Combating Corrosion in WTE Facilities: Theory and Experience

In boilers that use municipal solid wastes as fuel, metal wastage due to corrosion and erosion and tube fouling due to the buildup of deposits present serious problems to the system designer and operator. This study examines the corrosion mechanisms in Waste-To-Energy (WTE) boilers and summarizes the findings of a corrosion survey of several WTE facilities and of interviews with senior engineers in the WTE industry. In addition, this study examines the existing methods of reducing corrosion that are adopted in WTE plants. Finally, the study proposes experimental research on corrosion resistant materials to be carried in the near future.

Numerical Analysis of Size Reduction of Municipal Solid Waste Particles on the Traveling Grate of a Waste-to-Energy Combustion Chamber

The size reduction of municipal solid waste (MSW) particles on the reverse acting traveling grate of a waste-to-energy (WTE) combustion chamber was estimated by means of a numerical model combining the particle size distributions (PSD) of MSW and combustion residues and the Shrinking Core Model (SCM). This new integrated model was used to simulate the particle behavior on the grate. During their travel on the moving grate, the sizes of the particles are reduced by combustion, breakage, and compaction. This study shows the calculation of the particle size change using this model and comparison of the numerically derived PSDs of MSW and ash particles with experimental data. There is good agreement between calculated and measured values.

Pulse Combustion Technology

Pulse combustion has been used in a variety of ways since first being discovered in 1877. This a combustion process that occurs under oscillatory conditions with changing state variables, such as pressure, temperature and velocity. This paper looks at the historic uses of pulse combustion, and it provides an overview of this unique process. Pulse combustion has been used to amplify thrust power with the German V-1 rockets. Pulse combustion has been used to optimize flame efficiencies, and it is now reemerging in many new industrial applications including some for Waste to Energy.

Problem Solving Tools in Waste-to-Energy Systems

Covanta is using a multifaceted approach to problem solving in Waste-to-Energy systems which combines several types of computer modeling with physical cold flow models, field testing, and engineering experience. This problem-solving approach is applied to boiler corrosion, gas and particulate flow patterns, reagent injection, and APC system issues. Our goals are to bring the most appropriate tools to each issue and incorporate results back into the engineering approach in order to continually improve our technical capabilities. Several types of computer modeling are used. A commercially available energy balance program is used for steam cycle evaluations and boiler energy balance and heat transfer calculations. Computational Fluid Dynamics (CFD) models are developed to investigate temperature and flow patterns where local conditions must be understood in detail. We have made extensive use of cold flow models to improve performance of APC systems, and to evaluate overfire air mixing in furnaces, and flow distribution through tube banks in boilers. Field testing is used to investigate temperature fluctuations and distributions, flow stratification, corrosion rates, and to validate modeling or analytical results. Each of these approaches has its own set of advantages, disadvantages, and limitations, and must always be combined with a healthy dose of operating and engineering experience. Analytical work is done by, or in close cooperation with, our operations and engineering staff with many years of experience operating, designing, and modifying boilers, APC systems, and related equipment. This integrated approach has yielded significant improvements in many cases and is being used in increasingly complex applications.

New Waste-to-Energy Projects: Opportunity Knocks!

Never have conditions been more favorable for the development of new waste-to-energy projects. The record of operating waste-to-energy plants has dispelled all of the objections that had been raised by environmental activists with respect to emissions and residue toxicity. The economics have become positive due to the rising cost of disposal at distant landfills and the increased value of the recovered energy due to sharp increases in the cost of fossil fuels. The threat of global warming and the recognition of the need to reduce reliance on imported fuel sources have made the public aware of the need to make full use of all domestic sources of energy. Regardless of legal definitions, energy from wastes is renewable energy and established technology. Waste-to-energy plants are even now providing more energy then other renewable sources such as biomass, wind, and direct solar combined. What is needed now for the industry to look at the existing technology to see how it can optimize energy recovery, both in capital and operating costs, without compromising environmental performance. Above all, we need a major push to make our case with the public and the politicians who represent it to convince them that waste-to- energy is not only good economics, but good environmental policy as well.

