Fate of heavy metals and radioactive metals in gasification of sewage sludge

The fates of radioactive cadmium, strontium, cesium, cobalt, arsenic, mercury, zinc, and copper spiked into sewage sludge were determined when the sludge was gasified by a process that maximizes production of char from the sludge (ChemChar process). For the most part the metals were retained in the char product in the gasifier. Small, but measurable quantities of arsenic were mobilized by gasification and slightly more than 1% of the arsenic was detected in the effluent gas. Mercury was largely mobilized from the solids in the gasifier, but most of the mercury was retained in a filter composed of char prepared from the sludge. The small amounts of mercury leaving the gasification system were found to be associated with an aerosol product generated during gasification. The metals retained in the char product of gasification were only partially leachable with 50% concentrated nitric acid.     

Studies on the dechlorination and oil-production technology of waste plastics

 Recycle technology for waste plastics containing polyvinyl chloride (PVC) has been developed in the Hokkaido National Industrial Research Institute for the production of solid and liquid fuel, and has established a recycling process which includes a dechlorination process for PVC plastics, and a two-stage catalytic pyrolysis process for plastics using zeolite catalysts. The dechlorination equipment consists of a two-axis screw extruder with a heating element, which can remove chlorine up to 99.9 wt. % from PVC containing plastics as hydrogen chloride. The product had about 44 000 kJ/kg calorific value and was fed into the next oil production process, although it could also be used as a solid fuel. Natural and synthetic zeolite were used as catalysts for the two-stage catalytic process, which produced a light oil with a boiling point which was between those of kerosene and gasoline. The yield of this oil reached 82 wt. %. The chemical type was analyzed using liquid chromatography, and was found to have many aromatic compounds. These technologies make it possible to produce a nonpolluting, high-calorie solid fuel and a liquid fuel very efficiently. Received: July 19, 2000 / Accepted: September 21, 2000     

Carbon materials-catalyzed hydroliquefaction of flame-retardant plastics in organic solvents

Flame-retardant plastics, such as desktop and laptop personal computer bodies, could be completely liquefied by carbon materials-catalyzed hydroliquefaction in tetralin without using H₂ as a hydrogen source. Active carbons with larger surface areas (1450–3450 m²/g) acted as superior catalysts in transferring tetralin hydrogens to plastics. On the other hand, carbon blacks and fullerene-rich soot were less active catalysts. Graphite and mesocarbon microbeads did not show any catalytic effects. Benzene, toluene, and ethylbenzene were obtained as recyclable hydrocarbons; their total amounts varied from 4 wt% to 12 wt% depending on the types of plastics and the carbon materials used. Organic bromides such as polybromodioxins were not contained in the gases and oils of the product. Received: July 19, 2000 / Accepted: September 17, 2000     

Two-stage thermal gasification of polyolefins

Plastic pellets of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were gasified in a two-stage thermal degradation process. The first stage is the conversion of polyolefins to distilled oils using a melting vessel. In the second stage, the oils from the first stage are gasified using a tubular reactor. The distilled oil yields of PE, PP, and PS in the first stage were 84, 89, 92 wt%, respectively, each at 470°C. The total gas yields of PE, PP, and PS in the second stage were 80, 74, and 6.2 wt%, respectively, each at 800°C. The main components of the product gas for PE and PP were methane and olefins such as ethene and propene. Some aromatic oils, including benzene, toluene, and xylene, were also produced as by-products. The amount of carbonaceous residue, or coke, was very low (less than 1 wt%). By dividing the process into two stages, the coking rate was considerably reduced compared with direct gasification of the polyolefins. Received: July 19, 2000 / Accepted: September 17, 2000     

Gasification of waste plastics by steam reforming in a fluidized bed

The process of producing synthetic gas from waste plastics by steam reforming was investigated. To evaluate this process, the steam reforming of the oils derived from low-density polyethylene and polystyrene were carried out using a laboratory-scale fluidized bed of Ni-Al₂O₃ catalysts. The performance of gasification in terms of carbon conversion, gas yield, and gas compositions was examined. Although oils derived from plastics contain many kinds of heavy hydrocarbons and aromatics, they were well gasified at temperatures above 1023 K with a steam/carbon ratio of 3.5 and a weight hourly space velocity of 1 h⁻¹. The hydrogen content of the product gas was very high at approximately 72 vol% for polyethylene-derived oil and 68 vol% for polystyrene-derived oil. These compositions agreed well with the values calculated from chemical equilibrium.     

