By Ana McPhail, Robert Griffin, Mahmoud El-Halwagi, Kenneth Medlock and Pedro J.J. Alvarez

Abstract

Use of municipal solid waste (MSW) as fuel for electricity generation reduces landfill disposal and can mitigate air quality degradation associated with combustion of conventional fossil fuels. Co-combustion is a waste-to-energy technology that can use MSW and coal as co-fuels, offering potential energy recovery and reduced air emissions. This research discerns how MSW composition influences the heating value and air pollution for the co-combustion of coal with MSW using five MSW composition scenarios, four of which were derived by a reduction of plastics, organics, paper, or a combination thereof, as compared to the national average MSW composition. Numerous combustion products could be evaluated; this study focused on five high impact air combustion products: SO₂, CO, CO₂, NO, and NO₂. The moisture content was varied from ∼10% (considered dry) to 40% (average MSW moisture). AspenPlus software was used for the deterministic simulation modeling of incineration (MSW only) and parallel co-firing (co-combustion of coal and MSW) to determine theoretical heating values and pollutant effluent concentrations. The United States Environmental Protection Agency (U.S. EPA) models WAR and WARM were used to determine the potential environmental impacts (PEIs) and greenhouse gas emission equivalencies, respectively, for each MSW scenario. For the WAR model, values for each impact category parameter can vary, but each parameter is weighed equally. Of the MSW scenarios studied, the national average held the highest heating value with 8519 MBtu/lb and the lowest occurred for the MSW scenario with recycled paper and composted organics, with 8251 MBtu/lb. Results show that SO₂, CO, CO₂, NO, and NO₂ flue gas concentrations (and therefore PEIs) depend upon the composition and moisture of the MSW, in addition to the MSW/coal ratio. Approximate ranges for the WAR results (PEI/h) are 7410–7663 for NO, 4–8 for NO₂, 18–105 for CO, 30–46 for CO₂, and 89–2152 for SO₂. WARM results show lower net CO₂ emission equivalents to landfill MSW with reduced paper and organics, while combustion is preferred for MSW with paper reduction, organics reduction, and plastics reduction. The results for the national average MSW were independent of the disposal processing method. Reduction in pollutant concentrations did not yield overall cost savings for the electricity producer, as profit was reduced by ∼20–30%. There are savings associated with emission costs using MSW in lieu of coal: up to ∼3.3% for NO, ∼20–47% for NO₂, and ∼95% for SO₂. A hypothetical carbon dioxide tax was also imposed to realize the potential cost savings by reducing CO₂ emissions. In summary, the measurable impact MSW composition and moisture had on pollutant concentration, heating value, and economic parameters was important.

Read the full article in Energy & Fuels.