Steam gasification of tyre waste, poplar, and refuse-derived fuel: a comparative analysis

In the field of waste management, thermal disposal is a treatment option able to recover resources from "end of life" products. Pyrolysis and gasification are emerging thermal treatments that work under less drastic conditions in comparison with classic direct combustion, providing for reduced gaseous emissions of heavy metals. Moreover, they allow better recovery efficiency since the process by-products can be used as fuels (gas, oils), for both conventional (classic engines and heaters) and high efficiency apparatus (gas turbines and fuel cells), or alternatively as chemical sources or as raw materials for other processes. This paper presents a comparative study of a steam gasification process applied to three different waste types (refuse-derived fuel, poplar wood and scrap tyres), with the aim of comparing the corresponding yields and product compositions and exploring the most valuable uses of the by-products.     

Steam gasification of epoxy circuit board in the presence of carbonates

To recover useful metals from end-of-life electronic devices and to convert plastic components from these devices into clean fuel gas, steam gasification of epoxy board samples was carried out at 600–700?°C and 0.1?MPa pressure in the presence of a ternary eutectic carbonate (lithium carbonate, sodium carbonate, and potassium carbonate). Hydrogen and carbon dioxide were the main products, and methane and carbon monoxide were detected as minor products. The gasification proceeded in two steps: an initial rapid pyrolysis followed by secondary steam gasification of char produced from the pyrolysis. The ternary eutectic carbonate accelerated not only the latter steam gasification but also the initial rapid pyrolysis. The activation energy for the steam gasification of epoxy board samples in the presence of the carbonate was 122?kJ/mol.     

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.     

Influence of mixed molten carbonate composition on hydrogen formation by steam gasification

Steam gasification in the presence of carbonate compounds is an effective method to recover useful materials from electronic waste streams by converting plastics into gaseous products that can be used for energy production and avoiding the expensive manual disassembly process. We investigated steam gasification of activated carbon in the presence of various mixtures of lithium carbonate, sodium carbonate, and potassium carbonate. The activated carbon was almost completely converted into hydrogen and carbon dioxide at 700°C under 0.1 MPa pressure in the presence of carbonate mixtures. Carbon dioxide was also derived from partial decomposition of lithium carbonate. Steam gasification was accelerated in the presence of various carbonate mixtures and at increasing steam partial pressures. These experimental results show that fluidity of carbonates, the potassium content of the carbonate, and the steam partial pressure are important factors in accelerating steam gasification.     

Energy recovery from sewage sludge by means of fluidised bed gasification

Because of its potential harmful impact on the environment, disposal of sewage sludge is becoming a major problem all over the world. Today the available disposal measures are at the crossroads. One alternative would be to continue its usage as fertiliser or to abandon it. Due to the discussions about soil contamination caused by sewage sludge, some countries have already prohibited its application in agriculture. In these countries, thermal treatment is now presenting the most common alternative. This report describes two suitable methods to directly convert sewage sludge into useful energy on-site at the wastewater treatment plant. Both processes consist mainly of four devices: dewatering and drying of the sewage sludge, gasification by means of fluidised bed technology (followed by a gas cleaning step) and production of useful energy via CHP units as the final step. The process described first (ETVS-Process) is using a high pressure technique for the initial dewatering and a fluidised bed technology utilising waste heat from the overall process for drying. In the second process (NTVS-Process) in addition to the waste heat, solar radiation is utilised. The subsequent measures--gasification, gas cleaning and electric and thermal power generation--are identical in both processes. The ETVS-Process and the NTVS-Process are self-sustaining in terms of energy use; actually a surplus of heat and electricity is generated in both processes.     

Evaluating waste disposal systems

Waste disposal systems conventionally exhibit many problems, such as difficulties in finding final disposal sites for incinerator residues and the issue of how to recycle waste materials. Some new technologies have been developed to solve such problems, including ash melting and gasification melting. Furthermore, to improve the power generation efficiency of waste treatment facilities so that their energy is used more efficiently, combined stoker/gas turbine power generation (super waste power generation) technology has been developed. Through examination of two cases in this study, environmental impacts and costs were determined using lifecycle assessment (LCA) and lifecycle cost (LCC) methods in a model city. In case 1, a stoker furnace was compared to a combined stoker/gas turbine system. In case 2, a stoker furnace plus ash melting system was compared to a gasification melting system. The results demonstrate that the stoker furnace has a lower environmental impact than the combined stoker/gas turbine system in case 1, and that the stoker plus ash melting system costs less than the gasification melting system in case 2, but both systems had strong impacts on the environment.     

