes to separate and extract selected components from wood prior to pulping for use in producing liquid fuels and chemicals;
- using gasification technologies to convert biomass, including forest and agricultural residues and black liquor, into a synthetic gas, which is subsequently converted into liquid fuels, power, chemicals and other high- value materials;
- enhancing forest productivity, including developing fast-growing biomass plantations designed to produce economic, high-quality feedstocks for bioenergy and bio-products.
The development of technologies for production of biofuels from cellulosic sources holds great promise for the use of wood in energy production. The fact that advanced technologies will be required, however, places constraints on the global availability of systems to convert wood and other cellulosic feedstocks into liquid fuels. The Institute for Agriculture and Trade Policy has warned that patent policy and the cost of patent royalties and licensing fees will influence the adoption of biofuels (IATP, 2007). In addition to the technological and economic issues, an understanding of patent policy on biomass and biofuels production is of crucial importance in understanding how biofuel technologies might contribute to sustainable development.
Countries and private companies considering the production of second- generation liquid biofuels from cellulosic biomass face an uncertain, if potentially lucrative, future. The development of technologies for the competitive production of liquid fuels from wood will require time and significant investment in research. Considerable investment is also needed for large-scale facilities, especially for gasification. It should be noted that the high oil prices in the early 1980s resulted in a number of gasification plants for the production of methanol from wood, particularly in a number of European countries. These, however, were eventually undercut by lower oil prices (Faaij, 2003). The risks associated with investment in second-generation liquid biofuels are relatively high; therefore most developing countries will probably explore other options fully before embarking on this venture.