Wet Sludge Research Articles

Study on the Thermogravimetric Kinetics of Dehydrated Sewage Sludge Regulated by Cationic Polyacrylamide and Sawdust.

With the continuous increase in sewage-sludge production worldwide, the pyrolytic disposal of sludge has received great attention. To build knowledge on the kinetics of pyrolysis, first, sludge was regulated using appropriate amounts of cationic polyacrylamide (CPAM) and sawdust to study their enhancing effect on dehydration. Due to the effects of the charge neutralization and skeleton hydrophobicity, a certain dose of CPAM and sawdust reduced the sludge's moisture content from 80.3% to 65.7%. Next, the pyrolysis characteristics of the dehydrated sludge regulated by CPAM and sawdust were investigated at a heating rate of 10~40 °C/min by using TGA method. The addition of sawdust enhanced the release of volatile substances and reduced the apparent activation energy of the sample. The maximum weight-loss rate decreased with the heating rate, and the DTG curves moved in the direction of high temperature. A model-free method, namely the Starink method, was adopted to calculate the apparent activation energies, which ranged from 135.3 kJ/mol to 174.8 kJ/mol. Combined with the master-plots method, the most appropriate mechanism function ultimately obtained was the nucleation-and-growth model.

Analysis of the local growth and density evolution of soot deposits generated under hydrocarbon condensation: 3D simulation and detailed experimental validation

The utilization of the Exhaust Gas Recirculation (EGR) system during atypical engine operating conditions in order to meet future type-approval criteria exposes the internal surfaces of the devices to exhaust gas with elevated concentrations of particulate matter and greater amounts of hydrocarbon species, leading to the formation of dense and wet sludge deposits. To broaden the understanding of this phenomenon and contribute to the development of advanced EGR devices, this study presents an extended Computational Fluid Dynamics (CFD) model that, in addition to simulating the growth of fouling deposits caused by the accumulation of soot particles, also takes into account the condensation of hydrocarbons. Two scenarios with varying hydrocarbon concentrations in the exhaust flow are analysed, and the evolution of the deposit's thickness and density is determined. A sequential validation process is carried out by comparing the numerical results to actual deposit profiles at different stages of the fouling process. Additionally, hyperspectral images of the fouling layer have been acquired and analysed to validate the regions where hydrocarbon condensation is predicted to play a crucial role, enabling the verification of the hydrocarbon condensation phenomenon predicted by the numerical model. The results obtained under the studied conditions indicate that, on average, 77.4% of the analysed area exhibits a low level of relative error, demonstrating that the proposed model and the methodology used serve as a valuable tool for examining the propensity for deposit formation in devices subjected to fouling exacerbated by hydrocarbon condensation.

Molecular characterization of microorganisms with industrial potential for methane production in sludge from Kangemi sewage treatment plant, Nyeri county–Kenya

Microbial consortia under anaerobic conditions are involved in oxidizing organic matter in the sludge to produce methane gas. However, in developing countries like Kenya, these microbes have not been fully identified to target them for the efficient harnessing of biofuel. This study collected wet sludge from two anaerobic digestion lagoons 1 and 2 that were operational during sampling at Kangemi Sewage Treatment Plant, in Nyeri County, Kenya. DNA was extracted from samples using commercially available ZymoBIOMICS™ DNA Miniprep Kit and sequenced using Shotgun metagenomics. Samples were analyzed using MG-RAST software (Project ID: mgp100988), which allowed for identifying microorganisms directly involved in various stages of methanogenesis pathways. The study found hydrogenotrophic methanogens, such as Methanospirillum (32%), Methanobacterium (27%), Methanobrevibacter (27%), and Methanosarcina (32%), being predominant in the lagoon communities, whereas acetoclastic microorganisms such as the Methanoregula (22%) and the acetate oxidazing bacteria such as Clostridia (68%) were the key microbes for that pathway in the sewage digester sludge. Furthermore, Methanothermobacter (18%), Methanosarcina (21%), Methanosaeta (15%), and Methanospirillum (13%) carried out the methylotrophic pathway. In contrast, Methanosarcina (23%),Methanoregula (14%), methanosaeta (13%), and methnanoprevibacter (13%) seemed to play an important role in the final step of methane release. This study concluded that the sludge produced from the Nyeri-Kangemi WWTP harbors microbes with significant potential for biogas production. The study recommends a pilot study to investigate the efficiency of the identified microbes for biogas production.

