Conversion of a pulverized coal boiler into a torrefied biomass boiler

The aim of the work is to analyze the possibility of converting a pulverized coal (PC) type OP650 boiler to combustion of 100% biomass in the form of torrefied PKS (Palm Kernel Shell). The replacement of hard coal with torrefied biomass in a PC boiler practically eliminates CO2 emissions. It also reduces the problem of coal shortages in the current political situation in Europe. However, it is necessary to solve the problems of changing the fouling of the boiler, grinding new fuel in coal mills, changing NOx emissions, and changing the exhaust gas dew point. We do not deal with torrefaction, assuming that this problem is technically solved and that such fuel can be purchased. The assessment of the boiler conversion to new fuel was based on the measurements of the boiler in its current state during combustion of coal dust. The HVT (temperature measurement by means of high -velocity thermocouple) measurement of temperature was carried out at points in the area of superheaters, on the rear wall and on one of the sidewalls using a water-cooled aspiration probe. The measurements of the growth rate of high-temperature deposits in the area of platen superheater tubes were also performed. On their basis, a 0 -dimensional model of the boiler was created and the risk of slagging after replacing coal with torrefied biomass was assessed. Slagging and fouling tendency of heating surfaces during coal and PKS combustion were compared. The final evaluation of the possibility of converting the OP 650 boiler to combustion of 100% biomass in the form of torrefied PKS was performed. With the same ash fouling of the boiler as for coal, replacing coal with a fuel with a slightly lower calorific value and much lower humidity and ash content, such as torrefied PKS, causes a slight increase in the temperature of live and reheat steam, which in the conditions of the given calculation example gives practically nominal values of these temperatures. In the variant simulating increased fouling of the boiler during biomass combustion, the flue gas temperatures increase even more, which may lead to a strong increase in slagging and fouling. Calculations for wet PKS with significant fouling of the furnace still show high flue gas temperatures in the high temperature area of the boiler. In addition, for a fuel with such characteristics, the risk of slagging is very high. To mitigate the possible operating problems, a doping the PKS with Dunino halloysite from the Polish mine Dunino (DH) was considered. This aluminosilicate is recommended to protect against slagging and fouling. Important advantage of DH is the reduction of chlorine corrosion of the boiler surfaces. The results of the calculations show that during the combustion of PKS, the temperature of the flue gas outlet is higher than during the combustion of coal.

Automated summary

The aim of this work is to analyze the feasibility of converting a pulverized coal boiler to 100% biomass combustion using torrefied PKS. By replacing hard coal with torrefied biomass, CO2 emissions can be practically eliminated, and the problem of coal shortages in Europe can be reduced. However, there are challenges to address such as boiler fouling, grinding new fuel, changing NOx emissions, and exhaust gas dew point. Measurements and calculations were conducted to assess the risks of slagging and fouling and to evaluate the potential use of Dunino halloysite for protection against corrosion. The results show that the combustion of PKS leads to higher flue gas temperatures compared to coal combustion.