Control of dioxin emissions from alternative fuels via regulated activated carbon injection and fluegas recirculation

Effect of relative humidity on the desulfurization performance of calcium-based desulfurizer

Low desulfurization efficiency impedes the wide application of dry desulfurization technology, which is a low-cost and simple process, and one significant solution is the development and manufacture of high-performance desulfurizers. In this study, firstly, a steam jet mill was used to digest quicklime; then, we utilized numerical simulation to study the flow field distribution and analyze the driving factors of quicklime digestion; and lastly, the desulfurization performance of the desulfurizer was evaluated under different relative humidities. The results show that the desulfurizer prepared via the steam jet mill had better apparent activity than traditional desulfurizers. Also, the entire jet flow field of the steam jet mill is in a supersonic and highly turbulent flow state, with high crushing intensity and good particle acceleration performance. Sufficient contact with the nascent surface maximizes the formation of slaked lime. The experiments demonstrated that the operating time with 100% desulfurization efficiency and the "break-through" time for the desulfurizer prepared via the steam jet mill is longer than that of traditional desulfurizers, and has significant advantages, especially at low flue gas relative humidity. Compared with traditional desulfurizers, the desulfurizer prepared via steam jet mill expands the range of acceptable flue gas temperature, and the failure temperature is 1.625 times that of traditional desulfurizers. This work breaks through the technical bottleneck of low dry desulfurization efficiency, which is an important step in pushing forward the application of dry desulfurization.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

Curbing dioxin emissions from municipal solid waste incineration: China's action and global share

China generates the world's second-largest amount of municipal solid waste (MSW) and incinerates the largest quantity of MSW. However, data on the latest dioxin emissions from MSW incineration (MSWI) and the related global share were lacking. In the context of MSW classification, distinguishing the long-term MSW generation and incineration quantity, and dioxin emissions was necessary for macro-control and policy-making by the Chinese Government. By considering population size and GDP per capita, China's MSW generation toward 2050 was projected based on Monte Carlo simulation. Moreover, dioxin emission factors were also assumed based on the diffusion rate of four grades of air pollution control devices (APCDs). Finally, we show that the quantity of China's MSW generation in 2050 will be 363.50 million tonnes (Mt) with 341.06-382.45 Mt of 75% certainty. China's dioxin emissions from MSWI were approximately 15.46 g I-TEQ in 2019, which accounted for 26.1% of total emissions from global MSWI. We discuss dioxin emission reduction scenarios depending on MSW diversion and APCD upgrades. China's dioxin emissions will be 70.38 g I-TEQ for the business-as-usual scenario, and the dioxin emissions will be 9.29 g I-TEQ (within the range of 8.88-9.64 g I-TEQ) for the optimal scenario in 2050. Moreover, in 2050, the APCD diffusion rate will account for 98.8% of the sensitivity of dioxin emissions from China's MSWI. According to the assumed scenarios, there is a dioxin emission reduction potential of 18.6% and 86.8% in 2050 by MSW diversion alone and maximum APCD upgrades combined with food waste diversion, respectively.

Emission characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans from industrial combustion of biomass fuels

Although biomass fuel has always been regarded as a source of sustainable energy, it potentially emits polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). This study investigated PCDD/F emissions from industrial boilers fired with three types of biomass fuel (i.e., bagasse, coffee residue, and biomass pellets) via stack sampling and laboratory analysis. The measured mass concentrations of PCDD/Fs varied among the boilers from 0.0491 to 12.7 ng Nm^-3 (11% O₂), with the calculated average international toxic equivalent quantity (I-TEQ) from 0.00195 to 1.71 ng I-TEQ Nm^-3 (11% O₂). Some of them were beyond the limit value for municipal waste incineration. 2,3,4,7,8-PeCDF could be used as a good indicator of dioxin-induced toxicity of stack flue gases from biomass-fired boilers. The PCDFs/PCDDs ratios were more than 1, likely indicating the formation of dioxins in the boilers favored by de novo synthesis. The emission factor (EF) of total PCDD/Fs averaged 5.35 ng I-TEQ kg^-1 air-dry biomass (equivalent to 39.0 ng kg^-1 air-dry biomass). Specifically, the mean EF was 6.94 ng I-TEQ kg^-1 (52.6 ng kg^-1) for biomass-pellet-fired boiler, 11.8 ng I-TEQ kg^-1 (74.6 ng kg^-1) for coffee-residue -fired boiler, and 0.0277 ng I-TEQ kg^-1 (0.489 ng kg^-1) for bagasse-fired boilers. The annual PCDD/F emission was estimated to be 208 g I-TEQ in 2020 in China, accounting for approximately 2% of the total national annual emission of PCDD/Fs. The results can be used to develop PCDD/Fs emission inventories and offer valuable insights to authorities regarding utilizing biomass in industry in the future.