Adsorbable organic halides (AOX), AOX formation potential, and PCDDs/DFs in landfill leachate and their removal in water treatment processes

Adsorbable organic halides (AOX) and AOX formation potential (AOXFP) were investigated in 46 landfill lea-chates as potentially toxic parameters. AOX in landfill leachate was within the range <10–2200 μg Cl/l, and AOXFP was within 51–15 000 μg Cl/l. AOX and AOXFP correlated with chemical oxygen demand (COD). AOX discharge from closed landfills was generally lower than that from operating landfills. The molar ratio of AOXFP/total organic carbon (TOC) suggested that organic compounds in a leachate have a double bond every 15–190 carbons under the supposition that one chlorine would add to one double bond. The five landfills discharging high-level AOXFP (>4000 μg Cl/l) were all landfills where sludge had been dumped. The removal efficiencies of three parameters through leachate treatment processes were as follows: polychlorinated dibenzo-dioxins/dibenzo-furans (PCDDs/DFs) > TOC > AOX. PCDDs/DFs were substantially removed at p.p.t. levels, while AOX was hardly removed at relatively low levels. Received: February 14, 2000 / Accepted: January 9, 2001     

Biomass waste gasification - can be the two stage process suitable for tar reduction and power generation?

A pilot scale gasification unit with novel co-current, updraft arrangement in the first stage and counter-current downdraft in the second stage was developed and exploited for studying effects of two stage gasification in comparison with one stage gasification of biomass (wood pellets) on fuel gas composition and attainable gas purity. Significant producer gas parameters (gas composition, heating value, content of tar compounds, content of inorganic gas impurities) were compared for the two stage and the one stage method of the gasification arrangement with only the upward moving bed (co-current updraft). The main novel features of the gasifier conception include grate-less reactor, upward moving bed of biomass particles (e.g. pellets) by means of a screw elevator with changeable rotational speed and gradual expanding diameter of the cylindrical reactor in the part above the upper end of the screw. The gasifier concept and arrangement are considered convenient for thermal power range 100-350 kW_th. The second stage of the gasifier served mainly for tar compounds destruction/reforming by increased temperature (around 950 °C) and for gasification reaction of the fuel gas with char. The second stage used additional combustion of the fuel gas by preheated secondary air for attaining higher temperature and faster gasification of the remaining char from the first stage. The measurements of gas composition and tar compound contents confirmed superiority of the two stage gasification system, drastic decrease of aromatic compounds with two and higher number of benzene rings by 1-2 orders. On the other hand the two stage gasification (with overall ER = 0.71) led to substantial reduction of gas heating value (LHV = 3.15 MJ/Nm³), elevation of gas volume and increase of nitrogen content in fuel gas. The increased temperature (>950 °C) at the entrance to the char bed caused also substantial decrease of ammonia content in fuel gas. The char with higher content of ash leaving the second stage presented only few mass% of the inlet biomass stream.     

Co-gasification of wood and polyethylene with the aim of CO and H2 production

This article describes the gasification of polyethylene–wood mixtures to form syngas (H₂ and CO) with the aim of feedstock recycling via direct fermentation of syngas to ethanol. The aim was to determine the effects of four process parameters on process properties that give insight into the efficiency of gasification in general, and particularly into the optimum gasification conditions for the production of ethanol by fermentation of producer gas. Gasification experiments (fluidized bed, 800°–950°C) were done under different conditions to optimize the composition of syngas suitable for fermentation purposes. The data obtained were used for statistical analysis and modeling. In this way, the effect of each parameter on the process properties was determined and the model was used to predict the optimum gasification conditions. The parameters varied during the experiment were gasification temperature, equivalence ratio, the ratio of plastic to wood in the feed, and the amount of steam added to the process. The response models obtained proved to be statistically significant in the experimental domain. The optimum gasification conditions for maximization of carbon monoxide and hydrogen production were identified. The conditions are: temperature 900°C, equivalence ratio 0.15, amount of plastic in the feed 0.11 g/g feed, and amount of steam added 0.42 g/g feed. These optimum conditions are at the edge of the present experimental domain. The maximum combined CO and H₂ efficiency was 42%, and for the maximum yield of CO and H₂ it is necessary to minimize the polyethylene content, minimize the added steam and the equivalence ratio, and maximize temperature.     

