No pellet stove has yet been engineered specifically to deal with relatively high ash grass pellets. Few individuals in North America have pelleted cool-season grasses or attempted to burn such pellets in stoves. We tested a few stoves over the winter of 2004-05, funded by the Cornell Agricultural Experiment Station. Some stoves are capable of burning grass pellets, although the long-term effects are not known.
The Harman corn stove has now burned over a ton of grass pellets with an ash content of 4.8%. The Bixby biomass stove and the Country Flame Harvester stove have also burned grass pellets successfully. In addition, grass pellets were tested in a pellet boiler (courtesy of TARM, USA) and in a gasifier (courtesy of Community Power Corporation).
Detailed results follow:
- Pelleting process
- Demonstration facility
- Baxi Pellet boiler
- Community Power Corporation gasifier
Approximately 7.5 tons of grass hay in large square bales from the 7 fields harvested in 2004 was shipped to VIFAM ProServices, Inc. in Kirkland, Quebec. Pellets (1/4" dia.) were produced using a mobile pelleting facility. Optimum feedstock moisture for this process is 15%, moisture is a lubricant for the system. Pelleting generates heat up with pellets reaching 160 degrees F. A cooling process is necessary to minimize condensation. The pelleting process removes about 5%age units of moisture from the material.
Average pellet length varied with the lot of hay pelleted. All but one of the hay lots produced pellets with a low percentage of “fines". Pellets were shipped back to NY in 40 lb plastic bags. If you are interested in additional pelleting details please contact VIFAM directly at (514) 426-4482.
A former farm shop facility at the Cornell Mt. Pleasant research farm was remodeled for the demonstration. The room is approximately 1300 sq. ft., with a 15 ft. ceiling. Four pellet stoves at a time were installed in the facility, some stoves were rotated out after testing. Pellets were stored in an un-insulated portion of the building.
The Breckwell stove is an example of a pellet stove with a standard burn pot, representing the majority of pellet stove models on the market. Most of the ash is blown out of the burn pot into an ash drawer. We set up this stove to demonstrate that burning of high ash pellets in such a stove will not work. Grass pellets most likely will not work in any stove that does not have a specific adaptation to deal with some type of non-wood ash.
Grass pellets burned for an hour or two before filling the burn pot and extinguishing the fire.
Heating capacity: 0-61,000 BTU
Hopper capacity: 120 lbs of grass pellets (with hopper extension)
The burn pot area slopes up from the auger pellet feed exit and then flattens out. Pellets are pushed into the combustion bed and burned residue is pushed up over the lip into the ash drawer. This would have the potential to work with high ash materials, and has worked with 4% ash wood residues.
At a medium burn rate, the grass pellet mass is burned successfully and the partially-fused ash mat is pushed over the lip of the burn pot. After one to two days, a carbon buildup develops on the burn pot surface a short distance from the pellet feed auger. Sooner or later this buildup stops the flow going up, out and over. This plugs up the stove. This stove could work for grass pellets if the burn surface was cleaned daily. The heat exchanger and air channel under the burn pot and the glass door would need regular cleaning, more often than with wood pellets. The pellet feed system operated flawlessly. The ability of this stove to completely shut down and then start up on its own when heat is required is a nice feature.
Heating capacity: 8,000-50,000 BTU
Hopper capacity: 106 lbs of corn
The Bixby biomass stove has a unique self cleaning fire pot, the stove is designed for burning corn. The bottom one inch of the burn pot contents is periodically cut off and dumped in an ash drawer (bottom shown with burn pot removed). The hopper has a “ferris wheel” design, fuel drops into slots in the wheel and then is released one slot at a time into the burn pot. This design prevents jamming from pieces of cob and other foreign material in corn grain.
The frequency of ash dumping is set for corn grain, and dumps every 7 to 13 hours depending on the feed rate. With the assistance of Bixby engineers we were able to control the dumping process using computer software. For grass pellets that are 4-5% ash, the required dumping frequency at a medium pellet feeding rate is approximately every 45 minutes. With the ash dump frequency matched to the pellet feeding rate, the stove burns grass pellets nicely. At a medium pellet feed rate, the ash drawer needs to be dumped every couple of days or so.
There has been no build-up on the surface of the burn pot, the air circulation holes in the bottom and sides of the burn pot have not become plugged. Since a partially melted ash wafer of 1 inch thickness is cut off the bottom of the mat in the burn pot, there is very little ash material blown around in the stove. This makes it the cleanest stove we tested.
The ferris wheel feeding system specifically designed for corn grain had some trouble with pellets, however, particularly with longer pellets. The feeding system has been modified recently by Bixby Energy Systems to work more effectively with pellets.
The Bixby biomass stove was donated for use by Bixby Energy Systems, Rogers, MN.
Heating capacity: 0-61,000 BTU
Hopper capacity: 120 lbs of grass pellets (with hopper extension)
The Harman corn stove is very similar to the P61A pellet stove, only with a different burn pot. The “Sidewinder” burn pot has a mixing auger that connects directly to the pellet feed auger. When pellets are fed the mixing auger is rotated.