The Influence of Concrete Sludge from Residual Concrete on Fresh and Hardened Cement Paste Properties.

In the concrete manufacturing industry, a large amount of waste is generated. Such waste can be utilised in the production of more sustainable products with a low carbon footprint. In this study, concrete sludge, a difficult-to-utilise waste that is obtained from residual concrete by washing a concrete truck, was investigated. During washing, aggregates from the concrete mixture are separated, and the remaining insoluble fine particles combine with water to form concrete sludge. Dried and wet concrete sludge were used in the tests. Samples with different compositions were produced with dried and wet concrete sludge, cement, superplasticiser, and tap water. Seven cement pastes with different compositions were made by partially replacing cement with dried concrete sludge (0%, 5%, 10%, 15%, 20%, 25%, and 30%). In compositions with wet concrete sludge, cement was replaced by the same amounts as in the case of dried concrete sludge. The slump, setting time, and their changes with different amounts of concrete sludge were determined for fresh cement pastes. It was found that with different forms of concrete sludge, the technological properties of the mixtures change, and the setting time decreases. The density and compressive and flexural strength results were confirmed by SEM and XRD tests. The research results show that dry concrete sludge causes the deterioration of the mechanical properties of cement stone, while wet concrete sludge improves the mechanical properties of cement stone. However, it was found that replacing 5% cement with dry concrete sludge does not significantly affect the properties of hardened cement stone. In mixes with wet concrete sludge, the recommended amount of replaced cement is 10%, because the technological properties of the mixture are strongly influenced by larger amounts.

Preparation of CFB fly ash/sewage sludge ceramsite and the morphological transformation and release properties of sulfur

In order to improve the resource utilization of circulating fluidized bed (CFB) fly ash and sewage sludge, light-weight and high-strength CFB fly ash/sewage sludge ceramsite (CFBS ceramsite) was prepared by using CFB fly ash and original wet sludge as the main raw materials. The effects of dosages of CFB fly ash, sewage sludge, fluxing agent, and foaming agent and sintering temperature on the physical and mechanical properties of CFBS ceramsite were studied by single factor experiment method. Using SEM, XRD, sulfur analyzer, TG-DSC, flue gas analyzer, TG-MS and XPS analyses, the micro morphology and mineral composition of CFBS ceramsite were studied, and the morphological transformation and release properties of sulfur were also revealed. The results show that the optimal preparation conditions are: the mass ratio [CFB fly ash: sludge] = [65 %∼80 %: 35 %∼20 %], addition of 10 % glass powder, addition of 2.5 %∼10 % calcium carbonate, and sintering at 1200 ∼ 1250℃ for 15 ∼ 20 min. The resulting CFBS ceramsite can be obtained with bulk density of 750 ∼ 850 kg/m3, water absorption of 0.27 %∼0.94 % and cylinder compressive strength of 5.11 ∼ 7.86 MPa. Sulfur in the CFBS ceramsite is mainly derived from organic sulfur in the sewage sludge and inorganic sulfur in the CFB fly ash, and it exists in the form of sulfate and sulfide when the sintering temperature exceeds 400℃. The sulfur content of the final prepared CFBS ceramsite is 0.0217 wt%. Decomposition of SO2 from the anhydrite in CFB fly ash is the main reason for the swelling and foaming of the CFBS ceramsite. However, calcium carbonate, derived from either sulfur-fixing agent in CFB fly ash or auxiliary foaming agent, cannot play the role of foaming agent, while the CaO decomposed from calcium carbonate increases the content of fluxing component in the ceramsite and finally affects the properties of CFBS ceramsite.

