Process and technological aspects of municipal solid waste gasification. A review

The paper proposes a critical assessment of municipal solid waste gasification today, starting from basic aspects of the process (process types and steps, operating and performance parameters) and arriving to a comparative analysis of the reactors (fixed bed, fluidized bed, entrained bed, vertical shaft, moving grate furnace, rotary kiln, plasma reactor) as well as of the possible plant configurations (heat gasifier and power gasifier) and the environmental performances of the main commercially available gasifiers for municipal solid wastes. The analysis indicates that gasification is a technically viable option for the solid waste conversion, including residual waste from separate collection of municipal solid waste. It is able to meet existing emission limits and can have a remarkable effect on reduction of landfill disposal option.     

Production of high quality syngas from argon/water plasma gasification of biomass and waste

Extremely hot thermal plasma was used for the gasification of biomass (spruce sawdust, wood pellets) and waste (waste plastics, pyrolysis oil). The plasma was produced by a plasma torch with DC electric arc using unique hybrid stabilization. The torch input power of 100–110 kW and the mass flow rate of the gasified materials of tens kg/h was set up during experiments. Produced synthetic gas featured very high content of hydrogen and carbon monoxide (together approximately 90%) that is in a good agreement with theory. High quality of the produced gas is given by extreme parameters of used plasma – composition, very high temperature and low mass flow rate.     

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.     

Modeling and comparative assessment of municipal solid waste gasification for energy production

Gasification is the thermochemical conversion of organic feedstocks mainly into combustible syngas (CO and H₂) along with other constituents. It has been widely used to convert coal into gaseous energy carriers but only has been recently looked at as a process for producing energy from biomass. This study explores the potential of gasification for energy production and treatment of municipal solid waste (MSW). It relies on adapting the theory governing the chemistry and kinetics of the gasification process to the use of MSW as a feedstock to the process. It also relies on an equilibrium kinetics and thermodynamics solver tool (Gasify®) in the process of modeling gasification of MSW. The effect of process temperature variation on gasifying MSW was explored and the results were compared to incineration as an alternative to gasification of MSW. Also, the assessment was performed comparatively for gasification of MSW in the United Arab Emirates, USA, and Thailand, presenting a spectrum of socioeconomic settings with varying MSW compositions in order to explore the effect of MSW composition variance on the products of gasification. All in all, this study provides an insight into the potential of gasification for the treatment of MSW and as a waste to energy alternative to incineration.     

Co-gasification of municipal solid waste and material recovery in a large-scale gasification and melting system

This study evaluates the effects of co-gasification of municipal solid waste with and without the municipal solid waste bottom ash using two large-scale commercial operation plants. From the viewpoint of operation data, there is no significant difference between municipal solid waste treatment with and without the bottom ash. The carbon conversion ratios are as high as 91.7% and 95.3%, respectively and this leads to significantly low PCDD/DFs yields via complete syngas combustion. The gross power generation efficiencies are 18.9% with the bottom ash and 23.0% without municipal solid waste bottom ash, respectively. The effects of the equivalence ratio are also evaluated. With the equivalence ratio increasing, carbon monoxide concentration is decreased, and carbon dioxide and the syngas temperature (top gas temperature) are increased. The carbon conversion ratio is also increased. These tendencies are seen in both modes.Co-gasification using the gasification and melting system (Direct Melting System) has a possibility to recover materials effectively. More than 90% of chlorine is distributed in fly ash. Low-boiling-point heavy metals, such as lead and zinc, are distributed in fly ash at rates of 95.2% and 92.0%, respectively. Most of high-boiling-point heavy metals, such as iron and copper, are distributed in metal. It is also clarified that slag is stable and contains few harmful heavy metals such as lead. Compared with the conventional waste management framework, 85% of the final landfill amount reduction is achieved by co-gasification of municipal solid waste with bottom ash and incombustible residues. These results indicate that the combined production of slag with co-gasification of municipal solid waste with the bottom ash constitutes an ideal approach to environmental conservation and resource recycling.     