Modelling of a downdraft gasifier fed by agricultural residues

A non-stoichiometric model for a downdraft gasifier was developed in order to simulate the overall gasification process. Mass and energy balances of the gasifier were calculated and the composition of produced syngas was predicted. The capacity of the modeled gasifier was assumed to be 0.5 MW, with an Equivalence Ratio (EQ) of 0.45. The model incorporates the chemical reactions and species involved, while it starts by selecting all species containing C, H, and O, or any other dominant elements. Olive wood, miscanthus and cardoon were tested in the formulated model for a temperature range of 800-1200 °C, in order to examine the syngas composition and the moisture impact on the supplied fuel. Model results were then used in order to design an olive wood gasification reactor.     

Metallic-phase lead in slag of municipal solid waste incineration ash and leaching characteristics

Metallic phases in slags and their influence on the leaching characteristics were investigated. The proportions of metallic phase in four slags were 0.028%, 0.24%, 1.87%, and 3.05% by weight. The lead content was 10–248 mg/kg in bulk slag after metal removal, while in the metallic phase it was 579–7390 mg/kg. Lead concentrations in the metallic phase were more than ten times higher than in slags after metal removal. Lead was distributed in the metallic phase at 2.0%, 8.3%, 10.3%, and 47.4%. The concentrations of all metallic elements in metallic phases were much higher than in bulk slag. Iron, copper, and nickel had accumulated in magnetic metals, while aluminum and zinc were found in nonmagnetic metals. As regards chromium, manganese, lead, and tin, the proportion of metallic phases depended on the slag samples. By removing metallic phases, both water and pH 4 leachable lead decreased. The basic principles of melting residues containing lead are the separation of lead as a metal in reductive melting, and the containment of lead ions into uniform glassy particles in oxidization melting. Melting slag can be seen to contribute to environmental preservation by facilitating the recycling of materials through the separation of metals from melting slag. Received: February 21, 2000 / Accepted: July 27, 2000     

Simple removal of dioxins by injecting combustion gas into water

A simple, low-cost method for suppression of dioxins/furans (hereinafter referred to as dioxins) is required because many middle- and, especially, small-scale incinerators have fallen into disuse or have been dismantled because of the high running and system costs of measures for the suppression of dioxins. Therefore, the purpose of the present study was to develop a simple removal method for dioxins from combustion gas and to evaluate the basic removal rate of dioxins. The removal method for suspended matter in a gas mixture (cold model) and dioxins in exhaust gases (hot model) has been investigated by means of gas injection into water, the mechanism of which is that the suspended matter in the gas gathers at the gas–liquid interface. In the cold model, the removal ratio of fine particles (R_P) by gas injection into water was reproduced well by the following equation: R_P (%) = 100 × {1−exp(−0.8 · S_S · t_C)}, where S_S (cm²/cm³) is the specific surface area of bubbles and t_C (s) is the residence time of bubbles in water. The removal ratio of fine particles increased as the product S_s · t_C increased. In a hot model using the exhaust gas from combustion experiments of polyvinyl chloride, the removal ratio of dioxins (R_D) by injecting the exhaust gas into water was estimated by the following equation: R_D (%) = 100 × {1−exp(−0.8 · S_S · t_C · C_D0 ^0.07)}, where C_D0 [ng/cm³ (at standard temperature and pressure)] is the dioxins concentration in the exhaust gas before injection into water. R_D depends greatly on the specific surface area of bubbles and the residence time of the bubbles in water, and only weakly on the dioxins concentration in the exhaust gas. Injection of the exhaust gas into water has been shown to be effective and was evaluated as a simple method for the removal of dioxins from exhaust gas.     

Gasification characteristics of sewage sludge combined with wood biomass

The gas products from gasification processes have been considered to have some limitations in gas composition and heating value from the previous studies. Gasification characteristics of sewage sludge and wood mixture were investigated using different mixing ratios with the purpose of better quality of gas product suitable for energy/power generation. The gasification experiment was performed by an indirectly heated fluidized bed reactor. As reaction temperature increased from 600 to 900 °C, the yield of gas product increased with higher generation of CO, H₂ and CH₄ by more activated gas conversion reactions. As the equivalence ratio increased from 0.2 to 0.4, composition ratio of CO₂ increased while CO, CH₄, H₂ decreased as expected. Several operating variables including mixing ratio of wood with dried sludge were also tested. From this initial stage of experiment, optimal operating conditions for the bubbling fluidized bed gasifier, could be considered 900 °C in temperature; 0.2 in equivalence ratio and 40 % in wood mixing ratio within test variables range. These results will be more thoroughly investigated for the application to the larger scale pilot system.     

