| dc.description.abstract | The effects of inorganic salts on the yields and distribution of products from air gasificationof biomass in a fluidized bed were investigated. The main objective was to establishconditions for production of gas with minimum tar content suitable for engine applications. The study was conducted in five stages: (1) field survey to assess the availabilityof biomass fuels-in Kenya and their suitability for gasification; (2) design and construction of a laboratory scale bubbling fluidized bed gasifier, biomass feeder, gas cleaningand cooling systems, and the necessary measurement systems; (3) experimentation to investigate the effects of the experimental variables on the gas quality; (4) development andvalidationof a single-phase (CSTR) model to predict the composition and flow rate of product; and (5) assessment of the impact of the use of inorganic salts (as catalysts) on the economics of a gasification project
The experimental apparatus consisted of a fluidized bed gasifier of 0.15 m ID and
1.76m total length, a biomass feeding system, and a gas cleaning and sampling system. The biomass feeding system consisted of a hopper and a variable speed screw feeder. The screw was externally cooled and continuously introduced the biomass (1.8 - 5.0 kg/hr.)
just above the distributor plate. The gas cleaning system consisted of a cyclone, two
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condensersand filters.
Coffee husks, spruce sawdust, and spruce sawdust impregnated with inorganic salts were gasified with air at 600 - 800 DC at atmospheric pressure. Sodium tetraborate (borax) (3.02% and 5.8% loading), potassium chloride (2.21% and 3.35% loadings), and lithium chloride (4.8% loading) were used. At the end of each experiment the following
were assessed: (1) product distribution (char, tar, gas.es), (2) gas composition by gas
chromatography (Hewlett Packard Model 5890 Series II, with thermal conductivity detectors, Molecular Sieve SA 60/80, and Porapak Q), (3) energy content of the product
gas, and (4) the mass balance closure in each run. Only a small portion of the product gas was passed through the condensers and the filters; the rest was directed to the stack. The condensers and filters were washed with acetone or methylene chloride to recover tars.
Both the reactor temperature and the inorganic salts have a major influence on the
yield and distribution of products. Yields of tar, char, and water decreased while gas yield increased with increasing reactor temperature. For untreated sawdust, as the reactor temperature increased from 600 °C to 800 oC, the tar yield decreased from 86 g/Nm--dry gas to 5.5 glNm3-dry gas (i.e. by 94%), char and water yields decreased by 88% and 62% respectively, while the gas yield more than doubled. The heating value of the product gas increased with reactor temperature to a maximum of 5.3 MJ/Nm3_dry gas at 700 0C and then decreased with further increase in temperature. In the operating temperature range, HHV of the product gas ranged between 4.5 MJ/Nm3 and 5.3 MJ/Nm3 for untreated sawdust. Gasification of the coffee husks generates gas with slightly lower energy content but with less tar and less moisture compared to that from untreated wood sawdust.
All the inorganic salts investigated in this study increased the char yield but decreased the tar and moisture content of the product gas (compared with the gasification of untreated sawdust under similar conditions). The tar reducing effects of the the inorganic
salts was less pronounced at higher temperatures. Tar reduction ranged between 11 % and
89% while the tar content of the product gas ranged from 2.3 g/Nm3 - dry gas to 9.2 g/Nm3 -dry gas. The lowest tar content value was obtained with sawdust impregnated with
3.02% of borax and gasified at 800 °C. Overall, borax seems the best for tar reduction. On mole basis, the three inorganic salts can be classified in order of tar reducing effectiveness as:
Na2B40] > LiCl > KCl.
While the borax and LiCI slightly lowered the heating value of the product gas, KCI was found to enhance it by between 1% and 30%. The percent energy recovered in the dry tar free product gas increased with increasing gasification temperature; borax and LiCI did not significantly affect this parameter, but KCI increased it as a result of the enhanced energy content of the product gas. The maximum efficiency obtained was 61.4% with untreated sawdust at 8000 C and 66.4% with sawdust impregnated with 2.2% of KCI at the same temperature. Total carbon conversion ranged between 64% and 99%.
Treatment of sawdust with the inorganic salts caused some operational problems.
At high operating temperatures and higher salt loadings, agglomeration of silica sand was observed. This caused channeling of the bed and consequent defluidization. Agglomeration can be attributed to the presence of high content of inorganic elements of K and Na in the ash of the salt treated sawdust.
A two-stage, single phase (CSTR) model was developed and used to predict the gas
flow rate and composition in a fluidized bed reactor. The model assumes instantaneous devolatilization of the biomass and incorporates (i) char gasification and water-gas shift reactions as finite rate processes, (ii) gas phase products from the devolatilization stage of the biomass, and (iii) products from thermal cracking of tar. A thermodynamic equilibrium model was also derived and compared with the CSTR model. When compared with
experimental results, the model predictions were within ± 20% of the experimental values.
The CSTR model gave better predictions than the equilibrium model.
An economic evaluation of a medium-scale sawdust gasifier plant was conducted to assess the economic impact of iinpregnating sawdust with 3.0% borax prior to gasification. Two systems were compared: one, gasification of untreated wood sawdust in an FBG at
700 °C with associated gas cleaning systems, and the other, gasification of pre-treated
sawdust under similar conditions. The results of the analysis showed that the latter gave slightly better economics than the former.
From the field survey conducted in Kenya, the forest industries and the agricultural
sector both generate over 4.6 million tonnes of biomass fuels per annum that are suitable for gasification. No major competing uses were identified for the biomass fuels, and none is anticipated. | en |
