Comprehensive insight into co-pyrolysis of sewage sludge and tobacco stalk: Effects of mixing ratio and pyrolysis conditions on product distribution and characteristics.

Co-pyrolysis of sewage sludge (SS) and tobacco stalk (TS) was conducted, where TS are rich in inorganic elements. The effects of the mixing ratio and pyrolysis temperature on pyrolysis products were investigated through the tube furnace system. The components of gas and tar were determined. In addition, the N/S ratio, pore distribution characteristics, surface functional group characteristics, aromatisation characteristics and inorganic element content of char were studied. Results show that the addition of TS to SS has a significant synergistic effect. Particularly, when the ratio of SS to TS is 1:3, the pyrolysis gas yield is the largest. The inorganic elements with a catalytic effect can promote the conversion of char and tar into pyrolysis gas. However, with the increase in pyrolysis temperature, the synergistic effect will gradually weaken. The increase in pyrolysis temperature will promote the development of mesopores in char but will reduce the order degree of the char structure. The addition of TS in SS will further increase the number of mesopores in char, and the obtained char carbon structure is more ordered. When the pyrolysis temperature is 600 °C, the tar yield is the highest, and the higher the mixing ratio of SS, the better the formation of polyaromatic compounds and aliphatic hydrocarbons.

Prediction of CO/NOx emissions and the smoldering characteristic of sewage sludge based on back propagation neural network.

Smoldering can achieve effective disposal of sewage sludge (SS) with high moisture content at low energy input, providing social and economic benefits. However, smoldering is accompanied by the emission of high concentrations of CO/NOx, and thus, it requires sufficient attention. This study comprehensively investigates the effects of SS characteristics and experimental parameters on CO/NOx emissions and smoldering characteristics. Results showed that when the moisture content of SS increases from 35% to 50%, CO concentration increases while NOx formation is simultaneously inhibited. After airflow rate exceeds 5 cm/s, the concentrations of CO and NOx begin to decrease. When SS concentration is increased to 20%, the emission concentration of gas pollutants is directly increased. However, high temperatures inhibit the formation of NOx. When the particle size range is 180-270 µm, the formation of CO/NOx is promoted. Finally, a back propagation (BP) neural network model is constructed with SS characteristics and experimental parameters as input conditions, and CO/NOx emission concentration, smoldering velocity, and smoldering temperature as output parameters. The BP neural network model can effectively predict the emission concentration of CO/NOx and smoldering characteristics, providing support for intelligent control scenarios related to SS smoldering, it will help to further explore the great potential of smoldering treatment.

Experimental study and modeling analysis of sewage sludge smoldering combustion at different airflow rates.

Sewage sludge is a major by-product of wastewater treatment, and its unfavorable properties are frequently a key restriction of disposal technologies, resulting in high costs and ineffective waste management. Smoldering combustion is a new technique for disposing of organic solid waste with high moisture content, which efficiently recovers energy with minimal igniting energy requirements. The objective of this study is to investigate the effects of airflow rate on sewage sludge (SS) smoldering combustion by combining experimental and modeling analyses. Results show that air channeling easily forms at the reactor's edge, intensifying the smoldering reaction and forming a concave smoldering front. The minimum airflow rate required for self-sustaining smoldering is 0.3 cm/s. As the airflow rate increases, convective heat transfer becomes dominant over conduction and radiation, resulting in a surge in smoldering temperature and velocity at 0.6 cm/s, followed by a linear increase. The maximum airflow rate at which the smoldering process can propagate stably during SS disposal is 8 cm/s. The expressions of the smoldering characteristics are obtained by using the activation energy asymptotic approach, and the calculated and experimental values show the same trend of variation, with good agreement at low airflow rate conditions. Sensitivity analysis shows that porosity φ is the most crucial parameter affecting smoldering temperature and velocity.

Theoretical modelling of the chemical reactivity of fresh biomass chars under non-catalytic conditions.

This study developed a model for the chemical reactivity of fresh biomass chars, and built a calculation equation for the char gasification rate using simple gas-solid collision theory (SCT). The effects of pore breaks, pore collapse and thermal annealing on the char reactivity were considered in the modelling. Experimental tests for six acid-washed biomass chars were performed under a CO2 atmosphere and used a thermo-gravimetric analyzer (TGA) over the temperature range of 1073-1273 K. The results showed that the reactivity of fresh char could be predicted quantitatively by some characteristic properties of certain kind of biomass and their combined parameters. For the instability of the biomass char structure, the internal pore length and gasification temperature showed a good exponential relationship. Good agreement was achieved, and the applicability of the model was demonstrated by comparing the predicted results with experimental data.