Catalytic Use of Ash in NOx Reduction

Due to challenges associated with source reduction, reuse, and recycling, waste to energy (WTE) remains a valuable technology in a portfolio of solutions for managing municipal solid waste (MSW). Specifically, the U.S. produces ~14 million MWh of electricity from ~30 million tons of MSW annually.[i] This amounts to only 12% of MSW produced annually with landfilling remaining the most prevalent disposal method due to economics and a limited understanding of thermal conversion.[ii] Increasing reliance on WTE hinges on reducing the technology’s cost. Specifically, there has been a recent acceleration in research of innovations that convert one of WTE’s liabilities into an asset: elimination of the disposal cost associated with MSW incineration (MSWI) ash by conversion into value-added products. Existing research in this area is limited, with most focused on the use of ash as an additive to building materials.[iii] My research aims to investigate MSWI ash in a less well-studied, but promising high-value application: as a catalyst in NOx reduction reactions. MSWI ash has demonstrated catalytic activity comparable to that of well-designed industrial catalyst[iv], but no concerted effort has been dedicated to understanding how its variable composition affects its catalytic activity or how stable it is under various reaction conditions.

Specifically, I will develop a kinetic understanding of NOx reduction over ash produced in MSW gasification and combustion such that a comparison can be made to those catalysts being employed in NOx mitigation currently. For reference, typical industrial catalysts employed in the selective catalytic reduction (SCR) of NOx exhibit ~90% conversion of NO when tested at 400 ºC with a reactor space velocity of 60,000 h-1.[v] Thus, benchmarks for measuring MSWI ash’s catalytic activity exist, and the initial focus of my project will be to synthesize ash with variable properties and make this comparison. Thereafter, the focus will shift to understanding performance as a function of time on stream. Typical industrial catalysts are susceptible to sulfur-poisoning and fouling by particulates. If activity comparable to industrial catalysts can be demonstrated, an understanding of the stability limitations is then required. In addition to consideration for the specific application of NOx emission mitigation, my project lies at the forefront of MSWI waste residual research, so my work is expanding the body of knowledge about these materials in general.

Furthermore, beyond the societal benefits related to MSW management, my research endeavors to reduce NOx emissions via a less costly catalyst, making treatment economically feasible in processes that presently preclude the use of expensive catalysts due to harsh environments and thus reducing emission of a potent greenhouse gas.  

[i] Michaels, T.; Waste-to-Energy Production. Energy Recovery Council’s Directory of Waste-to-Energy Facilities2018, 5.

[ii] Duren, R.; California’s methane super-emitters. Nature 2019, 180.

[iii] Joseph, A.; The Use of Municipal Solid Waste Incineration Ash in Various Building Materials. Materials 2018, 1.

[iv] Klinghoffer, N.; Influence of char composition and inorganics on catalytic activity of char from biomass gasification. Fuel 2015, 37.

[v] Nakajima, F.; The state-of-the-art technology of NOX control. Catalysis Today 1996, 109.