Thermal treatment of waste using grate-based systems has gained world-wide acceptance as the preferred method for sustainable management of residual waste. However, in order to maintain this position and respond to new challenges and/or priorities, it is necessary to further develop innovative concepts that use safe process engineering technology in terms of climate and resource protection as well as reduction of environmental impacts. MARTIN, in collaboration with research institutes, successfully developed and optimized a multi-stage combustion process in the 1990s. Various pilot and full-scale studies and tests followed. Based on this knowledge, MARTIN and its cooperation partners COVANTA ENERGY (USA), CNIM (F) and Mitsubishi Heavy Industries (JP) developed the Very Low NO x (VLN) process as a large-scale primary measure for NO x reduction. MARTIN’s next step was to develop the Very Low NO x gasification mode (VLN-GM) process. This process has been implemented directly in continuous operation at an industrial-scale Energy-from-Waste (EfW) plant in Switzerland. In VLN-GM operation, the excess air rate in the gas above the grate is decreased from λ = 1.2 to about 0.8. The characteristics of municipal solid waste make it suitable for the generation of heat and power. While boiler concepts implemented in the past often focused on factors such as high availability, reduced downtimes and minimized maintenance costs, measures to increase the efficiency of the overall process are also growing in importance. Energy efficiency can be increased by optimizing boiler efficiency itself on the one hand, and on the other hand by improving peripheral plant devices, in particular by improving energy recovery through changes in the steam parameters. MARTIN has developed corrosion-protected wall and radiant superheater solutions, located in the upper furnace area, and installed these as prototypes in full-scale plants. As a result, steam can be heated about 35 °C (90 °F) in excess of the current state-of-the-art parameters without adversely affecting plant operation due to superheater corrosion. This paper documents that innovative concepts using MARTIN technology successfully provide solutions for a grate-based conversion technology (VLN-GM) as well as measures for increasing the energy efficiency of Energy-from-Waste plants.

For the waste disposal of urban areas and major cities at the North American market place rather large scale energy from waste (EfW) plants are needed. This implies a mechanical input of approx. 40 Mg/h [39.36 tn l./h] and thermal input by waste per unit of 110 MW [375.3 MBTU/h] and more. There are basic design criteria that feature large scale EfW plants: - Layout of boiler with horizontal or vertical orientation of convective part. - Top or bottom suspension of boiler. - Flexible design of stoker regarding large throughput figures and heating values of waste with water or air cooled grate bars. - Design and geometry of combustion furnace in order to optimize the flow pattern. - Optimization of boiler steel structure: integrated steel structure for boiler and boiler house enclosure. - Optimization of corrosion protection and maintainability of large scale boilers: cladding versus refractory lining. - Maintenance aspects of the boiler. The paper gives information on the pros and cons regarding the design features with special focus on optimized solutions for large scale EfW plants. For the core component of the combustion system — the grate — Fisia Babcock Environment (FBE) is using forward moving grates as well as roller grates. The moving grate in STEINMÜLLER design, which is used in the great majority of all our plants, has specific characteristics for providing uniform combustion and optimal burnout. The automatic combustion rate control system is the key component in the combustion process in order to receive good burn out quality in slag and flue gas as well as constant steam production and oxygen content of flue gas. This paper includes a detailed report on a modern control system with focus on a simple and efficient control structure. Besides these measures regarding the combustion process, this paper also reports about the respective aspects and concepts for the flue gas cleaning systems. In this field the FBE CIRCUSORB ® process was presented in previous papers and is now compared with a multistage wet flue gas cleaning system. The latter is relevant in case of very low emission requirements.