Process tracing of PCDD/Fs from economizer to APCDs during solid waste incineration: Re-formation and transformation mechanisms

The emission of PCDD/Fs is a crucial factor for the aggravation of the Not-In-My-Back-Yard (NIMBY) syndrome, especially for the incineration plants that fail to meet the emission standard. It is well known that physicochemical processes in the boiler can notably affect the discharge of dioxins, especially under transient, non-steady conditions. However, few studies paid attention to the important operational parameters that influence PCDD/Fs formation and transformation in the boiler when an incinerator is in its daily steady operation. In this study, 36 samples were analyzed to achieve process tracing of PCDD/Fs. The concentration, congener profile and vapor/solid partitions of PCDD/Fs from the economizer to air pollution control devices (APCDs) under two typical steady conditions were investigated. Results indicated that increasing air supply aggravated the formation of PCDD/Fs, disturbed the vapor/solid partitions, and triggered a substandard emission. Quantitative structure-activity relationship (QSAR) modeling was firstly performed for the formation mechanism and orbital energy factors were identified as dominating factors. Besides, the removal rates of PCDD/Fs significantly correlated with the saturated vapor pressure and proportions of different isomers. This study is beneficial for operators to optimize relevant operational parameters of the incineration plants so as to get rid of substandard problems.

Feasibility of using fly ash-slag-based binder for mine backfilling and its associated leaching risks

As a binder to completely replace Portland cement for mine backfilling, the use of clinker-free cementitious materials combined with municipal solid waste incineration (MSWI) fly ash is proposed to achieve the targets of low-cost green backfilling, safe disposal and resource utilisation of bulk urban hazardous waste and metallurgical solid waste. This study balances the positive and negative effects of adding MSWI fly ash to the backfill by controlling its quantity in the binders, thus establishing an optimal concentration of 49 wt.% steel slag (SS), 21 wt.% blast furnace slag (BFS), 10 wt.% MSWI fly ash and 20 wt.% flue gas desulfurisation (FGD) gypsum. It is also reported that the filling performance of slurry (A2) satisfied strength requirements and is very suitable for long-distance transportation according to filling parameters. The leaching levels of the target elements (Cr, Ni, Zn, As, Cd, Sb, Pb, Hg and dioxins) for A2 matrix are lower than the required maximum concentration limits for the underground class Ⅲ water standard. Furthermore, the risk of leaching harmful constituents is mainly controlled by the pH of the environmental and the excellent buffering capacity of the matrix can reduce the potential leaching risk. The encapsulation, precipitation and adsorption of low-solubility double salts, such as hydrate calcium chloroaluminate (HCC) and ettringite, are the solidification/stabilisation (S/S) mechanism of series A on harmful substances. In addition, the high degree of polymerization(Ca/Si = 1.18 < 1.2, at 90d), the formation of long-chain C-S-H gels in binder A2-2, the dense pore structure lead to very stable growth in strength and control of leaching risks in subsequent periods.

Preparation of a new high-performance calcium-based desulfurizer using a steam jet mill

Dry flue gas desulfurization is an increasingly attractive technique in SO₂ emission control. However, the low efficiency in dry desulphurization is the bottleneck of this technology. To find a high-performance desulfurizer is an urgent task. This research utilized a steam jet mill digestion to prepare a desulfurizer at steam temperature of 220 ℃ and pressure of 0.45 MPa, and compared this product with the conventional digestion desulfurizer. Our results show that the digestion in steam jet mill can transform all the calcium oxide into calcium hydroxide. The calcium hydroxide had good fluidity and with honeycomb morphological characteristics. The experiments of dry flue gas desulfurization demonstrated that under the relative humidity of 15, 30 and 45%, the total dead times were 340, 640 and 720 min, the working time for keeping a 100% desulfurization efficiency were 120, 420 and 580 min, and the total sulfur fixation were 124.05, 274.58 and 332.09 mg. Compared with the desulfurizer by conventional dry digestion, the desulfurizer prepared in this research had a significantly superior performance. This experiment provides a new method for high-performance desulfurizer via quicklime digestion, which is an important step in pushing forward the application of dry flue gas desulfurization.