Decomposition of mixed plastics consisting of polypropylene and polyethylene terephthalate into oils over titania/silica catalysts

The catalytic decomposition of mixed plastics consisting of polypropylene (PP) and polyethylene terephthalate (PET) has been investigated over titania/silica catalysts at 698 K. The yield of oil produced was about 70%, and the large amounts of C₁₈₊ hydrocarbons this contained was from the aromatics in PET. Gas was also produced, including C₃–C₅ hydrocarbons. The carbon-number fractions in the oil was influenced by the PET/(PP + PET) ratios and the catalyst weight. The titania/silica catalysts could be used repeatedly, and after they had been fouled, could be regenerated. From the Fourier Transform Infrared (FT–IR) spectroscopic data of adsorbed pyridine on the catalyst surface, most of the acid sites of the titania/silica catalysts were found to be Lewis sites where the hydride abstracted from PP pyrolysates react with PET pyrolysates to form oil and gas. Received: July 19, 2000 / Accepted: October 20, 2000     

Polyethylene conversion by partial oxidation in supercritical water

This paper gives the results of partial oxidation experiments of polyethylene (PE) in supercritical water (SCW). The experiments were carried out at a reaction temperature of 693K and a reaction time of 30 min using 6 cm³ of a batch-type reactor. The loaded sample weight was 0.3 g and there was 2.52 g water (0.42 g/cm³). The ratio of oxygen atoms to carbon atoms was 0.3. The results show a significant CO formation in O₂–SCW, and the 1-alkene/n-alkane ratio in partial oxidation was higher than that in SCW pyrolysis. These results suggest the possibility of the hydrogenation of hydrocarbon through partial oxidation followed by a water–gas shift reaction. Received: July 19, 2000 / Accepted: September 28, 2000     

Effect of sewage sludge content on gas quality and solid residues produced by cogasification in an updraft gasifier

In the present work, the gasification with air of dehydrated sewage sludge (SS) with 20wt.% moisture mixed with conventional woody biomass was investigated using a pilot fixed-bed updraft gasifier. Attention was focused on the effect of the SS content on the gasification performance and on the environmental impact of the process. The results showed that it is possible to co-gasify SS with wood pellets (WPs) in updraft fixed-bed gasification installations. However, at high content of sewage sludge the gasification process can become instable because of the very high ash content and low ash fusion temperatures of SS. At an equivalent ratio of 0.25, compared with wood pellets gasification, the addition of sewage sludge led to a reduction of gas yield in favor of an increase of condensate production with consequent cold gas efficiency decrease. Low concentrations of dioxins/furans and PAHs were measured in the gas produced by SS gasification, well below the limiting values for the exhaust gaseous emissions. NH(3), HCl and HF contents were very low because most of these compounds were retained in the wet scrubber systems. On the other hand, high H(2)S levels were measured due to high sulfur content of SS. Heavy metals supplied with the feedstocks were mostly retained in gasification solid residues. The leachability tests performed according to European regulations showed that metals leachability was within the limits for landfilling inert residues. On the other hand, sulfate and chloride releases were found to comply with the limits for non-hazardous residues.     

Reduction of tar using cheap catalysts during sewage sludge gasification

Solid-fuel conversion or gasification study of sewage sludge and energy recovery has become increasingly important because energy recovery and climate change are emerging issues. Various types of catalysts, such as dolomite, steel slag and calcium oxide, were tested for tar reduction during the sewage sludge gasification process. For the experiments on sewage sludge gasification reactions and tar reduction using the catalysts, a fixed bed of laboratory-scale experimental apparatus was set up. The reactor was made of quartz glass using an electric muffle furnace. The sewage sludge samples used had moisture contents less than 6%. The experimental conditions were as follows: sample weight was 20 g and reaction time was 10 min, gasification reaction temperature was from 600 to 800°C, and the equivalence ratio was 0.2. The quantity of catalysts was 2–6 g, and temperatures of catalyst layers were 500–700°C. As the reaction temperature increased up to 800°C, the yields of gaseous products and liquid products increased, whereas char and tar products decreased, showing effects on gas product compositions. These results were considered to be due to the increase of the water-gas reaction and Boudouard reaction. In the case of experiments with catalysts, dolomite (4 g), steel slag (6 g) and calcium oxide (6 g) were used. When the temperature of catalysts increased, the weight of the tar produced decreased with different cracking performances by different catalysts. Reforming reactions were considered to occur on the surface of dolomite, steel slag and calcium oxide, causing cracking of the hydrocarbon structure, which eventually showed reduced tar generation.     