Statistical modelling for the prediction and control of PCDDs and PCDFs emissions from municipal solid waste incinerators

The emissions of a full range of polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs) have become one of the most controversial issues in siting and building new municipal incinerators. The lack of comprehensive evaluation of the PCDDs/PCDFs formation and emission control technologies in earlier times resulted in ambiguity in the decision-making of incineration projects. Until the last decade, several emission tests of municipal solid waste incineration regarding to new combustion criteria, public regulations, and risk assessments were conducted and reported in the literature. However, only a few analyses of statistical prediction and control have been established. This paper presents a series of multiple linear regression models for PCDDs/PCDFs emission prediction and control corresponding to different types of incinerators. The data used in the regression analysis were integrated from several testing programmes held in North America. By applying these regression results, evaluation of various combustion criteria, public regulations, and environmental and health risk assessment can then be achieved. Such evaluation is valuable for some developing countries which do not have sufficient finance or engineering experience to pursue large scale emission tests, but need rapid promulgation of emissions control for municipal solid waste incinerators.     

Current EU-27 technical potential of organic waste streams for biogas and energy production

Anaerobic digestion of organic waste generated by households, businesses, agriculture, and industry is an important approach as method of waste treatment – especially with regard to its potential as an alternative energy source and its cost-effectiveness. Separate collection of biowaste from households or vegetal waste from public green spaces is already established in some EU-27 countries. The material recovery in composting plants is common for biowaste and vegetal waste. Brewery waste fractions generated by beer production are often used for animal feeding after a suitable preparation. Waste streams from paper industry generated by pulp and paper production such as black liquor or paper sludge are often highly contaminated with toxic substances. Recovery of chemicals and the use in thermal processes like incineration, pyrolysis, and gasification are typical utilization paths. The current utilization of organic waste from households and institutions (without agricultural waste) was investigated for EU-27 countries with Germany as an in-depth example. Besides of biowaste little is known about the suitability of waste streams from brewery and paper industry for anaerobic digestion. Therefore, an evaluation of the most important biogas process parameters for different substrates was carried out, in order to calculate the biogas utilization potential of these waste quantities. Furthermore, a calculation of biogas energy potentials was carried out for defined waste fractions which are most suitable for anaerobic digestion. Up to 1% of the primary energy demand can be covered by the calculated total biogas energy potential. By using a “best-practice-scenario” for separately collected biowaste, the coverage of primary energy demand may be increased above 2% for several countries. By using sector-specific waste streams, for example the German paper industry could cover up to 4.7% and the German brewery industry up to 71.2% of its total energy demand.     

Continuous in-line gasification/vitrification process for thermal waste treatment: process technology and current status of projects

The Thermoselect High Temperature Recycling process has been developed in order to make available a thermal waste treatment technology avoiding major problems as known from traditional techniques like landfills or ashes, filter dust and emission producing processes. It combines slow degassing with fixed bed oxygen blown gasification and mineral and metal residue melting in a closed loop system. Municipal, industrial and other kinds of waste are compacted to less than one fifth of their original volume by means of an armored hydraulic press, and then periodically pushed into an indirectly heated degasification channel. As the waste plugs are pushed down the channel in an oxygen-free environment, waste humidity is evaporated and the organic components in the refuse are partially degasified and to a certain extent converted into a carbon-like product as the temperature increases. This flaky product and the enclosed inorganic components such as metals and minerals are continuously fed into a high-temperature reactor (HTR). Pure oxygen is added in controlled quantities and reacts with the material following exothermic oxidisation reactions. Due to overall under-stoichiometric conditions, gasification products form a combustible synthesis gas. The heat of reaction leading to temperatures up to about 2000°C in the core of the lower HTR section acts to also smelt the metal and mineral components of the waste. Chlorinated hydrocarbons such as dioxins and furans are reliably destroyed along with other organic compounds in the gaseous and the liquid phase. Material conversion equilibria are assured due to high temperatures and sufficient residence times. The synthesis gas is purified before use as combustible or primary material. After long term operation of the industrial scale demonstration plant in northern Italy, recent orders of differently sized Thermoselect plants can be announced and are illustrated on the basis of three cases out of five [with Herten 225,000 Mg/a and Berlin 300,000 Mg/a] in Germany: (1) Karlsruhe plant, 3 lines, 225,000 Mg/a, under construction; (2) Ansbach plant, 1 line, 75,000 Mg/a, completely purchased in July 1997; (3) Hanau plant, 2 lines, 90,000 Mg/a, partially purchased in October 1997. The technical concepts of these projects are illustrated with special emphasis on the flexibility of tailor-made energy recovery solutions.     

