Enhanced mitigation of inhalable particles and fine particle-bound PAHs from a novel hazardous waste-power plant candidate

Emissions of the inhalable particle (dp < 10 μm, PM10) and their harmful compositions from combustion sources have high potential on health risk with nearly no regulation. This study investigates the particle size distribution (PSD), as well as the removal mechanism of PM10 and fine particle (FP)-bound polycyclic aromatic hydrocarbons (PAHs) from the flue gas of a hazardous waste thermal treatment system. It has ultralow regulated emission and becomes a candidate of power generation module. A series of the advanced scrubbers, cyclonic demister, and baghouse was equipped for multi-pollutant control. The moderate or intense low oxygen dilution (MILD) combustion effectively inhibited the PM2.5 generation by volumetric oxidation. Advanced scrubbers removed PM1, PM2.5, and PM10 by 85.24, 68.68, and 97.60%, respectively, which achieved by local supersaturation, heterogeneous condensation of water vapor, and the growth of fine PM. Moreover, the scrubbers effectively scavenged the course PM10 containing the high-molecular-weight PAH homologs onto the water phase but promoted the condensation and absorption of the lighter homologs onto the fine particle surface (dp ∼5.3 μm). The size window (dp = 0.3-1.0 μm) of the minimum efficiency reporting value of a BH filtration led to the peak of FP-PAH mass and BaP equivalent (BaPeq) toxicity at dp = 0.1-0.4 and 0.1-0.8 μm, respectively. Consequently, the synergy of MILD combustion and the SCB-CYC-BH system effectively inhibited the PM2.5, PM10, PM2.5-PAHs, and FP-PAH levels from a waste thermal treatment process and further mitigated the potential health risk.

Insight into the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans in hazardous waste incineration and incinerators: Formation process and reduction strategy

Incineration technology has been widely adopted to safely dispose of hazardous waste (HW). While the incineration process causes the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Due to its extreme toxicity, many scholars have been committed to determining the PCDD/F formation process and reducing emissions in incinerators. Previous studies ignored the impact of incineration and fluctuation of feeding materials on PCDD/F formation in hazardous waste incinerators (HWIs). In this study, differences in PCDD/F formation between HWIs and municipal solid waste incinerators (MSWIs) were pointed out. The incineration section in HWIs should be carefully considered. Laboratory experiments, conventional analysis and thermogravimetry experiments were conducted. An obvious disparity of PCDD/F formation between 12 kinds of HWs was found. Distillation residue was found with remarkably higher PCDD/F concentrations (11.57 ng/g). Except for the Cl content, aromatic rings and C-O bond organics were also found with high correlation coefficients with PCDD/F concentrations (>0.92). And PCDD/Fs were formed through a chlorination process and structure formation process. All of these are helpful to further understand the PCDD/F formation process during HW incineration, optimize the operation conditions in HWIs and reduce the emission pressure of PCDD/Fs in the future.

Inhibition of PCDD/Fs in a full-scale hazardous waste incinerator by the quench tower coupled with inhibitors injection

The control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the flue gas in hazardous waste incinerators (HWIs) is an intractable problem. To figure out the formation mechanism of PCDD/Fs and reduce the emission, a field study was carried out in a full-scale HWI. Ca(OH)₂ & (NH₄)H₂PO₄ or CH₄N₂S & (NH₄)H₂PO₄ were injected into the quench tower, and the detailed inhibition effect on PCDD/Fs formation by the inhibitors coupled with quench tower was studied. Gas and ash samples were collected to analyze PCDD/Fs. XPS, EDS characterization and Principal component analysis were adopted to further analyze the de novo and precursors synthesis. The PCDD/Fs emissions reduced from 0.135 ng I-TEQ/Nm³ to 0.062 or 0.025 ng I-TEQ/Nm³ after the injection of Ca(OH)₂ & (NH₄)H₂PO₄ or CH₄N₂S & (NH₄)H₂PO₄, respectively. The quench tower was found mainly hindering de novo synthesis by reducing reaction time. CP-route was the dominant formation pathway of PCDD/Fs in quench tower ash. Ca(OH)₂ & (NH₄)H₂PO₄ effectively inhibit precursors synthesis and reduce proportions of organic chlorine from 4.11% to 2.86%. CH₄N₂S & (NH₄)H₂PO₄ show good control effects on both de novo and precursors synthesis by reducing chlorine content and inhibiting metal-catalysts. Sulfur-containing inhibitors can cooperate well with the quench tower to inhibit PCDD/Fs formation and will be effective to reduce dioxins formation in high chlorine flue gas. The results pave the way for further industrial application of inhibition to reduce PCDD/Fs emissions in the HWIs flue gas.

