Thermochemical Conversion of Biomass

Biomass feedstocks offer a number of distinct advantages over coal. Biomass typically possesses a higher hydrogen content and larger volatile component, produces a more reactive char upon devolatilization, and exhibits lower ash and sulfur contents. Biomass, when grown and converted in a closed-loop feedstock production scheme, generates no net carbon dioxide emissions, thereby claiming a neutral position in the build-up of atmospheric greenhouse gases. Closed-loop systems offer additional benefits by providing more markets for agricultural producers and creating demand for services and infrastructure.

The Hawaii Natural Energy Institute's (HNEI's) program in biomass energy conversion includes experimental and modeling thrusts, including studies of biomass pretreatment, conversion processes, and downstream processing. Biomass pretreatment focuses on improving biomass material characteristics to facilitate use in target conversion processes. An example of this would be pretreating high-alkali-content, herbaceous plant species to remove inorganic elements and improve fuel characteristics for thermochemical applications. Studies of thermochemical conversion processes are part of the biomass energy program with emphases on combustion and gasification.

An instrumented, bench-scale, bubbling fluidized bed gasifier facility has been used to test numerous biomass materials and determine operating limits, fuel behavior, producer gas quality, and fuel element inventories. Evaluation of downstream gas processing options is also part of the biomass energy conversion program and has included unit technologies, such as filtration, catalytic conversion, and solid sorbent purification, for selective removal or conversion of producer gas components. In addition to these experimental activities, HNEI conducts modeling to broaden the application of experimental results.

Biomass Treatment

Biomass treatment to improve fuel characteristics is an often neglected activity in commercial, thermochemical, biomass utilization systems. HNEI researchers have performed treatment studies on banagrass and high-fiber cane using low-cost water treatment methods. These methods result in fuels with lower potassium, chlorine, oxygen, and ash content, greater heating value, and higher ash deformation temperatures. HNEI is beginning a new study of cost-effective methods to treat sugar cane trash for use in thermochemical conversion processes. Sugar cane trash includes dead leaves that have accumulated in the field during the cane's growing period and green leaves and tops attached to the plant at harvest time. The most prevalent disposal practice for sugar cane trash is to open burn the field, either before or after harvest. Developing treatment techniques for trash will aid in turning a disposal problem into a biomass resource.

Contact: Scott Q. Turn

Co-fired Coal and Biomass

The Hawaii Natural Energy Institute recently completed a cooperative project with the University of California-Davis and Sandia National Laboratories on testing coal and biomass blends for power generation. The work was performed for the Hawaiian Commercial & Sugar Company (HC&S) on Maui under a project funded by the U.S. Department of Energy. Research involved (a) the pilot-scale testing of a matrix of coal and biomass blends in the Multi-Fuel Combustor Facility at Sandia and (b) full-scale testing of a limited number of such blends in an HC&S boiler unit. These tests were conducted to investigate the combustion and fouling characteristics of the fuel blends. Members of the project team designed, fabricated, installed, and operated an alkali sampling system required for extractive sampling and a probe used to collect in situ deposit samples during the full-scale boiler tests. Test results are summarized in a report that is available by contacting a project team member.

Contact: Scott Q. Turn

UH students harvest banagrass at the Hawaii Agriculture Research Center's substation in Kunia, Oahu. This fast-growing, herbaceous plant has been considered as a dedicated feedstock for biomass energy. Banagrass grows to maturity in only seven months, making it ideal for an energy crop.

The banagrass plot, with Honolulu and Diamond Head in the background.

A technician grinds the banagrass in preparation for drying and eventual processing in a gasifier.

Hydrogen Production from Biomass Gasification

Thermochemical gasification is a process operated at elevated temperature that converts a solid fuel into a gaseous one, while maximizing the chemical energy content of the product gas. The fuel gas can be combusted for heat or power generation, or synthesized into specific chemical products. One of the fuel gas components is hydrogen, and previous work has shown that the hydrogen yield from biomass gasification is most sensitive to the reactor temperature and equivalence ratio, both of which depend upon the relative amounts of fuel and oxygen used in the reactor. The hydrogen content of producer gas can be increased by steam reforming methane and higher hydrocarbon species present from the gasification process. If used in fuel cells, trace contaminants must also be removed from the gas stream. HNEI is beginning an experimental program focused on gas upgrading and purification for fuel cell applications.

Contact: Scott Q. Turn


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