Martti Aho

Importance of solid fuel properties to nitrogen oxide formation through HCN and NH3 in small particle combustion

The formation of nitrogen oxides from fuel-nitrogen through intermediates was studied by measuring first fuel-Ofuel-N ratios and nitrogen functionality in selected solid fuels. Then the ratios of the yields (fuel-N → HCN)(fuel-N → NH3) in a nearly inert atmosphere at 800°C in an entrained flow reactor was measured and finally the ratio (fuel-N → N2O)(fuel-N → NO) in an oxidizing atmosphere at 800°C. The fuels studied were coal, brown coal, S- and C-type peat, fir bark, birch bark and pine bark, all milled to a particle size < 63 μm. The ratios of ON in the fuel, measured by elemental analysis, ranged from 7 to 150. Nitrogen functionality (mass percent of the total nitrogen content) was determined by XPS. The (fuel-N → NCN)(fuel-N → NH3) conversion ratio in the absence of O2, and also the (fuel-N → N2O)(fuel-N → NO) conversion ratio with O2 present, decreased with increasing ratio of fuel-Ofuel-N, but neither ratio decreased regularly with the increasing ratio of pyrrolic to pyridinic nitrogen in the fuel. Thus, fuel-oxygen plays a more important role than nitrogen functionality in the chemistry of nitrogen oxide formation. The strong effect of (fuel-Ofuel-N) ratio on the (fuel-N → HCN)(fuel-N → NH3) ratio may be due to the reaction between OH radicals and HCN to form NH3 near the fuel particle. The importance of this reaction is considered. Charring the fuel sample before combustion led to a sharp drop in the conversion of fuel-N to N2O compared with the virgin fuels. Thus, heterogeneous combustion reactions produced much less N2O than homogeneous combustion reactions.

Formation of nitrogen oxides from fuel-N through HCN and NH3: a model-compound study

The conversion of fuel-nitrogen to HCN and NH3 and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH3 was determined under inert conditions, and the formation of NO, N2O and NO2 was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH3. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH3. In oxidizing atmosphere, the conversions of model-compound-N to N2O were high, but the substituent groups had no well-defined effect on the ratio of N2O to NO. The formation of NO2 was insignificant.

Simultaneous pyrolysis and char combustion

Simultaneous pyrolysis and char combustion of < 500 μm coal and peat particles in an entrained-flow reactor was studied experimentally and theoretically at 700–1100 °C and at oxygen contents < 21 vol%. A method based on near infrared radiation was developed to measure the particle temperatures. Simultaneous pyrolysis and char oxidation occurs when oxygen reaches the char particle surface during the devolatilization stage. The overlap of homogeneous and heterogeneous combustion is favoured by low temperature and high oxygen content, as well as by a decrease in particle size. A simplified method of calculating simultaneous pyrolysis and char oxidation is presented. Model calculations show that under some conditions a maximum combustion rate may be achieved with a specific particle size, since the contribution of faster homogeneous combustion is increased relative to slower heterogeneous combustion.

Conversion of fuel nitrogen through HCN and NH3 to nitrogen oxides at elevated pressure

Abstract Reactions of fuel nitrogen during pyrolysis and combustion of pulverized HVb coal, two peats and fir back were studied experimentally in a pressurized entrained flow reactor at T = 1123 and 1273 K, p = 0.2, 0.4 and 0.8 MPa. Mass loss of fuel, release of C, N, H and O, and formation of NH3 and HCN were measured during pyrolysis (in N2 containing O2 H > C > N) was found to be independent of pressure. The HCN NH 3 ratio in the flame was dependent on the fuel-O fuel-N ratio and independent of pressure. Pressure did however increase the N 2 O NO ratio, because the concentrations of the key radicals in NO formation are decreased by pressure. With peats, the formation of N2O increased slightly with pressure. The emission of N2O however doubled with wood bark when the pressure increased from 0.2 to 0.8 MPa. Formation of NO2 increased distinctly with pressure, and was fuel-dependent. One peat sample produced three times as much NO2 as the other under identical conditions.

Effect of fuel composition on the conversion of volatile solid fuel-N to N2O and NO

Older fuels, which generally have a low fuel-O/fuel-N ratio, produce more N2O in fluidized bed combustion than younger fuels. Here, a proposal is made regarding the effect of fuel composition on the conversion of fuel-N to N2O and NO through HCN and NH3 at temperatures typical of fluidized bed combustion. Because earlier experiments have shown that the fuel oxygen plays an important role in fuel-N chemistry, fuel oxygen was considered together with fuel nitrogen. In model compound studies, phenolic OH-groups in particular were found to increase the conversion of HCN to NH3. In general, the abundance of phenolic oxygen in fuel follows the fuel oxygen concentration. The importance of reactions between OH radicals and HCN was therefore considered.

