The potential of SO2 for reducing corrosion in WtE plants

Due to the high-temperature boiler corrosion induced by chloride-rich fly ash deposits, steam generation in today’s Waste-to-Energy (WtE) plants is typically designed only for 40 bar/400 °C as an economic compromise between acceptable corrosion rate and maximum power generation. The high-corrosive metal chlorides in the fly ash can react with SO₂ forming low-corrosive sulfates. The sulfation efficiency is enhanced by high SO₂ levels and sufficient residence time of the flue gas at high-temperatures (700–900 °C). The fly ash sulfation was tested in full scale in a Swedish WtE plant by applying the economic sulfur recirculation method. Probes of several alloys (16Mo3, Inconel 625, Sanicro 28) were exposed for 1000 h at controlled material temperatures in the superheater position, at normal and during sulfating operation respectively. Analyses of the fly ash showed that the molar Cl/S was decreased to values well below 1 and the corresponding corrosion rates of the individual material samples were less than half when sulfur recirculation was in operation. These positive findings demonstrate that the sulfur recirculation process has high potential for low-corrosive high-temperature steam generation (T ≈ 500 °C) and improved electricity production. Further steam superheating can be realized by staged superheating using small amounts of secondary fuel.     

Hydrolysis of organic matter during autoclaving of commingled household waste

Commingled household waste (HW) that had a controlled composition was autoclaved at elevated pressures in the presence of saturated steam for one hour at the nominal temperature levels of 130 °C, 160 °C and 200 °C. The focus of this study was the impact of temperature/pressure on hydrolysis of organic matter during autoclaving and the extent of its hydrolysis. The pH decreased with autoclaving temperature with which it had a linear relationship, and ranged from 7.4 and 6 in floc, and 6.7 and 3.6 in steam condensate. Overall, organic matter solubilisation, as indicated by dissolved organic carbon, biological and chemical oxygen demands, and total dissolved solids, increased with temperature. Lignin did not appear to hydrolyse. Hemicellulose hydrolysed and degraded the most, followed by cellulose. The highest recoveries of hemicellulose and cellulose in solution were achieved at 160 °C, although the latter could be due to experimental error. The largest losses of hemicellulose and cellulose were recorded at 200 °C. The performance of the system in respect to hydrolysis was inferior compared to other hydrothermal systems, particularly those employing wet oxidation.     

Simultaneous recovery of vanadium and nickel from power plant fly-ash: Optimization of parameters using response surface methodology

Simultaneous recovery of vanadium (V) and nickel (Ni), which are classified as two of the most hazardous metal species from power plant heavy fuel fly-ash, was studied using a hydrometallurgical process consisting of acid leaching using sulfuric acid. Leaching parameters were investigated and optimized in order to maximize the recovery of both vanadium and nickel. The independent leaching parameters investigated were liquid to solid ratio (S/L) (5–12.5 wt.%), temperature (45–80 °C), sulfuric acid concentration (5–25 v/v%) and leaching time (1–5 h). Response surface methodology (RSM) was used to optimize the process parameters. The most effective parameter on the recovery of both elements was found to be temperature and the least effective was time for V and acid concentration for Ni. Based on the results, optimum condition for metals recovery (actual recovery of ca.94% for V and 81% for Ni) was determined to be solid to liquid ratio of 9.15 wt.%, temperature of 80 °C, sulfuric acid concentration of 19.47 v/v% and leaching time of 2 h. The maximum V and Ni predicted recovery of 91.34% and 80.26% was achieved.     

Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste

The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. In efforts to improve the treatment process, this study examined the effects of temperature on solubilization and acidogenesis. The solubilization rate of food waste, which was based on suspended solid removal, was 47.5%, 62.2%, 70.0%, 72.7%, 56.1% and 45.9% at 15 °C, 25 °C, 35 °C, 45 °C, 55 °C and 65 °C, respectively. Solubilization rate was accelerated from the middle to late experimental periods under mesophilic (35 °C and 45 °C) conditions. In contrast, overall solubilization rate was significantly lower under thermophilic (55 °C and 65 °C) conditions than under mesophilic conditions, although solubilization occurred rapidly in the early experimental period. The production of biogas was high under mesophilic conditions of 35 °C and 45 °C, at 64.7 and 62.7 mL/g-VS, respectively, while it was scarce under thermophilic conditions. Solubilization of food waste was accelerated under both mesophilic and thermophilic conditions; however, solubilization rate was observed to be particularly high under mesophilic conditions, and a shortening of the hydraulic retention time is expected under thermophilic conditions.     