Studies on the pyrolysis behavior of gasification and melting systems for municipal solid waste

 It is important to investigate the pyrolysis processes of municipal solid waste (MSW) in the same way as for any mixture comprised of multiple substances. In this article, a two-reaction model for a variety of MSW mixtures is proposed to predict mass changes due to pyrolysis. In order to formulate the model based on pyrolysis kinetics, we conducted experiments to determine the kinetic model parameters. By thermal analysis of the typical components of MSW, mass changes attributable to the pyrolysis reaction were found at about 350°C for paper, 400°–500°C for plastics, and 200°–400°C for garbage (dry condition). Activation energies were obtained by the Ozawa method based on the mass changes in pyrolysis. Thus, the pyrolysis behavior is formulated as a function of temperature. Then the pyrolysis mass change of the mixture can be predicted by using a weighted sum of the individual components. The model proved useful in experiments with real waste (refuse-derived fuels). Furthermore, the weight yields (pyrolysis gas, tars, solid residues) of the mixture can be calculated by their additive property after measuring the mass balance of each component. Received: May 11, 2001 / Accepted: November 16, 2001     

Investigation of integrated municipal solid waste management strategies for Oman: an assessment of waste diversion,electricity generation and greenhouse-gas emissions

Journal of Material Cycles and Waste Management - The need of an integrated municipal solid waste (MSW) management system to maximize resource recovery and simultaneously reduce greenhouse-gas...     

Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review

The world’s waste electrical and electronic equipment (WEEE) consumption has increased incredibly in recent decades, which have drawn much attention from the public. However, the major economic driving force for recycling of WEEE is the value of the metallic fractions (MFs). The non-metallic fractions (NMFs), which take up a large proportion of E-wastes, were treated by incineration or landfill in the past. NMFs from WEEE contain heavy metals, brominated flame retardant (BFRs) and other toxic and hazardous substances. Combustion as well as landfill may cause serious environmental problems. Therefore, research on resource reutilization and safe disposal of the NMFs from WEEE has a great significance from the viewpoint of environmental protection. Among the enormous variety of NMFs from WEEE, some of them are quite easy to recycle while others are difficult, such as plastics, glass and NMFs from waste printed circuit boards (WPCBs). In this paper, we mainly focus on the intractable NMFs from WEEE. Methods and technologies of recycling the two types of NMFs from WEEE, plastics, glass are reviewed in this paper. For WEEE plastics, the pyrolysis technology has the lowest energy consumption and the pyrolysis oil could be obtained, but the containing of BFRs makes the pyrolysis recycling process problematic. Supercritical fluids (SCF) and gasification technology have a potentially smaller environmental impact than pyrolysis process, but the energy consumption is higher. With regard to WEEE glass, lead removing is requisite before the reutilization of the cathode ray tube (CRT) funnel glass, and the recycling of liquid crystal display (LCD) glass is economically viable for the containing of precious metals (indium and tin). However, the environmental assessment of the recycling process is essential and important before the industrialized production stage. For example, noise and dust should be evaluated during the glass cutting process. This study could contribute significantly to understanding the recycling methods of NMFs from WEEE and serve as guidance for the future technology research and development.     

Landfill mining and waste characterization: a strategy for remediation of contaminated areas

This article describes landfill-mining tests, including excavation, screening, and fraction characterization, carried out in the Msalycke and Gladsax landfills for municipal solid waste (MSW) in Sweden. The excavated waste in these two sites was 17–22 and 23–25 years old, respectively. The main part of Msalycke was unaffected by degradation, and during excavations no substantial amount of biogas was detected. After screening, three size fractions were obtained: <18mm, 18–50mm, and >50mm. Soil amendment and anaerobic digestion with energy extraction are suggested for the first and second fraction, respectively. Incineration with energy recovery is possible with the third fraction after any coarse (inert) material is removed, and construction/demolition waste can easily be recycled provided that it is not contaminated by hazardous material. Excavated waste taken from different depths was also analysed and compared in relation to composition, calorific value, and leachate constituents.     

