Process Efficiency And Regulation Mechanisms Of Syngas Generation From Microwave Pyrolysis Of Sludge

Energy efficient recovery and resource efficient recycling of sludge are the focus of interest in the environmental field.Microwave pyrolysis technology has become a hot research topic in recent years because of its ability to rapidly convert the sludge into resource products.However,there are bottlenecks such as the lower biogas yield and the difficulty of regulating the ratio of syngas.This study aimed to produce the syngas by microwave pyrolysis of sludge.Different types of absorbers were used to regulate the microwave pyrolysis temperature field.The effect of different microwave pyrolysis modes on the efficiency of sludge pyrolysis to produce the biogas was investigated.The combined H₂O/CO₂reforming technology was introduced to regulate the syngas ratio,and the reaction mechanisms of the bio-oil reforming to syngas was analyzed.The feasibility of process energy consumption and the product resource utilization were calculated and evaluated,which provided a significant theoretical guidance and a potential application value for the preparation of syngas and the biochar from sludge.Mixed microwave absorbers can quickly start the microwave pyrolysis reaction of sludge,but the selective heating characteristics of microwaves lead to uneven distribution of local pyrolysis temperature and reduce the biogas yield.The distribution of temperature fields during microwave pyrolysis was simulated by constructing a multi-physics field coupling model using COMSOL Multiphysics software.The enhancement of microwave output power could rapidly increase the pyrolysis temperature,but the overall microwave absorption performance of the material increased as the formation of biochar,which easily led to the formation of a localized high-temperature hot spot in the central region.The gradient distribution of the temperature field led to the inhomogeneous pyrolysis of organic matter and reduced the biogas yield.The use of SiC crucible with microwave absorption function could homogenize the temperature field and improve the efficiency of microwave energy utilization.The SiC crucibles could increase the sample heating rate more consistently than mixed with SiC powder.The temperature difference between the inside and outside of the sample was reduced from 518°C to 275°C,and the average microwave energy utilization efficiency was increased from 28%to 48%.The stable and rapid heating process was conducive to faster conversion of solid-phase organic matter into the gaseous products,and solves the problem of difficult separation of biochar and powder absorbers,which supported the microwave pyrolysis of sludge to produce biogas.The SiC crucible was used in the batch microwave pyrolysis mode to improve the uniformity of pyrolysis temperature distribution.The effect of different process parameters on the efficiency of microwave pyrolysis of sludge to produce biogas was investigated.An increase in the pyrolysis temperature could increase the biogas yield by promoting the pyrolysis of bio-oil and the solid phase organic matter.The biogas yield increased from 32.34 wt.%to 45.55 wt.%as the temperature increased from 500°C to 900°C.The increase in the sludge moisture content resulted in a decrease in the biogas yield,but facilitated the generation of H₂.The replacement of conventional N₂carrier gas with CO₂ reaction gas could improve the biogas yield by promoting the reforming of bio-oil and biochar.However,some of the bio-oil was not completely cracked during the rapid heating process due to the small reaction chamber space in the batch microwave pyrolysis mode,resulting in difficulties in further increasing the biogas yield.To enhance the efficiency of bio-oil cracking and reforming reactions,a continuous microwave pyrolysis mode with larger cavity space was used to extend the cracking path of bio-oil.The SiC blocks were placed in the front stage of the quartz tube reactor for rapidly increasing the heating rate and shortening the sludge heating time,and the biochar was used at the latter stage as absorbers to maintain the stability of the pyrolysis temperature.Compared with the batch mode,the continuous microwave pyrolysis mode could increase the biogas yield of sludge pyrolysis to 46.82 wt.%at 600°C,achieving the generation of biogas from microwave pyrolysis of sludge.A combined H₂O/CO₂ reforming technology was introduced in the continuous microwave pyrolysis process to further increase the syngas concentration.According to the response surface optimization analysis,the increase in the pyrolysis temperature and the increase in the concentration of CO₂ helped to synergistically improve the CO concentration,but reduce the H₂ concentration.The increase of sludge moisture content favored the generation of H₂.The H₂concentration could reach 34.79～36.55 vol.%at 600°C when the sludge moisture content and the CO₂ atmosphere concentration were controlled at 20～40 wt.%and 0～10 vol.%.In the pure CO₂ atmosphere,the CO concentration could reach30.05～36.0 vol.%.During the microwave pyrolysis of sludge,the reforming reaction between bio-oil and H₂O/CO₂ was the main pathway to promote the syngas generation,and the decrease in hexadecanoic acid content was closely related to the increase in the syngas concentration.Based on density functional theory calculations,it was verified that the activated CO₂ at high temperature could be used as an oxidant to reform with the hexadecanoic acid molecules,resulting in more H₂ and CO.According to the process optimization,the syngas concentration of produced from microwave pyrolysis of sludge could reach up to83.85 vol.%,which was close to the crude syngas composition.The gaseous product produced by the process can be used as a fuel gas or as a feed gas for the preparation of chemical products and has a high economic potential.The microwave pyrolysis process contributes to the efficient energy recovery and resource recycling of sludge,contributing to the improvement of environmentally friendly treatment and resource utilization of sludge.