Torrefaction of Biomass Materials
One of the largest impediments to utilization of biomass as an augment to fossil fuels in larger-scale utility-scale boilers (cofiring) is the handling and conveyance properties of the biomass material. Biomass is derived from a plethora of plants and organic materials, therefore, as a fuel, it is unlike more common solid fuels such as coal (see Biomass Power & Thermal, May 2011 issue, Energy Review column by Carolyn Nyberg, manager of EERC's Analytical Research Laboratory).
The processing and handling of these various biomass types is not well-understood because such low volumes are consumed for utility power. In contrast, literally billions of tons of coal have been at the forefront of utility-generated electricity for more than a century. Utilities and coal companies are well-versed in mining, processing, handling and burning coal in boilers. Boilers are specifically designed for coals of certain ranks, such as bituminous, subbituminous and lignitic coal. There is no similar ranking for biomass. And since the physical properties of biomass materials are highly diverse, so are the costs for getting these fuels from the field or forest into the boiler.
Most raw biomass has a relatively low-energy density and high- and variable-moisture contents, can be very hygroscopic. Because of high moisture and hygroscopicity, Biomass tends to rot during storage and has a tendency to have a fibrous nature that can make it difficult to grind into small particles with the conventional sizing technologies that are incorporated at fossil-fired utilities. Sizing biomass for use as a fuel can be a major expense and difficult, with each decrease in size adding attendant cost.
Coarse biomass materials (greater than 1 inch), such as corncobs and wood chips, can be and are fired in stoker-feed and bubbling-bed systems, usually small industrial-sized systems. Circulating fluidized-bed systems and large industrial and small utility-sized systems require additional size reduction (typically 6.4 millimeter (one-fourth inch)). To use biomass in large industrial- and utility-sized, conventional suspension-fired systems or advanced entrained-flow gasification systems, the biomass must be significantly reduced in size. This facilitates efficient combustion, and the use of existing feed systems. For suspension-fired, coal-based power systems, the fuel is typically sized to 80 percent less than 200 mesh (0.003 inches). As most biomass material will have a lower density coupled with a higher reactivity rate, sometimes the biomass can be sized slightly larger, when compared to coal fired in the same system.
One processing method that can actually increase the usability of the biomass in conventional fossil systems is torrefaction, a technology that has been more popular in Europe than in the United States over the past decade. Torrefaction can reduce the moisture of the biomass, increase the energy density, improve the grindability, and enhance the ease of conveyance, all of which are necessary for increased utilization of biomass in large-scale utility systems. Most of the information coming to the public from torrefaction systems is derived from experimental or demonstration systems. Various entities, including the Energy & Environmental Research Center have developed these systems.
Next month, we will highlight a type of torrefaction system being developed at the EERC and give more specifics on how these torrefaction systems may improve the use of biomass as a boiler fuel in utility boilers.
Author: Bruce C. Folkedahl
Senior Research Manager
Energy & Environmental Research Center