Tar Formation and Destruction in a Fixed-Bed Reactor Simulating Downdraft Gasification: Equipment Development and Characterization of Tar-Cracking Products

Abstract

The aim of the present study is to examine operating parameters that would reduce the residual tar content in the fuel gas in downdraft gasifiers and eventually eliminate it altogether. A two-stage fixed-bed reactor has been employed to simulate elements of tar cracking in a downdraft gasifier. In this reactor, tar is generated by pyrolysis in the first stage and cracking and gasification take place in the second stage. Modifications to a previous configuration of this reactor are described, which have enabled the use of smaller char particle sizes in the second stage and the generation of a more complete inventory of the reaction products. In this work, the effect of the temperature and the presence of char on product distributions are reported. Increasing the temperature from 700 to 1000°C resulted in a decrease in the quantity of tar recovered and an increase in the total amount of CO released. The amount of CH 4 released increased between 700 and 800°C before remaining steady up to 1000°C. The CO 2 content of the gas was relatively constant between 700 and 800°C and increased as the temperature increased from 800 to 1000°C. The amount of water and light hydrocarbons (C 2 -C 5 alkanes and alkenes) sharply decreased at 1000°C. The presence of char in the second stage had significant effects on tar cracking and product distributions. These effects were more obvious on the concentrations of CO, CO 2 , and H 2 O, which may be a result of reduction reactions taking place with the carbon in the packed char bed. These reactions appear to be more significant at temperatures between 900 and 1000°C, where the rates of gasification are expected to increase.

Influence of temperature and particle size on structural characteristics of chars from Beechwood pyrolysis

“…This trend was reversed at higher temperatures (> 400 °C), with the smaller particles leading to less tar yields. This is associated with the greater surface area available to interact with the volatiles, leading to the occurrence of further tar cracking at high temperatures to form gas [24]. The lower tar yields observed for the 0.212 -0.5 mm particle size at high temperatures were in agreement with much greater gas yields than those obtained for the two larger particle size ranges (Figure 2(c)).…”

Nascent Biomass Tar Evolution Properties under Homogeneous/Heterogeneous Decomposition Conditions in a Two-Stage Reactor

“…15,16,24,25 Our previous study 14 shows that it is a good way to learn the pyrolysis and gasification properties in the twostage reactor. In order to accurately investigate the tar conversion, some improvements were made, such as extending the second stage, and quantitative analysis of the tar species, especially the one ring aromatic ones.…”

Investigations into the effects of volatile biomass tar on the performance of Fe-based CLC oxygen carrier materials

“…A 6 kW e , 2-stage fixed-bed reactor was designed and constructed, based on an update and upgrade of an existing reactor [35][36][37][38]. A number of limitations, were overcome, namely: (i) welded flange connections were replaced with twin-ferrule compression fittings to allow for operation at pressures up to 30 bar a and temperatures up to 1273 K; (ii) the reactor has a wider internal diameter to allow for solid fuel feeding during operation; (iii) inert quartz tubes incorporating sintered quartz disks, acting as bed supports, lined the inside of the reactor to prevent undesirable reactions between gaseous reactants and products with the potentially catalytically active reactor walls [39]; (iv) a new tar trap (described below) was designed to enable efficient tar trapping with an ice and salt water coolant as opposed to liquid N 2 -this means that N 2 could be used as the carrier gas instead of He, and CO 2 and steam could be introduced as a reactive gas.…”