Sludge Char Research Articles

Preparation of red mud-modified sludge char through microwave-assisted one-step pyrolysis and steam activation and its adsorption properties for hydrogen sulfide

Preparation of red mud-modified sludge char through microwave-assisted one-step pyrolysis and steam activation and its adsorption properties for hydrogen sulfide

Steam gasification of lime dried sewage sludge: Effects of temperature and addition of lime

Gasification shows great potential as an alternative method for energy utilization from sewage sludge. Lime (CaO) is a cheap and commonly used dewatering conditioner for sewage sludge. The calcium hydroxide (Ca(OH)2) generated during the drying process with lime can enhance the pH level of the sludge, thereby providing disinfection properties. This paper investigated enhancement mechanism of Ca(OH)2 on hydrogen gas generation and the effect on gas product yield during the steam gasification process of lime-dried sludge. The experiments were conducted in an electrically heated reactor at 500–800 °C. The forms of calcium present in the gasified sludge char samples were analyzed using X-ray diffraction (XRD) analysis to investigate their transformation process. Fourier transform infrared spectroscopy (FTIR) was utilized to obtain information regarding the chemical structure in the gasified sludge char samples, with the aim of exploring the mechanism behind hydrogen production from lime-dried sludge. The results indicate that the CaO generated from Ca(OH)2 dehydration acts both CO2 adsorbent and catalyst during the gasification process. The addition of Ca(OH)2 during the sewage sludge gasification process leads to a reduction in the aromatic CH content in the organic matter, while significantly increasing the content of reactive CH3-chains. It also promotes the ring-opening and rearrangement reactions of aromatic hydrocarbons and nitrogen-containing heterocycles and steam reforming reaction of tar. The combined effects of these factors contribute to an overall increase in hydrogen gas yield.

Absorbing oxygen carriers promotes phosphorus recovery from sludge via the microwave thermal conversion process

This study aims to apply the Absorbing oxygen carriers (AOCs) to induce the migration and transformation of phosphorus compounds during the microwave thermal conversion of sludge so the hard-to-extract organic phosphorus (OP) can be converted to easy-to-extract inorganic phosphorus (IP) and be enriched onto the sludge char. The AOCs were recycled by screen separation from the IP-rich sludge char, with the latter being a renewable phosphorus source from sludge. The AOCs in this novel process enhanced the conversion efficiency of OP into non-apatite inorganic phosphorus (NAlP), which was further converted to apatite inorganic phosphorus (AP). Most phosphorus in the sludge char is presented in the form of orthophosphate.

A novel adsorbent based electroplating sludge – rice husk char for removal of methylene blue and ciprofloxacin in aqueous solution

The adsorbent based electroplating sludge and rice husk char (CR) was prepared by ultrasonication method for adsorption of methylene blue (MB) and Ciprofloxacin (CIP) in water. The characteristics of adsorbent such as surface morphology, specific surface area, functional groups, element components was confirmed by SEM, BET, FT-IR, XRD and EDX. The results showed the adsorbent is porous structure with BET surface area of 175.22 m2/g, most of pore width belonged meso section. Removal effectiveness of MB and CIP were 99.89% and 74.96%, respectively after 90 min of adsorption. The experiment was implemented with the mockup model containing of methylene blue and ciprofloxacin in NH3.

Synergistic application of alum sludge and sequential extraction for phosphorus recovery from sewage sludge char

Phosphorus (P) recovery from sewage sludge (SS) (or its ash/char) is challenging due to the presence of heavy metals (HMs) in SS. This study proposed the integration of SS, alum sludge (AS), sequential wet extraction, and pyrolysis for P recovery without simultaneous HM extraction. SS + AS mixture (P:Al molar ratio = 3:1) was first acid pretreated to obtain amended SS, then pyrolyzed, and finally, P was extracted from the char using an alkali. Pyrolysis was conducted at low (400 °C), medium (600 °C), and high (900 °C) temperatures. The highest alkaline P recovery efficiency was 88 % from the amended SS char, while 32 % from the unamended SS char (both prepared at 400 °C). Overall, the highest P recovery in the amended process was 48 % higher than the unamended process despite 11 % P loss during acidic pretreatment. Standards measurements and testing (SMT) protocol and solid-state NMR (31P and 27Al) analyses revealed that acidic pretreatment of SS with AS induced apatite phosphate (AP) to non-apatite inorganic phosphate (NAIP) conversion in SS. As NAIP is more soluble in alkali than AP and HMs, higher alkaline P recovery was achieved for amended SS char than unamended SS char, without HM contamination. However, higher pyrolytic temperatures impaired P recovery due to the immobilization of P in the char carbon matrix and the formation of recalcitrant P phases. These findings were corroborated by complementary characterization techniques such as XPS, XRD, and FTIR. Some major HMs in SS and AS were concentrated in char, except Zn, which partially volatilized above 600 °C. This study demonstrated the applicability of AS for extracting P from SS, offering a potential pathway for the synergistic management of sludge waste streams.

