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This paper is the second of two papers, describing probe measurements of deposit buildup and removal (shedding), conducted in a 350 MW th suspension-fired boiler, firing straw and wood. Investigations of deposit buildup and shedding have been made by use of an advanced online deposit probe and a sootblowing probe. The influences of feedstock (i.e., straw share in wood), flue gas temperature (600−1050°C), probe surface temperature (500 and 600°C), and probe exposure time on deposit shedding have been quantified. Quantification of naturally occurring deposit shedding and deposit shedding during plant sootblowing was made via deposit mass uptake signals obtained from the deposit probe. The deposit shedding process was characterized by calculation of the amount of deposit removed at a shedding event (g/m 2) and the frequency of the shedding events (h −1). The results showed that the shedding process is stochastic and that the amount of deposit shed varies even at constant local conditions. However, the deposit shedding rates showed an increasing trend with increase in flue gas temperatures and probe deposit mass loads. The deposit shedding rate was in most cases higher at a probe temperature of 500°C than at a probe temperature of 600°C. A possible reason for this is partial melting and/or sintering of the innermost deposit layer (rich in K, Cl, and S) at higher probe surface temperature. This could cause the adhesion strength of the deposit to the probe to increase at the higher probe temperature. Quantification of the necessary peak impact pressure (PIP) needed to remove the deposit was also made by use of a sootblowing probe in conjunction with the deposit probe. Results of deposit removal by artificial sootblowing showed that the deposits formed on a 500°C probe temperature and at exposure times of <91 h can be removed with a PIP of <55 kPa. However, increase in probe exposure time and/or probe surface temperature (600°C) significantly increases the PIP needed to remove the deposits.

A number of full-scale deposit probe measuring campaigns conducted in grate-fired and suspension-fired boilers, fired with biomass, have been reviewed and compared. The influence of operational parameters on the chemistry of ash and deposits, on deposit build-up rates, and on shedding behavior has been examined. The firing technology and the fuel utilized influence the fly ash and deposit chemical composition. In grate-firing, K, Cl, and S are enriched in the fly ash compared to the fuel ash, while the fly ash in suspension-firing is relatively similar to the fuel ash. The chemical composition of the deposits formed is determined by the fly ash composition and the flue gas temperature; increases in the local flue gas temperature lead to higher contents of Si and Ca and lower contents of Cl in the deposits. The net deposit build-up rates in grate-fired and suspension-fired boilers are at similar levels, 0–100 g/m2·h, while the ash deposit propensity is an order of magnitude larger in grate fired boilers than in suspension-fired boilers. Deposit build-up rates were found to increase at flue gas temperatures close to the melting temperatures of the fly ash. Furthermore, the rate of deposit build-up increased with the K-content of the fuel ash and fly ash for grate-fired boilers. For suspension-fired boilers, deposition rates are comparatively low for wood-firing and increase with increasing fuel straw shares. Shedding of deposits occurs by melting during straw-firing on a grate at high flue gas temperatures (>900 °C). At lower flue gas temperatures, the deposits can be removed by soot blowing. The required soot blower impact pressure is strongly influenced by the surface temperature, such that a high surface temperature makes the deposit more difficult to remove. During straw/wood-firing in suspension-fired boilers, shedding occurred by debonding with incomplete removal at flue gas temperatures of 600–1000 °C and by debonding with complete removal during wood-firing in suspension-fired boilers at high flue gas temperatures (1300 °C). Shedding events were not observed during wood suspension-firing at low flue gas temperatures (<900 °C). Here, a steady-state mass of deposits on the probe was observed. Increased exposure times and probe temperatures lead to deposits that are difficult to remove. This was observed for grate-firing of straw and for straw/wood firing in suspension-fired boilers.

a b s t r a c tThis study aims to provide a geometrical description of biomass particles that can be used in combustion models. The particle size of wood and herbaceous biomass was compared using light microscope, 2D dynamic imaging, laser diffraction, sieve analysis and focused beam reflectance measurement. The results from light microscope and 2D dynamic imaging analysis were compared and it showed that the data on particle width, measured by these two techniques, were identical. Indeed, 2D dynamic imaging was found to be the most convenient particle characterization method, providing information on both the shape and the external surface area. Importantly, a way to quantify all three dimensions of biomass particles has been established. It was recommended to represent a biomass particle in combustion models as an infinite cylinder with the volume-to-surface ratio (V/A) measured using 2D dynamic imaging.