Non-slagging entrained flow gasification of pyrolysis oil from radiata pine woody biomass.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A pilot scale non-slagging entrained flow (EF) gasification system was developed to gasify biomass pyrolysis oil using oxygen as the gasification agent. The pyrolysis oil was derived from New Zealand radiata pine wood chips through a fast pyrolysis process. The oil was fed into the system through an external mix twin-fluid atomizer which is capable of generating fine oil droplets after impact with the oxygen gas. Gasification was conducted at atmospheric pressure while the operation temperatures were dependent on the heat generated from partial combustion of pyrolysis oil with oxygen upon leaving the atomizer. Cold model experiments were first performed to investigate the effect of pyrolysis oil and gas flow rates on atomization performance. The use of external mix atomizer in this work is advantageous as it allows superior control of atomization performance by independent adjustments of pyrolysis oil and oxygen flow rates. Results from this cold model experiments show both flow rates of pyrolysis oil and oxygen gas have distinct influence on the resulted spray characteristics and consequently gasification performance. Increase in pyrolysis oil flow rate has negative influence on spray characteristics as larger droplet size and less uniform spray are generated. On the other hand, atomization at high gas flow rate substantially improves spray characteristics by production of more uniform spray and smaller droplets. The combined effect of pyrolysis oil and gas flow rates is reflected by the gas-to-fuel ratio (GFR) where higher GFR values indicate higher atomization performance and therefore better spray characteristics. After the cold model experiments, entrained flow gasification of pyrolysis oil were conducted to investigate effects of different gasification conditions on producer gas composition, gas yield, tar species distributions and total tar content in the producer gas. When equivalence ratio (ER) was increased during gasification at constant oxygen flow rate, concentrations of H2, CO and CO2 in the producer gas changed in parabolic trends, which is unique compared to linear relationships usually reported in the literature. Below the critical ER value, the H2 and CO concentrations increase while the CO2 concentration decreases with increasing equivalence ratio. However when the equivalence ratio exceeds the critical value, the opposite trends are observed. The critical ER during gasification at oxygen feeding rate of 900 L/h occurred at equivalence ratio of 0.3. At this condition, the maximum concentrations of H2 and CO were at 22 vol% and 36 vol% respectively while the minimum concentration of CO2 was measured at 33 vol%. The changes in producer gas trends with equivalence ratio relates to the continuous improvements in the spray characteristics and increase in the process residence time as the equivalence ratio increases. At constant oxygen flow rate, higher ER value is obtained by decreasing pyrolysis oil feeding into the system, which corresponds to higher GFR value thus better spray characteristics. The decline in pyrolysis oil feeding also results production of less producer gas which reduces its velocity and consequently increases the process residence time. Investigation on influence of increasing oxygen flow rate during gasification at constant ER was also performed in which the oxygen flow rate was varied between 600 L/h, 900 L/h, 1500 L/h and 3000 L/h in separate sets of experiments. Results from these experiments proved that at a given ER, gasification with high oxygen flow rate is highly desirable due to the substantial improvements in the oil spray characteristics and mixing behaviour that consequently leads to better overall reaction kinetics in the system. In addition to that, increase in the gasification temperature during gasification at high oxygen flow rates also help drive the system closer to equilibrium state which favours H2 and CO concentrations in the product. Increase of oxygen flow rate from 900 L/h to 1500L/h during gasification was observed to shift the critical ER value from 0.3 to 0.5 at which the maximum H2 and CO concentrations were also increased from 22 vol% to 28 vol% and from 36 vol% to 41 vol% respectively. On the other hand CO2 concentration declined from 33 vol% to 27 vol% when the oxygen flow rate was increased. In general, at high ER values, higher oxygen and pyrolysis oil flow rates favour producer gas quality with higher concentrations of H2 and CO but lower concentrations of CO2. However, at low ER values, the producer gas from gasification at higher oxygen and pyrolysis oil flow rates consists of lower concentrations of H2 and CO but higher concentration of CO2. As the residence time increases at increasing equivalence ratio, pyrolysis oil conversion improves more rapidly during gasification at 1500 L/h than that at 900 L/h thus giving more rapid growth in H2 and CO concentrations in the producer gas. These results are related to the effects of varying oxygen and pyrolysis oil flow rates on both spay characteristics and residence time. From analysis of tar in the producer gas, it was found that the total tar content in the producer gas decreased as the equivalence ratio was increased. In all cases investigated in this work, light polyaromatic hydrocarbon (PAH) compounds form the largest fractions of the tar components which accounts up to 78 wt% of the total tar in the producer gas. Detailed investigation into the tar individual species revealed naphthalene as the single most abundant tar species in the system which contributed as much as 36 wt% of the total tar in the producer gas. Results from this analysis also confirm that the methods used for tar sampling and analyses adopted in this work are capable of capturing and analysing tar compounds in the producer gas.