Conversion of Condensable Hydrocarbons in a Dual Fluidized Bed Biomass Gasifier

Abstract

Biomass gasification is a primary process in the thermochemical conversion of biomass into biofuels, chemicals, and electricity. The produced raw gas consists of permanent gas species, such as hydrogen (H 2 ) and carbon monoxide (CO), with variable amounts of heavier/larger species, depending on the gasification technique and process conditions employed. These heavier species are often referred to as tar, which is herein defined as all species with boiling points that lie between the boiling points of benzene and coronene. In this work, experiments were conducted in the Chalmers 2-4-MW dual fluidized bed gasifier utilizing equipment that allows for simultaneous quantification of the cold gas and the tar species, together with the total raw gas yields of C, H, O, and N. The obtained results are used to describe the effects of temperature, steam-to-fuel ratio, residence time, and active materials on both the gas composition and the carbon balance of the system. Furthermore, as the carbon balance is fulfilled, the char conversion, oxygen transport, and amounts of carbon in unidentified condensable species can be determined. The unidentified condensable species comprise a group of compounds that are not measured as part of the other groups [cold gas and SPA tar, measured using the solid phase adsorption (SPA) method]. In addition, this group is shown to be readily converted into SPA tar, and cold gas as the severity of the gasifier, in terms of temperature and residence time, was increased.

Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite

“…For the olivine case after 1 week of operation, the total elemental yields of CHO in the hot dry gas were instead estimated by means of the permanent gas and the SPA-tar yields. This calculation underestimates the carbon conversion and oxygen transport due to the existence of undetectable condensable species [38], which are not accounted for.…”

“…In the case of quartz-sand, 363 the alkali species are most likely chemically bound to silicon, which cannot be desorbed with steam 364 [26]. Therefore, the trend in the quartz-sand case shown in Fig.11 is in line with the shorter residence 365 time of the fuel in the gasifier as fluidization velocity increases [38]. When alkali species are physically 366 bound to the material, as in the case of bauxite [20], a greater steam flow results in enhanced char 367 conversion, in line with the enhanced desorption of alkali species from the material.…”

“…Quartz-sand and ilmenite were used as reference materials for the dominant thermal effect and dominant oxygen transport effect, respectively. The complete carbon balances for quartz-sand and ilmenite were investigated previously [38], and therefore they are only used here as supporting cases to interpret the results related to olivine and bauxite.…”

“…Different steam-to-fuel ratios (S/F) were applied after 1 week of operation, as the materials at that stage are considered to represent more accurately those present in industrial applications, which implies longer exposure to biomass ash and operational times. The S/F ratio was here modified by changing the flow of fluidization steam, which also influenced the gas-solid mixing [39], as well as the residence times of the gases in the reactor [38]. The reference case with quartz-sand provided the basis to understand the impacts of S/F ratio and residence time, whereas the differences between the quartz-sand case and the cases with olivine and bauxite could be attributed to the catalytic activities of the materials.…”

Influence of In-Bed Catalysis by Ash-Coated Olivine on Tar Formation in Steam Gasification of Biomass

“…Benzene can be formed through the dealkylation of alkyl-aromatics [32,36] and the dehydroxylation of phenol [35,38]. However, Israelsson et al [45,49] have reported that at temperatures <830°C, the conversion reactions of toluene and phenol are not sufficient to explain the high yields of benzene obtained with an increase in process temperature. Similarly,…”