New Potential ‘Star’ in the Biomass Gasification Realm
More than a decade ago, the U.S. DOE, the Energy & Environmental Research Center and KBR partnered to build a 1-megawatt (MW) version of a transport reactor integrated gasifier (TRIG) for converting coal to natural gas and other chemicals. Gasifiers are essentially low-oxygen thermochemical coal or biomass conversion systems that can provide a well-contained and clean synthetic gas for power and liquid fuels. A TRIG behaves more like a catalytic cracker used in oil refining and was installed in a 70-foot tower facility at the EERC. The transport concept is similar to a fluidized-bed gasifier, but allows for more coal feed and power output. The system can be scaled up to 300 MW with respect to power output from its synthesis gas. Since then, the EERC has conducted several thousand hours of testing on this large pilot-scale system in Grand Forks, N.D., including some new work related to biomass gasification.
Over the past several months, the EERC’s TRIG system has undergone modifications to accommodate pressurized biomass feeding at a level of 500 to 600 pounds per hour. After the successful feed system upgrade, several hundred hours of gasification testing has been accomplished for 100 percent wood residue, switchgrass, and corn stover biomass and 20 weight percent blends of these biomass types with subbituminous coal. The reason for the blend testing is that coal-fired utilities, in the U.S. and abroad, are considering TRIG-type systems for converting mixtures of biomass and coal, since some regions may be up against challenges in finding sustainable biomass supplies. Unlike Europe, the U.S. does not have strong enough incentives to encourage using biomass in larger systems. Converting fossil feedstocks to energy, even using conversion systems that are fitted with clean emissions control systems, are still cheaper than utilizing biomass feedstocks, blending biomass with coal makes sense right now.
Results of the recent biomass testing in the TRIG system were positive, however. Biomass materials were processed through a hammer mill with a 1/8-inch screen before being blended with the coal or before being fed into the TRIG system. Testing was done using air-blown and oxygen-blown gasification modes. Oxygen-blown gasification decreases the volume of greenhouse gas emissions and provides for more efficient separation and capture of carbon dioxide and other effluents. A larger-scale biomass TRIG system could enable effective biomass conversion and carbon capture, effectively providing a negative sum gain of greenhouse gas emissions.
The operating temperatures of the TRIG system are between 870 and 925 degrees Celsius at circulating velocities of 35 to 42 feet per second. Temperature and oxygen/air input play a major role in the TRIG system’s carbon conversion efficiency, synthetic gas quality and decreased production of unwanted tars. Carbon conversion was higher for the biomass tests than for straight coal or the coal/biomass blends. The syngas quality was excellent, showing nearly even levels of carbon monoxide and hydrogen, with much lower levels of methane. The main reaction chamber of the TRIG system showed volatile matter, organic carbon, aliphatics and methane reaching peaks at the midpoint of the chamber and then decreasing by the filter outlet. Hydrogen and carbon monoxide increased through the reaction chamber to the filter outlet. These reactions were a sign of good “cracking” of the syngas components and one reason for the excellent carbon conversion.
The quality of the syngas was important to research sponsors who were interested in converting the syngas to electricity (to utilize as a boiler fuel) and for the potential of reforming the cleaned gas to liquid biofuels such as green diesel.
In the future, more work will be done to optimize the TRIG system and to determine the costs for scaling up the system. Right now, these initial results hold promise for an upcoming new “star” in biomass conversion technology.
Author: Chris Zygarlicke
Deputy Associate Director, Energy & Environmental Research Center