PCDD/F adsorption enhancement over nitrogen-doped biochar: A DFT-D study.

Polychlorinated dibenzo-p-dioxin/furans (PCDD/F) have a great threat to the environment and human health, resulting in controlling PCDD/F emissions to regulation far important for emission source. Considering 2,3,4,7,8-pentachlorodibenzo-p-furan (PeCDF) identified as the most contributor to international toxic equivalent, 2,3,4,7,8-PeCDF can be considered as the target molecule for the adsorption of PCDD/F emission from industries. With the aim to in-depth elucidate how different types of nitrogen (N) species enhance 2,3,4,7,8-PeCDF on the biochar and guide the specific carbon materials design for industries, systematic computational investigations by density functional theory calculations were conducted. The results indicate pristine biochar intrinsically interacts with 2,3,4,7,8-PeCDF by π-π electron donor and acceptor (EDA) interaction, six-membered carbon rings of PeCDF parallel to the biochar surface as the strongest adsorption configuration. Moreover, by comparison of adsorption energy (-150.16 kJ mol-1) and interaction distance (3.593 Å) of pristine biochar, environment friendly N doping can enhance the adsorption of 2,3,4,7,8-PeCDF on biochar. Compared with graphitic N doping and pyridinic N doping, pyrrolic N doping biochar presents the strongest interaction toward 2,3,4,7,8-PeCDF molecule due to the highest adsorption energy (-155.56 kJ mol-1) and shortest interaction distance (3.532 Å). Specially, the enhancing adsorption of PeCDF over N doped biochar attributes to the enhancing π-π electron EDA interaction and electrostatic interaction. In addition, the effect of N doping species on PeCDF adsorbed on the biochar is more than that of N doping content. Specially, the adsorption capacity of N doping biochar for PCDD/F can be improved by adding pyrrolic N group most efficiently. Furthermore, pyrrolic N and pyridinic N doping result in the entropy increase, and electrons transform from pyrrolic N and pyridinic N doped biochar to 2,3,4,7,8-PeCDF molecule. A complete understanding of the research would supply crucial information for applying N-doped biochar to effectively remove PCDD/F for industries.

PCDD/Fs from a large-scale municipal solid waste incinerator under transient operations: Insight formation pathways and optimal reduction strategies.

Polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) emissions from the transient operation of municipal solid waste incinerators can reach up to 690 ng/Nm3, as measured in this study. To control the extreme emissions to meet the national standard, the formation pathways of PCDD/F were investigated under transient operations (cold start-up, hot start-up, and after start-up) and normal operations. Compared with normal operations, transient operations facilitate the formation of low-chlorinated congeners rather than highly chlorinated congeners. Statistically, for transient operations, strong correlations were found among tetrachlorodibenzo-p-dioxin or tetrachlorodibenzofuran isomers. An abundant carbon matrix is an important carbon source for PCDD formation. Moreover, the comprehensive study revealed that the oxidation of deposited soot is the main source of PCDD/F emissions, relative to de novo synthesis, chlorobenzene-route synthesis, chlorophenol-route synthesis, and chlorination of dibenzo-p-dioxin/dibenzofuran. In addition, the optimal start-up procedure was constructed by analyzing main formation pathways and operating conditions. The relationship between the international toxic quantity (I-TEQ) values (CI-TEQ) and the reaction time can be assigned as CI-TEQ = 11.72t-0.65 (R2 = 0.97) for the circulating fluidized bed. The relationship of CI-TEQ = 4.61t-0.59 (R2 = 0.85) was also proven on the dataset with a grate furnace. Then, the optimal feeding rate of activated carbon was further proposed by the relationship between the reaction time and I-TEQ, and the semi-empirical equation for PCDD/Fs adsorption. Finally, the PCDD/Fs emissions can be reduced to 0.1 ng I-TEQ/Nm3 under transient operations according to the time since start-up.

Generalized prediction and optimal operating parameters of PCDD/F emissions by explainable Bayesian support vector regression.

