Determination of Design Parameters and Investigation on Operation Performance for an Integrated Gas Cleaning System to Remove Tars from Biomass Gasification Producer Gas.

Abstract

Determinations of design parameters and investigation on operation performance of a tar removal system for gas cleaning of biomass producer gas have been undertaken. The presence of the tars in the producer gas has been the major hindrance for the commercialisation of the biomass gasification technology for power generation, hydrogen production, Fischer Tropsch (FT) synthesis, chemical synthesis and synthetic natural gas (SNG) synthesis. The characteristic of the tars to condense at reduced temperatures cause problems in the downstream processing as the tars can block and foul the downstream process equipment such as gas engines reactor channels, fuel cells, etc. Considerable efforts have been directed at the removal of tars from the producer gas where the tars can be either chemically converted into lighter molecular weight molecules or physically transferred from gas phase to liquid or solid phase. In the former, the tars have been removed in a scrubber by transferring them from the producer gas to a scrubbing liquid and then removed from the liquid to air in a stripper and finally recycled them into air to a gasifier to recover their energy. A tar removal test system involving a scrubber and stripper has been designed based on the predicted tar solubility in canola methyl ester (CME) as the scrubbing liquid and its measured properties (CME is a type of methyl ester biodiesel). The tar solubility has been predicted to decrease with increasing temperatures and thus its value increases at lower temperatures. In designing the test system, the design parameters are needed including equilibrium coefficients of the gas-liquid system, molar transfer coefficient and the optimum liquid to gas flow rate ratio. The equilibrium coefficients have been predicted based on thermodynamic theories where the required data are determined from CME composition and known properties of each component of the CME as well as the properties of the model tar (naphthalene). The molar transfer coefficients are then experimentally determined and the correlations as a function of liquid and gas flow rates are proposed which are consistent with literature. The optimum liquid to gas flow rate ratios have been found to be 21.4±0.1 for the scrubber and 5.7±0.1 for the stripper. Using these optimum ratios, the tar removal efficiencies in the scrubber and the stripper are 77 and 74%, respectively. The analysis of the system performance has been achieved after an innovative method of determining tar concentrations in both the liquid and gas phase had been developed based on the concept of the density of liquid mixtures. However, these tar removal efficiencies are low due to the fact that the targeted tar concentration in the scrubber’s off-gas was large. As a result the system has been redesigned based on the determined design parameters and its operation performance retested. In the redesigned system, the tar removal efficiency in the scrubber and stripper is 99%. The redesigned system would be integrated with the UC gasifier for downstream gas cleaning. Since 1% of tars are not removed, a makeup tar free CME of 0.0375 litres per hour for the 100kW UC gasifier has been introduced in the recycle stream between the scrubber and stripper to avoid tar accumulation in the system.