Search

In this study, a catalytic system for N 2 O reduction by H 2 onto char decorated with transition metals is proposed, which have excellent reduction efficiency at low temperatures below 200 °C and efficiently utilizes pyrolysis char and hydrogen. Density functional theory (DFT) is used to reveal the reaction mechanism and found that transition metal decoration can significantly promote reaction rate and decrease the activation energy required for N 2 O reduction by H 2 on char, from 383.7 kJ/mol to 50–160 kJ/mol. Furthermore, transition states in the process of reducing N 2 O by H 2 over metal-char were observed, including reduction of N 2 O and the removal of the surface oxygen complex. Through kinetic analysis, the activation energy and reaction rate order was found as Ni-char (50.0 kJ/mol)>Co-char (95.1 kJ/mol)>Fe-char (151.0 kJ/mol)>Cu-char (158.8 kJ/mol). According to experiments phenomenon, the sequence of reduction efficiency over different metal-char was consistent with the simulation results and it was even possible to convert N 2 O to N 2 below 200 ℃. This research contributes to a more mechanistic understanding of the N 2 O-H 2 reaction over char and provides new insights for improving the design of catalysts for N 2 O abatement at low temperatures. [Display omitted] • Char loaded with transition metal can reducing N 2 O by H 2 below 200 ℃. • The activation energy decreased greatly after transition metal decoration. • The catalytic effect of transition metal decoration follows: Ni >Co>Fe>Cu. • The second N 2 O reduction is the rate-determining step on the Fe-char. • The removal of oxygen complexes is the rate-determining step on the Co, Ni, Cu-char. [ABSTRACT FROM AUTHOR]

The low quality municipal solid wastes (MSW) derived char has a potential to be used as a methanation catalyst to achieve low-cost methanation of the syngas derived from MSW, and its performance with varying reaction parameters should be explored for better application. In this study, a MSW char supported Ni-based catalyst (Ni-MSWC) was prepared by impregnation; the influences of CO 2 and CH 4 impurities in the raw syngas on methanation and the feasibility of conducting methanation in a low-pressure condition were investigated, then the catalyst's response to the changing parameters was identified. The results showed that the presence of low concentration CO 2 in the H 2 -rich syngas is favorable to the gasification of the char carrier and activates Ni-MSWC catalyst. However, it also promotes F–T synthesis reaction and leads to a decrease in the methane yield (Y CH4). The decrease in reaction pressure causes a decrease in Y CH4 and results in coke formation; but inhibits F–T synthesis reaction and increases methane selectivity (S CH4). A higher reaction pressure increases system complexity and accelerates char carrier consumption. Moreover, presence of methane by 2.8% (vol.) promotes the methanation of CO 2 through the methane dry reforming reaction, but it inevitably causes coke formation and affects the catalyst's stability. Based on Y CH4 and Ni-MSWC's responses, CO:CO 2 ≥ 3:2 and reaction pressure of 1 MPa (gauge pressure) are recommended, which help to inhibit the side reactions and maintain a high Y CH4 (>95%). [Display omitted] • Methanation performance of the syngas explored using the Ni-MSWC catalyst. • The ratio of CO/CO 2 in the syngas recommended to be not less than 3/2. • Lower CO 2 concentration in the syngas helps to activate the MSWC carrier. • 80% or more CH 4 yield from CO x obtained at atmospheric pressure. • The feeding CH 4 promotes the CO 2 conversion but causes carbon deposition. [ABSTRACT FROM AUTHOR]

