Challenges to Biomass Processing

By Cole Gustafson, Biofuels Economist, NDSU Extension Service

The demand continues to grow for cellulosic biofuel production, as well as blending biofuel with coal in electrical power and heating plants. New biomass harvesting projects being conducted by University of Wisconsin agricultural and biosystems engineers Kevin Shinners and Tom Hoffman and a Mississippi State University team lead by Jeremiah Davis highlight some of the challenges that farmers are going to face in processing and delivering biomass materials that will replace coal.

The goal of the Wisconsin project is to develop a one-pass biomass field machine that will both harvest a crop and produce a biomass cube. At the moment, most biomass is harvested by farmers and then transported to regional centers for further processing and densification. These centers may pellet or cube the material. The advantage of a one-pass, in-field machine is that biomass doesn’t have to baled, which saves both energy and time. Also, a regional processing center won’t have to de-twine the bales before processing.

The machine that both the Wisconsin and Mississippi research teams are using is a 1970s-era John Deere 425 hay cuber. These machines originally were produced for commercial alfalfa growers. John Deere only built 400 of these specialized machines and less than 100 are known to exist today. If anyone knows of one in existence in the Dakotas, I would like to see it!

Cubing alfalfa requires significant horsepower. The John Deere 425 cuber has a 250-horsepower engine. When these machines were manufactured, John Deere didn’t produce an engine large enough at the time, so they purchased engines from Detroit Diesel. Even with this horsepower, the machines only had capacity to travel 1 mph when harvesting.

Since biomass cubing requires substantial horsepower and energy, it reduces its appeal as a renewable energy source. One of the main interests that the coal industry has is reducing its carbon footprint. Cofiring with biomass that requires significant energy to produce doesn’t help the coal industry realize this goal because all energy expended in production is counted.

Moreover, biomass has several other limitations when compared with coal because cubes are not a perfect replacement. While coal contains 12,000 British thermal units (Btu) per pound, biomass alfalfa cubes only contain 7,350 Btu. Biomass cubes also are less dense. Therefore, they are more bulky and expensive to haul long distances. Finally, biomass cubes have a tendency to break apart. This makes handling the cubes with traditional coal industry equipment problematic.

Cubing technology is part science and part art. Biomass cubes are formed under high temperatures and pressure. In fact, the Wisconsin research team found that the temperature of the alfalfa was raised to near the boiling point of water while it was being processed. To start cubing, the John Deere machine had to “warm up.” Generally, it took about 20 minutes of operation for the machine’s internal temperature to rise enough to form stable cubes.

Another challenge is the moisture content of the alfalfa. The cubing operation requires that the biomass be as dry as possible. However, letting alfalfa lay outside increases the risk of rain damage and deterioration.

The Wisconsin and Mississippi research teams are confident these issues can be overcome. They are in the process of designing new equipment prototypes that are more tailored to cubing and densifying biomass instead of alfalfa. I wish them success in their pursuit!

Source: NDSU Agriculture Communication