No abstract

The tar problems are the major limit for development of biomass gasification. Biomass char has been proven to be an economical and effective catalyst of tar destruction for both utilizations inside the gasifier and in downstream processes after the gasifier. In order to investigate the mechanism of catalytic cracking of tar over a biomass char bed, experimental research was performed in a bench-scale tube flow reactor, choosing rice straw char as the catalyst bed, naphthalene as the model tar compound, and argon as the inert atmosphere. The effects of temperature (700−1000 °C), tar concentration, time on stream (0−330 min), presence of syngas, and pretreatment of char (treated with deionized water or Ni(NO 3 ) 2 solution) on tar conversion were evaluated. The variation of the inner pore structure of biomass char during the process of tar removal was also investigated. Results showed that the original biomass char exhibited good catalytic activity in tar cracking and better stability compared with Ni(NO 3 ) 2 pretreated char. However, because of the naphthalene cracking reaction, soot was formed on the active sites of the inner pore surface of the char, leading to the deactivation of the char. A rapid decline in specific surface area of the char was observed from 262 to 4.6 m 2 /g when the test had begun to run for 5 min with high tar concentration (25 g/Nm 3 ) and a temperature of 800 °C. The presence of syngas in the atmosphere could slow the process of deactivation of char.

In order to study the mechanism of biomass tar formation and elimination in a two-stage downdraft gasifier, the nascent rice straw pyrolysis tar evolution properties under homogeneous/heterogeneous decomposition conditions have been investigated in a constructed lab-scale two-stage reactor by varying factors as temperature, concentration and reforming agents of CO 2 /H 2 O/O 2 , and char bed heights. The nascent tar was produced in the first stage reactor and then decomposed in the second stage with different reforming agents or char beds. In the first stage, the results showed that nascent pyrolysis tar yields increased with increasing pyrolysis temperature, tar was mainly produced during 200À400°C, and 400À500°C would be a proper pyrolysis temperature range in commercial operation due to little effect on tar yields in higher temperature. In the second stage, it can be observed that nascent biomass tar was converted into polycyclic aromatic hydrocarbons (PAHs) (even soot), thermally stable one ring aromatics, and noncondensable gases in homogeneous conditions with increasing temperature. Different effects were obtained in varying tar species under different homogeneous reforming agents. However, benzene, toluene, styrene, phenol, and naphthalene are the most typical compounds, accounting for 50À75% in total tar concentration at 900°C in all decomposition conditions. Char bed can selectively reduce PAH species remarkably and increase the toluene yields. As for the three reforming agents, steam showed the highest efficiency in tar elimination, while CO 2 and O 2 present will induce OH, H, and O radicals formation, which increases hydrocarbon conversion. The mechanism of tar destruction in a two-stage downdraft gasifier can be concluded as follows: nascent tar yields from the pyrolysis stage will be first reformed into PAHs, thermally stable one ring aromatics and noncondensable gases in the throat region, and then PAHs species are almost completely decomposed by the char bed, which are the main troublesome tar components in syngas, and finally the syngas with low tar was obtained.

The formation and destruction of pyrolysis tar during the thermal cracking and fuel-rich oxidation have been investigated in a constructed test rig. Temperatures of 700À1100°C and equivalence ratios (ERs) of 0À0.403 were considered, and yields of gravimetric tar, gas, water, and soot were taken into account. In inert conditions, pyrolysis tar thermal cracking was greatly enhanced with the temperature increasing. CO and CH 4 increased almost linearly, and H 2 increased exponentially from 700 to 1100°C; meanwhile, oxygen-containing compounds or substituted 1-ring aromatics were converted into polycyclic aromatic hydrocarbons (PAHs). In the homogeneous reactor, the presence of oxygen induced more tar decomposition compared to inert thermal cracking. When the ER increased from 0 to 0.403 with a constant reactor temperature of 900°C, total tar yields reduced rapidly and reached a minimum value of 0.26 wt % at an ER of 0.34; meanwhile, the mass of noncondensable gases reached a maximum value. However, the mass of combustible gases, such as H 2 , CO, and CH 4 , were sharply reduced as the ER increased from 0.34 to 0.403. Although the aromaticity index increased gradually, most aromatic compound yields increased first and then decreased with the ER increasing, except naphthalene. It is considered that a proper oxygen/fuel ratio can promote the free-radical formation and accelerate the tar destruction, but excess oxygen will burn out most combustible gases.

No abstract