Research On The Control Of NH₃and HCN During Microwave Pyrolysis Of Sewage Sludge

Microwave pyrolysis technology for sludge has the advantage in thoroughly sludgereduction effect, high energy recovery rate, high organic conversion rate and effectivefixation of heavy metals. Besides these, the characteristics of the microwave improves theweakness of low heating rate, low heat transfer rate and uneven heating in the traditionalheating method, thus having a good application prospects. The organic matter in thesludge contains a large amount of N elements, and therefore will produce the NH₃andHCN when producing high amount of combustible gases, this will have potential damagefor the environment and limit the application of the technology. So, it is necessary tofocus on the transformation law of nitrogen in the pyrolysis process in order to control thegeneration of NH₃and HCN.This experiment is based on the dewatered sludge of Taiping Sewage TreatmentPlant, the subject of this research are the prepared dry sludge, de-ash sludge and sludgethat has addictive minerals. We conducted the research on the pollution control of NH₃and HCN generated from the microwave pyrolysis process applied in the municipalsewage sludge. First, we studied the factor affecting the generation of nitrogen pollutiongases in the microwave pyrolysis processing, including the final temperature, heating rate,sludge moisture content as well as the inherent minerals and mineral additive. Then, westudied the internal mechanism of how the mineral will affect the generating of nitrogengases in the sludge microwave pyrolysis processing. Based on the research result of theaffecting factors, we got the optimum conditions in controlling the generating of nitrogengases and we also found the best pollution inhibitors. Based on this research, weconducted further research on the internal transformation mechanism of nitrogen gasesgeneration and we also analyzed the liquid products and gaseous products in themicrowave processing of sludge. We continued to study the controlling mechanism andthe pathway of generating gaseous product.The production rate of NH₃and HCN is affected by the following factors: the finaltemperature of the sludge heating process, the sludge moisture content and the heatingrate in the pyrolysis process. The conversion rate of NH₃and HCN is increasing with theincreasing of the final temperature, sludge moisture content and heating rate. In addition,mineral has a significant effect on the generating rate of NH₃and HCN; The generatingrate of NH₃and HCN from the acid ash-off sludge is much higher than the originalsludge; adding20%of Fe₂O₃and CaO has the best effect inhibiting the generation ofNH₃and HCN. By detecting the liquid and gases production from the pyrolysisprocessing of both original sludge and sludge that has mineral addictive using GC/MS,we found that liquid products mainly contain amine state-N, nitrogen-containingheterocyclic-N and nitrile-N. As the pyrolysis temperature increases, inorganic nitrogen,protein-N, amine state-N, as well as nitrogen-containing heterocyclic-N and nitrile-N inthe sludge have a concessive thermal cracking, and accompanied by the formation ofNH₃-N and HCN-N. After adding minerals, the pyrolysis of amine-N is affected and alarge number of NH₃is generated, and make it generate N₂and H2, the addition ofmineral also undermine the generating process of HCN and make it breaking down to N₂.What’s more, iron elements can act as a catalyze of the generation of N₂and H2fromNH₃, calcium elements can make the HCN hydrogenated to produce NH₃, and continue to serve as a catalyze of the production of N₂and H₂from NH₃.