Middlebury College has focused on the sustainability of its biomass fuel supply since the initial conception of the biomass plant in 2004. In 2007 the College coordinated with the State University of New York School of Environmental Science and Forestry (SUNY-ESF) to fund and plant a nine acre test plot of rapid growth willow. This test plot was planted on College land located on Route 125 approximately ½ mile west of the main campus. The harvest schedule for this willow crop included a three year harvest cycle with an anticipated yield of 25 tons of biomass fuel per acre per harvest cycle. This biomass fuel crop was harvested the week of January 3, 2011 and stored outside in wood chip form. Total harvest yield was 120 tons of willow chips from 6 acres of land, or 20 tons / acre. Three acres did not yield willow. Conclusions of the willow harvest include acceptable chip quality in terms of size and profile, reasonable moisture content, and willow fuel energy content as expected.
The process of willow harvest to gasification for the production of steam defined the following goals:
- Understand the details of willow harvesting and equipment involved in this process.
- Understand the harvest delivery to storage, and from storage to the biomass plant.
- Evaluate the gasification and combustion of willow derived biomass fuel and record key process data.
- Accurately monitor and record carbon monoxide (CO) emissions data.
Willow fuel consumption in the College’s biomass gasification plant began the week of January 10, 2011 and concluded on January 21, 2011. The biomass plant baseline operation was confirmed and documented as acceptable with hardwood whole tree fuel prior to introducing willow fuel. Through a series of four distinct trials the willow was introduced to the gasification process. We learned that the volume of willow was not adequate to tune the gasifier for optimum efficiency and we were not able to obtain satisfactory emissions results. Conclusions and discussion points resulting from the willow fuel gasification trial are:
- Willow fuel in a frozen state has an immediate and significant negative impact on the gasification process. Increasing gasifier primary combustion air temperature improves this situation.
- The carbon monoxide emissions threshold of 150ppm or less on an hourly average basis was not achieved.
- The willow fuel as harvested contained a large amount of wet fines creating significant handling and conveying issues in the plant. This fuel characteristic was not apparent during harvest.
- Biomass fuel stored outside must be protected from weather.
Further research is currently under consideration utilizing up to 1,000 tons of willow from western New York. It is proposed that we would partner with a local agricultural products vendor to transport and prepare the fuel supply. This would likely complete the research required to arrive at a decision with respect to further willow cultivation, harvesting, processing, and gasification for Middlebury College.