Jouni P. Hämäläinen

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.

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.

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.

Determination of heats of pyrolysis and thermal reactivity of peats

Heats of pyrolysis of four Finnish peats were determined by differential scanning calorimetry (d.s.c.), and combustion enthalpies were compared by differential thermoanalysis (DTA) in an air atmosphere. Weight loss data obtained by thermogravimetric analysis (TGA) were combined with the enthalpy data. The heats of pyrolysis of the peats varied between 120 and 250 kJ kg−1. These values were used to model the combustion and pyrolysis of small peat particles (typical in pulverized fuel combustion) and large peat particles (typical in grate combustion). TGA and DTA together gave useful information about the reactivity of peat in an oxidizing atmosphere with changing temperature, despite the similar shapes of the derived TGA (DTG) and DTA-curves. The pyrolysis and combustion behaviour of the peat with the lowest degree of decomposition was similar to that of cellulose, and the differences in the peats were apparent with all the methods used (d.s.c., DTA and TGA). The initial temperatures of pyrolysis and combustion were lower with peats of high ash content (inorganic material) and the combustion proceeded through more steps than with the peats of low ash content.

Analytical solutions for steady and unsteady state particle size distributions in FBC and CFBC boilers for non-breaking char particles

Abstract Continuous analytical solutions for the particle size distributions of char in steady and unsteady states in fluidized beds, when the inlet fuel feed is presented by monosize, lognormal, Rosin-Rammler or gamma distributions, are derived from a population balance model. The stationary size distribution is directly related to the rate of reduction of the particle size. Combustion and attrition reduce the particle size. Thus, it is possible to extract the dependence of the rate of reduction of radius (affected by a fuel’s reactivity and attrition) on radius from a measured steady-state particle size distribution. Unsteady particle size distributions are derived for impulse, step and square pulse changes in the fuel feed, when the oxygen level in the reactor is maintained constant.

CATALYTIC EFFECTS OF METALS ON PEAT COMBUSTION

Abstract A comparison was made of the effects of Fe, Mn, Cr, Ni, Co, Zn, Al, Mg and Ca on slow and rapid peat combustion. The concentrations used were 100–200 mmol kg−1 and lower and higher concentrations were also used for Fe. The order of the catalytic effects of the metals on the combustion of peat particles (100–125 μm diameter) in an entrained flow reactor (particle heating rate 15000 ± 5000 °Cs−1) was: Cr >Mn, Fe >Co, Ni >Ca >Zn, Mg >Al. In the presence of 100 mmol kg−1 Cr, the combustion time decreased by 26% compared with acid-washed peat having a low content of inorganic material. The decrease was only 4% with Al. Thermogravimetric experiments predicted the order: Fe, Cr >Mn, Ni >Co >Ca >Mg >Zn >Al (heating rate 0.17 °Cs−1). Experiments with Fe in the entrained flow reactor showed an increase in concentration from 105 to 330 mmol kg−1 to have little effect, but a decrease from 105 to 42mmol kg−1 weakened the catalytic effect markedly. The thermogravimetric experiments predicted that concentrations between 105 mmol kg−1 and 330 mmol kg−1 would strengthen the catalytic effect. Catalytic effects of metals on combustion can be predicted on the basis of atomic structure. Transition metals are good catalysts and the best of them have five or six electrons in d orbitals, whereas metals with only one stable oxidation state and completely occupied or empty s, p and d orbitals are poor catalysts. These effects probably prevail in the combustion of coals. In the case of dried Finnish bog peat, the catalytic effects of cations are mainly due to Fe, because the contents of other strong catalysts (Cr and Mn) are very low relative to Fe; Ca causes some additional effects.

Importance of iron and aluminium in rapid and slow combustion of peat

Abstract The effects of Fe and Al on peat combustion and the formation of nitrogen oxides were studied under conditions of rapid combustion in an entrained flow reactor, and under conditions of slow combustion in a thermobalance and a differential scanning calorimeter. Iron had a strong catalytic effect on both the slow and rapid combustion, and it decreased the ratio of N2O to NO in the combustion gases. A decrease in the content of inorganic compounds decreased reactivity, probably because of the decrease in iron content. Aluminium did not have a detectable catalytic effect on the combustion rate but it depressed the formation of nitrogen oxides slightly. The chemical treatment to increase the contents of Fe and Al and decrease the ash content had little effect on the composition of the organic part of the peat.