Performance evaluation of a completely stirred anaerobic reactor treating pig manure at a low range of mesophilic conditions

Many Chinese biogas plants run in the lower range of mesophilic conditions. This study evaluated the performance of a completely stirred anaerobic reactor treating pig manure at different temperatures (20, 28 and 38 °C). The start-up phase of the reactor at 20 °C was very long and extremely poor performance was observed with increasing organic loading rate (OLR). At an OLR of 4.3 g ODM L^?1 d^?1, methane production at 28 °C was comparable (3% less) with that at 38 °C, but the risk of acidification was high at 28 °C. At low OLR (1.3 g ODM L^?1 d^?1), the biogas process appeared stable at 28 °C and gave same methane yields as compared to the reactor operating at 38 °C. The estimated sludge yield at 28 °C was 0.065 g VSS g^?1 COD_removed, which was higher than that at 38 °C (0.016 g VSS g^?1 COD_removed).     

Flue gas desulfurization gypsum and coal fly ash as basic components of prefabricated building materials

The manufacture of prefabricated building materials containing binding products such as ettringite (6CaO·Al₂O₃·3SO₃·32H₂O) and calcium silicate hydrate (CSH) can give, in addition to other well-defined industrial activities, the opportunity of using wastes and by-products as raw materials, thus contributing to further saving of natural resources and protection of the environment.Two ternary mixtures, composed by 40% flue gas desulfurization (FGD) gypsum or natural gypsum (as a reference material), 35% calcium hydroxide and 25% coal fly ash, were submitted to laboratory hydrothermal treatments carried out within time and temperature ranges of 2 h–7 days and 55–85 °C, respectively. The formation of (i) ettringite, by hydration of calcium sulfate given by FGD or natural gypsum, alumina of fly ash and part of calcium hydroxide, and (ii) CSH, by hydration of silica contained in fly ash and residual lime, was observed within both the reacting systems. For the FGD gypsum-based mixture, the conversion toward ettringite and CSH was highest at 70 °C and increased with curing time. Some discrepancies in the hydration behavior between the mixtures were ascribed to differences in mineralogical composition between natural and FGD gypsum.     

Effects of thermobarical pretreatment of cattle waste as feedstock for anaerobic digestion

Lab-scale experiments were conducted to assess the impact of thermobarical treatment of cattle waste on anaerobic digestion. Treatment was at temperatures of 140–220 °C in 20 K steps for a 5-min duration. Methane yields could be increased by up to 58% at a treatment temperature of 180 °C. At 220 °C the abundance of inhibitors and other non-digestible substances led to lower methane yields than those obtained from untreated material. In an extended analysis it could be demonstrated that there is a functional correlation between the methane yields after 30 days and the formation rate and methane yield in the acceleration phase. It could be proved in a regression of these correlation values that the optimum treatment temperature is 164 °C and that the minimum treatment temperature should be above 115 °C.     

Tar-free fuel gas production from high temperature pyrolysis of sewage sludge

Pyrolysis of sewage sludge was studied in a free-fall reactor at 1000–1400 °C. The results showed that the volatile matter in the sludge could be completely released to gaseous product at 1300 °C. The high temperature was in favor of H₂ and CO in the produced gas. However, the low heating value (LHV) of the gas decreased from 15.68 MJ/N m³ to 9.10 MJ/N m³ with temperature increasing from 1000 °C to 1400 °C. The obtained residual solid was characterized by high ash content. The energy balance indicated that the most heating value in the sludge was in the gaseous product.     