Ash formation and the inherent heavy metal partitioning behavior in a 100 t/d hazardous waste incineration plant

Hazardous waste incineration (HWI) ash is also defined as hazardous waste and its disposal performance depends largely on the ash compositions as well as the potential environmental risk of heavy metals. In this work, HWI ashes of four sampling sites were collected in a 100 t/d hazardous waste incineration plant with rotary kiln over three consecutive days. The formation characteristics of ash samples including heavy metal partitioning were given, with further discussions on the melting disposal of HWI ash mixtures. Results showed significant differences in the ash compositions among the sampling sites. Caused by NaHCO₃ injection as de-acidizing adsorbent, the sum of Na, S and Cl content in bag filter ash even exceeded 70%. Cu/Mn/Cr tended to transfer into the bottom ash due to low volatilities, while Zn/Pb/Cd/Se/As were more likely to be enriched in the ash particles. In particular, chemical adsorption at medium- to high- temperature range was dominant for As enrichment in the waste heat boiler ash. Despite the complexity and diversity of raw hazardous wastes, little difference was found in the melting temperature of bottom ash during the sampling period. However, it could vary by more than 200 °C for fly ash due to the fluctuation of alkali components in raw wastes. Moreover, slagging medium was encouraged in order to achieve rapid and complete melting of ash mixtures. The objective of this study is to gain knowledge on the HWI ash formation and inherent heavy metal partitioning behavior, expecting to provide guidelines on the deep harmless disposal of HWI ash in future.

A review on emergency disposal and management of medical waste during the COVID-19 pandemic in China

The surge of medical waste (MW) generated during the COVID-19 pandemic has exceeded the disposal capacity of existing facilities. The timely, safe, and efficient emergency disposal of MW is critical to prevent the epidemic spread. Therefore, this review presents the current status of MW generation and disposal in China and analyzes the characteristics and applicability of emergency disposal technologies. The results show that movable disposal facilities can dispose of infectious MW on site, even though most of their disposal capacity is at a low level (<5 t/day). Co-disposal facilities need to be reformed completely for emergency MW disposal, in which separate feeding systems should be taken seriously. Specifically, municipal solid waste (MSW) incineration facilities have great potential to improve emergency MW disposal capacities. For hazardous waste incineration facilities, compatibility of the wastes must be matched to the composition and calorific value of the waste. As for cement kiln, MW can only be used as an alternative fuel instead of a raw material for cement. Based on the environmental risk and technical adaptability, the six emergency MW disposal technologies are recommended to be prioritized as follows: movable microwave sterilization, movable steam sterilization, movable incineration, co-incineration with hazardous waste, co-incineration with MSW and co-disposal in cement kilns. Infectious MW, especially COVID-19 MW, should be prioritized for disposal by centralized and movable disposal facilities, while non-infectious MW can be disposed of using co-disposal facilities. All stakeholders should strengthen the delicacy management of the end-of-life stage of MW, including collection, classification, packaging identification, transportation, and disposal. Currently, it is necessary for centralized disposal enterprises to follow the emergency disposal operation flowchart. From a long-term strategic perspective, making full use of regional movable and co-disposal facilities in the megacities can effectively enhance the emergency MW disposal capacity.

