Investigation of Nitrogen and Phosphorus Bearing Species in Steam Gasification of Poultry Litter

The production of broiler chickens has become one of the largest sectors in United States agriculture and the growing demand for poultry has led to an annual production growth rate of five percent. Over 70 percent of the production can be traced to the Southeastern United States. With increased demand for poultry, poultry litter management has become a major challenge in the agriculture industry and for the United States Department of Agriculture.

One of the options being considered for chicken litter management is steam gasification of the litter. Detailed study and research of this option has already been carried out at the University of Tennessee Space Institute. Although the steam gasification method has been accepted as a possible and feasible method for litter management, apprehension has been expressed at various forums about the liberation of nitrogen and phosphorus containing species along with the fuel gas and/or the final residue. The possible liberation of phosphorus as phosphine gas with the fuel gas will have an adverse impact on the environment and would pose an unacceptable feature of the steam gasification of litter. Possible liberation of ammonia from the nitrogen containing species is also not acceptable, unless means are developed to capture or control it. Hence, the present study was conducted to study the fate and the environmental impact of the nitrogen and phosphorus containing species released during steam gasification of the poultry litter.

From various preliminary tests carried out with poultry litter, it was concluded that most of the phosphorus remained in the residue and the nitrogen ended up as ammonia in the fuel gas. The effects of temperature and catalyst loading on ammonia liberation were studied in a muffled furnace setup, where the pressure was atmospheric under all the experimental conditions. The ammonia liberated was collected in a scrubbing solution containing dilute hydrochloric acid. The unreacted excess HCl solution was titrated with a caustic solution to indicate the amount of ammonia that had reacted with acid under various experimental conditions.

The amount of ammonia liberation was found to decrease with an increase in temperature during pyrolysis and gasification. It also decreased with an increase in additional catalyst loading. Additional testing was then carried out in a high-pressure fixed bed reactor to determine the preliminary kinetics of the ammonia liberation reaction.

The specific liberation rate measured in the differential fixed bed reactor was then used to design a scrubbing system and to revise (as necessary) the economic feasibility study of chicken litter gasification already performed earlier at UTSI.