Optimization of nitrogen-doped sludge char preparation and mechanism study for catalytic oxidation of NO at room temperature

Catalytic oxidation of NO at room temperature was carried out over nitrogen (N)-doped sludge char (SC) prepared from pyrolysis of municipal sewage sludge, and urea was adopted as nitrogen source. The effects of different N-doping methods (one-step and two-step method), dried sludge (DS)/urea mass ratios (5:1, 4:1, 3:1, 2:1, and 1:1), SC preparation procedures (pyrolysis only, pyrolysis with acid washing, and pyrolysis with KOH activation and acid washing), and different pyrolysis temperatures (500, 600, 700, and 800°C) on the catalytic oxidation of NO were compared to optimize the procedure for SC preparation. The results indicated that N-doping could obviously promote the catalytic performance of SC. The one-step method with simultaneous sludge pyrolysis (at 700°C), KOH activation, and N-doping (DS/urea of 3:1) was the optimal procedure for preparing the N-doped SC with the NO conversion rate of 54.7%, whereas the optimal NO conversion rate of SC without N-doping was only 47.3%. Urea worked both as carbon and nitrogen source, which could increase about 2.9%-16.5% of carbon and 24.8%-42.7% of nitrogen content in SC pyrolyzed at 700°C. N-doping significantly promoted microporosity of SC. The optimal N-doped SC showed specific surface areas of 571.38 m2/g, much higher than 374.34 m2/g of the optimal SC without N-doping. In addition, N-doping also increased amorphousness and surface basicity of SC through the formation of N-containing groups. Finally, three reaction paths, i.e. microporous reactor, active sites, and basic site control path, were proposed to explain the mechanism of N-doping on promoting the catalytic performance of NO.

Hydrogen production and coke resistance characteristic during volatile reforming over Fe2O3–Ce2O3/sludge char catalyst

The development of sludge to green hydrogen is a win-win solution for alleviating the environmental pollution caused by sludge and low-carbon hydrogen production. A series of iron-cerium bimetallic oxide catalysts on sludge char (SC) were prepared for catalytic volatile reforming in this work. The physicochemical properties of the catalysts were adjusted by doping with different metals and changing the proportion. The results showed that 50Fe50Ce/SC possessing the minimum coke deposition rate (12.79 mg h−1·gcat−1) obtained a hydrogen yield of 280.55 ml g−1 from catalytic reforming at 800 °C with a steam-to-carbon ratio of 4:1. Analysis of the catalyst structure and activity suggested that the excellent activity mainly originated from the large number of oxygen vacancies. Oxygen vacancies promote the migration of lattice oxygen inside the catalyst bulk to the surface to react with the coke deposition, while the missing oxygen of the catalyst is replenished by steam, and oxygen vacancies play an important role in the enhancement of the catalyst activity and the elimination of coke deposition. The positive potential of this study in hydrogen production and carbon reduction were assessed. The study could provide new ideas for organic solid waste to generate green hydrogen and the development of efficient and low-cost catalysts.

Increasing phosphorus plant availability from P-rich ashes and biochars by acidification with sulfuric acid

Biochars and ashes derived from thermal treatment of P-rich wastes could be used as bio-based fertilizers to improve P recycling. However, thermal treatments often result in low plant P availability. Acidification of these materials before soil application could potentially increase plant P availability. Based on the water-extractable P levels obtained in titration experiments, sulfuric acid concentrations between 2.5 M and 10 M were applied to digestate solids char and ash (DS-C, DS-A), poultry litter ash (PL-A), insect frass char (IF-C), sewage sludge char and ash (SS-C, SS-A) and meat and bone char (MB-C). Acidified and untreated materials were applied in a pot experiment with maize in 33P labeled soil to determine fertilizer P uptake. The acidification resulted in a significant increase in P solubility. The amount of acid required depended on the materials’ buffer capacity and P speciation. Based on XRD analysis, we assume that mainly Ca-associated P was solubilized. In the pot experiment, acidified materials outperformed untreated materials and the unfertilized control in terms of biomass and P uptake. The P recovery from the acidified materials ranked in the order DS-C > SS-A > PL-A > IF-C > DS-A > SS-C > MB-C. The acidification did not significantly decrease soil pH, nor was there an effect on plant heavy metal availability. In conclusion, acidification increased plant growth and P uptake without affecting plant heavy metal uptake and soil pH. Therefore, acidification to increase the P fertilizer value of ashes and chars is a promising approach to facilitate P recycling.

