Gradient Technology conducts research in the renewable energy arena with an emphasis on lignocellulosic biomass torrefaction and gasification. This research is conducted in conjunction with the State of Minnesota, the University of Minnesota, and the Natural Resources Research Institute, an affiliate of the University of Minnesota Duluth. We maintain a variety of research tools to investigate the torrefaction process and its application in adding value to underutilized wood species.
Renewable Energy— Lignocellulosic biomass, especially wood species, is an abundant resource in Minnesota and presents itself as a potential feedstock for energy generation and renewable chemicals. Both torrefaction and gasification are potential technologies that can be applied to biomass en route to renewable energy.
Torrefaction is a thermal treatment technology that produces a solid biofuel product with excellent handling, milling, and heating value properties. Torrefaction increases carbon content, reduces oxygen content, and eliminates storage hygroscopicity and degradation. Energy content per unit mass of torrefied product is increased; this is achieved at a cost of 10 to 20 % of the energy content of the incoming biomass. Much of the lost energy is in an off-gas that contains combustibles, which can be burned to provide some of the heat required by the torrefaction process. Hence, the net energy loss is small.
Gradient Technology has focused on developing torrefaction processes that utilize a moving bed reactor design with superheated steam for thermal treatment. These investigations have occurred at the laboratory and pilot scales. Alternative reactor designs for torrefaction include rotary kilns and fluidized beds. Our moving bed reactor has the potential to be more efficient than a rotary kiln or fluidized bed for biomass conversion because the gas and solid are in continuous direct contact so that temperature gradients required to drive heat transfer between the gas and solid can be smaller. In addition, there is no back mixing in a moving bed so that material with a very tight residence time distribution can be produced. This is important for the conversion process if the material produced will be subsequently densified in a pelleting or briquetting process.
Currently, a process demonstration unit is being commissioned at the Natural Resources Research Institute in Coleraine, Minnesota. This research unit operates as a moving bed and processes approximately 500 pounds of wood chips per hour. The resulting torrefied material is anticipated to be fed to a steam boiler for power generation. Alternatively, the torrified product can be gasified to produce syngas; torrefaction is an attractive pre-treatment technology for the gasification of lignocellulosic biomass since torrefaction yields a more uniform feed to a gasification process especially when varying wood species are utilized.
Gasification technology has been used to produce transportation fuels by way of syngas for decades. Gasification of coal is established technology and is used to produce synthesis gas that can be converted to liquid fuels. Methane reforming is another route to syngas. Both of these also use fossil fuel resources. In order to produce renewable, carbon-neutral transportation fuels, biomass must be used as a feedstock.
However, biomass gasification carries significant energy debits compared to coal and petroleum based materials due to the relatively low carbon content of cellulose. The gasification reactions are complicated by the presence of relatively high oxygen content, resulting in a significant amount of CO2 within the produced synthesis gas. Most biomass gasifiers currently in use or under commercial development operate at low pressures (< 100 psig) in order to achieve the desired thermal flux necessary to achieve high gasification yields and minimize the formation of undesired tar and soot. Most biomass gasifiers operate with significant temperature gradients (> 200°F) due to the need to supply heat for the endothermic reaction which produces syngas. Operations with direct firing often lead to localized high temperatures resulting in slag. Operations below the slagging temperature limit the amount of heat which can be brought into the system and often lead to excessive residence times to achieve the desired conversion.
As a result of the myriad of complications introduced by the use of lignocellulosic biomass in gasification, Gradient Technology has conducted research into the impact of pressure, temperature, heat and mass transfer, kinetics, residence time, and other factors on the performance of fluidized bed gasifiers that utilize torrefied biomass as a feedstock. Both theoretical and empirical models as well as laboratory-scale experimental investigations augment our research.
Gradient Technology has designed, fabricated, integrated, and operated a variety of pilot-scale systems for the investigation of torrefaction of various lignocellulosic biomass species including alfalfa, cellulose, corn cobs and stover, miscanthus, lignin, prairie grass, and straw. Wood species investigated have included balsam, black ash, tamarac and mixed tree tops. Operation of these processes is supported by our laboratory for the analysis of reaction products and final torriefied materials.
Gradient Technology also utilizes an electrically-heated sandbath with the ability to investigate up to four fixed bed reactors simultaneously that experience identical heating profiles. As a result, multiple biomass species can be investigated accurately and efficiently.
Lastly, Gradient Technology has the capability of investigating the grindability of biomass both before and after torrefaction. Our Thomas Model 4 Wiley mill has been instrumented so the grinding energy can be accurately quantified.