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.     

Gasification of dried sewage sludge: status of the demonstration and the pilot plant

The disposal of sewage sludge from municipal waste water treatment plants is suffering from raising costs.The gasification is an alternative way of treatment, which can reduce the amount of solid residues that must be disposed from a water treatment plant. The produced gas can be used very flexible to produce electrical energy, to burn it very cleanly or to use it for upgrading.The gasification in the fluidised bed and the gas cleaning with the granular bed filter has shown successful operation. A demonstration plant in Balingen was set up in 2002 and rebuilt to a larger throughput in 2010. As a next step a demonstration plant was built in Mannheim and is now at the end of the commissioning phase. Nowadays the product gas is blended with biogas from sludge fermentation and utilized in a gas engine or combustion chamber to produce heat. In the future the process control for a maximized efficiency and the removal of organic and inorganic impurities in the gas will be further improved.     

Hydrogen-rich synthesis gas production from waste wood via gasification and reforming technology for fuel cell application

The purpose of this study was to establish a fuel process for an advanced power generation system in which hydrogen-rich synthesis gas, as the fuel for the molten carbonate fuel cell (MCFC), can be extracted from biomass via gasification and reforming technologies. Experiments on waste wood gasification were performed using a bench-scale gasification system. The main factors influencing hydrogen generation in the noncatalytic process and in the catalytic process were investigated, and temperature was identified as the most important factor. At 950°C, without employing a catalyst, hydrogen-rich synthesis gas containing about 54 vol% hydrogen was extracted from feedstock with appropriately designed operation parameters for the steam/carbon ratio and the equivalence ratio. However, by employing a commercial steam reforming catalyst in the reforming process, similar results were obtained at 750°C.     

Biocrude oils from the fast pyrolysis of poultry litter and hardwood

The safe and economical disposal of poultry litter is becoming a major problem for the USA poultry industry. Current disposal methods such as land application and feeding to cattle are now under pressure because of pollution of water resources due to leaching, runoffs and concern for mad cow disease contamination of the food chain. Incineration or combustion is potentially applicable to large scale operations, but for small scale growers and EPA non-attainment areas, this is not a suitable option because of the high cost of operation. Thus, there is a need for developing appropriate technologies to dispose poultry litter.Poultry litters from broiler chicken and turkey houses, as well as bedding material were converted into biocrude oil in a fast pyrolysis fluidized bed reactor. The biocrude oil yields were relatively low ranging from 36 wt% to 50 wt% depending on the age and bedding material content of the litter. The bedding material (which was mostly hardwood shavings) biocrude oil yield was 63 wt%. The higher heating value (HHV) of the poultry litter biocrude oils ranged from 26 MJ/kg to 29 MJ/kg while that of the bedding material was 24 MJ/kg. The oils had relatively high nitrogen content ranging from 4 wt% to 8 wt%, very low sulfur (<1 wt%) content and high viscosity. The viscosities of the oils appeared to be a function of both the source of litter and the pyrolysis temperature. The biochar yield ranged from 27 wt% to 40 wt% depending on the source, age and composition of the poultry litter. The biochar ash content ranged from 24 wt% to 54 wt% and was very rich in inorganic components such as potassium and phosphorous.     

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.     

Gasification of refuse derived fuel in a fixed bed reactor for syngas production

Steam gasification of two different refuse derived fuels (RDFs), differing slightly in composition as well as thermal stability, was carried out in a fixed-bed reactor at atmospheric pressure. The proximate and ultimate analyses reveal that carbon and hydrogen are the major components in RDFs. The thermal analysis indicates the presence of cellulose and plastic based materials in RDFs. H2 and CO are found to be the major products, along with CO2 and hydrocarbons resulting from gasification of RDFs. The effect of gasification temperature on H2 and CO selectivities was studied, and the optimum temperature for better H2 and CO selectivity was determined to be 725 degrees C. The calorific value of product gas produced at lower gasification temperature is significantly higher than that of gas produced at higher process temperature. Also, the composition of RDF plays an important role in distribution of products gas. The RDF with more C and H content is found to produce more amounts of CO and H2 under similar experimental conditions. The steam/waste ratio showed a notable effect on the selectivity of syngas as well as calorific value of the resulting product gas. The flow rate of carrier gas did not show any significant effect on products yield or their distribution.     