Effects of inherent components and disposal temperature on the melting behavior of petrochemical sludge char during CO2 gasification

Pyrolysis-coupled gasification-melting is a promising technology as it can dispose of the petrochemical sludge (PS) and recover the leftover energy. Unfortunately, there has been little research investigating the effects of pyrolysis degree on melting characteristics of the pyrolysis residue (PR) and the transformation properties of the heavy metal (HM). In this study, the function of inherent components and disposal temperature were elucidated. The results show that the moisture and light volatile could disperse the melting residue (MR) during gasification-melting treatment, causing different morphology and color of the MR. In addition, as pyrolysis temperature increased, the HMs speciation (e.g. Zn, Cu, and Cr) in the PR was transformed from bioavailable to a stable state, and the yield of PR decreased from 66.8% to 36.5%. The PR produced at 800 °C could decrease about 0.9 ∼ 1.9 potential ecological risk of releasing substances during the subsequent high-temperature gasification-melting owing to its stable HMs state and less char composition. Moreover, the gasification at 1250 °C could realize the safe disposal of the PR. Further increasing the gasification temperature to 1450 °C could not improve the acid-leaching resistance of the HMs, although the apparent concentration of C and the acid dissolution proportion of slag decreased by 6.3% and 1.7%, respectively.

Catalytic degradation of crystal violet and methyl orange in heterogeneous Fenton-like processes

Metals-loaded (Fe3+, Cu2+ and Zn2+) activated carbons (M@AC) with different loading ratios (0.1%, 0.5%, 1%, 5% and 10%) were prepared and employed for catalytic degradation of dye model compounds (crystal violet (CV) and methyl orange (MO)) in wastewater by heterogeneous Fenton-like technique. Compared with Cu@AC and Zn@AC, 0.5% Fe3+ loaded AC (0.5Fe@AC) had better catalytic activity for dyes degradation. The effects of dyes initial concentration, catalyst dosage, pH and hydrogen peroxide (H2O2) volume on the catalytic degradation process were investigated. Cyclic performance, stability of 0.5Fe@AC and iron leaching were explored. Degradation kinetics were well fitted to the pseudo-second-order model (Langmuir-Hinshelwood). Almost complete decolorization (99.7%) of 400 mg L−1 CV was achieved after 30 min reaction under the conditions of CV volume (30 mL), catalyst dosage (0.05 g), H2O2 volume (1 mL) and pH (7.7). Decolorization of MO reached 98.2% under the same conditions. The abilities of pyrolysis char (PC) of dyeing sludge (DS) and metal loaded carbon to remove dye pollutants were compared. The intermediate products were analyzed and the possible degradation pathway was proposed. This study provided an insight into catalytic degradation of triphenylmethane- and aromatic azo-based substances, and utilization of sludge char.

Soil Remediation after Sewage Sludge or Sewage Sludge Char Application with Industrial Hemp and Its Potential for Bioenergy Production

Soil Remediation after Sewage Sludge or Sewage Sludge Char Application with Industrial Hemp and Its Potential for Bioenergy Production

Potential use of faecal sludge derived char briquettes as an alternative cooking energy source in Dar es Salaam, Tanzania: “Nexus between Sanitation and Energy (SDG 6 & 7)”

Coprophobia characterizes faecal sludge (FS) as an unpleasing raw or partially digested faecal waste with no recycling value. However, this so-called waste is claimed to have tremendous recycling values after being pyrolyzed. This study hypothesized that char briquettes made from FS might have the potential of replacing charcoal consumption in Dar es Salaam, Tanzania. Quantitative methods were used to examine the proximate, emissions and environmental benefits of faecal sludge char briquettes in comparison to wood charcoal from Dalbergia melanoxylon (locally known as Mpingo). Results revealed that FS collected during the dry season exhibited a higher total solids content (26%) compared to that collected during the rainy season (20%). This difference was statistically significant (p = 6.12405E-14 at α=0.05). Dewatering and further drying the FS to a minimum of 90% total solids yielded an energy value of 11-12 MJ/kg, which was lower than that of uncarbonized wood (firewood) at 19 MJ/kg. Production of FS char briquettes bonded by paper waste has a lower energy value (13 MJ/kg) than that bonded by cassava starch (14.6 MJ/kg). The average energy value of FS char briquettes is almost half that of wood charcoal (26 MJ/kg), meaning that the replacement of 1 kg of charcoal consumption would require 2 kg of FS char briquettes is required. Nevertheless, due to an ineffective collection of FS to be used in the production of char briquettes, only 8% of charcoal consumption in the city can be replaced. It is recommended to use the produced briquettes in open kitchens with ample oxygen circulation, as they emit carbon monoxide levels exceeding the allowable limits set by the Tanzania Bureau of Standards.

