November 19, 2008 | 15 Comments
Torrefaction is a thermo-chemical treatment of biomass in the 200 to 340 degrees Celsius range. In this process the biomass partly (especially the hemi-cellulose) decomposes, giving off various types of volatiles. The remaining torrefied biomass (solid) has approximately 30% more energy content per unit of mass.
Annually, photosynthesis is said to store 5-8 times more energy in biomass than humanity currently consumes from all sources. Biomass is currently the fourth largest energy source in the world – primarily used in less developed countries and could in principle become one of the main energy sources in the developed world.
Higher temperature gasification of biomass began in about 1800 and by about 1850 gas lighting for streets was commonplace. Before the construction of natural gas pipelines, there were many “gasworks” serving larger towns and cities in Europe and the US. During the petroleum shortages of World War II, almost a million gasifiers were used to run cars, trucks, and buses using primarily wood for a fuel.
With long experience with biomass gasifiers, reliable and large-scale operations continue to suffer from several problems. It’s difficult to scale-up fixed-bed gasifiers to a large capacity, most current large scale biomass gasifiers are based on bubbling fluidized bed or circulating fluidized bed technology. Sand is used as a heat carrier and the operating temperature is limited to 900-950°C to prevent ash sintering problems. These relatively low gasification temperatures form tar components. Then when cooling of product gas, condensation of these tars can lead to choking of equipment and piping. Although many technologies have been developed to handle the tar problem, such as physical separation, thermal and catalytic cracking, these methods add to the complexity and the investment cost.
The gas clean-up problems would be largely avoided if biomass were gasified in an entrained flow or flame gasifiers used elevated temperatures up to 1700°C. But that requires the use of pure oxygen and the removal and handling of molten ashes. The fibrous structure of wood and many other biomass feed stocks makes it difficult to grind the processed material to the desired size. For co-feeding of biomass with coal in thermal electric generating plants, a separate feed route is needed because the biomass cannot be ground small enough.
Torrefaction of biomass is being researched and developed as a certain amount of the energy potential previously fixed by biomass photosynthesis decreases with every conversion step. The goal is to develop optimized processes, in which the energy potential is largely retained, so that a high proportion of useful work is delivered from the biologically reformed energy while solving the production, transport and handling problems.
Torrefied biomass has excellent combustion properties; the fuel can be readily co-fired with coal, further gasified or fed to pyrolysis units. Biomass is typically thermally unstable which usually leads to formation of those condensable tars in gasifiers, making problems in down-stream equipment such as choking and blockage of piping. Just the thing to give engineers fits in designing processes competitive to biomass processing with biological tools.
Torrefaction is done in the temperature zone about a roasting temperature a kind of mild pyrolysis process that improves the fuel properties of biomass. At lower temperatures, now developed between 200°C and 300°C, torrefied products and volatiles are formed resulting in hardened, dried and more volatile free solid product. The product is at a much higher energy density than the raw biomass, increasing the distance over which the biomass can be transported to plants for use or processing the products further because of lower weights and volumes. Torrefied biomass is also hydrophobic, meaning it can be stored in the open for long periods without taking up water, similar to the infrastructures used for coal. Torrefied biomass requires less energy to crush, grind or pulverize and the same tools to crush coal can be used.
So what do you get in a torrefaction process? The solid product retains a high percentage of the energy content of the biomass feedstock, condensable gases such as water vapor, acetic acid and other oxygenates and non-condensable gases mainly carbon dioxide, carbon monoxide and small amounts of hydrogen and methane but also furfural, formic acid, methanol, lactic acid, phenol and others All useful things when a catalytic converter is used to convert the CO to CO2.
Biomass to combustion processes and chemical process to fuel may well benefit from introducing a torrefaction step as it can solve many problems from transport to process issues. What end products can be competitive besides just burning torrefied biomass? Can the solid products be used for making more hydrogen rich products such as methane to butane or prove useful in other ways in using biomass to recycle carbon into an endless loop of fuels powered by the sun’s solar radiation? More research would be beneficial.
In the U.S. Integro Earthfuels is focused on wood torrefied to displace coal in electrical generation and has announced it is finalizing off-take agreements with local utilities and Universities with their own heat and power plants to provide them with a majority of the supply beginning in 2009. Integro will build 10 additional facilities over the next 6 years to meet the demand from coal-fired electricity producers.
Meanwhile, the Energy Research Center of the Netherlands, sustainable energy consultancy Econcern and engineering and industrial investor Chemfo announced they have agreed to build a first commercial scale biomass torrefaction plant that will produce second generation biomass-based pellets for multiple applications.
There are several university studies done and in others need of further grant funding. It’s clear that from forestry waste products to wide scale carbon farming, torrefaction has yet to be fully considered for a role in energizing the future.
We’re seeing more steps to work out the path from sunlight and plants taking up CO2 to fuels that are used giving the CO2 back to the carbon cycle. If anything is clear, all the possible steps may not be known, nor are the possible paths fully explored and considered in an economic context. But one thing is certain; carbon will play an important role for centuries to come.