This paper is based on data compiled in the course of developing, for InterAmerican Development Bank (IDB), a WTE Guidebook for managers and policymakers in the Latin America and Caribbean region. As part of this work, a list was compiled of nearly all plants in the world that thermally treat nearly 200 million tons of municipal solid wastes (MSW) and produce electricity and heat. An estimated 200 WTE facilities were built, during the first decade of the 21st century, mostly in Europe and Asia. The great majority of these plants use the grate combustion of as-received MSW and produce electricity. The dominance of the grate combustion technology is apparently due to simplicity of operation, high plant availability (>90%), and facility for training personnel at existing plants. Novel gasification processes have been implemented mostly in Japan but a compilation of all Japanese WTE facilities showed that 84% of Japan’s MSW is treated in grate combustion plants. Several small-scale WTE plants (<5 tons/hour) are operating in Europe and Japan and are based both on grate combustion and in implementing WTE projects. This paper is based on the sections of the WTE Guidebook that discuss the current use of WTE technology around the world. Since the beginning of history, humans have generated solid wastes and disposed them in makeshift waste dumps or set them on fire. After the industrial revolution, near the end of the 18 th century, the amount of goods used and then discarded by people increased so much that it was necessary for cities to provide landfills and incinerators for disposing wastes. The management of urban, or municipal, solid wastes (MSW) became problematic since the middle of the 20 th century when the consumption of goods, and the corresponding generation of MSW, increased by an order of magnitude. In response, the most advanced countries developed various means and technologies for dealing with solid wastes. These range from reducing wastes by designing products and packaging, to gasification technologies. Lists of several European plants are presented that co-combust medical wastes (average of 1.8% of the total feedstock) and wastewater plant residue (average of 2% of the feedstock).

In 1989, United Power Association (now Great River Energy) and Northern States Power (now Xcel Energy) formed a partnership and entered a 20 year contract with five local counties to turn MSW (municipal solid waste) into RDF (Refuse Derived Fuel) and combust the RDF in converted grate-fired boilers in Elk River, MN. Great River Energy owned and operated the Energy Recovery Station (ERS) and Xcel Energy operated the Resource Processing Plant (RPP) a few miles away. The Resource Processing Plant processed 400,000 tons/year of MSW into RDF for the Energy Recovery Station and other RDF plants owned by Xcel Energy. The project was successful, but required significant subsidies from the counties to maintain competitive tipping fees. At the end of the original 20 year contract, a number of the counties wanted to reduce or end any subsidies and restructure the contracts. In the fall of 2009, lack of contracted MSW created difficult financial conditions that threatened to end the project and divert 400,000 tons/year of MSW to area landfills. In May of 2010, Great River Energy purchased the Resource Processing Plant and reorganized the project to be able to better control operating costs and maintain competitive electric rates for its customers. In 2011, Great River Energy restructured processing contracts with three of the original counties and also directly contracted with the regional MSW haulers while implementing sweeping changes in the processing of MSW. A cleaning system was installed to increase the value of the ferrous material collected during the production of RDF. The installation of a bulky waste shredder and processing changes increased the efficiency of converting MSW to RDF. In addition, the recovery of non-ferrous materials from the MSW and heavy residue was optimized. In one year of operation, the Resource Processing Plant has increased RDF production from 84% to over 95% and decreased landfilling to near zero while increasing the revenue from recovered materials. County subsidies have been significantly reduced and will phase out after 2015, tipping fees have been adjusted to be competitive with local landfills, and electric costs have been stabilized at comparable renewable energy rates.

Wheelabrator Technologies Inc. (WTI) operates a waste-to-energy facility in Portsmouth, Virginia. At full capacity, a total of 2,000 tons/day of refuse derived fuel (RDF) can be fired in four identical boilers to generate a total of 600,000 lb/hr of steam and 60 MW of electricity. The boilers were originally designed to co-fire RDF and coal; however, coal burning capability was removed a few years after commissioning. The plant provides all of the process/heating steam and the majority of the electrical power to the nearby Norfolk Naval Shipyard. Historically, the boilers had not been able to reliably achieve carbon monoxide (CO) emissions compliance. CO emissions experienced during normal boiler operation would be more than twice the mandated emission limit. WTI’s goal was to improve the boilers’ CO emissions performance while achieving sustained boiler operation at higher steam generation and RDF firing rates. WTI contracted Jansen Combustion and Boiler Technologies, Inc. (JANSEN) to evaluate the operation of the boilers, to assess the overall feasibility of meeting WTI’s goals, and to develop design concepts to overcome boiler limitations. The project was initiated by an engineering site visit where boiler operating data was collected and evaluated to develop a baseline of boiler operation. Current and new combustion system arrangements were evaluated with Computational Fluid Dynamics (CFD) modeling. The results confirmed that the root cause of the poor CO emissions performance was the inadequate penetration and mixing of the original overfire air (OFA) system (comprised of multiple rows of small ports on the front and rear furnace walls). CFD modeling also showed increased CO emissions to result from non-uniform RDF delivery profiles generated by the original fuel distributors that were installed at a high elevation over the grate. Modeling of the furnace with larger and fewer OFA nozzles placed on the side walls in an interlaced pattern, and the installation of “new-style” RDF distributors at a lower elevation where the boiler’s original coal distributors formerly were located was shown to significantly improve CO burnout. From December 2010 to May 2011, the new combustion systems were installed on all four boilers. Subsequent testing has shown that CO levels have been lowered by more than 70% and boiler availability has been significantly improved. Nitrogen oxides (NO x ) emissions, although slightly higher following the upgrade, are still within the NO x compliance limit. This paper describes the process that led to a successful project, including: data collection and analyses, CFD modeling, equipment design and supply, operator training, and start-up assistance.

