The characteristics of biomass torrefaction and nitrogen oxide emission during co-firing with coal

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Abstract

Torrefaction can effectively improve biomass physicochemical properties and mitigate adverse effects during coal co-firing, while nitrogen migration differences between woody and herbaceous biomass during torrefaction remain unclear. This work investigates torrefaction effects on woody Salix psammophila (SP) and herbaceous Arundo donax (AD), focusing on NOx emission and fuel nitrogen migration during co-firing. The results show that torrefaction temperature significantly affects the product distribution. Considering low heating value (LHV) and energy yield, 270 °C is the optimal torrefaction temperature. 

Under N2 atmosphere, the N-A functional groups in SP mainly transform into N-5 and tar-N, and the oxidative atmosphere inhibits this transformation process. In contrast, under N2 atmosphere, the N-A functional groups in AD convert into tar-N and N-Q, and the oxidative atmosphere promotes the formation of N-Q. The co-firing mode has a significant impact on NOx emissions during the co-firing of raw/torrefied biomass and coal: under the premixed co-firing mode, NOx emissions decrease monotonically with the increase of biomass mass blending ratio, while under the staged co-firing mode, NOx emissions first decrease and then increase with the biomass mass blending ratio. The fundamental reason behind this phenomenon is the difference in NH3 conversion.

In addition, there is no difference in NOx conversion rate between raw and torrefied AD, while torrefied SP exhibits slightly higher NOx conversion rate than raw SP, and it is related to the nitrogen-containing functional groups. N2-torrefied SP has the highest content of N-5 functional groups, leading to the highest NOx conversion rate. This work reveals the intrinsic difference in nitrogen transformation pathways between woody and herbaceous biomass during torrefaction, clarifies the core regulatory mechanism of NOx emission during co-firing, and provides critical theoretical support and basic data for the industrial application of biomass torrefaction and low-nitrogen co-firing in coal-fired boilers.

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