Vitrification of Bottom Ash From AVR MSW Incinerators

During incineration of municipal solid waste (MSW), various environmentally harmful elements and heavy metals are liberated either into bottom ash, or carried away with the off-gases and subsequently trapped in fly-ash. If these minor but harmful elements are not properly isolated and immobilized, it can lead to secondary environmental pollution to the air, soil and water. The stricter environmental regulations to be implemented in the near future in the Netherlands require a higher immobilization efficiency of the bottom ash treatment. In the present study, MSW incinerator bottom ash was vitrified at higher temperatures and the slag formed and metal recovered were examined. The behaviour of soluble elements that remain in the slag is evaluated by leaching extraction. The thermodynamics of slag and metal formation is discussed. The results obtained can provide a valuable route to treat the ashes from incinerators, and to make recycling and more efficient utilization of the bottom ash possible.

Waste-to-Energy Project Economics and Financing: A Look Into the Factors Influencing the Future

This paper focuses on significant changes in the overall economics of waste-to-energy (WTE) during the last 30 years. The WTE industry in this country has seen several different business cycles occur since 1975, as different market drivers have caused the industry to rise and fall. This paper compares: (1) those economic factors that were in play in 1975, when the first WTE facility in the United States was built, and the industry was in its infancy; (2) the factors at play when the WTE industry was at its height in 1990; and (3) some of the factors that caused the industry’s steep downward trend since 1994, when the last greenfield WTE facility in the United States was built. The paper will identify changes that have occurred with regard to the pricing of electricity and the ability of public sectors to charge non-market-based tipping fees. The paper discusses the drivers of 2006 and focuses on completed economic factors to be considered when comparing WTE with other waste disposal means. The paper discusses the drivers of 2006 and whether the industry is finally poised to begin an upward turn in the cycle. The paper focuses on the impact of the cost of diesel fuel oil on the overall economics of long-haul transfer, and how that is likely to impact the future development of WTE facilities. The paper also presents a case study of a recent analysis that was undertaken for two counties that were evaluating the financial viability of WTE as compared to other disposal options.

Maximizing Energy Revenues: Providing the Best Incentive to the Contract Operator

Communities that own waste-to-energy (WTE) facilities rely heavily on the revenues generated by their facility to help pay for the costs to finance, operate and maintain these facilities. The two primary revenue streams are tipping fees and energy sales, generally in the form of electricity. While communities often retain all of the tipping fee revenue, revenue from the sale of energy is nearly always shared with the contract operator. In some cases the shared energy revenues include both capacity and electricity payments. The basis of this strategy is to offer the contract operator an added incentive to maximize this revenue stream through more efficient operation and, in the case of capacity payments, to meet certain capacity commitment criteria required by the energy purchaser. This strategy recognizes that the contract operator has some degree of control over the factors that affect energy production. Under most existing service agreements, which date back to the 1980s, energy revenues are shared on a 90/10 basis, with 90 percent going to the community. Now that many of these service agreements are coming up for renewal or are expiring, communities will need to revisit how best to share energy revenues with the contract operator in order to maximize the total revenues retained by the community. This paper analyzes several different approaches to sharing energy revenues in light of the operational experience gained over the past 20 plus years and concludes that, while energy revenue sharing is still in the best interest of the community, the widely employed strategy of a 90/10 split may not offer the best incentive, and therefore may not lead to the maximization of energy revenues to the community.

