Summary: Toasting wood can create torrefied wood for energy and biochar for soil improvement. Markets for these products have lagged, but if nations put a price on carbon, interest in these products is likely to grow rapidly.
Toasting wood is an ancient industrial process that can create a wide variety of products, from the lightly toasted inside of a bourbon barrel, to charcoal and the manufacture of advanced materials using activated carbon. The most widespread use is for cooking, with about 3 billion people dependent on charcoal for cooking.
Toasted wood products are all manufactured by pyrolysis – heating wood at low oxygen concentration. The process can be as simple as heating wood in a kettle that excludes fresh air, and as complex as a major industrial plant. The final product depends on the temperature of the toasting process. Low temperature toasting produces torrefied wood (the process is called torrefaction), with low moisture and high energy content. High temperature toasting produces charcoal, including the form of charcoal known as biochar, with a lower, but still substantial, energy content. Pyrloysis of wood is an ancient technology, but is now producing new materials and entering new markets.
Torrefaction is a familiar process to anyone who drinks coffee. Coffee is roasted at low temperatures in a low-oxygen roasting machine to produce the characteristic shades of brown that we are used to. You can do the same thing with wood: roast it at 200-300 C without oxygen and the wood turns a rich brown. It loses most of its moisture and some low-molecular weight molecules are lost. Lignin, the stuff that glues the cell walls of trees together, melts and migrates to the outside of the wood, making it water resistant. The volatile materials produced in the toasting process can be fed back into the torrefaction machine as fuel, offsetting some of the need for fossil fuels for toasting.
Torrefaction of solid wood products produces a highly stable product that is almost completely waterproof and decay resistant. A floor, door or paneling of torrefied wood will last far longer than a raw-wood product.
The greatest interest in torrefaction right now is for fuel pellets, sometimes called bio-coal. The US currently exports over 5 million tons of wood pellets per year for fuel, mostly from southern pine plantations. The pellets are almost entirely going to Europe. Wood pellets can be a sustainable fuel from renewable resources, but they have some disadvantages. The energy density of a wood pellet is about half that of a piece of coal. Wood pellets fall apart when wet, which makes them expensive to store.
Torrefaction increases the energy density of wood pellets and makes them water repellent. They can be stored outside, like coal, and have an energy density equivalent to some lower-grade coals. If done properly, using waste wood or wood from well-managed forests, torrefied wood pellets can be carbon neutral or nearly so.
Currently, US production of torrefied wood pellets for fuel is less than 5% of total pellet production. Technological impediments and market forces have kept the market from growing as rapidly as expected. However, several companies, including Solvay, and New Biomass Energy have moved out of the pilot plant and into industrial scale production, with contracts for shipments overseas.
Whether energy from wood is a sustainable, carbon-neutral process is still controversial. Our economy produces a large amount of waste wood. Logging for solid wood products leaves 30-40% of the wood – tops, twigs and poor stems – behind. Some of this needs to be left behind for soil enrichment, but a considerable amount can be harvested for wood energy. Land clearance also produces a large amount of wood that is not otherwise usable. Certification by third parties, such as the Forest Stewardship Council or the Sustainable Forest Initiative, is a critical part of creating a sustainable wood-based energy system.
For several thousand years, Indians of the Amazon enriched their soil with charcoal, creating a highly fertile soil know as terra preta de Indio (dark soil of the Indians). Under the name biochar, this ancient invention is now a potential tool for increasing agricultural productivity and sequestering carbon in soil.
Biochar can increase plant growth in some soils, probably by increasing the surface area available to soil microorganisms. Biochar can also sequester carbon in the soil. The terra preta de Indio has lasted thousands of years, representing a large amount of carbon sequestered in soil for a long time.
When initially rediscovered in the 1980’s terra preta received a lot of press attention and a great deal of enthusiasm. From backyard gardeners to agriculture policy makers, enthusiasts for biochar created a wave of interest, attracting investment capital, engineering skill and more than a few crooks. Growth of the biochar industry has been held back by widespread perceptions of fraud, overenthusiastic claims about the benefits of biochar, and the lack of a coherent climate change policy in the US.
Since then, biochar has developed a host of enthusiasts and backyard experimenters, as well as some serious science. More and more solid research is showing that biochar does enhance agricultural productivity in selected soil. The soils of Amazonia are oxisols, highly weathered tropical soils, and there is little doubt that biochar enhances oxisols throughout the tropics. For other soil tops, the results are less clear. Also less clear is whether biochar stays in the soil long enough to provide carbon sequestration benefits. Whether biochar sequesters carbon in the soil long enough to be considered an offsetting solution to fossil fuel carbon emissions is still an open question.
Analysis of multiple biochar studies suggests that biochar generally has a positive effect on soil quality and agricultural productivity, but the results are highly variable. The largest effects are in degraded soils, especially in the tropics. Because biochar is easy to make, agricultural villages in the tropics may be able to boost their farm productivity at low cost.
In the developed world, the level of enthusiasm for the material remains high, supporting a modest manufacturing industry. Many home gardeners and some commercial farms are using biochar as a soil amendment.
Climate Policy and Toasted Wood
Torrefied wood and biochar may be poised for a new surge of interest and investments in 2015 and beyond for one simple reason: the United States and other nations finally appear prepared to take concrete action to reduce fossil fuel greenhouse gas emissions. Placing a price on carbon emissions will stimulate interest in carbon-neutral fuels and carbon sequestration. Whether as an energy source or a method of sequestering carbon in the soil, torrefied wood and biochar may have a bright future.
If the market for torrefied wood and/or biochar does begin to grow, then questions will arise as to the total life cycle assessment of the materials and the sustainability of forests use to produce the feedstock.
Nature has a recent review of biochar research. A recent column in Forbes addresses the potential demand for toasted wood as fuel.