Energy-from-Waste plants using grate-based systems have gained world-wide acceptance as the preferred method for the sustainable treatment of waste. Key factors are not only the reduction of waste volume and mass and the destruction or separation of pollutants but also the efficient production and use of energy (electricity, district heating/cooling, process steam), compliant disposal and the recovery of resources from combustion residues (e.g. metals, rare earths). International requirements relating to energy efficiency and materials recovery by means of thermo-recycling in Energy-from-Waste plants call for the continuing development and optimization of existing technologies and concepts. The technologies and processes for the recovery of reusable materials from dry-discharged bottom ash and from filter ash point to the key role that Energy-from-Waste plants are able to play in the efficient conservation of resources. It is primarily thermal treatment with dry discharge and subsequent processing of the bottom ash fractions that enables Energy-from-Waste plants to justify their status as universal recyclers. In addition to recovery of the energy inherent in the waste, the treatment of dry-discharged bottom ash is an important contribution to compliance with raw material and climate policies and to the promotion of closing the material cycle in general. Furthermore, dry bottom ash discharge represents a further step towards waste-free operation and “after-care-free” landfills. This paper documents the potential of Energy-from-Waste plants for the recovery of resources and provides examples of recent developments and large-scale implementations of innovative recovery technologies in Europe.

China has the largest population (1.33 billion) on Earth and a 2010 GDP of $5.4 trillion. This nation has experienced rapid economic growth in the last decade that has been accompanied by the generation of an enormous amount of municipal solid wastes. From 2000 to 2009, the reported MSW increased by 33% to 157 million tons. This paper presents the current situation in MSW generation, characterization, and means of disposal, based on the results of studies by WTERT (www.wtert.org) in China. The landfills serving the large cities of China are reaching or have already reached full capacity and there is strong government support for the waste to energy (WTE) alternative, resulting in over 90 WTE plants built or under construction. The thermal treatment technologies are based mostly on imported or domestic grate combustion technologies and on fluid bed combustion of shredded wastes. Of particular interest to the WTERT studies have been the Air Pollution Control systems used in Chinese plants and their performance, in particular the dioxin and furan levels attained, in view of continuing public opposition to WTE in Beijing and some other cities. The cities of Guangzhou, Shanghai, and Beijing were visited to examine any obstacles to further expansion of the WTE industry in China. There are extreme differences in the composition of MSW as well as waste management from region to region. It is believed that one of the reasons for public opposition to WTE projects is inadequate transparency as to the emissions of WTE plants. Also, it appears that some WTE facilities tend to cut down costs at the expense of adequate emission control. The paper concludes with discussion of the economics of Chinese WTE plants built in the last six years.

The grate systems of waste-to-energy (WTE) mass-burn combustion chambers, which historically stem from coal combustion technology, have an important role in controlling the mixing of heterogeneous MSW during the combustion process. They are designed for providing efficient flow and mixing of Municipal Solid Waste (MSW) in the combustion chamber. This study presents results from a numerical analysis for grate design and chamber operation, i.e., number of reciprocating bars and reciprocation speed that influence the degree of mixing and residence time of MSW particles. A particle-based bed model of MSW and a physical model of reverse-acting grate were used in order to quantify the mixing diffusion coefficient of MSW particles. We analyzed the particle mixing with different parameters: particle size (d = 6–22 cm diameter), reciprocation speed of moving bars (R r = 0–90 recip./h), and number of moving bars (N b = 1–16 bars). According to the size segregation in the particle mixing process in the MSW bed, the undersized waste particles in the MSW bed on the reverse-acting grate have a higher diffusion coefficient than those of oversized and mean size particles. Also the number of moving bars, N b , as well as reciprocation speed, R r , were quantitatively related to a diffusion coefficient equation for MSW particle mixing.