Emissions of nitrogen oxides in pulverized peat combustion between 730 and 900 °C

Abstract Five Finnish peats were burned in an entrained flow reactor at temperatures between 730 and 900 °C to study the conversion of peat nitrogen to nitrogen oxides (N 2 O, NO and NO 2 ) in conditions simulating low-temperature, unstaged pulverized fuel combustion. Oxygen content was varied between 1 and 18 %. The major product was NO (conversion of fuel-N to NO varied between 10 and 43%), but the concentration of N 2 O was high at low temperatures and at 730 °C the degree of conversion of fuel-N to N 2 O sometimes exceeded that of fuel-N to NO. Unlike NO, the formation of N 2 O was not sensitive to oxygen content of the reaction environment, and it decreased with increasing O N ratio in the peat. This led to marked differences in the molar concentrations of N 2 O in the flue gas from different peats between 730 and 800 °C. Particle sizes in the range 16–205 μm and moisture contents of 5–20% had no observed effect on fuel nitrogen conversions. Conversion of fuel-N to NO 2 was

The effects of pressure, oxygen partial pressure, and temperature on the formation of N2O, NO, and NO2 from pulverized coal

The main features of a new, pressurized, entrained-flow reactor are described and results presented of experiments investigating the formation of nitrogen oxides (N2O, NO, and NO2) from pulverized Polish coal, burned in the reactor at temperatures (T) 800–1300°C, pressures (p) 1–20, bar and oxygen partial pressures (pO2) 0.05–2.4 bar. The experimental results are compared with the results of detailed gas-phase kinetic calculations at 850°C, where HCN was used as the source of coal-nitrogen, and H2, H2O, CO and C2H4 were used to describe the gaseous products of pyrolysis and char combustion. The new reactor made it possible to control the experimental conditions with high precision. Regression equations were obtained between the dependent, y-variables (conversions of fuel-N to N2O, NO, and NgOy) and independent, x-variables (p, pO2 and T). NO formation decreased sharply with pressure, and increased, but not as strongly, with oxygen partial pressure and temperature. Total pressure and oxygen partial pressure did not affect N2O formation in the pO2 range 0.15-0.6 bar. At higher pO2 the conversion of fuel-N to N2O decreased with both total pressure and oxygen partial pressure. An increase in temperature strongly reduced N2O formation, independently of pressure and pO2. No N2O was found at or above 950°C. NO2 was formed in sufficient concentrations to find a regression model at high partial pressures (> 0.5 bar) of oxygen. Like N2O formation, the yield of NO2 decreased with temperature. But like NO, and in contrast to N2O, the formation of NO2 increased with pO2. NO was the only nitrogen oxide produced above 1000°C at 4–16 bar pressure. Under these conditions its formation obeyed a simple regression equation. Concentrations of NO, NO2 and N2O obtained in kinetic computations showed similar trends to the measured values. Calculations also showed the concentrations of O, OH and H radicals to decrease with pressure, and also that HO2 becomes the dominating radical at high pressures. These changes probably originate mostly from the three-body reaction H + O2 + M → HO2 + M, which at 850°C begins to compete with and finally dominates over the reaction H + O2 → OH + O as the pressure increases. The decrease in NO formation with increasing pressure follows as a consequence, because O and OH are key radicals in the production of NO.

Conversion of peat and coal nitrogen through HCN and NH3 to nitrogen oxides at 800°C

The conversion of fuel nitrogen through HCN and NH3 to nitrogen oxides (N2O, NO and NO2) was studied using an entrained-flow reactor at 800 °C with one coal and four peats at 5 and 1% O2. The ON ratios of the fuels were between 7 and 20. A clear dependence was found between the HCNNH3 ratio measured just after the vigorous pyrolysis step and the N2ONO ratio in the flue gas when these were plotted as a function of the initial ON ratio of the fuel.

Formation and destruction of N2O in pulverized fuel combustion environments between 750 and 970 °C

Abstract Formation and destruction of N2O were investigated under dilute pulverized coal combustion conditions in an entrained-flow reactor. The effects of temperature, atmosphere (oxidizing and reducing) and reaction environment (homogeneous and heterogeneous) were studied. N2O was measured on-line by FT-i.r. spectrometry. No N2O evolution occurred above 950 °C. At oxygen concentrations below 1.5%, in the presence of CO, the N2O already formed was rapidly destroyed above 900 °C and fairly rapidly even at 830 °C. At high oxygen concentrations, destruction occurred more slowly, and not at all below 900 °C. Heterogeneous reactions did not noticeably increase the destruction rate of N2O in a dilute fuel-air suspension. Low O N ratio of coal favoured N2O formation. The results should assist the design of low-temperature combustors that generate only low levels of N2O.

Separate effects of pressure and some other variables on char combustion under fixed bed conditions

Abstract The effects of pressure, oxygen partial pressure, oxygen flow rate, total gas flow rate and temperature on the combustion rates of chars from three different coals were measured by means of a small fixed bed, linked to a high pressure thermobalance at initial sample temperatures of 450–750 °C and total pressures of 0.1–2.1 MPa. The combustion rate was estimated with the help of three different functions: the time required for total oxidation (tr), the maximum sample temperature Tsmax, and the maximum mass loss rate (dm dt) max . The results were interpreted with statistical models, using factor analysis combined with a partial least square method. This allowed determination of the separate influence of each variable. These models indicated that combustion rate increased strongly with oxygen partial pressure and oxygen flow rate. The char type also influenced the combustion rate. The effect of initial temperature was weaker, and that of total flow rate was the weakest of the variables. These data can be used when developing pressurized fixed bed gasifiers and combustors for combined cycles.