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This work presents detailed measurements of fouling deposits of a 100 kW (nominal thermal input) pilot-scale combustor operated over different tests with different combustion settings. After the tests, the heat exchangers of the convective section were removed for deposit examination. The results offer information regarding the fouling behavior during periods that lasted more than 20 h, starting from clean tubes. These long periods yielded valuable information about the bulk deposit buildup and final shapes as well as the effects of the oscillating flow patterns that occur in the staggered-plate tube banks. Special consideration was given to the deposit thickness profile around the tubes, final deposit shapes, and deposit mechanisms. Two different sub-bituminous coal test campaigns have been considered in this study. Typical bulky deposits were observed for a Wyoming coal (first test), where the deposits were about 4–6 mm in the tube winds and about 1 mm in the lees. These results were significantly diffe...

A method for predicting the conditions that lead to plugging of the boiler bank by submicrometer condensation aerosols of alkali-metal salts in kraft recovery boilers has been developed and evaluated against field observations. The method is based on recent data on the phase behavior and sintering characteristics of compacts of dusts from kraft recovery boilers, both of which are dependent on the inorganic composition of the particles. The method also takes into account the thermal load on the boiler and the specific design characteristic of the gas temperature entering the boiler bank versus thermal load on the boiler. The prediction method presented in this paper has been tested against data from 20 different recovery boilers. It correctly predicted whether the boiler did or did not encounter problematic fume deposits for 80% of those boilers.

The composition and sintering characteristics of deposits of alkali metal salt particles formed in-situ in a kraft recovery boiler were investigated. The deposits were formed on air-cooled probes inserted into the convective gas passages of an operating kraft recovery boiler. Their microstructure was characterized by scanning electron microscopy. The results show that, in regions of the boiler where the alkali metal salt particles are no longer molten, two types of deposits can form. The first is composed almost entirely of submicron aerosol (fume) particles. The second contains a mixture of fume and larger particles, with dimensions typically 5 to 70 μm. Two types of larger particles were identified: spherical particles of solidified ash from combustion of black liquor, and irregular chunks from upstream deposits disintegrated by soot blowing. The structure of deposits formed from fume particles was one of branched dendritic growths. The deposits as formed were relatively low density. Sintering proceeded via growth of nodes and thickening of branches, but with slow densification of the deposits. All of the deposits observed appeared to harden via sintering of the much finer fume particles. The larger size particles, when present, appeared to be bound in the deposits by the matrix of finer particles that was produced as the finer particles sintered.

In this work comparisons and validations of several simplified ammonia oxidation mechanisms at conditions relevant for combustion of biomass have been performed. The aim of the investigation was to clarify the weakness and the strength of the available simplified mechanisms for computational fluid dynamics (CFD) based prediction of NOx emissions. The mechanisms have been tested at typical conditions for biomass combustion, considering temperature and gas composition. All the simplified mechanisms have been compared against a validated comprehensive elementary reaction mechanism containing 340 reversible reactions between 56 chemical species. In comparing the simplified mechanisms, the mixing pattern between the species was described by a continuous stirred tank reactor, with typical residence times in eddy dissipation concept fine structure. The results show that, for substoichiometric conditions at low temperatures (

Combustion aerosol measurement methods were introduced and applied for extensive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pressure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 μm and the coarse mode above 3 μm aerodynamic diameter. According to SEM images, fine ash mode consists of individual, almost spherical 0.3−0.7 μm alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode primary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3−0.7 μm alkali salt primary particles and spherical silica particles. Ash particle main component was sodium sulfate as determined with X-ray diffraction. Sodium-to-sulfur molar ratio of ash particles calculated on the analyses results with an ion chromatography decreased from the upper furnace sampling point to electrostatic precipitator inlet conditions, indicating sulfation of ash particles within the heat exchanger section. Chlorine in ash was bound as sodium chloride, no potassium chloride was detected with X-ray absorption fine structure spectroscopy. Furnace measurements showed that fine mode ash particles are formed already in the furnace via vapor condensation. The extents of release of 12% for Na, 24% for S, and 48% for Cl were determined on the basis of ion concentrations in fine particles and the mass balance calculation on the recovery boiler. Coarse particles observed downstream the heat exchangers are proposed to form mainly via entrainment of large agglomerates of fine ash particles deposited on the heat exchangers. The fine mode particle size was insensitive to the furnace conditions although the particle concentration increased when the furnace gas temperature increased. This and the increase of Na/S molar ratio in the particles indicates that Na volatilization increases with the increasing furnace temperature, whereas S release is less sensitive to the temperature increase.