Dioxin Formation in Biomass Gasification: A Review

A Study of Parallel and Competitive Reaction Schemes in Kinetic Modeling of Plastic Pyrolysis

Pyrolysis is a technology capable of harnessing energy from challenging-to-recycle plastics, thus mitigating the necessity for incineration or landfill disposal. To optimize the plastic pyrolysis process, reliable models for product yield prediction are imperative. This study endeavors to determine the suitability of lumped models, a widely used approach for modeling biomass and coal pyrolysis, in accurately estimating product yields in the context of plastic pyrolysis. To address this question, three lumped models with parallel and competitive reaction mechanisms were compared and fitted to experimental data collected across a broad temperature range. The aim is to identify which models can elucidate the most appropriate reaction pathway for the plastic pyrolysis process. The first model in this study assesses whether the commonly employed wood pyrolysis kinetic models can effectively fit the experimental data from plastic pyrolysis. Subsequently, the final two models introduce additional reactions into the pyrolysis process, prompting the authors to investigate the necessity of these supplementary reaction pathways for accurately predicting plastic pyrolysis outcomes. This investigation seeks to pinpoint the essential terms and discern which ones may be safely omitted from the models. The results of the study reveal that the model incorporating secondary tar reactions with gas, tar, and char is the most precise in predicting the products of plastic pyrolysis, surpassing all other combinations evaluated in this research.

[Burning of sugarcane biomass and hospitalizations of children and older adults for respiratory problems in Pernambuco State, Brazil]

This study aimed to analyze the relationship between hospitalizations for respiratory problems and the regular burning of sugarcane in Pernambuco State, Brazil. This is an ecological time series study corresponding to the period from 2008 to 2018. The rates of hospitalizations for respiratory diseases in children aged under 5 years and in adults older than 60 years in sugarcane-producing and non-producing municipalities were compared using nonparametric Mann-Whitney statistical analysis. Together, we observed the monthly distribution of the hot spots occurrences in the case and control municipalities and applied Pearson's correlation to analyze the association between both variables. For both age groups, hospitalization rates are higher in sugarcane-producing municipalities, with a statistically significant difference p < 0.005. The rate of hospitalization in older adults is 28% higher in the case municipalities, and is even higher in children aged under 5 years whose ratio of the medians is 40%. However, the seasonal behavior of hospitalizations for respiratory diseases differs from that observed in the monthly distribution of hot spots, without statistically significant correlation. These findings suggest a possible association with chronic exposure to particulates emitted by biomass burning, compromising the health of vulnerable groups, and endorse the need to replace fires in the monoculture of sugarcane and to structure public policies to protect human and environmental health.

Polychlorinated dibenzo-p-dioxins/dibenzofurans and mercury in vegetable of the contaminated Ya-Er Lake area: Concentrations, sources, and health risk

The Ya-Er Lake is a seriously polychlorinated dibenzo-p-dioxins/dibenzo-furans (PCDD/Fs) and mercury (Hg)-contaminated lake by pesticide and chlor-alkali plants in China. The oxidation pond method has been conducted to control pollution, moreover, the contaminated sediment was dredged and stacked, becoming a sediment stack yard for vegetable cultivation. To assess effects of oxidation pond method and dredging programme on pollution management and long-term risks of PCDD/Fs and Hg, the concentrations of PCDD/Fs, total Hg (THg), and methylmercury (MeHg) in soil and vegetable sampled from the sediment stack yard were measured and analyzed. Significantly positive relationships between concentrations of PCDD/Fs (p < 0.01), THg, and MeHg (p < 0.05) in edible parts of vegetable and soil were found, suggesting that bioaccumulation from contaminated soil derived from sediment dredging is important sources of PCDD/Fs and Hg in vegetable. Much higher PCDD/Fs (12 ± 9 pg/g dw) and Hg (THg, 0.14 ± 0.23 μg/g dw; MeHg,12.63 ± 13.31 ng/g dw) levels in vegetable were found compared with those from other contaminated regions, indicative of serious PCDD/Fs and Hg pollution in vegetable harvested from contaminated soil. Finally, the calculated provisional tolerable daily intake (PTDI) values showed higher health risk of PCDD/Fs and Hg exposure to local residents through consumption of purple and white flowering stalk, and oilseed rape. Our study established a good model to evaluate the long-term risks of PCDD/Fs and Hg. Moreover, the results indicate that the oxidation pond method and dredging programme were not effective to remove PCDD/Fs and Hg in sediment, which shed new light on management strategy of PCDD/Fs and Hg pollution in contaminated regions.

Mathematical Modeling and Experiments on Pyrolysis of Walnut Shells Using a Fixed-Bed Reactor

Pyrolysis is a low-emission and sustainable thermochemical technique used in the production of biofuels, which can be used as an alternative to fossil fuels. Understanding the kinetic characterization of biomass pyrolysis is essential for process upscaling and optimization. There is no accepted model that can predict pyrolysis kinetics over a wide range of pyrolysis conditions and biomass types. This study investigates whether or not the classical lumped kinetic model with a three-competitive reaction scheme can accurately predict the walnut shell pyrolysis product yields. The experimental data were obtained from walnut shell pyrolysis experiments at different temperatures (300–600 °C) using a fixed-bed reactor. The chosen reaction scheme was in good agreement with our experimental data for low temperatures, where the primary degradation of biomass occurred (300 and 400 °C). However, at higher temperatures, there was less agreement with the model, indicating that some other reactions may occur at such temperatures. Hence, further studies are needed to investigate the use of detailed reaction schemes to accurately predict the char, tar, and gas yields for all types of biomass pyrolysis.

Revealing the Mechanism of Dioxin Formation from Municipal Solid Waste Gasification in a Reducing Atmosphere

Gasification is an effective technology for the thermal disposal of municipal solid waste (MSW) with lower dioxin emission compared to the prevailing incineration process. Nevertheless, the mechanism of dioxin formation in the reducing atmosphere during the gasification process was seldomly explored. Herein, the effects of the atmosphere, temperature, and chlorine source were systematically investigated in terms of dioxin distribution. With CO₂ and H₂O as gasification agents, a reducing reaction atmosphere was formed with abundant H₂ which effectively suppressed the generation of C-Cl, contributing to a substantial decrease of dioxin concentration by ∼80% compared to the incineration process. The formation of dioxin was favored at temperatures below 700 °C with its peak concentration achieved at 500 °C. It was unveiled that inorganic chlorine played a dominant role in the reducing atmosphere, with a lower proportion of C-O-C/O-C═O on residual slag compared to an oxidizing atmosphere. Additionally, the generated H₂ reduced the concentration of dioxins by attacking C-Cl and inhibiting the crucial Deacon reaction for dioxin formation, validated by density functional theory calculation. Eventually, the formation route paradigm and the reaction mechanism of dioxin formation from MSW gasification were revealed, facilitating and rationally guiding the control of dioxin emission.