The current derived models for predicting polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) emissions from incineration can only be applied to a specific incinerator due to high deviation or systematic errors. And the models fail to provide quantized guidance for the operation of full-scale municipal solid waste incinerators. To address the problem, explainable Bayesian support vector regression (E-BSVR) has been established to generalized predict and maximumly reduce the PCDD/F emissions. First, forty-two PCDD/F samples were determined from a whole year experiment in a full-scale incinerator. Meanwhile, 1,2,4-trichlorobenzene(1,2,4-TrCBz), carbon monoxide, sulfur dioxide, oxynitride, particulate matter, fluoride, and hydrogen chloride were measured, as input features. Second, after box-cox transformation normalization, and hyperparameters tuning, the R-Squared and root mean square error (RMSE) of the proposed method are 0.983 and 0.044, exhibiting high accuracy. The high accuracy (R-Squared = 0.992) and generalization are also proven on the dataset with high PCDD/F emissions. Then, the performances of BSVR are compared with kernel ridge regression, multiple linear regression, and unary linear regression, indicating afar smaller RMSE of BSVR. Finally, the optimal operating parameters are calculated through local interpretable model-agnostic explanations and the partial dependence plot. Results indicate that reducing the content of organic chlorine in municipal solid waste and inhibiting the deacon reaction are important methods for reducing PCDD/F emissions. The optimal operating parameters for the maximal reduction of PCDD/F emissions are 1,2,4-TrCBz < 0.098 ug/m3, fluoride > 0.452 mg/m3. As a whole, the E-BSVR method can be used as a reliable and accurate approach for the prediction and reduction of PCDD/F emissions.

Stable and Effective Online Monitoring and Feedback Control of PCDD/F during Municipal Waste Incineration.

For the long-term operation of municipal solid waste incineration (MSWI), online monitoring and feedback control of polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) can be used to control the emissions to national or regional standards. In this study, 500 PCDD/F samples were determined by thermal desorption gas chromatography coupled to tunable-laser ionization time-of-flight mass spectrometry (TD-GC-TLI-TOFMS) for 168 h. PCDD/F emissions range from 0.01 ng I-TEQ/Nm3 to 2.37 ng I-TEQ/Nm3, with 44% of values below 0.1 ng I-TEQ/Nm3 (the national standard). In addition, the temperature of the furnace outlet, bed pressure, and oxygen content are considered as key operating parameters among the 13 operating parameters comprising four temperature parameters, four pressure parameters, four flow parameters, and oxygen content. More specifically, maintaining the furnace outlet temperature to be higher than 800 °C, or bed pressure higher than 13 kPa, or the oxygen content stably and above 10% are effective methods for reducing PCDD/F emissions. According to the analysis of the Pearson coefficients and maximal information coefficients, there is no significant correlation between operating parameters and PCDD/F I-TEQ. Only when there is a significant change in one of these factors will the PCDD/F emissions also change accordingly. The feedback control of PCDD/F emissions is realized by adjusting the furnace outlet temperature, bed temperature, and bed pressure to control the PCDD/F to be less than 0.1 ng I-TEQ/Nm3.

Online predicting PCDD/F emission by formation pathway identification clustering and Box-Cox Transformation.

The composition of the fuel and operational conditions change dramatically under the long-term operation of municipal solid waste incineration (MSWI). Therefore, it is difficult to provide effective rapid feedback to control PCDD/F emissions, presenting as International Toxic Equivalent Quantity (I-TEQ). To address this problem, a PCDD/F emission prediction method is developed, based on formation pathway identification clustering (FPIC) and Box-Cox transformation (BCT). Meanwhile, 1,2,4-trichlorobenzene is measured by the thermal desorption gas chromatography coupled to tunable-laser ionization time-of-flight mass spectrometry (TD-GC-TLI-TOFMS). In the method, FPIC includes de novo synthesis, chlorobenzene(CBz)-route synthesis, chlorophenol (CP)-route synthesis, and the chlorination of dibenzofuran (DD) or dibenzodioxin (DF). The PCDD/F emission data was divided into Cluster 1 (I-TEQ>0.1 ng/Nm3) and Cluster 2 (I-TEQ<0.1 ng/Nm3) by FPIC due to PCDD/F in Cluster 1 main from CP-route and PCDD/F in Cluster 2 main from de novo synthesis and CBz-route synthesis. Also, the BCT was used to transform the I-TEQ and 1,2,4-trichlorobenzene data and to construct effective models. The accurate and precise PCDD/F emissions are predicted with the vast majority of error percentage within [ -40%, 40% ], and errors within [ -0.126, 0.016 ] I-TEQ (ng/Nm3). The absolute value of the relative difference between predicted I-TEQ and measured I-TEQ (|RD|) of the linear model constructed by the method has a significant reduction to 20.28%. FPIC and BCT can be used as an effective method to online predict PCDD/F emission in long-term operation thereby allowing the rapid operational feedback to control PCDD/F emission from the incinerator.