The focus of this work was on the influence of process parameters like feed gas temperature, feed gas volume flow rate, feed gas composition (air or mixture of air and steam), feedstock particle size and feedstock type on fuel gas quality and distribution, during gasification of biomass on high temperature air and steam in an updraft fixed bed gasifier. This was studied, based on the fuel gas produced with respect to: composition, lower heating value, cold gasification efficiency, mass flow rate, specific gasification rate and tar composition. The results obtained allow drawing the following major conclusions:
1. Experiments conducted in the updraft fixed bed batch type gasifier with use of high temperature air and steam as feed gas have shown that the feedstock type and composition has influence on the performance of the gaisfier. Although all feedstocks have shown similar behaviour with respect to improved quality of the fuel produced, the batch mode operation of the gasifier did conditioned the mass of charge, being lower for low bulk density biomass fuels and for charcoal. Owing to its composition, gasification of charcoal, however, did yield fuel gas with similar magnitude of quality as was with wood pellets.
2. Preheating of air used as feed gas provided conditions for the temperature of the gasifier bed to be kept above 1000oC (with spots of approx. 1300oC) with resulting creation of higher heating rates, that favoured fuel gas produced having high molar fractions of H2, CO and CH4, but also noticeable quantities of higher hydrocarbons, particularly increasing with the temperature.
3. An increase of the feed gas temperature reduces production of tars and increases calorific value of the fuel gas produced, cold gasification efficiency and specific gas production rate.
4. The average lower heating value of the fuel gas produced with wood pellets is in the range 4.7 MJ/Nm3 to 7.6 MJ/Nm3. The highest value obtained with increasing temperature of the feed gas (7.6 MJ/Nm3) is notably higher than those obtained in more conventional air
gasifying low bulk density feedstock like bark and wood chips was as low as for conventional air blown gasification of biomass.
5. An increase in the feed gas temperature allows the fuel gas to be less diluted by nitrogen and therefore the higher calorific value fuel gas. The percentage of N2 would depend on the temperature in the gasifier and on the feedstock type. For the conditions of experiments used in this work, nitrogen decreases with temperature.
6. An increase of steam fraction in relation to the high temperature air in the feed gas decreases the temperature of the gasification but increases molar fraction of H2 while decreasing that of CO. The effects originated by the steam are greater than in the case of the low temperature air gasification process. It has been noticed that preheating of air and steam requires an additional energy, which has to be supplied to enhance thermal cracking and reforming.
7. At maximum molar fraction of steam in the feed gas (83%), the decrease of the gasifier bed temperature results in decreased heating rate with consequent decrease in LHV, mass flow rate of fuel gas, cold gasification efficiency and specific gas production rate.
8. Globally, gasification of feedstocks with mixture of air and steam has clearly shown that the H2 and CO2 contents in the fuel gas increase and the CO content decrease, as a result of shift reaction. It was seen that the water-gas shift reaction is more favoured by highly preheated feed gas temperature.
9. Thermal cracking of biomass tar compounds requires feed gas temperature above 850oC in order to induce process temperature above 1000oC. Tar amount shows a decrease trend with feed gas temperature, and its composition is also influenced with high temperature with higher molecular weight organic compounds, of the class of neutral fractions (mainly tertiary products), prevailing in the fuel gas. The high amounts of H2 and CO has shown the potential of the preheated feed gas on promoting thermal cracking in a one step updraft fixed batch type gasifier.
10. Increase of volume flow rate has shown quick heat release, thus favoring the attainment of successively higher gasifier bed temperature, with consequent fast production of high molar fraction of combustible products, particularly H2 and CO.
11. A mathematical model has been formulated for predicting all the main chemical and physical processes taking place during the fixed-bed gasification of biomass fuels with high temperature air. It was seen that this model was able to give good agreements of the fuel gas’ species profiles and their maximum values with that obtained experimentally.
12. This work provided important information that led to designing and construction of an updraft continuous gasifier at the same laboratory premises from the Royal Institute of Technology. First trial of experiments was performed and results did show promising trends on the gasification profile. For instance, carbon monoxide/hydrogen ratio for gasification with preheated air reaches as high values as 2.0-2.2 while for gasification with mixture of air and steam it may be found within the range of 0.8-1.3.