The efforts for reducing the emissions into the atmosphere start already in the furnace and are completed by an effective flue gas cleaning system. This implies the necessity for design developments of key components for a modern EfW plant. For the core component of the firing system — the grate — Fisia Babcock Environment (FBE) is using forward moving grates as well as roller grates. The moving grate, which is used in the great majority of all our plants, has specific characteristics for providing uniform combustion and optimal burnout. These include, amongst others: - Uniform air supply by means of specific grate bar geometry. - Two grate steps in direction of waste transport for optimum burnout. - Flexible adaptation of the combustion process to the respective conditions and requirements by zone-specific air distribution and transport velocity of waste on grate. - Combustion control adapted to the specific plant for ensuring a consistent combustion process and production of energy. In addition to these features influencing the emissions the moving grate exhibits also specific characteristics regarding the mechanical aspects allowing low-maintenance and reliable operation. For optimum flue gas burnout a good oxygen distribution after leaving the combustion zone is required. For ensuring this, the injection of secondary air is designed to produce a double-swirl, developed by FBE. Final reduction of the nitrogen constituents NO and NO 2 to the stipulated emission value is achieved by the SNCR process. As well in this respect, there is a great amount of experience available. Besides these measures regarding the combustion process, this paper also reports about flue gas cleaning systems. In this field the FBE CIRCUSORB ® process is presented and compared with the known dry absorption process. CIRCUSORB ® is a lime-based flue gas cleaning process with continuous recirculation of the moistened reaction product and simultaneous addition of fresh hydrated lime. The flue gas temperature downstream of the economizer can be selected very low and permits in this way maximized utilization of the energy. The evaporation of the moisture from the reaction product (flash evaporation) effects final cooling down of the flue gas to optimum process temperature and improves at the same time SO 2 separation. This reduces the technical investment required for the flue gas cleaning process. The total of all measures taken and the robust design of all components permit economical plant operation while complying with the stipulated emission limit values.

This investigation has been undertaken to better understand pollutant formation in Waste-to-Energy (WTE) systems by using Computation Fluid Dynamics (CFD). An above-grate gas phase only model was built and calculated in FLUENT™ with the intent of specifically studying the factors that influence the formation of NO x . Results are shown for a typical reciprocating-grate WTE boiler operating on municipal solid waste (MSW). Contours of velocity, temperature, CO 2 , CO, H 2 O, and O 2 agree well with previous modeling and data resulting in a high fidelity model that can be implemented in the next phase of this research. Preliminary data is shown for thermal NO x and the results are promising. The next phase of this research will include the development and implementation of detailed kinetic mechanisms (DKM) to model NO x formation with the current boiler presented as well as others with varying fuels.

The dominant technology for large Waste-to-Energy (WTE) facilities is combustion on a moving grate of “as-received” municipal solid wastes (MSW). However, there are circumstances where a low-capacity plant (<100,000 tons per year) is required. This study examines the technical, economic, and environmental aspects of some small-scale WTE technologies currently in operation. The Energos technology was developed in Norway, in order to provide relatively small communities with an economically efficient alternative to mass-burn incineration with equally low emissions to the atmosphere and flexibility in feedstock. All operating plants treat MSW plus additional streams of commercial or industrial wastes. Prior to thermal treatment, the materials are shredded in a high-torque, low-rpm shredder and ferrous metals are removed magnetically. The feedstock is partially oxidized on a moving grate in the gasification chamber where the fixed carbon is completely burnt off. The volatilized gases are fully combusted in a second chamber and the heat is transferred to a heat recovery system for steam generation. The Energos gasification technology is currently in operation at six plants in Norway, one in Germany, and one in the UK. As expected, the capital cost per ton of annual ton of capacity increases with decreasing plant capacity, while there is a linear relationship between energy recovery and capacity. Some other small-scale technologies are investigated in this study and will be reported at the NAWTEC meeting. Low capacity (<80,000 tons) WTE facilities require a relatively small footprint (1.5 to 2 acres; <1 hectare) and it is believed that these facilities can be built at a capital cost per ton that is as low, or lower, than that of large mass burn WTE facilities.