Formation characteristics of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and coplanar PCBs in fly ash from a gasification–melting process of municipal solid waste

PCDDs/DF and Co-PCB (dioxin) formations were studied with ash from a newly developed gasification and melting process for municipal solid waste. Ash samples were heated in a laboratory-scale fixed-bed reactor. Emphasis was placed on the effects of the type and composition of ash, temperatures, gas residence time, and gaseous organic precursors. Investigations using macroscopic and homologue distribution analyses led to the following conclusion. The ash from the gasification–melting process had the ability to generate dioxins in flue gas. A possible carbon source is unburned carbon in the ash samples, although this was very low (less than 0.01%). An experimental result that the level of dioxins generated from preheated fly ash obtained from a conventional incinerator was much lower than that from nonheated fly ash supported this conclusion. Dioxin concentrations obviously showed temperature dependence and peaked at 350°C. Dioxins formed in a gasification–melting process ash were readily desorbed from the surface, probably because of the low carbon content of the ash. There was no experimental evidence that gaseous organic precursors fed to the reactor generated dioxins. Therefore, an organic precursor was not essential for the formation of dioxins. A good linear relationship obtained between PCDDs/DFs and gas residence time also supported the assumption. Received: February 14, 2000 / Accepted: June 30, 2000     

Development of a purpose built landfill system for the control of methane emissions from municipal solid waste

In the present paper, a new system of purpose built landfill (PBLF) has been proposed for the control of methane emissions from municipal solid waste (MSW), by considering all favourable conditions for improved methane generation in tropical climates. Based on certain theoretical considerations multivariate functional models (MFMs) are developed to estimate methane mitigation and energy generating potential of the proposed system. Comparison was made between the existing waste management system and proposed PBLF system. It has been found that the proposed methodology not only controlled methane emissions to the atmosphere but also could yield considerable energy in terms of landfill gas (LFG). Economic feasibility of the proposed system has been tested by comparing unit cost of waste disposal in conventional as well as PBLF systems. In a case study of MSW management in Mumbai (INDIA), it was found that the unit cost of waste disposal with PBLF system is seven times lesser than that of the conventional waste management system. The proposed system showed promising energy generation potential with production of methane worth of Rs. 244 millions/y ($5.2 million/y). Thus, the new waste management methodology could give an adaptable solution for the conflict between development, environmental degradation and natural resources depletion.     

钙基固定剂对制氢和污染性气体减排的影响

在未来相当长的一段时间内,煤气化仍是大规模制取氢气的主要途径。目前,常规煤气化过程得到的是H2、CO和CO2为主的混合气,需要通过净化、变换和分离工艺才能得到洁净的氢气,工艺过程复杂。采用连续式超临界水反应装置,以质量分数为20%的水煤浆为反应原料,考察了Ca/C摩尔比和温度对褐煤制氢系统的影响。试验结果表明：Ca（OH）2不仅可以很好地固定气相中的CO2和硫化物,而且对煤气化过程也表现出较好的催化作用。反应温度600℃,压力为25MPa的条件下,与未加Ca（OH）2相比,Ca/C摩尔比为0.45时,气体中CO2的体积分数由50.7%降至1.0%,趋于完全固定;硫化物浓度由10 878mg/m3降至807mg/m3;H2的体积分数由32.4%增至73.3%。Ca（OH）2对煤气化的催化作用在高温下更加明显。     