Investigation on hydrogen-rich syngas preparation from high wet sludge mixed with sawdust based on iron oxygen carrier

In this study, chemical looping gasification (CLG) of high wet sludge (HWS) mixed with sawdust was carried out based on red mud and Fe/Al composite oxygen carrier (OC) prepared by impregnation method. Further, the optimal gasification conditions in a fixed-bed reactor and the migration path of water from HWS were investigated for the preparation of hydrogen-rich syngas. Results show that the reaction temperature significantly affects gasification, and the higher temperature is conducive to the progress of sludge gasification. Addition of sawdust changes the volatile content of mixture fuel, thus affecting the composition of syngas. Utilization of appropriate amount of OC can promote the gasification rate. The syngas with calorific value of 11.16 MJ⋅Nm−3 was obtained under conditions including temperature of 800 °C, ratio of sludge to sawdust of 3:1, and O/C ratio of 1:10. Owing to the catalyst and adsorbent dual function, addition of CaO into bed material led to the significant increase in the H2/CO ratio of syngas; however, addition of excessive CaO affected the contact between sludge volatile and OC, resulting in the increase of CO2 fraction and the decrease of the calorific value of syngas. A proportion of the vapor released from HWS reacted with carbon and OC and water–gas shift reaction with CO in gas to produce more H2 and improved the syngas quality, which thus proves the advantages of wet sludge CLG to obtain higher energy yield than those of the dried sludge.

Biodiesel production from wet sewage sludge and reduced CO2 emissions compared to incineration in Tokyo, Japan

Sewage sludge has potential use as a feedstock source for biodiesel production, but pretreatment such as drying it before the conversion has made it an uneconomical process. In this study, sewage sludge samples were collected from two different wastewater treatment facilities in Tokyo to develop an efficient process for extracting the saponifiable lipids and preparing biodiesel. A direct extraction method was chosen and performed in multi-stages by mixing hexane with sewage sludge in a jar at room temperature using various process conditions. Results showed the saponifiable lipids‘ extraction in the range of 19–21.5 wet wt. % (38–41 wt% DM), and it was possible to extract saponifiable lipids in an energy-efficient and economical manner. Furthermore, these extracted saponifiable lipids were converted into biodiesel through acid/base transesterification, resulting in an approximately 98 % and 99 % biodiesel yield using 1 wt% NaOH and 2 wt% H2SO4, respectively with a 94% lipid conversion efficiency. The biodiesel's high heating value, low cost, and low sulfur content make it a candidate for a carbon–neutral fuel and its production can reduce CO2 emissions in comparison to sludge incineration.

Investigation on Co-combustion characteristics and NOx emissions of coal and municipal sludge in a tangentially fired boiler

Using the current coal-fired power plant to treat sludge with large annual production is an efficient sludge treatment method. Nevertheless, the majority of the previous studies concentrated on sludge co-firing with low blending ratio and moisture content in utility boiler. Further analysis and discussion are necessary regarding the impact of sludge with high blending ratio and high moisture content on co-firing performance as well as the potential interactions between high moisture content sludge and coal during the co-firing process. The simulation results of the verified computational fluid dynamic (CFD) model can be regarded as numerical experiments. In the present research, CFD was used to analyze the co-firing characteristics of municipal sludge and coal in a 600 MW tangentially fired utility boiler. The impacts of sludge moisture content, blending ratio, blending strategy, and secondary air (SA) distribution on co-combustion characteristics and NOx emissions were investigated, and their potential synergistic effects were assessed. The co-firing of coal and sludge has little impact on the furnace outlet temperature while potentially lowering NOx emissions, provided that the net calorific value of the feeding of the mixture of sludge and coal is kept constant. The reduction of sludge moisture content can significantly improve co-combustion performance, but it also facilitates NOx production. Blending sludge from the position of the medium-upper burners can reduce the number of layers needed to burn the sludge in practical coal-fired boiler. Furthermore, the SA distribution has a major impact on the furnace outlet flue gas temperature and NOx content. The shrunk-middle type is the optimum scheme for SA distribution. This study contributes to a deeper knowledge of the actual co-firing of sludge and coal in the utility boiler and provides a possible theoretical foundation for the widespread, clean, and effective use of sludge.