[Display omitted] • MSW pyro-gasification performed with HT-CO 2 gasifying char and volatiles. • Contact modes of HT-CO 2 , char and volatiles assessed to capitalize on char catalyst. • The maximum gas yield of 0.71 Nm3/kg MSW and highest CGE obtained in M3. • The char contacting HT-CO 2 alone firstly in M3 contributed its highest activity. • HT-CO 2 and volatiles contact must be delayed when design a reforming reactor. The volatiles and char derived from municipal solid waste (MSW) pyrolysis can be catalytically reformed and gasified using high-temperature CO 2 (HT-CO 2) as gasifying agent and char as a catalyst simultaneously to obtain high quality synthesis gas, but the reactor's design for this purpose is still a question. In this research, the contact configuration between the HT-CO 2 , the volatile compounds, and the char from MSW pyrolysis were studied to understand the relevant reaction behaviors and to establish guidelines for the reactor's design. Three contact modes were designed, including: M1, where volatiles and HT-CO 2 contact first, then contact the char; M2, where volatiles, CO 2 , and char contact simultaneously at the bottom of the char layer; and M3, where CO 2 contacts with the char first, then the volatiles contact in the middle of the char layer. The temperature evolution in the char layer, the yields and properties of the resultant combustible gases, used char, and tar were investigated. Experimental results revealed that the contact mode significantly affected the levels of char gasification and volatiles' reforming. For M1, intense thermal cracking of volatiles occurred and 65.41% of the input heat of HT-CO 2 was consumed for thermal cracking, resulting in substantial carbon deposition and limited energy transfer from char to the synthesis gas. While, the char contacting HT-CO 2 firstly in M3 improved its catalytic activity, causing 73.33 % of the input heat utilized for gasification and reforming; as a result, the maximum synthesis gas yield of 0.71 Nm3/kg MSW and gas energy ratio of 76.3 % were obtained respectively in M3 with the lowest tar yield of 5.45 %; additionally, the used char corresponded to the highest specific surface area of 10.12 m2/g. Ultimately, M3 is constructive and recommended, and the findings of this study offer helpful guidance for the design of pyro-gasification reactors. [ABSTRACT FROM AUTHOR]

Syngas methanation was studied using the regenerated Ni-based catalyst supported on municipal solid waste-derived char (Ni/MSWC). Catalysts were prepared by recovering Ni from the spent Ni/MSWC via an acid-assisted process and their performance evolution was analyzed by experiments and density functional theory (DFT) calculations. The regenerated catalysts showed similar methanation activity to the original with a CH 4 yield of ∼90 % but repeated acid-assisted recovery carried the risk of deactivating the catalyst, due to the enrichment of acid-soluble minerals from the char carrier. The ash composition of char carriers quantified with an alkali-carbon index of ∼20 could be an indicator for assessing the char carriers. DFT insights proved defect-induced Ni particle adsorption on MSWC and the improved electronic structure of Ni particles induced by the active acid-soluble minerals. This research highlights the potential of utilizing MSWC and recovered Ni for sustainable methanation catalysts and provides implications for choosing qualified waste-derived char carriers. [Display omitted] • Repeated Ni recovery and reuse to prepare Ni/MSWC for methanation. • The catalyst prepared with recovered Ni showed activity comparable to the original catalyst. • The number of acid-assisted recoveries depended on the char carrier's ash content. • H-assisted CO&CO 2 dissociation was determined to be the pathway for CO x methanation. • Enhanced performance of catalyst due to the interactions between char components and Ni. [ABSTRACT FROM AUTHOR]

Sewage sludge char (SC) reduces NO to N 2 at high temperatures thus acting as a potential reducing agent in flue gas cleaning systems. However, SC needs to be modified to enhance the carbon-deNO x performance. In this study, coal char (CC) and different types of SCs, i.e., original (SC-R), pyrolytic volatiles activated (SC–V) and KOH activated (SC–K), were compared in terms of their carbon-deNO x performance, including NO removal rate and secondary pollution discharged. The results showed that when the oxygen content in the flue gas was 5–6%, the carbon-deNO x efficiency of the three types of SCs was greater than 70%, which was higher than that of the CC. SC-V has lower emissions of CO and gaseous nitrogen-containing compounds (NH 3 , HNCO, HCN) among the three types of SCs. For the oxygen content of 8–11% in the flue gases, the NO conversion performance was found in the order of SC-K > SC-R > SC-V > CC. The physical and chemical characterization of activated carbon shows that pyrolytic-volatile activation increases the ratio of C–O and C=O functional groups on its surface of SC-V, which not only facilitates the chemisorption of NO but are also easily converted under high oxygen conditions. SC-V is found as a suitable reductant for carbon-deNO x within the temperature range of 300–350 °C. [Display omitted] • SC-V activated by pyrolytic volatile are used for De-NO x at 300–600 °C. • CO emission from SC-V during carbon-deNO x process is effectively inhibited. • High De-NO x performance of SC-V at 300–350 °C can be achieved. • Physical and chemical property characterisations indicate high SC-V activity. [ABSTRACT FROM AUTHOR]

