Controlled Spontaneous Reactor
Last month, we highlighted an old technology that is resurging as a potential answer to technical issues involved in processing biomass into a suitable cofiring boiler fuel. It is called torrefaction. The largest impediments to the utilization of biomass as a cofiring fuel with fossil fuels in larger-scale utility boilers are the handling and conveyance properties of the biomass material. Torrefaction may provide an answer.
The Energy & Environmental Research Center and other entities have developed and patented various systems. The EERC’s technology is called the controlled spontaneous reactor (CSR). The CSR was initially a type of torrefaction technology designed for high-moisture, low-rank coals where moisture and volatiles were driven off at lower temperatures, leaving an appropriately sized char that burned, releasing more heat, then reignited above the coal burners, yielding lower-nitrogen-compound pollutants. The EERC has now adapted the technology for biomass in order to convert higher moisture and low-density biomass into char biomass materials that have similar physical properties as crushed coal particles and blend effectively with coal in a cofiring scenario for conventional large-scale utility boiler systems. The added major benefit of upgrading biomass instead of coal, of course, is the lower carbon footprint that results (lower net greenhouse gas emissions) compared to coal.
The CSR technology is a fluidized-bed technology that utilizes a proprietary fluidization plate to effectively control the velocity profile across the bed for well-controlled sizing of the produced fuels. The CSR is an atmospheric system that can be utilized as a stand-alone torrefaction system. Another rendition of the technology may be to integrate it with an existing boiler and utilize available low-grade heat, such as the exiting flue gas, to provide heat for the torrefaction process. If no source of low-grade heat is available, a portion of the biomass feed material is consumed to produce the required heat, lowering the overall efficiency of the process.
One of the major advantages of the CSR over other torrefaction technologies is its ability to handle a wide variety of feedstocks. The fluidized-bed system is forgiving for a range of biomass sizes from several centimeters to a few millimeters. The system can be operated in an optimized mode for a specific biomass’s physical and chemical properties, such that the produced fuels have consistent and improved grindability and handling properties along with low moisture, low volatiles, and a much-improved heat content. Additionally, because of its design, the system acts as a classifier to remove tramp metals (unwanted nails, fence wiring, machinery parts or other metals that can be associated with wood resources) and other unwanted materials carried in with the feedstock.
In conclusion, although biomass is simply very, very young coal, it does not have the high density and heating properties that coal has, which contributes to a significant economic challenge. Torrefaction may be one solution for processing biomass into a usable fuel for utility boilers. The EERC’s CSR technology, a pseudotorrefaction system, and several other torrefaction systems in development around the globe might provide solutions for cofiring. Torrefaction systems appear to have a lower cost and smaller design footprint, they allow for a wide range of biomass feedstock types, and they produce a consistent fuel product. Several different demonstrations are ongoing with limited reportable insights at the moment. Stay tuned for more factual torrefaction results in the near future.
Author: Bruce C. Folkedahl
Senior Research Manager
Energy & Environmental Research Center