Treatment of Mine Water with Reverse Osmosis and Concentrate Processing to Recover Copper and Deposit Calcium Carbonate

Mine water usually contains heavy metals and other inorganic and organic pollutants that contaminate water bodies. Reverse osmosis (RO) techniques are capable of producing purified water that meets discharge regulations. However, the problem of RO concentrate disposal and utilization is still not solved. The well-known zero liquid discharge (ZLD) process provides total concentrate utilization at the power industries but seems unreasonably expensive for the treatment of large amounts of mine water due to required chemical softening and the evaporation of concentrate. In the present article, a new approach to increase the recovery of reverse osmosis and to avoid high operational costs is demonstrated and discussed. The new technique involves radical RO concentrate flow reduction and withdrawal, together with dewatered sludge. The idea to "hide" concentrate in dewatered sludge is proposed and demonstrated during experiments. The article demonstrates results of the conducted experimental program aimed at reduction of volumes of all liquid wastes produced during mine water treatment using a new approach to concentrate it with a cascade of nanofiltration membranes and to reach a TDS value of 110-120 g per liter. The obtained concentrate is mixed with the wet sludge, which is further dewatered and withdrawn together with the dewatered sludge. Experiments are conducted that demonstrate a reduction in calcium in the concentrate due to deposition of calcium carbonate on the "seed crystals" in the circulation mode. Another distinguishing feature of the new technique is the separation of concentrate into two streams containing high concentrations of monovalent ions (sodium and ammonium chlorides) and divalent ions (calcium, magnesium and copper sulphates). Flow diagrams of the processes are presented to demonstrate the water treatment technique used to produce deionized water and two types of sludges: sludge after clarification and sludge after calcium carbonate deposition.

Production of Drinking Water with Membranes with Simultaneous Utilization of Concentrate and Reject Effluent after Sludge Dewatering.

A new technology is described that enables us to completely exclude liquid discharges during production of drinking water from surface sources. The proposed described technological scheme separates the natural water into a stream of purified drinking water and dewatered sludge. The sludge moisture has a value of 80 percent. The experimental program is described to treat the natural water with nanofiltration membranes and to produce a drinking-quality water with recovery value of 0.99 and higher. Concentrate of membrane plant is mixed with the wet sludge and the reject effluent after sludge dewatering is again treated by reverse osmosis membranes and returned back to the sludge thickening tank. Results of experiments to treat reject water after sludge dewatering are presented. The use of nanofiltration membranes provides reduction in the Total Dissolved Solids content (TDS), aluminum, color and oxidation to meet drinking water standards. Experimental plots are presented that can be used to select membrane characteristics and to predict product water chemical composition at each stage of the membrane treatment scheme. Concentrate of membrane treatment plant is mixed with the wet sludge in the thickening tank. The sludge, after the thickening tank, is dewatered using either filter-press or centrifugal equipment. The reject (or fugate), after sludge dewatering, is treated by membrane facility to separate it into deionized water stream and concentrate stream. The deionized water can be mixed with the feed water or drinking water and the concentrate stream is returned back to the thickening tank. Thus, the salt balance is maintained in the thickening tank, whereby all dissolved salts and impurities that are rejected by membranes are collected in the thickening tank, and then are withdrawn together with the dewatered sludge. Based on the results of experimental data processing, balance diagrams of the sludge dehydration process with waste water purification at the membrane plant and with the addition of the membrane plant concentrate to the sludge thickener are presented, according to which all contaminants removed by the membranes are removed together with the sludge.

Development and prototype testing of a novel small-scale pyrolysis system for the treatment of sanitary sludge

An innovative twin auger pyrolysis reactor for the intermediate conversion of partially wet sanitary faecal sludge generated on train toilets (mobile applications) is designed and successfully commissioned. The effects of operating temperature, feedstock feeding rates and moisture content and their interaction on the distribution and properties of product biofuels with the use of no carrier gas were assessed. Approximate yields of 50 %, 40 % and 10 % were observed for the biooil, syngas and biochar respectively at 500 °C. Meanwhile, the highest calorific value (HHV) of the syngas did not exceed 10 MJ/Nm3. Acidic compounds (10 % RPA) and nitrogen-containing hydrocarbons (18 % RPA) substituted the abundant species in biooil fraction at 500 °C with the lowest feed volumes of feedstock. Biochar presented the highest fixed carbon (58 %) and ash content (37 %) upon higher operating temperatures for the minimum and maximum volumes of feedstock. Interestingly, the retention of inorganics within the structure of char were observed. Meanwhile, the calorific value of the biochar reduced due to intensive reduction reactions at higher temperatures ranging to a minimum of 19 MJ/kg. Effects of reactor design and configuration on the pyrolytic products are discussed accordingly and further implications are provided. Overall, extensive further research for the end-use of pyrolytic products and the investigation of emissions, upon energy recovery or land-application of by-products are suggested.