The pyrochar of sewage sludge can be exploited for catalytic tar reforming since the developed pore structure and high metal content. However, Fe in the raw sludge tends to form Fe-S and Fe-P during pyrolysis, which inhibits catalytic activity of pyrochar. Here, the wet sludge was conditioned with quicklime, and the pyrochar was used to reform sewage sludge pyrolysis volatiles. The results showed that H 2 generation from pyrochar reforming increased by 55.2% after conditioning. In addition, the tar elimination achieved the highest ratio (79.6%) compared with unconditioned char and quicklime, while PAHs and N-compounds were concentrated in tar with the relative content 46.9% and 37.1% due to the significant removal of other tar compounds such as oxygenates. During pyrolysis, a proportion of added CaO captured P and S anions which favored the formation of Ca 2 Fe 2 O 5 and Fe-O, while the remaining CaO (21.9%) was highly dispersed on pyrochar. This promoted both the catalytic tar cracking effects and the CO 2 absorption of the char. These findings provide a novel method to produce H 2 -rich gas from sewage sludge pyrolysis with less tar yield. • The reforming of volatile from sewage sludge pyrolysis by its pyrochar is improved. • Pyrochar obtained from CaO conditioned sewage sludge was used as reforming catalyst. • The tar cracking and H 2 generation of sludge volatile were significantly promoted. • The CaO conditioning promoted both the Fe-O formation and high dispersion of CaO. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni/MSWC has a higher catalytic activity than Ni/Al 2 O 3 for syngas methanation. • Shape change of Ni particles caused decreases in CO conversion and CH 4 selectivity. • Whisker carbon deposition caused decrease in CH 4 selectivity. • Amorphous carbon deposition had negligible influence on CH 4 selectivity. Municipal solid waste pyrolysis char (MSWC) has none suitable high-valued applications. In this research, a Ni/MSWC catalyst was prepared by loading Ni on to MSWC through impregnation for syngas methanation. Both continuous and intermittent operations were carried out to investigate activity and stability of the Ni/MSWC catalyst. Results showed that the catalyst exhibited high activity with CO conversion reaching 100%, CH 4 yield higher than 100% in the beginning. SEM images of the fresh Ni/MSWC demonstrated that Ni particles were highly dispersed on the MSWC surface. In the continuous operation, the CH 4 yield maintained above 90% for 29 h. In the 60 h's intermittent operation with 10 h as a cycle, CH 4 yield decreased rapidly from 102.4% in the first cycle to 78.0% at the start of the second cycle, and it decreased to around 37% at the end of the third cycle but maintained this minimum till the end; while sharp rises appeared at the start of each cycle. Decreases in CO conversion and CH 4 selectivity are found to be closely related to aggregation of Ni particles, carbon deposition and collapse of the char structure. However, carbon deposition in the intermittent operation could be alleviated by N 2 purging, while whisker carbon deposition was observed in the used Ni/MSWC after 39 h's continuous operation. Highly dispersed Ni particles on the MSWC surface and proper interaction between Ni and MSWC were favorable to the activity of the catalyst, but its stability should be further improved. [ABSTRACT FROM AUTHOR]

Chars from wastes/biomass pyrolysis are widely used as catalysts. But their catalytic mechanism is not clear enough. To explore the role of inorganics in char-based catalysts, in this research, chars from municipal solid wastes (MSW) and wheat straw (WS) were compared for their catalytic performances and structure changes in reforming volatiles from MSW and WS pyrolysis. Results showed that MSW char (ash: 43 wt%) was more active than WS char (ash: 16 wt%) in reforming both volatiles within 500–700 °C, which facilitated higher syngas yield. 74% of the metals existed as non-evaporable acid-soluble inorganics in MSW char, which favored the transformation of microcrystalline carbon into amorphous carbon by carbon gasification yet increased aromaticity of its carbon matrix; while 27% of metals in WS char were organically bound alkali and alkaline-earth metals (AAEMs), and aromaticity of its carbon lattice increased from 0.39 to 0.67 with increase in reforming temperature. Additionally, WS char was characterized with larger aromatic ring size after reforming temperatures. Rich acid-soluble inorganics in MSW char were found to be favorable to its high activity; while migration of AAEMs caused the lower activity of WS char. These findings will help to improve catalytic activity of char-based catalysts. [Display omitted] • Role of inorganics in different speciation in char-based catalysts explored. • MSW char with more inorganics showed better catalytic performance than porous WS char. • Acid-soluble inorganics hinder microcrystallization and enrich functional groups on the surface. • Migration of organically bound and water-soluble AAEMs led to larger aromatic ring size. [ABSTRACT FROM AUTHOR]

