Suspension-Firing of Biomass. Part 2: Boiler Measurements of Ash Deposit Shedding

Abstract

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.

Impact of coal fly ash addition on ash transformation and deposition in a full-scale wood suspension-firing boiler

“…Ash deposition probe system 2.3.1. Probe system at Location A An advanced deposit probe system developed in our earlier studies [21][22][23] was adopted in this study. A drawing of the probe system used at Location A of AVV2 is shown in Fig.…”

“…With the deposit mass uptake data, the surface area of the probe exposed to the flue gas and the corresponding exposure time, the ash deposition rate (g/m 2 /h) can be calculated. In this study the derivative based-deposit formation rate (DDF-rate) defined in our earlier work [22,23] is used. The detailed procedure for calculating the DDF-rate is described elsewhere [22].…”

Complementary Methods for the Characterization of Corrosion Products on a Plant-Exposed Superheater Tube

“…Figures 4, 5 and 10a, b). Deposit shedding is a common phenomenon occurring naturally or artificially induced in power plants, and may cause descaling of the initially formed oxides, thereby exposing the internal oxides to the corrosive atmosphere [35]. This may be responsible for the observed deviation of oxide composition from the ideal segregation into an external Fe-rich and internal Cr-rich oxide.…”

Modeling of ferric sulfate decomposition and sulfation of potassium chloride during grate‐firing of biomass

“…[10][11][12] One of the major technical challenges associated with the use of biomass in these applications is that the combustion of biomass may result in a considerable amount of alkali chlorides in the flue gas and subsequently lead to severe ashdeposition and corrosion problems in the boilers. 6,7,10,11,[13][14][15][16][17] In order to mitigate the alkali chloride-induced problems in biomass combustion, a feasible method is to use additives to convert the alkali chlorides to less harmful alkali species (such as sulfates or aluminosilicates) and release the chlorine as HCl.…”

“…[10][11][12] One of the major technical challenges associated with the use of biomass in these applications is that the combustion of biomass may result in a considerable amount of alkali chlorides in the flue gas and subsequently lead to severe ashdeposition and corrosion problems in the boilers. 6,7,10,11,[13][14][15][16][17] In order to mitigate the alkali chloride-induced problems in biomass combustion, a feasible method is to use additives to convert the alkali chlorides to less harmful alkali species (such as sulfates or aluminosilicates) and release the chlorine as HCl. 5,9,[18][19][20][21][22][23][24][25][26][27] In biomass combustion, it is desirable to convert the fuel-chlorine to HCl, as it is not condensable on heat transfer surface and the partial pressure of HCl in biomass-derived flue gas is not high enough to cause severe gas-phase corrosion.…”