Shedding light on the governing mechanisms for insufficient CO and H2 burnout in the presence of potassium, chlorine and sulfur

Teresa Berdugo Vilches,Wubin Weng,Peter Glarborg,Zhongshan Li,Henrik Thunman,Martin Seemann

doi:10.1016/j.fuel.2020.117762

Teresa Berdugo Vilches, Wubin Weng + Show 4 more

Open Access

https://doi.org/10.1016/j.fuel.2020.117762

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Abstract

Based on the experiences of insufficient burnout in industrial fluidized bed furnaces despite adequate mixing and availability of oxidizer, the influence of potassium on CO and H2 oxidation in combustion environments was investigated. The combustion environments were provided by a laminar flame burner in a range relevant to industrial furnaces, i.e. 845 °C to 1275 °C and excess air ratios ranging from 1.05 to 1.65. Potassium, in the form of KOH, was homogeneously introduced into the hot gas environments to investigate its effect on the radical pool. To quantitatively determine key species that are involved in the oxidation mechanism (CO, H2, KOH, OH radicals, K atoms), a combination of measurement systems was applied: micro-gas chromatography, broadband UV absorption spectroscopy and tunable diode laser absorption spectroscopy. The inhibition effect of potassium on CO and H2 oxidation in excess air was experimentally confirmed and attributed to the chain-terminating reaction between KOH, K atoms and OH radicals, which enhanced the OH radical consumption. The addition of chlorine or sulfur could reduce the concentrations of KOH and K atoms and consequently eliminated the inhibition on CO and H2 oxidation. Existing kinetic mechanisms underestimate the inhibiting effect of potassium and they fail to predict the effect of temperature on CO and H2 concentration when potassium and sulfur co-exist. This work advances the need to revise existing kinetic mechanisms to fully capture the interplay of K and S in the oxidation of CO and H2 in industrial fluidized bed furnaces.

Highlights

Combustion technologies are important sources of heat and elec tricity in today’s society, and they are likely to play a role in the future energy system as a stabilizing complement to intermittent energy sources
The measurements are presented and discussed in two parts corresponding to the two sets of experiments in Table 2: those related to the influence of potassium on CO oxidation at different operating conditions; and those related to the influence of sulfur and chlorine in co-existence with K
The results prove the interplay between potassium and sulfur in the CO oxidation under the conditions tested, and confirm the hypothesized mechanism involved in the large scale tests at the Chalmers unit [3]

Summary

Introduction

Combustion technologies are important sources of heat and elec tricity in today’s society, and they are likely to play a role in the future energy system as a stabilizing complement to intermittent energy sources. CO should be kept below 150–500 ppm at the stack for a given excess air concentration of usually 6% for biomass and 11% for waste. A common reason for CO emissions is a local deficit of O2 due to mixing limitations between fuel and air. The problem of CO emissions is exacerbated in facilities that combust biomass and inhomogeneous solid fuels such as waste streams. In those cases, moderate temperatures (typically 750–900 °C) are applied to reduce the risk of uncontrolled ash melt and corrosion issues derived from the impurities in the fuel, as well as prevent overheating of the furnace material; a measure that can challenge the complete oxidation of CO into CO2. Even trace levels of active species like alkali com pounds can drastically influence the combustion chemistry in thermal conversion processes [2]

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