Characterization of products obtained from pyrolysis and steam gasification of wood waste,RDF, and RPF

Pyrolysis and steam gasification of woody biomass chip (WBC) obtained from construction and demolition wastes, refuse-derived fuel (RDF), and refuse paper and plastic fuel (RPF) were performed at various temperatures using a lab-scale instrument. The gas, liquid, and solid products were examined to determine their generation amounts, properties, and the carbon balance between raw material and products.The amount of product gas and its hydrogen concentration showed a considerable difference depending on pyrolysis and steam gasification at higher temperature. The reaction of steam and solid product, char, contributed to an increase in gas amount and hydrogen concentration. The amount of liquid products generated greatly depended on temperature rather than pyrolysis or steam gasification. The compositions of liquid product varied relying on raw materials used at 500 °C but the polycyclic aromatic hydrocarbons became the major compounds at 900 °C irrespective of the raw materials used. Almost fixed carbon (FC) of raw materials remained as solid products under pyrolysis condition whereas FC started to decompose at 700 °C under steam gasification condition.For WBC, both char utilization by pyrolysis at low temperature (500 °C) and syngas recovery by steam gasification at higher temperature (900 °C) might be practical options. From the results of carbon balance of RDF and RPF, it was confirmed that the carbon conversion to liquid products conspicuously increased as the amount of plastic increased in the raw material. To recover feedstock from RPF, pyrolysis for oil recovery at low temperature (500 °C) might be one of viable options. Steam gasification at 900 °C could be an option but the method of tar reforming (e.g. catalyst utilization) should be considered.     

Hydrodechlorination/detoxification of PCDDs,PCDFs, and co-PCBs in fly ash by using calcium polysulfide

Dioxins like polychlorinated dibenzo-p-dioxins (PCSDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are mainly emitted from waste incinerators (WIs) and have become an international research focus because of its serious concerns over the adverse health effects. The detoxification of PCCDs/Fs and PCBs is very difficult because of their stable chemical structure. A significant hydrodechlorination/detoxification of polychlorinated 1-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) were achieved in fly ash by using an aqueous mixture of calcium hydroxide and sulfur. Two different fly ashes were studied: originating from municipal waste incinerator (FA1) and industrial waste incinerator (FA2). They were heated with the aqueous mixture at 150 °C for 30 or 60 min with agitation. Higher decomposition (87%) and detoxification (87.7%) of PCDD/Fs and PCBs were achieved at 150 °C with two runs; every run was for 30 min, compared to one run for 60 min. FA2 gave higher decomposition and detoxification as compared to FA1, which might be due to higher metal content that played a catalytic role to decompose and detoxify the PCDDs, PCDFs and PCBs. The decomposition and detoxification of PCDFs in fly ash was higher than PCDDs and was augmented with increasing number of chlorides on aromatic compounds. As the highly significant decomposition and detoxification of higher concentration of PCDD/Fs and PCBs were achieved in 1 hour without additive catalyst and at low temperature of 150 °C, therefore, the developed method is cost effective and most suitable to apply on commercial/industrial level. The detail results of hydrodechlorination/detoxification of PCDD, PCDFs at different conditions are described and its mechanism is discussed.     

Effects of temperature on the release of nutrient elements of solid organic materials under conditions of oversaturation

The effects of temperature on the release of chemical components of six solid organic materials under conditions of oversaturation were investigated in this paper. The six materials were peat moss (PM), weathered coals (WC), charred rice husks (CRH), sawdust (Sd), turfgrass clippings (TC), and chicken manure (CM). Significant differences were observed in the available nitrogen and phosphorus content of the aqueous extracts of organic materials at different temperatures. The available nitrogen content in aqueous extracts of PM and WC at 25 °C was higher than that registered at 15 °C and 35 °C. Available nitrogen content in the aqueous extracts of CRH, Sd, TC, and WC at 35 °C was higher than at 15 °C and 25 °C. The available phosphorus content in the aqueous extracts of organic materials at 35 °C was higher than that available at 15 °C and 25 °C, with the exception of Sd. In addition, the release of available phosphorus in the aqueous solution of organic materials at different temperatures varied constantly for 108 h. The release of potassium (K⁺) and sodium (Na⁺) ions in the aqueous extracts of organic materials was basically steady over time, with the exception of CM. High temperature (35 °C) may significantly hasten the release of K⁺ from organic substrates (except for WC) with low temperatures significantly inhibiting release of K⁺ in Sd and CRH. High temperatures (35 °C) might significantly facilitate the release of Na⁺ in CM and TC. However, no significant differences were manifested in the release of Na⁺ from organic substrates at different temperatures, with the exception of CM and TC. Moreover, no significant differences were observed in the release of calcium, magnesium and iron ions with time, nor were there any significant differences in the contents of iron ions in the aqueous extracts of organic materials at different temperatures. The results indicate that multiple mediums should be pretreated in water for a week before being used for planting. They should be used when all mineral elements of organic materials are steady and ignoring the effect of organic mediums.     

