WASTE/ENERGY: Costly Biomass-To-Energy Process Holds Promise

William D. Siuru Jr.

Currently, a vast potential energy resource - biomass waste - is going up in smoke or heading to the local landfill. These materials range from pits, hulls and shells discarded by food processors to tree branches and lumber from demolished buildings. Agricultural waste, now burned in fields, is another abundant biomass fuel.

Converting biomass to energy has several benefits. For example, it reportedly results in a near-zero net increase in carbon dioxide, a greenhouse gas which some believe leads to global warming. In biomass-to-energy combustion, the carbon dioxide produced is recaptured in nearly equal amounts as new biomass fuels are grown. Also, compared to field burning, combustion of straw, orchard prunings and other green wastes in a biomass power station reportedly reuces carbon monoxide, nitrous oxides and particulate pollutants by substantial amounts.

However, the biomass power production process has yet to be perfected and proven economically viable. Currently, California leads the nation in energy production from biomass - approximately one gigawatt annually from more than 70 biomass power stations. About 2 percent of the state's electrical energy production is generated by these projects. Price guarantees and other economic incentives enable California biomass power producers to compete with more traditional, economically viable energy sources. However, these will expire within a few years.

The high costs can be attributed to the ash deposits which form when biomass materials are burned and which can quickly foul the boilers. Corrective action requires significant downtime and labor intensive cleaning. For instance, when straw is burned, a glassy, gray deposit forms and sticks to the interior surfaces of the boilers. (Straw is a byproduct of rice cultivation - an important biomass crop in California.)

To help make biomass a more cost-effective energy source, a cooperative effort was formed by Sandia National Laboratories, Livermore, Calif., the University of California-Davis, engineering consultants Thomas R. Miles & Thom-as R. Miles Jr., Portland, Ore., and the Bureau of Mines.

The first phase of the project identified the mechanisms of ash deposit formation such as impaction and adhesion of particles on surfaces, thermophoris, condensation and chemical reaction. This included investigating contributing factors such as fuel properties, operating conditions and boiler design. The fuels included wood, straw, nut shells, fruit pits, nut hulls and non-recyclable pa-per, all obtained from either urban or agricultural sources. Boiler de-signs included stoker-moving grates, bubbling fluid beds, circulating fluid beds, auger moving grates and cigar burners.

The tests demonstrated that straw presents the greatest deposition problem because up to 20 percent of the material can be composed of inorganic materials excluding dirt. These inorganic materials - silica and potassium - are ingredients found in glass. While silica alone poses no real problem, the potassium decreases the temperature at which the ash melts and causes the ash deposits which require frequent cleaning. To re-duce ash deposit problems, facilities can decrease operating temperatures, improve boiler design and find fuel blends that minimize potassium levels, the researches concluded.

In contrast, non-recyclable pa-per, which is now usually landfilled, was one of the best fuels tested. If non-recyclable paper were used as a biomass fuel, large fractions could be diverted from the waste stream. In addition, municipalities may receive recycling credit for the materials.

In the next phase of the project, the researchers will develop quantitative computer models to predict ash deposit formation in boilers and to study corrosion issues. Models also will allow researchers to predict the performance of fuel blends and therefore to avoid the high costs of designing by trial and error. Also, the technical data compiled in the second phase will assist regulatory agencies in determining which materials can be burned in states' biomass powerplants.

The project includes laboratory-scale tests in the Sandia Multifuel Combustor Laboratory as well as tests in eight commercial biomass-fired boilers including two Denmark and six California powerplants. Sponsors of the project include the U.S. Department of Energy's Energy Efficiency and Renewable Energy Office and the National Renewable Energy Laboratory, Golden, Colo. In addition, 10 industrial sponsors contributed financial support and personnel and assisted in field testing.