Composting Corn for Heat Recovery
Corn grain and stover and the distiller’s grains from ethanol production have the potential to be a much less expensive space heating fuel than conventional heating fuels like LP, natural gas, fuel oil, and electricity in the Corn Belt. The purpose of this research project is to determine their heating potential and the efficacy of heat recovery under optimum composting conditions, as an alternative to combustion, and produce an organic residue conserving the nutrients that could be returned to the soil.
Project activities have so far been preparatory. We have had several planning meetings with Kapil Arora, the consulting agricultural engineer from Iowa State University, to design the experiments. I attended the national SARE meeting in Oconomowoc, WI presenting the project idea in a poster to solicit feedback and ingenuity. Both small square and large round cornstalk bales were made this fall. The small square bales will be used to construct temporary bunks for the static pile experiments. Kept dry under cover, the bales should provide ample insulation for composting to occur even during cold weather.
For the tumbling reactor, it became apparent that modifying a clothes dryer might be easier than building from scratch. A commercial size dryer (one or more) that is destined for the scrap heap is being sought currently. None has been found locally, but a very promising lead at the state level was recently found. If after the first of the year, nothing materializes from this lead, a tumbling reactor will be built from scratch. The more energy-rich materials such as grain and distiller’s grains will be composted in the tumbler in order to better oxygenate and remove heat from the material. An exercise treadmill has been modified using bicycle wheels to achieve a drive shaft revolution speed reduction to drive the tumbler at a very low, but adjustable, RPM. The commercial dryer drive motor will be removed and linkage made with the treadmill contraption. The drum in the dryer will be covered with sheet metal so that air from the fan will no longer blow through the dryer, but will blow around it to collect compost heat. This heat will exhaust from the port for the dryer hose. A cold air return to the housing and a means of air supply to the compost mixture will also be installed.
We decided that conducting screening trials first in 30-gallon insulated containers would allow us to increase the number of treatments to evaluate various composting condition parameters more quickly than would be possible in the larger bunkers. Inside each vessel there will be a 3-inch diameter galvanized heat duct to allow conductive heat exchange from the compost. Heat flux will be monitored throughout the trial by measuring air velocity and temperature increase. Airflow velocity will be adjusted to maintain desired temperatures in the compost. Air passing through the heat duct will not be in contact with the composting material, but will only conduct heat away from it. The insulated vessels will be plumbed separately for air supply to maintain oxygen levels in the composting mixture. Construction materials are currently being gathered and space prepared. Composted materials will include shredded corn stover, grain, and distiller’s grain with/without oil removed. Materials will be composted alone and in mixtures.
Most of the necessary monitoring instruments are modestly priced and are being purchased with grant funds. The exception is for monitoring the oxygenation status of the compost. Arrangements have been made to borrow oxygen/carbon dioxide sampling sensors from Iowa State University Department of Agricultural Engineering. Compost trial timing will need to accommodate their need for the instruments.
Although most of the work so far has been preparatory, some initial exploration has been done. I made a small drum tumbler for a 30-gallon drum using an ice cream maker drive motor on a shaft fitted with a rubber hose to drive the drum by friction. The drum was mounted horizontally on rollers. Several mixtures of ground corn grain, shredded cornstalks, water, and a small amount of manure for inoculant was added to the drum. It became very apparent that without tumbling the material would quickly turn to silage. But slow periodic tumbling was adequate to keep the mixture aerated. Heat escaped from the open topped drum by convection. Temperature was monitored but air volume was not. Similarly, wetted and shredded corn stover was composted in a drum without turning or oxygenation. Temperature climbed rather quickly, but declined after about a week illustrating the need for monitoring and controlling the composting conditions.
Kapil Arora has requested information from another composting research project currently underway on the heat retention characteristics of various composting materials.
WORK PLAN FOR 2007
Optimum composting mixtures and conditions will be evaluated systematically through a series of research trials. Trials will begin February 1, 2007 after construction of the composting vessels is complete. Initial trials will be conducted in the small vessels, beginning with varying air supply rates to the compost materials. Other variables that will be evaluated will include moisture content, C/N ratio, temperature maintenance or heat removal rate, inoculant effectiveness. In addition, tumbling rate or frequency will be evaluated in the tumbling reactor. Heat recovery from air exhausted from the compost mixtures during aeration will also be attempted if the volume of air required warrants it. All possible variable combinations may lead to an unmanageable number of trials. New trials will be results driven using information gleaned from previous trials to work toward optimum mixtures and conditions for composting. After the screening trials in small vessels have suggested some optimum mixtures and conditions, larger trials in the bunks constructed from cornstalk bales for the static windrows and in the tumbling reactor will begin and include evaluation of the capacity to recover usable heat for space heating. In the static windrow, double-wall polycarbonate greenhouse glazing (Lexan) will be used to line the walls for removal of heat conducted from the compost. The Lexan consists of continuous hollow cells that can be collected at the end of a sheet into a manifold. One end can be used for cold air return; the other for warm air supply to the heated space. Heat transfer in the compost mixture will indicate the optimal windrow dimensions or the necessary spacing of any means of heat removal used, such as ducts within the windrow.
There are no results to share yet. Plans for how to disseminate what we learn have not been discussed yet. Kapil Arora’s ISU extension connections will undoubtedly be used. But a plan has not been made.