Biomass is a renewable, carbon-neutral resource that produces minimal pollution when used to generate electricity, fuel vehicles, and provide heat for industry. Every year in Iowa, millions of bushels of treated seed corn go unused, and are wasted (sent to the landfill). Old treated seed corn goes unplanted because of low germination rates, but it goes unused because of the toxicity associated with the pesticides and fungicides applied to it. If the toxic additives could be destroyed through gasification with a long, high-temperature residence time, the producer gas from treated seed corn could then be used as a fuel source in regular power plants. The temperature and reactivity required to destroy these chemicals is best achieved in a reactive bed, like one formed by carbon char. This makes a char producing combustion system an ideal candidate for this type of fuel.
In this work, a char-producing downdraft gasification system is used to examine system behavior for seed corn fuel. The system is pilot-scale and the producer gas is of primary interest for power production. Both experiments and numerical simulations are carried out and a range of parameters are examined, including the thermal profile, equivalence ratio, bed depth, and producer gas composition. A second downdraft gasifier, with two-stage gasification, is also studied to compare the systems’ behaviors. From these results, a 1-d hybrid model was developed and utilized to predict optimal gas production in these systems. Results show that above the minimal char bed level, higher equivalence ratio (ER) value results in a higher combustion zone temperature and a higher gas yield while leading to a lower CO concentration in the producer gas. Bed height consumes more heat in the combustion zone which brings about a lower combustion zone temperature. In general, ER plays a more dominating role in determining gas yield and combustion zone temperature. The two-stage system, which expands the combustion zone, effectively increases carbon conversion rate and hence generates a producer gas with high cold gas efficiency, although this makes maintaining sufficient char depth difficult.