A new measurement method to analyse the thermochemical conversion of solid fuels

Detta är en avhandling från Stockholm : Materialvetenskap

Sammanfattning: The firing of fuel wood has been identified as one of themain causes of pollutant emissions from small-scale (<100kW) combustion of wood fuels. The emissions are a result ofinsufficient combustion efficiency. This thesis presents a newmeasurement method to analyse the thermochemical conversion ofbiofuels in general, as well as to explain the main reason ofthe inefficient combustion of fuel wood in particular.In general, small-scale combustion of biofuels are carriedout by means of packed-bed combustion (PBC)technology. Acomprehensive literature review revealed that textbooks,theories, and methods in the field of thermochemical conversionof solid fuels in the context of PBC are scarce. This authorneeded a theoretical platform for systematic research on PBC ofbiofuels. Consequently, a new system theory - the three-stepmodel - was developed, describing the objectives of, theefficiencies of, and the process flows between, the leastcommon functions (subsystems) of a PBC system. The three stepsare referred to as the conversion system, the combustionsystem, and the heat exchanger system (boiler system). A numberof quantities and concepts, such as solid-fuel convertibles,conversion gas, conversion efficiency, and combustionefficiency, are deduced in the context of the three-step model.Based on the three-step model a measurement method washypothetically modelled aiming at the central physicalquantities of the conversion system, that is, the mass flow andstoichiometry of conversion gas, as well as the air factor ofthe conversion system. An uncertainty propagation analysis ofthe constitutive mathematical models of the method was carriedout. It indicated that it should be possible to determine themass flow and stoichiometry of conversion gas within the rangesof relative uncertainties of ±5% and ±7%,respectively. An experimental PBC system was constructed,according to the criteria defined by the hypothetical method.Finally, the method was verified with respect to total massflow of conversion gas in good agreement with the verificationmethod. The relative error of mass flow of conversion gas wasin the range of ±5% of the actual value predicted by theverification method.One experimental series was conducted applying the newmeasurement method. The studied conversion concept correspondedto overfired, updraft, horizontal fixed grate, and verticalcylindrical batch reactor. The measurements revealed newinformation on the similarities and the differences in theconversion behaviour of wood chips, wood pellets, and fuelwood. The course of a batch conversion has proven to be highlydynamic and stochastic. The dynamic range of the air factor ofthe conversion system during a run was 10:1. The empiricalstoichiometry of conversion gas during a run was CH3.1O:CH0O0. Finally ,this experimental series revealed one ofthe main reasons why fuel wood is more difficult to burn thanfor example wood pellets. The relatively dry fuel wood (12-31g/m2,s) displayed a significantly lower time-integratedmean of mass flux of conversion gas than both the wood pellets(37-62 g/m2,s) and the wood chips (50-90 g/m2,s). The higher the mass flux of conversion gasproduced in the conversion system, the higher the combustiontemperature for a given combustion system, which in turn ispositively coupled to the combustion efficiency.In future work the method will be improved so thatmeasurements of combustion efficiency can be carried out. Othertypes of conversion concepts will be studied by the method.Keywords: Packed-bed combustion, thermochemical conversionof biomass, solid-fuel combustion, fuel-bed combustion, gratecombustion, biomass combustion, gasification, pyrolysis,drying.