Minn. company builds 'Grassifier' prototype

A Buffalo, Minn., company is looking to scale up a residential-sized biomass energy system and develop turn-key power plants that could supply grid power and hot water for space heating, process heat or other applications.

Development of the “Grassifer” began more than two years ago, when Mel Moench, president of Moench Inc. became interested in combusting biomass—in particular, warm season grasses. Moench, who has more than 40 years of mechanical engineering experience and 35 years in the renewable energy field, said he knew there was a future in fast-growing, carbon-neutral feedstocks. “Wood is slow to grow and there is a lot of interest in energy grasses such as switchgrass and miscanthus,” he said. “There are also huge supplies of corn stover, wheat straw, flax straw and many other crop industrial and agricultural residues that simply go unused and other residues are available depending on the geographical location.”

Moench determined that the major challenges for biomass combustion systems were moisture content, particle size and efficiency. “I decided to eliminate the densification process and combust the material without preprocessing to save energy,” he said. “I wanted the unit to be automated, compact and highly efficient.”

Moench also sought to eliminate any special equipment needed to prepare the feedstocks for combustion. “We’ve combusted grassy materials with moisture content of up to 25 percent, which is close to ‘as harvested’ condition,” he said. “Our unit also contains a cyclone to extract excess water vapor in the flue gas.”

After three prototypes and scores of modifications, Moench completed a unit that achieved two of his top priorities—reaching more than 1,850 degrees Fahrenheit consistently, and overcoming the issue of grate clogging by using a patent-pending, grateless configuration. “I also had to use a proprietary fuel feeding system that virtually eliminates issues with fuel compaction and subsequent auger jamming,” he said. “The prototype can also utilize multiple, dry coarse-chopped materials interchangeably.”

After the chopped biomass is conveyed to the in-feed auger, it drops into a screw auger and is pushed into the combustion chamber as needed to maintain the proper temperature. Negative pressure is supplied by a vacuum blower so no smoke or gasses escape, and primary and secondary burning occur at a high temperature, ensuring the complete combustion of all particles and gases. While the ash continues forward and drops into the ash collection drum, the hot gases leave the heat containment chamber and enter the heat exchanger that transfers the heat to the antifreeze solution surrounding the tubes. A circulating pump keeps the hot antifreeze solution moving through insulated underground pipes to a plenum heat exchanger, baseboard unit, in-floor radiant tubing or other heating systems.

The Grassifier is also designed for automatic-feed operation, so it can be unattended for three to 10 days depending on heat demand.

A 100,000 Btu unit with a footprint of 6 x 12 feet would currently cost about $19,500 and consume less than 200 watts of energy, according to Moench. “It’s installed the same as an outdoor wood boiler, but doesn’t smolder and cause excessive smoke emissions,” he said.

Moench said he believes the company’s core combustion technology is much more efficient than straw-fired power plants operating in countries such as China, Spain, Denmark, Germany, Sweden and Italy. “These units tumble the whole bales onto a grate system to separate the fuel and be more efficient,” he said. “These units fail to allow the air to penetrate completely into the feedstock, as baled grasses are compacted and don’t easily allow complete combustion. We burn from the inside-out instead of from the outside-in. Our exhaust temperature, after the heat exchanger, is less than 200 degrees F and the process can be modulated easily.”

Moench said that although his company believes the Grassifer is technologically and environmentally advanced, it only has a working prototype at this point. “We have not been successful at obtaining funding for scale-up or extensive U.S. EPA testing,” he said. “We’re searching for partners to continue improvements and other applications, including sizing up the unit for trigeneration, district heating or combined-heat-and-power applications. The larger capacity will allow use of flue gas cleaning technology for both compliance and higher efficiency, and our system makes it much easier to utilize the hot exhaust gases for sequential uses such as torrefaction, air conditioning, or gasification. This design could be the core for tailoring a renewable energy plant for industries/municipalities with varied process heating and cooling needs.”