HELGA Fluidised Bed Cooler

摘要

Biomass gasification attracts a lot of attention. It not only can offer a way to produce electricity efficiently, also gaseous and liquid energy carriers as well as chemicals can be synthesized from the gas produced by a gasifier. Large hydrocarbon molecules in the product gas however, are considered to be a major problem. It easily causes fouling by condensation on cold surfaces. The gas cooler therefore is prone tar fouling. From earlier work, it has become clear that the presence of a minimum amount of large particles can prevent the formation of an insulating fouling layer. This observation has been the basis for the development of the HELGA cooler concept. HELGA is a fluidized bed cooler that uses the abrasive forces of the fluidizing bed material to constantly clean the cooler surfaces. Within the present project, a lab-scale HELGA, cooler has been designed, constructed and tested downstream a 7 m (sub n) (sup 3) h fluidized bed gasifier. It is concluded that a very high steady state heat transfer can be maintained of approximately 250 W/m (sup 2) (bed-side) despite the fact that a thin tar coating appears on the cooler surfaces. The presence of the coating proves that erosion of the cooler tubes is not an issue. Moreover, the coating serves as chemical and physical barrier for corrosion. During each test, the bed material in the fluidized bed cooler was coated with a thin layer of solidified tar. Furthermore, fouling and blocking of the gas distributor of the HELGA fluidized bed heat exchanger appeared to be a serious problem. This was solved by allowing enough particles hitting the gas distributor. This could be realized by allowing entrainment of bed material from gasifier to heat exchanger. The required addition of particles upstream the HELGA cooler nicely coincides with the necessity to refresh the HELGA bed regularly because of the tar coating formation on the bed particles. If particle entrainment is not an option (which is the case for certain gasification technologies), sand can be added from an external source. A first design has been made of a HELGA-cooler downstream the existing 0.5 MW (sub th) CFB-gasifier at ECN. A compact reactor of only 0.3 m diameter and 0.5 bed height seems to be sufficient to cool the gas to 400 degrees C oil (or steam).