• Obvious kinetic compensation effects were found during hydrochar gasification. • Formation of fixed carbon by HTC lowered char-CO 2 gasification reactivity. • HTC treatment had determinant impact on chemical composition of pyrolytic volatiles. • Positive correlation of carbon functionality and volatile aromatics was found. • Mild HTC pretreatment is encouraging in combination with gasification. Hydrothermal carbonization (HTC) combined with subsequent thermochemical conversion is an effective approach for sewage sludge (SS) to energy. In this work, HTC pretreatment of SS was performed. Then the properties of released volatiles and the remaining char-CO 2 gasification after pyrolysis were evaluated via the multiple kinetic model and combined instrumental analyses. The gasification behavior of these samples exhibited slightly different due to high ash content and limited fixed carbon, although the aromatic structure in the hydrochars developed as HTC proceeded. Kinetic analysis indicated that volume reaction model (VRM) with chemical reaction order of 1.5 showed high correlation coefficients (R 2>0.99) for all carbonized samples. There was an obvious kinetic compensation effect in the char gasification step, and activation energy (E) and pre-exponential factor (A) of these samples followed a positive correlation relationship as lnA = 0.1037 × E + 0.3579. The formation of fixed carbon in the hydrochar samples was conserved during pyrolysis, which lowered the average gasification reactivity of char. Py-GC/MS demonstrated that the chemical composition of pyrolytic volatiles was found to be mainly determinant by HTC conditions. The share of aromatics in the volatiles was in a positive correlation with the evolution of carbon functionality in the hydrochar samples. Above in all, HTC temperature exerted a more significant effect on the fuel properties of hydrochar samples than HTC duration did, which also indicated by their subsequent thermochemical conversion. These findings can provide essential knowledge on the thermochemical conversion of hydrochar samples and favor the designing, optimization, and scaling up of the process. [ABSTRACT FROM AUTHOR]

Sewage sludge (SS) char can be potentially applied to De-NO x processes but it should be active enough and the relevant CO emissions should be controlled. In this paper activated SS chars have been prepared by using a simple KOH impregnation-carbonisation method and the activated chars are applied to remove NO x from flue gases within temperature range of 100–250 °C, acting as both reductants and catalysts. Special attention is paid to inhibiting CO emission in the process. Four reductive agents are adopted to check the catalytic effect of the activated SS chars. The results show that the activated SS chars (i.e. SC-KOHs) present a higher adsorption-reduction ability with much lower CO emission than that of non-activated SS chars (i.e. SC-Raws). SC-KOH produced at 800 °C (SC–KOH-800) shows obvious chemical adsorption behaviors, its adsorption capacity for NO is higher than 8.06 mg.L−1, superior to the coal-based activated carbon. Hydrazine hydrate is found to enhance De-NO x performance with SC-KOHs acting as catalysts, especially at higher temperatures. However, the De-NO x efficiency decreased when ammonia, urea and urea involved reductants were used. The physicochemical structure of the SS chars was characterized to show that the SC-KOHs are more porous with higher BET areas and pore volumes. Simultaneously, the SC-KOHs are crystallized to much lower extent, less graphitised, but have richer O-containing functional groups and zeolite structure on the surface when compared to the SC-Raws, which contributed to their high activities. Moreover, SC-KOH-800 can be recommended as the suitable adsorbent/catalyst for De-NO x within 100–250 °C. • Sewage sludge chars (SC-KOHs) have been prepared for De-NO x at 100–250 °C. • CO emission from SC-KOHs in De-NO x process is effectively inhibited. • SC-KOH-800 is superior to coal-based activated carbon due to chemical adsorption. • SC-KOH-800 is effective both as absorbent and as catalyst for hydrazine reductant. • Characterisation of SC-KOHs supported their high activity. [ABSTRACT FROM AUTHOR]