The reaction of NO with chars of sewage sludge, refuse derived fuel (RDF), and straw was investigated in a fixed bed reactor at temperatures from 800 to 900 degrees C and NO inlet concentrations from 400 to 1500 ppmv. The effect of ash forming elements in the chars was examined by comparing the reactivity of raw and demineralized chars. Compared to straw char, the reaction rates of sewage sludge and RDF char, at 800 degrees C and 400 ppmv inlet NO, was an order of magnitude and a factor of six higher, respectively. The very high reactivity of the two waste chars was attributed to the catalytic effect of their large content of calcium and iron. A simple first order globalized rate expression was employed to describe the reactivity of waste chars toward NO, which predicted reasonably well the the conversion of char nitrogen to NO during char combustion at 800 degrees C in 10% O-2. A comparison with literature data revealed a higher reactivity of the waste chars towards NO compared to that of coal and biomass chars. The results in this work provide a simple and validated rate expression to simulate waste char-NO reaction in boilers, and moreover facilitate a potential utilization of waste chars as primary or secondary measures for NO reduction.

It is still in discussion to what extent alkali sulfate aerosols in biomass combustion are formed in the gas phase by a homogeneous mechanism or involve heterogeneous or catalyzed reactions. The present study investigates sulfate aerosol formation based on calculations with a detailed gas phase mechanism. The modeling predictions are compared to data from laboratory experiments and entrained flow reactor experiments available in the literature. The analysis support that alkali sulfate aerosols are formed from homogeneous nucleation following a series of steps occurring in the gas phase. The rate-limiting step may be the oxidation of sulfite to sulfate, rather than the oxidation of SO2 to SO3 proposed previously. Even though the proposed model is consistent with experimental observations, experiments in a rigorously homogeneous system are called for to test its validity. (C) 2007 Elsevier Ltd. All rights reserved.

A counter-flow reactor setup was designed to investigate the gas-phase sulfation and homogeneous nucleation of potassium salts. Gaseous KOH and KCl were introduced into the post-flame zone of a laminar flat flame. The hot flame products mixed in the counter-flow with cold N-2, with or without addition of SO2. The aerosols formed in the flow were detected through Mie scattering of a 355 nm laser beam. The temperature distribution of the flow was measured by molecular Rayleigh scattering thermometry. From the temperature where nucleation occurred, it was possible to identify the aerosols formed. Depending on the potassium speciation in the inlet and the presence of SO2, they consisted of K2SO4, KCl, or K2CO3, respectively. The experiments showed that KOH was sulphated more readily than KCl, resulting in larger quantities of aerosols. The sulfation process in the counterflow setup was simulated using a chemical kinetic model including a detailed subset for the Cl/S/K chemistry. Similar to the experimental results, much more potassium sulfate was predicted when seeding KOH compared to seeding KCl. For both KOH and KCl, sulfation was predicted to occur primarily through the reactions among atomic K, O-2 and SO2, forming KHSO4 and K2SO4. The higher propensity for sulfation of KOH compared to KCl was mostly attributed to the lower thermal stability of KOH, facilitating formation of atomic K. According to the model, sulfation also happened through SO3, especially for KCl (KCl -> KSO3Cl -> K2SO4).

The KOH-capture reaction by coal fly ash at suspension-fired conditions was studied through entrained flow reactor (EFR) experiments and chemical equilibrium calculations. The influence of KOH-concentration (50-1000 ppmv), reaction temperature (800-1450 degrees C), and coal fly ash particle size (D-50 = 6.03-33.70 mu m) on the reaction was investigated. The results revealed that, at 50 ppmv KOH (molar ratio of K/(Al + Si) = 0.048 of feed), the measured K-capture level (C-K) of coal fly ash was comparable to the equilibrium prediction, while at 250 ppmv KOH and above, the measured data were lower than chemical equilibrium. Similar to the KOH-kaolin reaction reported in our previous study, leucite (KAlSi2O6) and kaliophilite (KAlSiO4) were formed from the KOH-coal fly ash reaction. However, coal fly ash captured KOH less effectively compared to kaolin at 250 ppmv KOH and above. Studies at different temperatures showed that, at 800 degrees C, the KOH-coal fly ash reaction was probably kinetically controlled. At 900-1300 degrees C it was diffusion limited, while at 1450 degrees C, it was equilibrium limited to some extent. At 500 ppmv KOH (molar ratio of K/(Al + Si) = 0.481), and a gas residence time of 1.2 s, 0.063 g K/(g additive) and 0.087 g K/(g additive) was captured by coal fly ash (D-50 = 10.20 mu m) at 900 and 1450 degrees C, respectively. Experiments with coal fly ash of different particle sizes showed that a higher K-capture level were obtained using finer particle sizes, indicating some internal diffusion control of the process.

Emission of nitrogen oxides (NOx) is a major challenge for combustion of solid fuels. Strategies for emission control can be developed from computational fluid dynamics (CFD) simulation. This, furthermore, requires a computational efficient kinetic model that is able to capture both formation and destruction of NOx in a wide range of conditions. In this work, three skeletal mechanisms with varying degrees of reduction were developed based on a detailed kinetics model proposed recently (148 species and 2764 reactions). By preserving all major reaction pathways of NO formation, the most comprehensive skeletal mechanism Li45 (45 species and 788 reactions) behaved very similar compared to the base mechanism with regard to the prediction of NO. The more compact skeletal mechanism Li37 (37 species and 303 reactions) was generated specifically for the conditions relevant to large scale industrial combustion of solid fuels. The Li37 mechanism is capable of predicting NO formation as well as simulating common measures of NOx reduction such as the staged combustion and selective non-catalytic reduction (SNCR). Without the consideration of SNCR, the smallest skeletal mechanism Li32 (32 species and 255 reactions) still maintained a good predictability over broad temperature and excess air ratio ranges. Compared to the base mechanism, the skeletal mechanisms achieved over 70% reduction in species. Furthermore, the computational cost was lowered to a large extent, particularly with Li37 and Li32. This makes the developed skeletal mechanisms very suitable to be implemented in CFD simulations.

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 H-2 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 degrees C to 1275 degrees 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, H-2, 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 H-2 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 H-2 oxidation. Existing kinetic mechanisms underestimate the inhibiting effect of potassium and they fail to predict the effect of temperature on CO and H-2 concentration when potassium and sulfur coexist. 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 H-2 in industrial fluidized bed furnaces.