The introduction of wet dyeing sludge pellets in the production of clay brick: A novel approach to promote the disposal efficiency

The introduction of dyeing sludge in the fired brick production is an economic and practical disposal approach. Wet dyeing sludge with high moisture usually produces block mass during mixing process of raw materials, not only reduces the production efficiency but negatively impacted products engineering properties. This study proposed a highly efficient method, wet sludge firstly being pelleted and then introduced in the production of fired bricks, to reduce the negative impacts on mixing process and the products quality. It was proved that the introduction of dyeing sludge pellets remarkably improved physical and mechanical properties of fired bricks. Especially, water absorption declined by 12% and compressive strength increased by 35% comparing with dyeing sludge without pelleting. The engineering properties of fired bricks containing 25 wt% sludge pellets are similar with that containing 15 wt% sludge without pelleting. The decrease in water absorption and porosity is mainly attributed to the reduction in number and volume of pores with diameter below 100 nm. It was considered that pellets with greater density and higher strength might play a role as aggregate to improve compressive strength. However, the introduction of sludge pellets with particle size over 8 mm is adverse to improving properties of products, because a large amount of macropore with diameter over 100 μm is produced, and destroys the homogeneous and compact structure of products. The optimal sludge pellet size is in the range of 2–6 mm based on products performance. The leaching toxicity of heavy metals from products is far lower than regulatory limitation. This study provides an advanced method to promote the disposal efficiency of solid waste and improve the engineering quality of products.

Real-time measurement of the moisture contents of dewatered sewage sludge during thermal drying using a low-cost ECH2O EC-5 soil moisture sensor

Real-time measurement of sewage sludge moisture content renders it possible to optimize the consumption of auxiliary fuel and the thermal treatment. Thus, we explored whether a dielectric soil moisture sensor could be used to measure real-time changes in sludge moisture content. This was possible with the aid of a unified calibration curve. The explanatory variable was the third-order sensor output voltage. The calibration curve well-fitted the data; the mean measurement error was 6.14 wt.%. The use of individual calibration curves for each sludge batch reduced the mean measurement error to 3.09 wt.%. The mean measurement error improves when multiple sensors are simultaneously used. The sensor operates very well when the moisture content is low; the results are minimally affected by the bulk density and Fe level. Considering feed-back to the process, it is ideal to measure sludge moisture content near the outlet of the thermal treatment.

Wet dyeing sludge being pelleted and introduce d in clay brick production to promote product quality and sludge consumption dosage

The introduction of dyeing sludge in the production of fired brick is an economic and practical disposal approach. However, wet dyeing sludge is easy to form block mass during the mixing process of raw materials due to the high moisture content, which affected the production efficiency and destroyed the homogeneous structure of the fired brick, posing a negative impact on the engineering performance of the fired brick. In this study, wet sludge firstly is firstly pelleted and then introduced in the production of fired bricks to reduce the negative impacts on mixing process and the products quality. The results showed that the introduction of dyeing sludge pellets significantly improved the physical and mechanical properties of fired bricks. Compared with bricks incorporating dyeing sludge, water absorption is reduced by 24 % and compressive strength is increased by 101.6 %. The physical and mechanical properties of fired bricks incorporating 30 wt% of pelleting sludge are similar with bricks incorporating 20 wt% dyeing sludge without pelleting. The reduction in water absorption and porosity compared to that incorporating sludge without pelleting was mainly due to the reduction in the number and volume of pores of the same size. However, the introduction of more than 30 wt% dyeing sludge pellets is not conducive to improving the performance of the product, because a large number of macropores with size larger than 100 μm will be generated, which destroys the uniform and dense structure of the product. To ensure product quality meet the standard, the maximum dosage of dyeing sludge with pelleting is controlled below 30 wt%. This study provides an advanced method to improve the treatment efficiency of solid waste and the quality of products.

