Deposit Probe Measurements in Danish Grate and Pulverized Fuel Biomass Power Boilers

Several measuring campaigns with focus on deposition behavior have been conducted at full-scale power plants firing biomass in Denmark. These measuring campaigns have been reviewed in this work. The focus of the review is the obtained experiences on deposit formation, chemistry and shedding.
Corresponding samples of fuels, ash deposits and fly ash have provided information on the transformation of inorganics in the boiler. Generally, grate fired boilers provide a fly ash containing high contents of K, Cl and S compared to the fuel ash, while suspension fired boilers fly ash has a composition nearly similar to the fuel ash. Inner most biomass deposits are always salt-rich, while thicker deposit layers also contain some Si and Ca.
Deposit probe formation rate measurements have been performed in different ways on several boilers. Grate and suspension fired boilers seems to cause similar deposit formation rates. Suspension fired boilers generate more fly ash, while grate boilers form a fly ash with a higher fraction of melt formation (and thereby a higher sticking probability) at similar temperatures. For suspension fired units it is observed that wood with a lower ash content than straw gives rise to lower deposit formation rates.
The flue gas temperature is an important parameter in the deposit behavior of biomass-fired boilers. Increases in the flue gas temperature of both grate and suspension fired boilers leads to increased deposition rates and deposits with higher contents of Si and Ca. This can be explained by increased deposition by inertial impaction due to higher melt fractions of ash particles (containing Si and Ca) at increased temperatures.
The shedding behavior is influenced by exposure time, probe surface temperature, flue gas temperature and deposits chemistry. The influence of temperature on the degree of sintering varies with the fuel type. Possibly, increased contents of Si and Ca will lead to increases in the sintering temperature and thus to altered shedding behavior. It is recognized that the exposure time of the deposits in the flue gas influences the removability of the deposits.