8. Experimental Study on Removal of Low Concentration Coal Tar in Syngas by Mg-Ca Composite Catalyst

The catalytic cracking of low concentration coal tar in syngas was studied using Mg-Ca composite catalyst. The reaction was carried out in a two-stage fixed bed reactor. The effects of bed temperature (400–800 ℃), catalyst particle size (400–600 μm, 200–400 μm, 100–200 μm, and <75 μm) and catalyst layer height in bed reactor on tar conversion, pyrolysis gas products (CO, H₂, CO₂, CH₄), and the CO₂ absorption by CaO in catalyst were investigated. The results showed that the cracking rate of coal tar vapor increased with the increasing of bed temperature, till reaching a maximum of 94% at 700 ℃. The production of H₂ also increased with temperature, and the amount of CO₂ in the gaseous products decreased between 500 and 700℃ responding to the CO₂ absorption capacity of CaO reaching its peak in this temperature range. The absorbed CO₂ subsequently released at higher temperatures. The catalytic cracking rate was highest when the catalyst particle size was 100–200 µm, and the catalytic cracking efficiency increased with the increasing of catalyst layer height and reaction time. These results demonstrate that when the catalyst particle size is too small (<75 µm) or the reaction temperature is too high (>700 ℃), the catalyst activity for coal tar cracking is reduced.