Analysis of the combustion of sewage sludge-derived fuel by a thermogravimetric method in China

The treatment and disposal of sewage sludge are significant environmental problems in China. The reuse of sewage sludge for fuel could be an effective solution. The aim of this study was to characterize the behavior of sludge-derived fuel during combustion by a thermogravimetric method. The combustion profiles obtained showed four obvious weight loss regions. The results of dynamics analysis showed that first-order reactions together with Arrhenius’ law explained reasonably well the different stages of weight loss in the samples. Three temperature regions (162–327 °C, 367–445 °C, and 559–653 °C for sawdust and 162–286 °C, 343–532 °C, and 609–653 °C for coal) in each derivative thermogravimetry (DTG) curve corresponded well with the Arrhenius equation. The reactivity of sludge was lower than that of samples containing sawdust, but higher than that of coal-containing samples. These data demonstrate that sludge-derived fuel has better combustion characteristics than sludge, sawdust, or coal.     

Recovery of lithium and cobalt from waste lithium ion batteries of mobile phone

In view of the stringent environmental regulations, availability of limited natural resources and ever increasing need of alternative energy critical elements, an environmental eco-friendly leaching process is reported for the recovery of lithium and cobalt from the cathode active materials of spent lithium-ion batteries of mobile phones. The experiments were carried out to optimize the process parameters for the recovery of lithium and cobalt by varying the concentration of leachant, pulp density, reductant volume and temperature. Leaching with 2 M sulfuric acid with the addition of 5% H₂O₂ (v/v) at a pulp density of 100 g/L and 75 °C resulted in the recovery of 99.1% lithium and 70.0% cobalt in 60 min. H₂O₂ in sulfuric acid solution acts as an effective reducing agent, which enhance the percentage leaching of metals. Leaching kinetics of lithium in sulfuric acid fitted well to the chemical controlled reaction model i.e. 1 ? (1 ? X)^1/3 = k_ct. Leaching kinetics of cobalt fitted well to the model ‘ash diffusion control dense constant sizes spherical particles’ i.e. 1 ? 3(1 ? X)^2/3 + 2(1 ? X) = k_ct. Metals could subsequently be separated selectively from the leach liquor by solvent extraction process to produce their salts by crystallization process from the purified solution.     

Enrichment of PCDDs/PCDFs in peripheral utilities of the municipal solid waste incineration facility

This study was performed to suggest the improvements through measuring the amounts of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), re-synthesized in peripheral utilities (PUs) of a commercial-scale municipal solid waste incineration facility (MSWIF) where a few research results existed. The PUs examined in this study consisted of air pre-heaters (APHs) and gas/gas re-heater (GGRH) and kerosene-fired duct burner for selective catalytic reduction (SCR) process. PCDDs/PCDFs in flue gas were simultaneously measured at the inlet and outlet of PUs.Flue gas was cooled down from 380 °C to 249 °C by exchanging the heat with fresh air in APHs, and then heated up to 383 °C by GGRH and duct burner from 164 °C at the outlet of bag filter. The results showed that PCDDs/PCDFs were 3–4 times higher within this temperature range of PUs. In comparison of PCDDs/PCDFs concentrations at the inlet with those at the outlet of PUs, particulate-phase PCDDs/PCDFs were about 9.5–10 times enriched while gaseous-phase ones were decreased by about 33–41%. The PCDDs/PCDFs re-synthesized in the PUs, where PCDDs were relatively higher than PCDFs, showed somewhat different patterns compared to those formed at incinerators and emitted at stack. Through the investigations for PUs, we conclude that the PUs used in MSWIFs was a potential source for de novo synthesis of PCDDs/PCDFs.     