Catalytic performance of sewage sludge char and ash in the decomposition of NH3

The present study was conducted to study the catalytic performance of sewage sludge char and ash on NH3 decomposition by using a fixed bed reactor. The results indicated that sewage sludge char exhibited strong activity towards NH3 decomposition, which was highly influenced by temperature and atmosphere. The influence of atmosphere on NH3 decomposition was investigated at 973 K, it showed that the char activity was greatly promoted with the presence of O2, and the major product of NH3 oxidation catalyzed by relatively fresh char was N2 rather than NO. In contrast, the NH3 decomposition was suppressed with H2O introducing, and there was a significant amount of NO being produced. Compared to char, the in-situ combusted ash showed relatively poor catalytic performance with respect to NH3 conversion. At 973 K, the NH3 conversion catalyzed by ash was dominated by the inhabitation effect of H2O with the presence of both O2 and H2O. However, when the temperature increased to 1128 K, O2 tends to take a dominant role to promote NH3 conversion. By XRD analysis of the fresh and spent char, it was proposed that iron phosphide (Fe2P) may be responsible for the high activities towards NH3 decomposition, following a cycle mechanism.

Comparative study of carbon-deNOx process by different sewage sludge chars

Sewage sludge char (SC) reduces NO to N2 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-deNOx 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-deNOx 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-deNOx 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 (NH3, 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-deNOx within the temperature range of 300–350 °C.

Enhanced Sewage Sludge Disintegration and Nutrients Release by Catalytic Microbubbles Ozonation Using Sewage Sludge-Based Char as Catalyst

Using microbubble ozonation (MO) technique to disintegrate sludge is a promising sludge treatment process. To enhance the lysis and reduction of sludge, the catalytic ozonation consisting of MO and sewage sludge derived char (SC) were combined. Total solids (TS), volatile solids (VS), total nitrogen and phosphate (TN and TP) were selected as main parameters for evaluating the treatment performance both in solids and supernatant. With the utilization of the catalytic MO, the ozone utilization and sludge reduction were largely improved. At a reaction time of 90 min, an ozone utilization efficiency exceeding 99% was achieved by using a MO system. The optical ozone and sludge char dosages of 150 mg/g suspended solids (SS) and 1 g/L were found for sludge lysis, respectively. TS and VS concentrations decreased by 43% and 56%, respectively, as compared to those of 16.7% and 17.9% obtained by the treatment with MO alone under the condition of sludge solution pH 4. The supernatant soluble chemical oxygen demand (SCOD), TN, TP, NH4+-N and NO3−-N increased by 1750%, 205%, 25%, 31% and 43%, respectively. A small amount of additional SC exhibited strong catalytic activity on dissolving organic matter of the sludge, demonstrating the positive effect caused by the heterogeneous catalytic ozonation on sludge disintegration.

Co‐Combustion of Oil Sludge Char and Brown Coal in a Continuous Fluidized‐Bed Combustor

AbstractFor developing technologies for innocent disposal of/energy recovery from high‐ash oil sludge char (OSC), it was continuously combusted in a 10 kg h−1 fluidized‐bed combustor. Addition of brown coal (BC) improved its combustion, but excessive BC was prone to cause slagging. Fine coke particles in OSC and volatiles in BC led to staged temperatures along the fluidized bed through air‐staging co‐combustion, enabling excellent combustion performance as well as efficient NOx reduction, especially when coupled with selective non‐catalytic reduction. Thus, there is a potential to cleanly recover energy from OSC by its co‐combustion with BC.

