Measurement and Control of Metal Vapours during Fluidized Bed Combustion of Biomass Fuels

Doctoral thesis, 2004

Increased combustion of biomass fuels for heat and power production has created a need to recycle nutrient elements in biomass ash to soils to make the biomass consumption sustainable. It is, however, difficult to recycle the total amount of biomass ash produced due to the high concentrations of heavy metals and their readily soluble forms in some ash fractions. In addition, alkali metals in biomass fuels are directly responsible for problems of fouling and bed agglomeration during the combustion of biomass fuels in fluidized bed combustion (FBC) boilers. Therefore, to understand and control the fate of both alkali and heavy metals in the process is of great importance and the central issue in this thesis. The first part of the thesis deals with further developments of the gas quenching (GQ) probe for measurement of metal vapours in FBC conditions. A new version of the probe with a sintered quartz filter incorporated in a quartz tube module for excluding solid particles from the sampled gas has been constructed and evaluated through field measurements of cadmium in a full-scale FBC research boiler. The results are promising, indicating that the technique can be a valuable tool to characterise the presence of gaseous metal species and obtain mass balances in FBC processes. In addition, a novel particle impactor has been developed for use with the GQ probe instead of the filter. The impactor has been tested at room temperature and gave very sharp cut-off curves with a maximum collection efficiency of about 0.9 and a cut-off size square root of Stk50 of about 0.22. The efficiency of the impactor was compared with the efficiency of a sintered quartz filter of the same type as the filter previously used with the GQ probe. The difference is very small, indicating that the impactor can be used with the GQ probe in FBC conditions. In the second part of the thesis, capture of alkali and/or heavy metals on kaolin in oxidizing or reducing conditions has been investigated in a fixed bed reactor equipped with an on-line alkali detector. Kaolin was found to capture potassium vapours irreversibly and effectively in both oxidizing and reducing conditions. Kaolin captures cadmium in air, but not in reducing conditions. In air, the process was determined to be of a reaction order 1.5 with a rate constant k = 5.2 x 108 cm3 gas/cm3 solid-hr at 850°C, with respect to the KCl concentration in the gas phase. A reducing atmosphere appears to promote the reaction between gaseous potassium species and kaolin, compared to the oxidizing atmosphere. Co-presence of potassium and cadmium compounds promotes the capture of both metals on kaolin in oxidizing conditions. This promotion was more pronounced for potassium than for cadmium, indicating that potassium capture by kaolin is more favourable than cadmium capture. Overall, kaolin is suitable for in-situ and selective removal of alkali during combustion of biomass fuel in FBC conditions.