Thermal plasma torches convert electricity to high-temperature thermal energy by applying a high voltage across a flowing gas stream. Plasma torches are used extensively for producing metallic and ceramic coatings and also for vitrifying hazardous materials, such as asbestos-contaminated wastes. In the last decade, several thermal plasma processes have been proposed for treating municipal solid wastes (MSW). This research is based on a critical analysis of previous work by the Earth Engineering Center and on published reports and examines the possibilities for the proposed thermal plasma (TP) processes to be recover energy from MSW as an alternative to the conventional waste-to-energy (WTE) by grate combustion. In particular, this study will investigate two prominent thermal plasma technologies that are presently under development: The Alter NRG “Westinghouse” process in the U.S. and the Europlasma process in France. The environmental impacts and the technical economic aspects of plasma-assisted WTE processes will be compared to the traditional process of MSW combustion on a moving grate.

Over the last few years an increase in the calorific value of the waste has been observed at our waste-to-energy facilities. Wheelabrator Technologies, Inc. in conjunction with Von Roll/Inova decided to install a zone of water-cooled grate blocks at the Millbury Massachusetts waste-to-energy facility as a pilot program. Common in Europe these water-cooled grate blocks address the issue of higher BTU waste and increase the overall life expectancy of the blocks compared to regular air-cooled grate blocks. This technical paper provides an overview on the installation, operation, and maintenance of a zone of water-cooled grate blocks. Discussed are the procedures for evaluating the overall project and some of the challenges we resolved.

Thermal treatment of waste using grate-based systems has gained world-wide acceptance as the preferred method for sustainable treatment of waste. It is therefore necessary to develop innovative processes with safe process engineering technology that guarantee the treatment of waste in accordance with ecological and economic constraints in addition to complying with legal requirements. This paper documents successful use of industrial-scale R&D using MARTIN technology in providing solutions for optimizing grate-based Energy-from-Waste technologies in terms of protection of climate and resources, reduction of environmental impacts as well as political, regulatory and market aspects.

A new WTE (Waste-to-Energy) power plant configuration combining municipal solid waste and gas turbines or landfill gas engines is proposed. The system has two objectives: increase the thermodynamic efficiency of the plant and avoid the corrosion in the MSW (Municipal Solid Waste) boiler caused by high tube metal temperatures. The difference between this concept and other existing configurations, such as the Zabalgarbi plant in Bilbao, Spain, is lower natural gas consumption, allowing an 80% waste contribution to the net energy exported or more. This high efficiency is achieved through four main steps: 1 . introducing condensing heat exchangers to capture low temperature heat from the boiler flue gases; the stack temperature can drop to 70°C; 2 . high steam temperatures in external superheaters using hot clean gases heated with duct burners; 3 . mixing the exhaust gases of a small gas turbine with hot air preheated in a specially designed heat exchangers. The resulting temperature of this gas mixture is almost the same as a standard gas turbine but with the flow similar to that of a large machine with a higher O 2 content; 4 . After the duct burner and heat exchangers, the oxygen content of the clean gas mixture is still high, nearly 18%, and the temperature is approximately 200°C. The gas is then used as combustion air to the MSW boiler such that all the energy stays in the system. The efficiency can be as high as 33% for the MSW part of the plant and 49% for the natural gas system. Since the natural gas consumption is almost ten times less than the existing designs, it can be replaced by landfill gas or gasified ethanol or biodiesel. Currently an 850 ton/day plant is being designed in Brazil in partnership with a large power company. Other advantages include, self generation of internal power and lower steam superheating temperatures in the MSW boiler. This concept can be used with any grate design.