Study on the influence of various factors in the hydrometallurgical processing of waste printed circuit boards for copper and gold recovery

The present lab-scale experimental study presents the process of leaching waste printed circuit boards (WPCBs) in order to recover gold by thioureation. Preliminary tests have shown that copper adversely affects gold extraction; therefore an oxidative leaching pre-treatment was performed in order to remove base metals. The effects of sulfuric acid concentration, hydrogen peroxide volume and temperature on the metal extraction yield were studied by analysis of variance (ANOVA). The highest copper extraction yields were 76.12% for sample A and 18.29% for sample D, after leaching with 2 M H₂SO₄, 20 ml of 30% H₂O₂ at 30 °C for 3 h. In order to improve Cu removal, a second leaching was performed only on sample A, resulting in a Cu extraction yield of 90%. Other experiments have shown the negative effect of the stirring rate on copper dissolution. The conditions used for the process of gold extraction by thiourea were: 20 g/L thiourea, 6 g/L ferric ion, 10 g/L sulfuric acid, 600 rpm stirring rate. To study the influence of temperature and particle size, this process was tested on pins manually removed from computer central processing units (CPUs) and on waste CPU for 3½ h. A gold extraction yield of 69% was obtained after 75% of Cu was removed by a double oxidative leaching treatment of WPCBs with particle sizes smaller than 2 mm.     

Effect of microwave irradiation on anaerobic degradability of model kitchen waste

High temperature and pressure microwave (MW) irradiation was investigated as a pre-treatment to enhance anaerobic biodegradability and methane production from a model kitchen waste (KW). Heating rates of 7.8, 3.9 and 1.9 °C/min from room temperature to a final pre-treatment temperature of 175 °C with 1 min temperature holding time were tested. MW irradiation was successful in solubilization of particulate chemical oxygen demand (COD) resulting in higher soluble COD, protein and sugar concentrations in the supernatant phase (<0.45 μm) as well as in the whole fraction of pretreated KW compared to controls (not pretreated). Anaerobic biodegradability of the supernatant and whole fractions of pretreated KW was assessed by using a batch biochemical methane potential assay (BMP) at 33 °C. Although the highest level of solubilization was achieved at a heating rate of 1.9 °C/min, improvement in anaerobic biodegradability was observed only at the fastest heating rate of 7.8 °C/min for whole waste and for all conditions with the supernatant phase. BMP indicated increased biodegradability of between 5% and 16% for the supernatant fraction relative to controls. For the whole fraction, anaerobic biodegradability improved by 9% at a heating rate of 7.8 °C/min.     

Energy-efficient modification of reduction-melting for lead recovery from cathode ray tube funnel glass

Lead can be recovered from funnel glass of waste cathode ray tubes via reduction melting. While low-temperature melting is necessary for reduced energy consumption, previously proposed methods required high melting temperatures (1400 °C) for the reduction melting. In this study, the reduction melting of the funnel glass was performed at 900–1000 °C using a lab-scale reactor with varying concentrations of Na₂CO₃ at different melting temperatures and melting times. The optimum Na₂CO₃ dosage and melting temperature for efficient lead recovery was 0.5 g per 1 g of the funnel glass and 1000 °C respectively. By the reduction melting with the mentioned conditions, 92% of the lead in the funnel glass was recovered in 60 min. However, further lead recovery was difficult because the rate of the lead recovery decreased as with the recovery of increasing quantity of the lead from the glass. Thus, the lead remaining in the glass after the reduction melting was extracted with 1 M HCl, and the lead recovery improved to 98%.     