Incorporation of gypsum waste in ceramic block production: Proposal for a minimal battery of tests to evaluate technical and environmental viability of this recycling process

Civil engineering-related construction and demolition debris is an important source of waste disposed of in municipal solid waste landfills. After clay materials, gypsum waste is the second largest contributor to the residential construction waste stream. As demand for sustainable building practices grows, interest in recovering gypsum waste from construction and demolition debris is increasing, but there is a lack of standardized tests to evaluate the technical and environmental viability of this solid waste recycling process. By recycling gypsum waste, natural deposits of gypsum might be conserved and high amounts of the waste by-product could be reused in the civil construction industry. In this context, this paper investigates a physical property (i.e., resistance to axial compression), the chemical composition and the ecotoxicological potential of ceramic blocks constructed with different proportions of clay, cement and gypsum waste, and assesses the feasibility of using a minimal battery of tests to evaluate the viability of this recycling process. Consideration of the results for the resistance to axial compression tests together with production costs revealed that the best formulation was 35% of plastic clay, 35% of non-plastic clay, 10% of Portland cement and 20% of gypsum waste, which showed a mean resistance of 4.64 MPa. Energy dispersive X-ray spectrometry showed calcium and sulfur to be the main elements, while quartz, gypsum, ettringite and nacrite were the main crystalline compounds found in this formulation. Ecotoxicity tests showed that leachate from this formulation is weakly toxic toward daphnids and bacteria (EC_20% = 69.0 and 75.0, respectively), while for algae and fish the leachate samples were not toxic at the EC_50% level. Overall, these results show that the addition of 20% of gypsum waste to the ceramic blocks could provide a viable substitute for clay in the ceramics industry and the tests applied in this study proved to be a useful tool for the technical and environmental evaluation of this recycling process, bacterial and daphnid tests being more sensitive than algae and fish tests.     

Fabric-filter-supported catalyst for removal of harmful solid and gaseous compounds in municipal waste incinerator exhaust gas: characteristics of NO x reduction

We have investigated a fabric-filter-supported catalyst for removing harmful solid and gaseous compounds simultaneously from municipal incinerator exhaust gas. We studied the ways in which the efficiency of NO _x removal is influenced by filter temperature, gas flow rate, and catalyst content. The reduction of the catalyst content by mechanical reverse washing was investigated, and the state of the catalyst supported in the fabric filter was also observed using a scanning electron microscope. The catalyst supported by the fabric filter showed a removal efficiency above 75% when the filter temperature was 200°C, the gas flow rate was 1 m/min, and the catalyst content of the fabric filter was above 300 g/m². The catalyst was supported uniformly on the fibers of the fabric filter, and fine catalyst particles remained on the fibers after mechanical reverse washing. Received: October 15, 1997 / Accepted: March 19, 1999     

Fluid bed gasification – Plasma converter process generating energy from solid waste: Experimental assessment of sulphur species

Often perceived as a Cinderella material, there is growing appreciation for solid waste as a renewable content thermal process feed. Nonetheless, research on solid waste gasification and sulphur mechanisms in particular is lacking. This paper presents results from two related experiments on a novel two stage gasification process, at demonstration scale, using a sulphur-enriched wood pellet feed.Notable SO₂ and relatively low COS levels (before gas cleaning) were interesting features of the trials, and not normally expected under reducing gasification conditions. Analysis suggests that localised oxygen rich regions within the fluid bed played a role in SO₂’s generation. The response of COS to sulphur in the feed was quite prompt, whereas SO₂ was more delayed. It is proposed that the bed material sequestered sulphur from the feed, later aiding SO₂ generation. The more reducing gas phase regions above the bed would have facilitated COS – hence its faster response. These results provide a useful insight, with further analysis on a suite of performed experiments underway, along with thermodynamic modelling.