In March 2008, Keppel Seghers started the engineering, supply, construction and commissioning of a Combined Heat and Power (CHP) Waste-to-Energy (WtE) plant in A˚motfors (Sweden). When completed in 2010 the plant will process close to 74,000 tons per year of household waste (average LHV = 10.5 MJ/kg) and limited quantities of (demolition) wood resulting in a yearly production of about 108,700 MWh of steam, 12,100 MWh of heat and 13,400 MWh of electricity. Herewith, the A˚motfors WtE-CHP is sized to meet the joined energy needs of the local paper production, neighboring industries and buildings at an overall net plant efficiency of almost 65%. The WtE-CHP will offer state-of-the-art combustion and energy recovering technology, featuring Keppel Seghers’ proprietary Air-Cooled Grate, SIGMA combustion control and integrated boiler. Waste is fed into the combustion line with an automatic crane system. To surpass the stringent EU emission requirements, a semi-dry flue gas cleaning system equipped with Keppel Seghers’ Rotary Atomizer was selected as economic type of process for purifying the combustion gas from the given waste mixture. Furthermore a low NOx-emission of 135 mg/Nm 3 (11%O2, dry) as imposed by Swedish law is achieved by SNCR. The plant engineering is described with a focus on the overall energy recovery. As stable steam supply to the paper mill and the district heating system needs to be assured under all conditions the design includes for supporting process measures such as combustion air preheating, steam accumulation, turbine bypassing, buffering of the main condenser and back-up energy supply from an auxiliary fuel boiler. Additionally, external conditions can trigger distinct plant operation modes. A selected number of them are elaborated featuring the WtE-plant’s capability to conciliate a strong fluctuating steam demand with the typical intrinsic inertia of a waste-fired boiler. With prices for fossil fuels increasing over the years, the cost for generating process steam and heat has become dominant and for paper mills even makes the overall difference in viability. As will be documented in this paper, the decision to build the A˚motfors WtE-CHP was taken by Nordic Paper after a quest for significant cost-cutting in the production of process energy. Moreover, the use of industrial and household waste as fuel brings along the advantage of becoming largely independent from evolutions on the international oil and gas markets. By opening up the possibility for a long-term secured local (waste) fuel supply at fixed rates, WtE-technology offers a reliable alternative to maintain locally based industrial production sites. The Nordic Paper mills in A˚motfors are therefore now the first in Sweden to include a waste-fired CHP on a paper production site.

Flow, mixing, and, size segregation of Municipal Solid Waste (MSW) particles on the traveling grate of a mass-burn waste-to-energy (WTE) combustion chamber is analyzed for understanding those parameters that control the combustion processes and for designing the chamber. In order to quantify these phenomena, a full-scale physical model of the reverse acting grate was built and used for investigating the effects of the motion of the reverse acting grate under a MSW packed bed with tracer particles ranging from 6 – 22 cm in diameter. Based on these experimental data, a stochastic model of MSW particle within the packed bed on a traveling grate was applied for simulating the MSW particle behavior. The result shows that the motion of the traveling grate, whose speed ranged from 15 to 90 reciprocations/hour, increases the mean residence time of small and medium particles by 68% and 8%, respectively, while decreasing the mean residence time of large particles by 17%. This is because of size segregation of particles known as the Brazil Nut Effect. When the ratio of particle diameter to the height of moving bar, d/h, increases from 0.46 to 1.69, the mixing diffusion coefficient, De at 60/hour., decreases from 96 to 38.4. This indicates that the height of the moving bars should be greater than the diameter of targeted particles.

Chemical rate and heat transfer theory indicates that the combustion performance and productivity of a moving grate waste-to-energy boiler should be enhanced by means of pre-shredding of the MSW, thus reducing the average particle size, homogenizing the feed, and increasing its bulk density by an estimated 30%. However, the capital, operating and maintenance costs of the shredding equipment should be low enough so that existing or new WTE facilities consider pre-shredding of the MSW. In cases where MSW is transported to a central WTE from a number of Waste Transfer Stations (WTS), pre-shredding may take place at the WTS, thus increasing density and decreasing transportation costs. This is a mechanical engineering study that examined the evolution and present state of shredding equipment since 1994 when the last WTE shredder in the U.S. was installed at the SEMASS facility. The quantitative benefits realized through the pre-processing of MSW by means of modern shredding equipment are evaluated both for the traditional high speed hammermills and the new generation of low-rpm, high-torque shredders. The combustion characteristics of shredded MSW were analyzed and compared to those of the “as-received” material that is presently combusted in mass burn WTEs. The emphasis of the project has been on equipment that can be integrated in the traditional flowsheet of a WTE and serviced readily. The most important criterion in the final design will be that the economic and energy benefits of pre-shredding be clearly greater than the conventional operation of combusting as received MSW.

