The present study has examined the behaviour of a range of biomass/waste derived fuels under downdraft gasification conditions. The aim has been the identification of operating conditions leading to the complete destruction ofthe tars. A two-stage 'hot-rod' fixed-bed reactor has been developed to simulate the operating conditions in a commercial downdraft gasifier. The upper 'hot-rod' reactor simulates the pyrolysis zone of the commercial gasifier. A 1 g batch of fuel is electrically heated from ambient to 500°C at a rate representative of conditions in the upper zone of the commercial gasifier, i.e. few °C/min. In the commercial device, air is injected through a throat part-way down the shaft permittJllg gasification to take place in the lower regions of the gasifier. In the laboratory experiment, the throat has been simulated by a flange with three built-in nozzles through which helium or helium-diluted air may be admitted. This flange joins the upper pyrolysis 'hot-rod' reactor to the lower, tardestruction reactor. A systematic study has been made of the factors that will destroy tar in the lower reactor; these are temperature, presence of a char bed, type of char bed, char particle size, gas residence time and addition of oxygen (as air) at the throat. In the first part of the experimental programme, the emission of tars from a range of fuels pyrolysed in the upper reactor alone has been measured. The fuels used were plastic waste, eucalyptus wood, paper sludge, pine wood and silver birch wood. These were pyrolysed under similar operating conditions, using helium as sweep gas, at atmospheric pressure.,. The reactor was operated at two different heating rates, representing the likely extremes in a commercial gasifier, i.e. 1 °C/s and 10 °C/s, over the temperature range 200 - 500°C, with holding times of 300 s and 900 s at peak temperature and with a helium gas velocity of 0.1 mfs. The results indicate that the pyrolysis-derived tar, char and gas yields varied between the samples, but the trends were similar with increasing temperature; the tar yield increased with increased temperature. It was observed that silver birch wood gave the highest tar yield (at 47% by weight of the fuel fed) in the hot-rod reactor, whilst paper sludge gave the lowest tar yield (19%). As a result of this first phase of experiments, silver birch wood, pyrolysed to 500°C, has been chosen as the standard for the later tar destruction experiments, in the second stage. In the second phase of experiments, tar-laden gas from the upper stage was fed into the lower stage reactor, over the temperature range 700 - 1000°C. The sweep gas velocity was 2 mfs and initially helium, then a airlhelium mixture was added via three injector nozzles. Initial experiments showed that significant extents of tar destruction occur even in the absence of a packed bed in the heated, second stage reactor. Tar yield reduction from the initial value of47% down to 5% has been observed at 800°C. This figure fell to 0.7%, when the reactor temperature was raised to 1000°C. Addition of a char bed to the second-stage reactor achieved further tar reduction, compared to the empty reactor. The size of char particles in~roduced was observed to
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:444142 |
| Date | January 2007 |
| Creators | Nunes, Stianeth Monteiro |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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