An innovative example of herb residues recycling by gasification in a fluidized bed

A utilization way of herb residues is designed to convert herb residues to gas fuel in industrial-scale by a circulating fluidized bed gasifier in this paper. The product gas is used in the production of Chinese medicine, and the heat of the flue gas from the boiler can be used in herb residues drying to realize the energy recycling and no herb residues discharge. The gasification characteristics of herb residues in the circulating fluidized bed of 300 kg/h were investigated for about 200 h. The results indicated that the gas composition and tar yield were affected by biomass flow rate, equivalence ratio (ER), moisture content and char circulating. The lower heating value of product gas was 4–5 MJ/m³ using herb residues as feedstock. When mean biomass flow rate was at 5.5 kg m^?2 s^?1 and ER at 0.35, the product gas reached a good condition with lower heating value of 4.89 MJ/m³ and cold gas efficiency of 62.36%. When the moisture content changed from 12.5% to 18.7%, the concentrations of H₂, CO and CO₂ changed from 4.66% to 6.92%, 11.23% to 10.15%, and 16.55% to 17.82% respectively, and the tar content in gas decreased from 15.1 g/m³ to 14.4 g/m³ when the moisture content increased from 12.5% to 15.4%. There are metal oxides in the ash of herb residues, especially CaO, MgO, K₂O, Al₂O₃, and Fe₂O₃ which have obvious function on tar catalytic decomposition. The ash that attaches to the char particles can decrease the tar yield and improve the quality of gas after returning to the gasifier.     

Porous structure of activated carbon prepared from waste newspaper

In an earlier report, we presented a method for preparing activated carbon from waste newspaper as a way to recycle used paper for a purpose other than producing raw material for paper-making. In this article, we consider the porous structure of the activated carbon that affects its adsorption properties for various substances. The specific surface area of activated carbon prepared from waste newspaper was 838 m²/g, the micropore volume was 0.368 ml/g, and the mesopore volume was 0.138 ml/g, which is about the same as commercially available activated carbon. The activated carbon prepared from waste newspaper usually has a high content of ash, which makes little contribution to the adsorption properties. In particular, as compared with reagent-activated carbon, the quantities of mesopores with a pore radius of 3–25 nm and macropores with a pore radius of 25–250 nm were 8 and 14 times larger, respectively. Activated carbon prepared from waste newspaper has a well-developed porous structure, particularly in the mesopore and macropore ranges. Received: July 12, 1999 / Accepted: March 8, 2000     

Improved quartz furnace method for chlorine and sulfur determination in municipal solid waste

 A method of determining the chlorine (Cl) and sulfur (S) in municipal solid waste (MSW) was studied. The quartz furnace method was improved in two ways: recovery from ash by hot extraction with dilute nitric acid, and avoidance of the volatilization of alkali (earth) metal chlorides by setting the sample combustion temperature at 600°C. In a comparison with the bomb method, using nine sets of kitchen garbage and waste plastics, the bomb method yielded a 15%–25% lower value than the improved quartz furnace method. Combustion in the bomb was frequently incomplete, resulting in recovery losses of Cl and S. The average kitchen garbage involved 5.2 mg Cl/g, of which at least 24.1% would be converted to HCl. Plastics contained 23 mg Cl/g generating 88.1% HCl on average. In the same way, kitchen garbage contained 3.0 mg S/g, generating 52.3% SO _x , whereas plastics contained 1.1 mg S/g with 55.1% SO _x formation. Received: March 20, 2002 / Accepted: October 13, 2002     

Behavior of bromine on the thermal decomposition of paper-based phenolic laminate wastes

We investigated the thermal properties and behavior of bromine on the thermal decomposition of paper-based phenolic laminate wastes containing polybrominated flame retardants. The thermal properties of the phenolic laminate wastes were measured with a thermogravimeter and a conduction-type scanning calorimeter (TG-CSC). The weight loss of the wastes on thermal decomposition was mainly in three phases between 40°C and 600°C. The enthalpy (ΔH) of the thermal decomposition was 104 cal/g. The material balance of the decomposition components was measured with batch-type thermal decomposition equipment. The ratios of carbon residue, liquid, and gas on decomposition at 800°C in a vacuum were 37 wt. %, 48 wt. %, and 15 wt. %, respectively. The bromine contents in the carbon residue and liquid were less than 0.02 wt. % and 10 wt. %, respectively. These results are useful both in the carbonization process of these wastes and in the application of carbon residue as carbon materials. Received: August 11, 2000 / Accepted: March 7, 2001