This paper discusses the retrofit injection of humid gas from sludge dryer with secondary air in the WTE furnaces in Que´bec City. In 1992, a municipal sludge treatment plant was added in the WTE building. Three sludge dryers, each connected to a furnace, were added. Direct contact with hot furnace gas was used to dry sludge in a rotary drum. Humid gas from the dryer was returned to the rear wall of the furnace just above the finishing grate. CFD modeling showed cold flow of humid gas on the rear furnace wall, restriction of the combustion area on the principal grate, and stratification of the flow inside the boiler. A retrofit of the first chamber of the boiler was designed using injection of humid gas from the sludge dryer with secondary air on the front and rear walls. The main purpose of the retrofit was to maintain CO levels of under 57 mg/m 3 on a 4 hour mobile average. The first boiler was retrofitted in winter 2008 and results have been very encouraging.

The dominant waste-to-energy technology is combustion of “as-received” municipal solid wastes (MSW) on a moving grate. By far, the largest cost item in the operation of such plants is the repayment of the initial capital investment of $600 to $750 per annual metric ton of capacity which results in capital charges of $60–75 per ton of MSW processed. On the average, such plants generate about 650 kWh of electricity per metric ton of MSW combusted. Therefore, on the basis of 8,000 hours of operation per year (90% availability), the capital investment in WTE facilities ranges from $7,500 to $9,000 per kW of electric capacity. This number is three times higher than the present cost of installing coal-fired capacity (about $2,500 per kW). Of course, it is understood that WTE plants serve two purposes, environmental disposal of solid wastes and generation of electricity; in fact, most WTE plants would not exist if the fuel (i.e. the MSW) had to be paid for, as in the case of coal, instead of being a source of revenue, in the form of gate fees. However, the question remains as to why WTE plants are much more costly to build, per kWh of electricity generated, than coal-fired plants, even when the coal supply is lignite of calorific value close to that of MSW (about 10 MJ/kg). This study intends to examine the possible contributing causes, one by one, in the hope that the results may lead to the design of less costly WTE plants. Some of the factors to be examined are: Feed-stock handling; heat generation rate per unit volume of combustion chamber; heat transfer rate per unit area of boiler surfaces; % excess air and, therefore, volume of gas to be treated in Air Pollution per kW of electricity; differences in gas composition and high temperature corrosion in boiler that limit steam temperature and pressure and thus thermal efficiency; cost of APC (air pollution control) system because of the need to remove volatile metals and dioxin/furans from the process gas; and the handling of a relatively large amount of ash. In seeking the answers to the above questions, the study also compares the operational performance characteristics and engineering design of various existing WTE plants. This study is at its very beginning and it is presented at NAWTEC 17 in the hope of generating useful discussion that may lead to significant improvements in the design of future WTE facilities. The WTEs built in the U.S. until 1995 were designed for efficient and environmentally benign disposal of MSW, with energy recovery being a secondary consideration. There have been three principal changes since then: (a) the capital cost of WTEs, per daily ton of capacity has doubled and in some cases nearly tripled, (b) energy recovery per unit of carbon dioxide emitted has become an important consideration, and (c) the price of renewable electricity has increased appreciably. All these three factors point to the need for future WTEs to become more compact, less costly to build, and more energy-efficient. It is believed that this can be done by combining developments that have already been tested and proven individually, such as shredding of the MSW, higher combustion rate per unit surface area of the grate, oxygen enrichment, flue gas recirculation and improved mixing in the combustion chamber, superior alloys used for superheaters, and steam reheating between the high-pressure and low-pressure sections of the steam turbine. For example, oxygen enrichment is practiced at the Arnoldstein, Austria, WTE where parts of the primary air stream are enriched between 23% and 31% oxygen; steam reheating has been proven at the Waste Fired Power Plant of AEB Amsterdam where electricity production for the grid has been increased to over 800 kWh per ton MSW.