Hydrothermal liquefaction of sewage sludge and model compound: Heavy metals distribution and behaviors

Hydrothermal liquefaction (HTL) is an attractive thermochemical method for the conversion of wet sewage sludge (SS) into bio-oil. However, the bio-oil suffers from the contamination of heavy metals (HMs). In this study, the effects of reaction conditions on the HMs (Cu, Zn, Cd, Cr and Pb) migration during HTL of SS was investigated. Soybean protein was used as the typical model compound of SS to reveal the interactions between HMs and organic matters in the HTL process. Results showed that higher temperatures and longer reaction times promoted HMs enriched into the solid residues with more stable forms, while the proportion of HMs in the bio-oil was also increased. XPS analysis showed that the coordinate bond was formed between HMs and N during HTL of soybean protein at 180 °C. At higher temperature of 280 °C, metal oxides appeared in residues, while Cu and Cd led to the formation of nitrates. GC–MS analysis showed that Cu reduced the aromatic content by 52.8 %, while other HMs (Zn, Cd, Cr and Pb) reduced the oxygenated compounds content by 21.6 % at 180 ℃. In addition, the HMs had little effects on the bio-oil compositions at 280 °C due to the more stable forms of HMs at higher temperatures.

Numerical Simulation of Heat and Mass Transfer in Sludge Low-Temperature Drying Process.

Based on the sludge mass transfer flux model, this paper conducts a simulation study on the drying characteristics of sludge under low-temperature environment and compares it with the previous experimental results. It is found that when the sludge moisture content is low, the change of its drying curve is basically consistent with the experimental results, but there is a large error when the sludge moisture content is 0.4-0.6. In order to better simulate sludge drying characteristics, a model of cracking and shrinkage coefficients based on sludge moisture content is proposed, and the effective diffusion coefficient and mass transfer coefficient are modified. The maximum error between simulation and experiment is reduced to 23.78%. Based on this model, the sludge drying mechanism was studied. It was found that heat transfer and diffusion played a major role in the initial stage of sludge drying, and diffusion played a major role in sludge drying 30 min later.

Sequential Extraction and Characterization of Nitrogen Compounds after Hydrothermal Liquefaction of Sewage Sludge.

Organic solid wastes such as sewage sludge are potential feedstocks for the production of drop-in biofuels. Hydrothermal liquefaction (HTL) is a process that converts the wet sewage sludge into an organic biocrude. To fulfill industrial fuel standards, the considerable heteroatom content of the biocrude needs to be lowered by downstream processes. Nitrogen (N) contained in several compounds poses a challenge and yet, the complex chemical composition of HTL-biocrude samples has hindered detailed analysis and understanding. In particular, N-containing aromatic substances appear very persistent in biocrude. In the present work, two alkaline (NaHCO3 and NaOH) and one acidic (HCL) aqueous solutions were subsequently applied to extract and recover polar N-containing compounds from the biocrude matrix with an N-content of 3.8 wt %. Gas chromatography-mass spectrometry, atmospheric pressure chemical ionization in positive mode, and Fourier transform ion cyclotron resonance mass spectrometry were applied for their characterization and results show that a large share of N-compounds with an aromatic, pyridinic structure was found in the acidic extracted fraction with an N-content of 9.5 wt %. Aliphatic N-compounds were less affected by the separation and ended in the residual fraction. N-compounds with multiple oxygen functionalizations are enriched in the alkaline extracted fractions. This showed that N-compounds with an aromatic structure are strongly affected by polar groups and can potentially be extracted by downstream processes with appropriate solvents.

