Tar Formation and Destruction in a Simulated Downdraft, Fixed-Bed Gasifier:  Reactor Design and Initial Results

Abstract

A suite of improved technologies is being developed to minimize the environmental impact of biomass/waste fired gasification processes. Downdraft, fixed-bed reactors are particularly favored because of their ability to destroy the majority of tars produced from the fuel volatiles. However, there is some concern about the impact of the low residual tar concentration on the long-term operational reliability. A two-stage laboratory scale fixed-bed reactor has been constructed for studying the release and destruction of tars in downdraft gasifiers. The reactor has been commissioned and its performance demonstrated using several biomass feedstocks. Experiments using the first stage only have shown that as the temperature is raised from 250 to 450 °C, the gas and tar yields increase at the expense of the char residue. Four different biomass/waste materials (eucalyptus wood, sludge, plastic waste, and silver birch wood) showed qualitatively similar behavior. Volatile yields appear to stabilize around 450 °C. With silver birch wood, the tar yield reached 47% of the initial fuel. Preliminary tests using a char bed in the second stage have been completed. The presence of the throat and the second-stage char bed results in a substantial reduction in the quantity of tar leaving the reactor. With a hot empty second stage (at 800 °C), the tar content was reduced to 5.3% (by wt of initial fuel charge) in the exit gas from the reactor. Packing the second stage with char (at 800 °C) further decreased the tar content to less than 0.1%. Gas analyses have been performed, showing that some of the initial tar is broken down mainly to CO and CH4 in the second stage of the reactor. Further work is in progress to study the impact of the operating conditions in the second stage on the residual tar concentrations and gas analysis.

On the suitability of thermogravimetric balances for the study of biomass pyrolysis

“…This reactivity of tar in the presence of char has been demonstrated by numerous researchers. [11][12][13][14][15] In this way, the more time gas-phase pyrolysis products spend in close proximity to char particles (the sample bed), the greater char yield will be, and any experimental parameter that affects this residence time, will thus affect char yield. [16][17][18][19] To minimise the effect of repolymerisation on char yield, inert gas is often flowed through the sample bed to sweep away gas-phase tars, but in fixed-bed reactors the effects of particle stacking are unavoidable, as gas-phase pyrolysis products do not move as quickly through the sample bed as does sweep gas.…”

Influence of Reactor Design on Product Distributions from Biomass Pyrolysis

“…A second stage may be added, when catalytic or thermal post-treatment of the product stream is required. Nunes et al, Dabai et al, and Volpe et al have studied the cracking reactions of biomass pyrolysis tars/oils, by injecting tar/oil vapors generated in the first stage into a solids-packed second-stage fixed-bed reactor.…”

Experimental Tests and Modeling on a Combined Heat and Power Biomass Plant

“…m H20 UT 2 = . m smokes C p _smokes_8_8 (T 8 − T 8 ) C p H20 T H20 UT 2out − T H20 UT 2in = 1.1317 kg s = 4071.2 kg h (10) Finally, the thermal power for the two thermal users is obtained as:…”

“…A proper analysis of the specific operating conditions of the combined heat and power (CHP) plants is a priority to maximize the primary energy savings. Further developments are needed in order to optimize the efficiency of the process and to ensure a reduced environmental impact [10].…”