Sulfur Abatement in Pyrolysis of Straw Pellets

摘要

A batch-type reactor was used to study the sulfur release from pellets made of Danish wheat straw during inert conditions. In addition, two means of sulfur abatement were investigated, the use of calcium-based additives for sulfur retention and the use of nonthermal plasma (NTP) for the treatment of the devolatilized products. The results were interpreted by quantifying the releases of hydrogen sulfide (H_2S) and carbonyl sulfide (COS) to the gas phase for the pyrolysis temperatures of 400-800 ℃. In experiments where additives were used, straw was mixed and pelletized with either calcium oxide (CaO) or calcium hydroxide [Ca(OH)_2] at different calcium/sulfur ratios. The effect of additives on the sulfur release was difficult to assess by solely investigating the gaseous products. However, a comparison of the sulfur content in the starting material and the char residues after pyrolysis showed evidence of minor sulfur retention. The nonthermal plasma reactor system was set up to process the produced gas either before or after the removal of the liquid fraction from the devolatilized products. Furthermore, carbon dioxide was mixed with the nitrogen carrier gas to achieve a more complex gas composition for the nonthermal plasma. For comparative reasons, some preliminary experiments were performed to study the removal efficiency of H_2S in pure nitrogen. The removal efficiency for sulfur under the effect of nonthermal plasma in pure N_2 increased with an increasing H_2S concentration and plasma power. The highest registered removal efficiency was close to 96%. The removal efficiency of H_2S in the pyrolysis experiments was highest at a pyrolysis temperature of 400 ℃, with the nonthermal plasma reactor placed downstream of liquid removal. At this configuration, 86% of H_2S was removed from the devolatilized products. The best plasma reactor placement was proved to be downstream of liquid removal for both H_2S and COS. Increasing the CO_2 amount in the carrier gas has improved the removal efficiency of H_2S at the cost of increased COS formation.