Thermo-chemical conversion analysis on dairy manure-based biomass through direct combustion

Growing rates of manure produced from large dairies and repeated application of manure on the same parcels of land have increased concerns for the environmental quality of nearby streams and watersheds. This paper deals with energy conversion analysis using a "black box" approach of supplying flushed dairy biomass (DB), combusting the DB solids, exhausting the combustion products to the atmosphere, returning liquid water back to the stall and thus eliminating (ideally) the need for lagoons and land applications. Minimum allowable moisture and ash percentages, as well as heat outputs and flame temperatures are presented for a DB direct firing system. Fuel analyses yield that flushed DB contained 93% moisture (%M), 3% ash (%A), 4% combustibles (%Cb), and a dry, ash free higher heating value, HHVDAF=20,505 kJ/kg while separated DB solids contained 81%M, 2%A, 17%Cb, and HHVDAF=20,494 kJ/kg. In order to fire separated DB solids in a combustor or furnace, the solids must have a minimum combustible percentage to vaporize all moisture in the fuel and burn at a minimum prescribed flame temperature. For example, DB containing 70%M requires 9.74%Cb to vaporize all moisture and produce gaseous products of combustion at 373 K, but requires 17.82%Cb to burn in a regenerative combustor with a flame temperature of 1200 K. Separated solids that are pressed in an auger to 70%M (3%A and 27%Cb) can burn at 1200 K with exhaust temperatures of up to 1130 K and a minimum required regenerative heat exchanger effectiveness of 15%. Pressed solids can thus be fired in a boiler, where the remaining separated liquid (waste water) can be used as boiler water. Depending on the boiler pressure and the %M of the original flushed manure, the pressed solids can only release about 30% of the required heat to vaporize the remaining waste water. However, pressed DB solids can be blended and fired with drier fuels to vaporize almost all the waste water. The low quality steam produced from the waste water may be used in thermal processes on the farm.