Characterization of trace elements in chicken and duck litter ash

For safe and sustainable management of poultry litter, it is important to evaluate and understand the chemical forms and concentrations of their constituent trace elements during treatment for disposal. This experiment was carried out to compare changes in metal (Cu, Mn, Zn, Pb and Ni) fractions in chicken and duck litter after incineration at temperatures ranging from 200 to 900 degrees C. The metals were stepwise fractionated into exchangeable, adsorbed, organically bound, carbonate precipitated and residual forms by extracting with 0.5M KNO3, de-ionized water, 0.5M NaOH, 0.05M Na2 EDTA and 4M HNO3, respectively. The content of total metal and other elements (i.e., Ca, Mg and K) were was also determined. Results showed an increasing trend in the total concentrations of metals with increasing temperature with higher amounts in chicken litter ash (CLA) than duck litter ash (DLA). Higher temperatures significantly reduced the levels of H2O-soluble Mn, Zn and Ni and enhanced those of Cu and Pb. The metal fractions extracted by EDTA and HNO3 increased directly with increasing temperature while the fraction extracted with KNO3 and NaOH decreased with ashing. For Cu, Mn, Pb and Ni, the amount extracted varied in the order EDTA>HNO3>NaOH>KNO3>H2O, but the absolute amounts differed between CLA and DLA. Peak concentrations of the total metals were achieved at the highest burning temperature. The amount of H2O soluble Ca and Mg decreased and K increased in both CLA and DLA with temperature. Total and exchangeable forms of cations increased with increasing temperature. Total Ca was highest in DLA, whereas total Mg and K were higher in CLA. This study indicated that incinerating poultry litter before soil application may have mixed effects on the vulnerable metal fractions by increasing or decreasing some fractions, depending on poultry type.     

Clay-sewage sludge co-pyrolysis. A TG-MS and Py-GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Wastewater sewage sludge was co-pyrolyzed with a well characterized clay sample, in order to evaluate possible advantages in the thermal disposal process of solid waste. Characterization of the co-pyrolysis process was carried out both by thermogravimetric-mass spectrometric (TG-MS) analysis, and by reactor tests, using a lab-scale batch reactor equipped with a gas chromatograph for analysis of the evolved gas phase (Py-GC).Due to the presence of clay, two main effects were observed in the instrumental characterization of the process. Firstly, the clay surface catalyzed the pyrolysis reaction of the sludge, and secondly, the release of water from the clay, at temperatures of approx. 450-500 °C, enhanced gasification of part of carbon residue of the organic component of sludge following pyrolysis.Moreover, the solid residue remaining after pyrolysis process, composed of the inorganic component of sludge blended with clay, is characterized by good features for possible disposal by vitrification, yielding a vitreous matrix that immobilizes the hazardous heavy metals present in the sludge.     

Landfill gas upgrading with countercurrent water wash

A pilot-scale countercurrent absorption process for upgrading landfill gas to produce vehicle fuel was studied using absorption and desorption units and water as absorbent. The height-to-diameter ratio of the absorption column used was 3:1 instead of the more conventionally used 20:1 ratio, and a higher pressure was used along with a lower water flow rate. The effects of pressure (10-30bar) and water and gas flow-ratios on the upgrading process were studied. Methane content in the product gas increased to near or above 90% with both gas flows (50 and 100l/min) used at over 20bar pressure with 10l/min water flow and at 30bar pressure with 5l/min water flow. Carbon dioxide content with these upgrading parameters ranged from 3.2% to 4.8%. The remaining fraction of the product gas was nitrogen (from 6% to 7%), while hydrogen sulphide was removed to below the detection limit with all of the upgrading parameters used. The methane content of exhaust gas increased with increasing pressure. In conclusion, the pilot-scale gas upgrading process studied here appears to be able to produce gas with high energy content (>90% methane), apparently suitable as vehicle fuel, from landfill gas.     

Demonstration study of high temperature melting for asbestos-containing waste (ACW)

The service life of many buildings and houses built using asbestos-containing materials is coming to an end and their demolition will lead to a great deal of asbestos-containing waste (ACW). Conventionally, the disposal of such waste is conducted by isolation under controlled landfill procedures; however problems with this method exist, such as the risk at the time of re-utilization of landfill sites and the depletion of lands to be reclaimed. Melting treatment is a promising technology that could be used to solve these problems; a thermal process involving temperatures exceeding the melting points of asbestos, it transforms them into non-hazardous minerals. This technology may be applicable not only for friable ACW but also for nonfriable ACW. We performed a demonstration test of melting treatment of nonfriable ACW using a gasification and melting furnace of the shaft furnace type, which is a typical method for high temperature melting. Detailed observation using transmission electron microscopy as well as general analyses verified that the same level of asbestos remained in the slag obtained by high temperature melting as that of the background soil. In addition, the asbestos concentration in the exhaust gas and the dust from the facility were at sufficiently low levels, and it was thus verified that the asbestos concentration in the atmosphere in the vicinity of the facility during the melting treatment was comparable to that of Japan’s background level reported by the Ministry of the Environment in 2007.     

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.