Models To Predict Kinetics of NO_x Reduction by Chars as a Function of Coal Rank

Abstract

During coal combustion, NO x reduction occurs by two possible routes: homogeneous reduction by hydrocarbons and heterogeneous reduction by char formed during coal devolatilization. This paper investigates the latter route, which also has potential as the basis for post-combustion NO x clean-up processes, including reburning. The purpose of this investigation is to develop a kinetic model for the reduction of NO x by char during coal combustion and to understand the role of coal rank and char surface area on the resulting char reactivity. This investigation reports original kinetic data for nine char samples including graphite, coconut char, and five coal chars ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3). An empirical kinetic model with six universal (not char-specific) parameters reproduces the experimental data for all chars. The investigation also presents an alternative and simpler model with only two parameters that differ for each char type. Correlations for the two model parameters were then developed as a function of two char surface areas: (1) active mineral matter surface area measured using CO2 titration after high-temperature exposure and (2) total sample surface area measured using CO2 at room temperature and Dubinin–Polanyi theory. Predictions of the rate constant values over a wide range of temperature using this universal approach with only the surface areas differing among the six chars generally fit the experimental data within ±50%.

Effects of High-Temperature Burnout Level and Devolatilization/Oxidation Conditions on Char Properties for Low Rank Coal Chars: Correlation of CaO Surface Area with Intrinsic Char Oxidation Rates

“…This investigation follows previous studies by some of the current authors; 8,38,39 pore surface area variations led to greater reactivity; however, they did not consider the Ca surface area in their study. 8 Cope et al measured the CaO surface area for Beulah Zap chars and observed that, as the intrinsic oxidation rate decreased, the CaO surface area also decreased for the Ca-containing char series.…”

“…This investigation follows previous studies by some of the current authors; ,, however, it covers different aspects of the overall work. McDonald et al studied chars made by different preparation methods in three different environments and measured their TGA reactivities between 550 and 950 K. They concluded that a higher H/C ratio rather than larger char pore surface area variations led to greater reactivity; however, they did not consider the Ca surface area in their study .…”

“…The question still remained, however, as to whether catalytic activity is important during high-temperature oxidation. Related publications have addressed whether the high-temperature rates and activation energies of these chars are affected by CaO . While additional work is needed to provide definite proof, these results are probably not applicable to high-rank coals where calcium is less dispersed and associated much differently.…”

“…Cope et al measured the CaO surface area for Beulah Zap chars and observed that, as the intrinsic oxidation rate decreased, the CaO surface area also decreased for the Ca-containing char series. , Guo et al studied the reduction of NO x by nine coal chars of different ranks, correlating the NO x reduction rate with the surface area of both CaO and char pore structure. He proposed a kinetic model to accurately predict rate as a function of these two surface areas . The present study uses three low rank char series to determine if variations in intrinsic char oxidation reactivity (obtained by varying the extent of high-temperature oxidation or “burnout’’) correspond with changes in CaO surface area.…”

Catalytic Mechanism of Calcium on the Formation of HCN during Pyrolysis of Pyrrole and Indole: A Theoretical Study

“…Thermal conversion of coal is regarded as one of the main sources of NO x in the atmosphere. , Nitrogen in coal mainly exists in three forms, i.e., pyrrole N, pyridine N, and quaternary N, − among which pyrrole N is the most abundant, accounting for around 50–80 wt % of total nitrogen. During the thermal conversion process, coal is usually decomposed into various products in the absence of oxygen, which then undergo diverse reactions (oxidation, cracking, reforming, etc.)…”