Experimental co-digestion of corn stalk and vermicompost to improve biogas production

Anaerobic co-digestion of corn stalk and vermicompost (VC) as well as mono-digestion of corn stalk were investigated. Batch mono-digestion experiments were performed at 35 ± 1 °C and initial total solid loading (TSL) ranged from 1.2% to 6.0%. Batch co-digestion experiments were performed at 35 ± 1 °C and initial TSL of 6% with VC proportions ranged from 20% to 80% of total solid (TS). For mono-digestion of corn stalk, a maximum methane yield of 217.60 ± 13.87 mL/g TS_added was obtained at initial TSL of 4.8%, and acidification was found at initial TSL of 6.0% with the lowest pH value of 5.10 on day 4. Co-digestion improved the methane yields by 4.42–58.61% via enhancing volatile fatty acids (VFAs) concentration and pH value compared with mono-digestion of corn stalk. The maximum biogas yield of 410.30 ± 11.01 mL/g TS_added and methane yield of 259.35 ± 13.85 mL/g TS_added were obtained for 40% VC addition. Structure analysis by X-ray diffractometry (XRD) showed that the lowest crystallinity of 35.04 of digested corn stalk was obtained from co-digestion with 40% VC, which decreased 29.4% compared to 49.6 obtained from un-treated corn stalk. It is concluded that co-digestion with VC is beneficial for improving biodigestibility and methane yield from corn stalk.     

Impact of nitrate-enhanced leachate recirculation on gaseous releases from a landfill bioreactor cell

This study evaluates the impact of nitrate injection on a full scale landfill bioreactor through the monitoring of gaseous releases and particularly N₂O emissions. During several weeks, we monitored gas concentrations in the landfill gas collection system as well as surface gas releases with a series of seven static chambers. These devices were directly connected to a gas chromatograph coupled to a flame ionisation detector and an electron capture detector (GC-FID/ECD) placed directly on the field. Measurements were performed before, during and after recirculation of raw leachate and nitrate-enhanced leachate. Raw leachate recirculation did not have a significant effect on the biogas concentrations (CO₂, CH₄ and N₂O) in the gas extraction network. However, nitrate-enhanced leachate recirculation induced a marked increase of the N₂O concentrations in the gas collected from the recirculation trench (100-fold increase from 0.2 ppm to 23 ppm). In the common gas collection system however, this N₂O increase was no more detectable because of dilution by gas coming from other cells or ambient air intrusion. Surface releases through the temporary cover were characterized by a large spatial and temporal variability. One automated chamber gave limited standard errors over each experimental period for N₂O releases: 8.1 ± 0.16 mg m^?2 d^?1 (n = 384), 4.2 ± 0.14 mg m^?2 d^?1 (n = 132) and 1.9 ± 0.10 mg m^?2 d^?1 (n = 49), during, after raw leachate and nitrate-enhanced leachate recirculation, respectively. No clear correlation between N₂O gaseous surface releases and recirculation events were evidenced. Estimated N₂O fluxes remained in the lower range of what is reported in the literature for landfill covers, even after nitrate injection.     

An advanced study on the hydrometallurgical processing of waste computer printed circuit boards to extract their valuable content of metals

This study refers to two chemical leaching systems for the base and precious metals extraction from waste printed circuit boards (WPCBs); sulfuric acid with hydrogen peroxide have been used for the first group of metals, meantime thiourea with the ferric ion in sulfuric acid medium were employed for the second one. The cementation process with zinc, copper and iron metal powders was attempted for solutions purification. The effects of hydrogen peroxide volume in rapport with sulfuric acid concentration and temperature were evaluated for oxidative leaching process. 2 M H₂SO₄ (98% w/v), 5% H₂O_2, 25 °C, 1/10 S/L ratio and 200 rpm were founded as optimal conditions for Cu extraction. Thiourea acid leaching process, performed on the solid filtrate obtained after three oxidative leaching steps, was carried out with 20 g/L of CS(NH₂)₂, 6 g/L of Fe³⁺, 0.5 M H₂SO₄, The cross-leaching method was applied by reusing of thiourea liquid suspension and immersing 5 g/L of this reagent for each other experiment material of leaching. This procedure has lead to the doubling and, respectively, tripling, of gold and silver concentrations into solution. These results reveal a very efficient, promising and environmental friendly method for WPCBs processing.