Preparation of activated sludge char through microwave-assisted one-step pyrolysis and activation for gaseous H2S removal

A one-step method which combined microwave pyrolysis and CO2/steam activation to prepare activated sludge char (ASC) for H2S adsorption was investigated. The adsorption properties of ASC prepared under different pyrolysis temperature, CO2/steam flow rate, residence time, and sludge moisture content were investigated by the characterization of proximate analysis, H2S adsorption kinetics, and iodine adsorption values (IAV) through orthogonal method. Results showed that steam-ASC showed better H2S adsorption performance than CO2-ASC at the pyrolysis/activation temperature of 500–700 °C. The optimal saturated H2S adsorption capacities (SHAC) of steam-ASC and CO2-ASC were 9.149 and 3.451 mg/g, respectively. There was a strong correlation (R2 = 0.964) between SHAC and IAV of steam-/CO2-ASC. H2S adsorption related to pore diffusion, but less significant than physisorption and chemisorption. According to the H2S adsorption characteristics and textural characterization of the optimal steam-/CO2-ASC, the decrease of adsorption temperature, the increases of H2S concentration and ASC moisture content, and N2 atmosphere (compared with air) could enhance SHAC. Sulfide, sulfate, and sulfone were formed in ASC during H2S adsorption, and the possible formation pathways were discussed. The results of this study indicated that one-step of simultaneous sludge pyrolysis and steam activation is a promising ASC preparation method for H2S adsorption.

Medium-Low Temperature Conditions Induce the Formation of Environmentally Persistent Free Radicals in Microplastics with Conjugated Aromatic-Ring Structures during Sewage Sludge Pyrolysis.

Medium-low temperature pyrolysis is an effective method of retaining active components in sludge char. However, we found that incomplete cracking reactions resulted in residues of microplastics (MPs) remaining in the char; moreover, high levels of environmentally persistent free radicals (EPFRs) were detected in these MPs. Here, we investigated the temperature-dependent variations in the char-volatile products derived from sludge and MPs under different pyrolysis scenarios using multiple in situ probe coupling techniques and electron paramagnetic resonance spectroscopy, thereby identifying the sources of EPFRs and elucidating the corresponding formation-conversion mechanisms. The temperature was the key factor in the formation of EPFRs; in particular, in the 350-450 °C range, the abundance of EPFRs increased exponentially. Reactive EPFR readily formed in MPs with conjugated aromatic-ring structures (polyethylene terephthalate and polystyrene) at a temperature above 350 °C; EPFR concentrations were 5-17 times higher than those found in other types of polymers, and these radicals exhibited half-lives of more than 90 days. The EPFR formation mechanism could be summarized as solid-solid/solid-gas interfacial interactions between the polymers and the intermediate products from sludge pyrolysis (at 160-350 °C) and the homolytic cleavage-proton transfer occurring in the polymers themselves under the dual action of thermal induction and acid sites (at 350-450 °C). Based on the understanding of the evolution of EPFRs, temperature regulation and sludge components conditioning may be effective approaches to inhibit the formation of EPFRs in MPs, constituting reliable strategies to diminish the environmental risk associated with the byproducts of sludge pyrolysis.

In-situ recycling strategy for co-treatment of antimony-rich sludge char and leachate: Pilot-scale application in an engineering case

A novel in-situ recycling strategy is proposed for the co-disposal of antimony-rich dewatered sludge char (ADSC) and sludge leachate, and this strategy is further applied in a pilot-scale pre-treatment system. The preparation and operating conditions of lightweight micro-electrolysis ceramic fillers (LMCF) for ADSC were systematically studied, and LMCF characterisations and working mechanisms were scrutinised using SEM-mapping, XRD, XPS, EPR, BET, GF-AAS and CV examination. The results indicated that optimal preparation parameters for LMCF operation were 35% iron scrap content, a 1:2 vol ratio for Fe/ADSC, which was sintered at 900 ℃ for 2 h. Under the initial pH was 3.0 and the aerating intensity was 1 cm3·cm−2·s−1, the Sb-species were concentrated on the porous surface of the LMCF, degraded by various reactive oxygen species (ROS) released from Fe-C cells, and co-precipitated with FeOOH on the used LMCF surface. Thirty-one days of pilot-scale operations indicated that backwash was essential to maintain the LMCF with high efficiency regarding the total Sb removal and the biodegradability improvement of the actual sludge leachate. Lastly, preliminary economic assessment and suggestions for engineering-scale application were proposed.

Pore development during CO2 and H2O activation associated with the catalytic role of inherent inorganics in sewage sludge char and its performance during the reforming of volatiles

We present a highly-detailed discussion on the pore development in sewage sludge char to better understand its transformation and subsequent role in volatiles reforming. Interactions with the pyrolytic gas can change the porosity of biochar that subsequently participates in the volatiles reforming, thus affecting the product yield and quality. The results demonstrated that H2O activation was more susceptible to the catalytic effects of inorganics, pointing out the more selective nature of CO2. Interestingly, we found important difference between ultra- and supermicropores role in the CO2 activation and toluene conversion. Thus, we suggest distinguishing the micropore types in char porosity assessment.