Sequential hydrothermal carbonization and CO2 gasification of sewage sludge for improved syngas production with mitigated emissions of NOx precursors

Due to high moisture and protein contents in sewage sludge (SS), conventional thermal treatment of SS is energy-intensive and a large amount of harmful nitrogen-containing gases are emitted. In this study, a sequential hydrothermal carbonization (HTC) and CO2 gasification system has been proposed to effectively improve the gasification efficiency and considerably reduce the emissions of NOx precursors (i.e., NH3 and HCN) in SS treatment. Yield and quality of syngas were comprehensively investigated during CO2 gasification in terms of various temperatures with different dosages of CaO additive in HTC, and varied reaction temperature and CO2 percentage in gasification process. On the whole, CO2 gasification of SS derived hydrochar (HC) demonstrated obvious reduction of tar formation from 10.9 to 6.8 % and enhancement of calorific value of formed syngas from 14.1 to 15.7 MJ/Nm3. Production of NOx precursors was greatly reduced due to formed non-active quaternary-N in HC, while HTC with CaO favored the mitigated NOx precursors in syngas owing to enriched pyrrole-N and quaternary-N therein. Catalytic tar decomposition and Boudouard reaction in CO2 gasification were responsible for the distinct reduction of tar formation, and notable increase of carbon conversion ratio and syngas yield. Although facilitated catalytic gasification in CO2 atmosphere can maximize syngas yield, lower portion of CO2 (ca. 20 %) was more beneficial to drastically mitigate emissions of NOx precursors, especially the reinforced transformation of NH3 to N2. The fundamental knowledge could ultimately help to achieve significant abatement of NOx precursors during CO2 gasification for improved syngas production.

Effect of calcium hydroxide on the characteristics of volatile organic compounds emission during sewage sludge incineration

AbstractEmission of volatile organic compounds (VOCs) during sludge incineration was explored after the addition of calcium hydroxide (Ca [OH]2), and the effects of incineration temperature, sludge particle size, and moisture content were investigated in a lab‐scale tubular furnace. Further, the contribution of the emission of VOCs in ozone formation potential (OFP) without Ca (OH)2 was calculated. The results indicated that incineration temperature in the range of 550–950°C can inhibit the emission of VOCs, with concentrations of the emissions varying from 3076.91 to 1277.11 μg/m3. Ca (OH)2 content had pronounced effects on the emission of VOCs during sludge incineration. The emission concentration of the VOCs decreased from 2421.16 to 1226.68 μg/m3 with Ca (OH)2 content increasing from 0 to 0.8 g. Sludge particle size and moisture content had little effects on the total amount of emitted VOCs during sludge incineration. Meanwhile, the contribution of the components of VOCs from sludge incineration in OFP was analyzed, which exhibited a decreasing trend: aromatic compounds > other substances > alkanes and olefins > chlorinated organic compounds. Aromatic compounds displayed the highest contribution rate at 92.72% because of the high concentration of benzene (1033.43 μg/m3). Our study provided theoretical data and a strong evidence of reduction in the emission of VOCs, OFP, and environmental regulation during sludge incineration.

Thermodynamic and economic analyses of sewage sludge resource utilization systems integrating Drying, Incineration, and power generation processes

In an effort to beneficially utilize sewage sludge as a resource, a process system integrating sewage sludge drying, incineration, and power generation are proposed and subjected to thermodynamic and economic analysis. The proposed system comprises wet sewage sludge drying using a low pressure steam, dry sewage sludge incineration in a boiler to raise steam for power generation in a steam turbine whose exhaust facilitates the drying of the wet sewage sludge. Three alternative schemes are considered. Scheme 1 features higher temperature and pressure of the turbine exhaust for drying without auxiliary fuel; Scheme 2 includes an external steam input while Scheme 3 incorporates firing of coal as auxiliary fuel. The results indicate that the energy efficiency of Schemes 2 and 3 can reach up to 49.93 % and 48.69 %, respectively. The economic analysis suggests that Scheme 2 outperform Schemes 1 and 3. The cost of electricity for Schemes 2 and 3 are as low as 0.037 and 0.039 USD/kWh, respectively, with corresponding dynamic payback period being 13.63 and 13.70 years. The net present value attains 48.67 million and 48.62 million USD, respectively. Therefore, the proposed schemes 2 and 3 are suitable and promising for sewage sludge resource utilization from the viewpoint of thermodynamics and economics. Moreover, a conceptual proposal for renewable energy dispatch is presented and analyzed, which would provide a reference for practical engineering application.