Integrated Solid Waste Management in Northwest Minnesota

In the early 1980’s Polk County and four other partner counties in rural Northwest Minnesota made the decision to incorporate a waste to energy (WTE) plant into their solid waste management program. This decision was made to comply with the Minnesota hierarchy for solid waste management, to extend the life of the Polk County landfill, and to recover valuable energy from the waste. The plant was constructed in 1987 and began burning MSW in 1988. The processing technology consisted of two starved air mass burn municipal solid waste combustors each with a combustion capacity of 40 tons of MSW per day, and produced energy in the form of saturated steam for customers in the adjacent industrial park. Initially each train utilized a two field electrostatic precipitator (ESP) as the air pollution control (APC) device. In 1996, a materials recovery system (MRF) was constructed in front of the waste combustors to remove problem/objectionable items most of which are recyclable. This facility has been a tremendous success providing many benefits including reduced stack emissions, lower O & M costs for the WTE units, and revenues from the sales of extracted recyclables. In 1998 Polk began injecting powdered activated carbon (PAC) into the flue gas of each unit upstream of the ESP to attain compliance with new State limits for dioxin/furans and mercury. Then in 2000 Polk County proceeded with an APC retrofit project designed to meet revised EPA emission guidelines which set more stringent limits for pollutants currently regulated and added limits for several other pollutants previously unregulated. In 2001 and 2004 Polk County performed research demonstration projects substituting screened WTE combined ash for a portion of natural aggregate in two asphalt road construction projects. Both projects passed stringent environmental testing and demonstrated superior strength and flexibility performance compared to conventional asphalt. Polk County is now proceeding with the installation of a turbine/generator to produce renewable electricity with excess steam. The electricity produced will be used to reduce the demand for incoming power from the local utility. Initially this may be only a twenty-five percent reduction but has the potential to be more in the event one or more of the steam customers reduces their dependence on steam from the WTE plant. All of these projects received funding assistance from the State of Minnesota in the form of Capital Assistance Grants. In 2003 the WTE plant and MRF became debt free and Polk County lowered the tip fee resulting in a disposal rate that is fairly competitive with that of most out of state landfills. This paper will discuss the development, success, and benefits of this completely integrated solid waste management system for these five counties located in Northwest Minnesota.

Pyrolysis in Waste to Energy Conversion (WEC)

Solid waste (SW), mostly now wasted biomass, could fuel approximately ten times more of USA’s increasing energy needs than it currently does. At the same time it would create good non-exportable jobs, and local industries. Twenty four examples of wasted or under-utilized solids that contain appreciable organic matter are listed. Estimates of their sustainable tonnage lead to a total SW exceeding 2 billion dry tons. Now usually disposal problems, most of these SW’s, can be pyrolyzed into substitutes for or supplements to expensive natural gas. The large proportion of biomass (carbon dioxide neutral plant matter) in the list reduces Greenhouse problems. Pyrolysis converts such solid waste into a medium heating value gaseous fuel usually with a small energy expenditure. With advanced gas cleaning technologies the pyrogas can be used in high efficiency gas turbines or fuel cells systems. This approach has important environmental and efficiency advantages with respect to direct combustion in boilers and even air blown or oxygen blown partial combustion gasifiers. Since pyrolysis is still not a predictive science the CCTL has used an analytical semi-empirical model (ASEM) to organize experimental measurements of the yields of various product {CaHbOc} yields vs temperature (T) for r dry ash, nitrogen and sulfur free (DANSF) feedstock having various weight % of oxygen [O] and hydrogen [H]. With this ASEM each product is assigned 5 parameters (W, T0, D, p, q) in a robust analytical Y(T) expression to represent yields vs. temperature of any specific product from any specified feedstock. Patterns in the dependence of these parameters upon [O], [H], a, b, and c suggest that there is some order in pyrolysis yields that might be useful in optimize the throughput of particular pyrolysis systems used for waste to energy conversion (WEC). An analytical cost estimation (ACE) model is used to calculate the cost of electricity (COE) vs the cost of fuel (COF) for a SW pyrogas fired combined cycle (CC) system for comparison with the COE vs COF for a natural gas fired CC system. It shows that high natural gas prices solid waste can be changed from a disposal cost item to a valuable asset. Comparing COEs when using other SW capable technologies are also facilitated by the ACE method. Implications of this work for programs that combine conservation with waste to energy conversion in efforts to reach Zero Waste are discussed.