Abstract
It is estimated that California’s dairies have the potential to produce an estimates 40 million cubic feet of biogas per day, representing a potential capacity of about 140 mW of energy. Due to the distributed nature of these resources, reciprocating engines coupled to generators are a common means of extracting power from the digester gases. One of the most significant challenges facing the combustion digester biogas is high NOx emissions. To address this challenge, an integrated pollution capture and microwave system has been developed to reduce NOx emissions from biogas engines. The feasibility of reburning the captured NOx was assessed and the various operating parameters, including temperature, pressure and reactants composition were determined using chemical equilibration, kinetic modeling and a Computerized Fluid Dynamic simulation using ANSYS FLUENT software. The NOx removal system is a pair of alternating carbon adsorption vessels installed on the engine exhaust pipe. The carbon will capture cooled exhaust gas NOx and is expected to pass through cleaned exhaust having NOx levels in the range of 1-5 ppm. Each carbon adsorber is equipped with a microwave generator that desorbs the NOx and regenerates the carbon in place when it is isolated from the engine exhaust. Desorbed NOx is collected in a concentrated small volume sweep gas and reacted in a separate microwave activated reactor with consumable carbon to produce CO2 and N2. This system is currently being demonstrated at a dairy facility in Northern California. While the amount and cost of the carbon consumed during the microwave activated NOx reduction step is small, it would be desirable to eliminate this step with another means of NOx destruction. One potential alternative is to use the engine to destroy NOx by dissociation at high temperatures. Traditionally, NOx is formed in the engine by the thermal or prompt mechanisms. However, high concentrations of NOx in the sweep gas would approach equilibrium from the other direction (high concentration to low) and could yield a net NOx reduction. Since the carbon absorber can accept high concentrations of NOx, the absorbers could be used in a potentially sustainable process where NOx is continuously accumulated and destroyed. The objective of the study is to assess the feasibility of this alternate method of NOx destruction using chemical equilibrium, kinetics and computerized fluid dynamic simulations.