This stove mechanically deals with ash/clinker issues. The mixing auger keeps large clinkers from forming, and if they do form it breaks them up. This process generates fly ash that settles on all flat areas in the burn chamber, most ends up in the ash drawer. Burning grass pellets in this stove will be messier than wood pellets to clean up. None of this material escapes while the stove is running, but may escape during clean up. It also can be noisy when clinkers are broken up. The noise was bothersome for a while, but it grows on you.
Nevertheless, this stove very effectively burns grass pellets with a minimum of maintenance. We have burned over 1 ton of grass pellets with 4.8% ash content. The pellet feed system operated very well. The ash drawer will need to be dumped every 2-3 days, and the burn pot/air exchanger area will need to be cleaned at least once weekly. Different grasses were burned across our range of ash content. The stove appears to be relatively insensitive to fuel ash content. It is likely that grass with ash content exceeding 5.2% would work.
This stove has been used to burn switchgrass pellets in Iowa. It has a mixing auger in the burn pot similar to the Harman corn stove, but the auger runs independently of the pellet feed auger. This stove has a manual start-up.
The principle is similar to the Harman corn stove. The mixing auger grinds up clinkers and keeps the ash loose enough to be blown out of the burn pot. Ash is deposited everywhere in the burn chamber, even stacked up on the glass door, making it the messiest stove to clean up. As with the Harman, however, it does successfully burn grasses, and does not appear to be greatly impacted by fuel ash content.
This stove makes the most background noise of any stove tested. Set on maximum feed rate, it still does not deliver grass pellets at a very fast rate, this would need to be adjusted for grass pellets. Feed rate probably works fine for corn. It also would be useful to be able to speed up the rate of turning for the mixing auger. Ash is deposited on a relatively small portion of the ash drawer surface, so that even when burning corn it is necessary to dump ash every day, to avoid hindering the combustion air flow.
The Europa is the first gasifier pellet stove, which means gasses are released by controlled thermal degradation and then burned above the residue mass. Three augers at the bottom of the burn pot remove ash at controlled rates. Like the Bixby stove, pellet feed rate must be synchronized with ash removal.
The Europa is currently burning a 50:50 mix of switchgrass pellets and wood pellets in Canada. At this time the burning of cool-season grass pellets results in a bridging of the ash mass above the ash removal augers, resulting in burn pot overflow and shutdown. Mixing cool-season grass pellets with wood pellets 50:50 does not solve this problem. Dell Point Industries will be addressing this issue with design modifications in the future.
The Dell Point Europa stove was donated for use by North Shore Oil Company Inc., West Monroe, NY.
Baxi in Denmark produces a range of boilers for domestic heating and domestic hot water. TARM USA, Inc. in Lyme, NH distributes three Baxi models, a 50,000 BTU wood boiler and 85 or 155,000 BTU boilers for wood or corn. The larger boilers have a stirring paddle in the combustion bed. Pellets are fed horizontally and loose ash is blown away and manually removed. These boilers are designed for and work very well with wood pellets. The two larger boilers work OK for corn, but need regular attention. These boilers are not designed to handle high ash fuels such as grasses, but we tried it anyway.
We tested about 500 lbs of grass pellets in two Baxi boiler models, courtesy of TARM USA, Inc. in Lyme, NH. The stirring paddle did not help with residue management in the burn chamber. Both boilers burned grass pellets OK, but grass burning resulted in the formation of a large clinker every 6-8 hours, which would have to be manually removed from the burn chamber. Domestic boilers designed to cope with higher residues are being tested with grass pellets in Europe.
Community Power Corporation of Littleton, CO operates several BioMax Small Modular Biopower System gasifiers. The feedstock needs to be in some type of densified form, such as pellets or chips. As with all the combustion appliances tested, it is best to have a feedstock below 15% moisture. Gasification is controlled thermal decomposition of organic matter in an oxygen deficient atmosphere, producing combustible gases.
Timothy grass pellets were sent to the CPC testing facility in Colorado and tested in the Biomax 15 unit, courtesy of Community Power Corporation. The Biomax 15 shown here is a 15 kW system. Softwood chips and commercially available wood pellets were gasified prior to the timothy grass pellet test. About 64 lbs of grass pellets were used for the test. Results are in the Table. Two gas composition readings were taken for timothy pellets and they were variable, but in this test the heating value of producer gases derived from grass was higher than that from wood pellets. The preliminary test produced very positive results.
Longer term testing is required to determine the impact of ash residue on the grate system and to determine the weight of grass pellets required per kW hour. The high ash content of grasses may require an active grate for facilitating ash removal. Larger biopower units (Biomax 50) have an active grate. Producer gas composition and engine emissions, particularly NOx, also need to be evaluated under a constant load flow rate. For this test, our timothy pellets were lower in nitrogen content than premium hardwood pellets.