Pollution emission characteristics, distribution of heavy metals, and particle morphologies in a hazardous waste incinerator processing phenolic waste

Secondary pollution emitted from hazardous waste incinerators (HWIs) can pose potential risks to the surrounding populations and environment. An investigation was conducted on pollutant emission status in a HWI combusting homogenized phenolic waste, woodchips, and electroplating sludge during the sampling period. Morphologies and elemental compositions of particles in flue gas and indoor air of the incinerator were characterized by TEM-EDS. Eight types of single particles were classified, as organic, soot, K-rich, S-rich, Na-rich, Fe-rich, mineral and fly ash particles. In the indoor air near the fly ash collector, organic and S-rich particles were the two most observed particles, taking 56 % and 30 %, respectively. While near the bottom ash collector, Fe-rich particles took approximately 30 %. Besides, the partitioning behavior of heavy metals in the incinerating process were investigated. Hg, Cd and Pb were mainly enriched in fly ash through evaporation, condensation, and adsorption; while Cr, Cu, Mn, and Ni were mostly remained in the bottom ash due to their low volatilities. This study provides information for regional air pollution source apportionment, but also helps understand the partitioning behavior of heavy metals for the secondary pollution control. Meanwhile, the visualized micro-compositions of indoor particles pave a way for occupational exposure risk assessment.

PCDD/F emissions during startup and shutdown of a hazardous waste incinerator

Compared with municipal solid waste incineration, studies on the PCDD/F emissions of hazardous waste incineration (HWI) under transient conditions are rather few. This study investigates the PCDD/F emission level, congener profile and removal efficiency recorded during startup and shutdown by collecting flue gas samples at the bag filter inlet and outlet and at the stack. The PCDD/F concentration measured in the stack gas during startup and shutdown were 0.56-4.16 ng I-TEQ Nm^-3 and 1.09-3.36 ng I-TEQ Nm^-3, respectively, far exceeding the present codes in China. The total amount of PCDD/F emissions, resulting from three shutdown-startup cycles of this HWI-unit is almost equal to that generated during one year under normal operating conditions. Upstream the filter, the PCDD/F in the flue gas is mainly in the particle phase; however, after being filtered PCDD/F prevails in the gas phase. The PCDD/F fraction in the gas phase even exceeds 98% after passing through the alkaline scrubber. Especially higher chlorinated PCDD/F accumulate on inner walls of filters and ducts during these startup periods and could be released again during normal operation, significantly increasing PCDD/F emissions.

The emission of fluorine gas during incineration of fluoroborate residue

The emission behaviors of wastes from fluorine chemical industry during incineration have raised concerns because multiple fluorine products might danger human health. In this study, fluorine emission from a two-stage incineration system during the combustion of fluoroborate residue was examined. In a TG-FTIR analysis BF3, SiF4 and HF were identified as the initial fluorine forms to be released, while fluorine gases of greenhouse effect such as CF4 and SF6 were not found. Below 700 °C, NaBF4 in the sample decomposed to generate BF3. Then part of BF3 reacted with SiO2 in the system to form SiF4 or hydrolyzed to HF. At higher temperatures, the NaF left in the sample was gradually hydrolyzed to form HF. A lab-scale two-stage tube furnace is established to simulate the typical two-stage combustion chamber in China. Experimental tests proved that HF was the only fluorine gas in the flue gas, and emissions of BF3 and SiF4 can be negligible. Thermodynamic equilibrium model predicted that all SiF4 would be hydrolyzed at 1100 °C in the secondary-chamber, which agreed well with the experimental results.

Suppression of dioxins in waste incinerator emissions by recirculating SO2

Sulphur is an effective inhibitor of the formation of Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-furans (PCDD/Fs), as was proven in laboratory and pilot plant studies. In this study, a pilot-scale system with capacity 300 N m(3) h(-1) was situated at the bypass of an actual hazardous waste incinerator (HWI) and tested to reduce the emission of PCDD/Fs. Activated carbon was used as a medium to adsorb SO2 from flue gas and release it again at the higher temperature of filtered ash detoxification to achieve SO2 circulation in the system. Most PCDD/Fs in the filtered ash are decomposed by thermal treatment. Experimental results indicate that the system is capable of stable operation with SO2 accumulation at a high level of concentration and a high reduction efficiency of PCDD/Fs. A reduction of more than 80% was already achieved without addition of other sulphur compounds. When pyrite (FeS2) was added the reduction of PCDD/Fs could reach 94%, with a residual PCDD/Fs concentration in the flue gas as low as 0.13 ng TEQ N m(-3). This SO2 recirculating and suppression technology potentially provides significant progress for dioxin emission control in waste incineration and could be useful for controlling emissions of PCDD/Fs and other chlorinated organic chemicals in China.