This study was undertaken to better understand the governing processes and reaction conditions under which NO x is produced in Waste to Energy (WtE) boilers. A three dimensional CFD model was created and calculated using the GRI 3.0, 50 species, 309 step detailed chemical kinetic model (DCKM) for methane/ethane combustion. Model results for primary NO x emissions and other pollutants agree well with collected data, proving the fidelity of the model. NO was the primary pollutant accounting for approximately 99% of the total NO x emissions. Fuel bound nitrogen was found to be the main source of NO produced in the boiler with thermal and prompt mechanisms having lesser impacts. Three principal intermediates were identified in the formation of NO; NH, HNO, and NCO. The assumption of fuel nitrogen conversion to either NH 3 or HCN is an unknown parameter that was shown to have a small impact on NO emissions, indicating that this is an area that should not be explored further in this continuing study. Furthermore, varying the boiler pressure had a small impact on final NO emissions, indicating that this is not a condition that should be considered for plant operation. The next phase of this research will include the development of a reduced DCKM in order to expedite the running of new scenarios for future studies as well as optimization of boiler geometry and combustion mixing to achieve the lowest possible NO x emissions.

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.

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.

Biomass is one of the most important sources of renewable energy. One aim of Biomass gasification is to convert a solid feedstock into a valuable syngas for electricity or liquid fuel production. Actual industrial auto-thermal gasification processes achieve a production of syngas mainly polluted by products such as dust, nitrogen oxides, sulfur dioxide and tars. Tars remain, one of the main drawbacks in using the gasification process since they are capable of condensing at low temperature. This could lead to fouling, corrosion, attrition and abrasion of downstream devices such as gas turbines or engines. Tars are often removed from the syngas, decreasing the internal energy of the syngas itself. These tars are heavy aromatic hydrocarbons whose treatment remains difficult by thermal, catalytic or even physical methods. They can condense or polymerize into more complex structures, and the mechanisms responsible for their degradation are not completely identified and understood. Turboplasma© is a thermal process, proposed by Europlasma. The main principle of operation relies on the use of thermal plasma for the cracking of tars inside a syngas produced in an auto-thermal gasification step. Basically, it consists of a degradation chamber where the syngas is heated by a plasma torch. The plasma plume provides a high temperature gas (around 5000K) to the system and enables heating of the incoming stream (above 1300K) and also generates high temperature zones (above 1600 K) inside the device. Due to both high temperature and long residence times of the syngas in the vessel, cracking of the tars occurs. Finally, the species released are mainly CO and H 2 , leading to an increase in the Lower Heating Value of the syngas. The work presented here describes the design of a high temperature gasification system assisted by thermal plasma. It was performed using a CFD computation implemented with a full chemical model for the thermal degradation of tars. The objectives were to understand the aerodynamic behavior of the vessel and to propose enhancement in its design. We present here some results of this study.

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 amount of municipal solid waste is still increasing and the calorific value of the waste is steadily growing. The combined result is an increasing demand for new thermal treatment capacities. An alternative solution to new waste-to-energy projects is an expansion and technical upgrade of existing incineration plants. This is an advantageous option for waste management companies because they avoid the NIMBY syndrome and the difficulties in getting permits for green field projects. Furthermore, the investment cost per tonne burned waste is less than that for a new incineration line. This paper will present the basic ideas and principles used in upgraded projects. The core of the technology is a combination of a new furnace design, new water cooled wear zones and combustion grates and new control systems. Moreover, CFD modelling is an important tool in the design phase, and the paper gives a demonstration of the flow design process applied at Babcock & Wilcox Vo̸lund. CFD gives the designer the possibility of checking the design for a large number of critical factors such as velocities, mixing of combustion products and secondary air, oxygen and CO concentration, temperature, surface temperature, corrosion etc. This ability is extremely valuable in the case of expansion of existing incineration plants because many of the process parameters have to be within the limits of the old plant.

Great River Energy operates a waste-to-energy plant in Elk River, Minnesota. The plant burns 850 tons per day of refuse derived fuel (RDF) in three boilers, and its three steam turbines can produce 32 MW of electricity. In the largest of the three units, the No. 3 Boiler, steam generation was restricted by carbon monoxide (CO) and nitrogen oxides (NO x ) emission limits. The plant had an interest in improving the combustion performance of the unit, thereby allowing higher average RDF firing rates while staying within emissions compliance. The project was initiated by an engineering site visit and evaluation. The boiler had a history of unstable burning on the stoker grate, which required periodic natural gas co-firing to reduce CO levels. As an outcome to the evaluation, it was decided to install a new overfire air (OFA) system to improve burnout of combustible gases above the grate. Current and new OFA arrangements were evaluated via Computational Fluid Dynamics (CFD) modeling. The results illustrated the limitations of the original OFA system (comprised of multiple rows of small OFA ports on the front and rear furnace walls), which generated inadequate mixing of air and combustible gases in the middle of the boiler. The modeling illustrated the advantages of large and fewer OFA nozzles placed on the side walls in an interlaced pattern, a configuration that has given excellent performance on over 45 biomass-fired boilers of similar design upgraded by Jansen Combustion and Boiler Technologies, Inc. (JANSEN). Installation of the new OFA system was completed in April of 2008. Subsequent testing of the No. 3 Boiler showed that it could reliably meet the state emission levels for CO and NO x (200 ppm and 250 ppm, respectively, corrected to 7% dry flue gas oxygen) while generating 24% more steam than a representative five month period prior to the upgrade. This paper describes the elements that led to a successful project, including: data collection, engineering analyses, CFD modeling, system design, equipment supply, installation, operator training, and startup assistance.

Energy recovery from waste is an efficient way to reduce emissions of greenhouse gas and other gaseous, liquid and solid pollutants and thereby to contribute to a sustainable development. Waste fired power plants are an important part of the European waste management system, and the demands made to modern waste fired power plant are very focused on high electrical output. Lately, Babcock & Wilcox Vo̸lund (BWV) and the CHEC research centre at the Technical University of Demnark has developed a new technology and received a world patent. The basic idea is to improve the electrical efficiency by increasing the steam data. Especially, increasing the steam temperature without the risk of superheater corrosion. The new concept is fully integrated in the boiler and from the outside the waste fired power plant has the same layout as the classic waste fired power plant. The goal is to achieve an increase between 50 °C to 100 °C in the superheated steam temperature and a total increase of electrical efficiency up to 30% without any influence on the normal operation. This paper presents the basic ideas that provide a basis for the patent. The core of the technology is a combination of a new furnace design and a new control system. At the moment, test results from an operating plant support the principal idea. Furthermore, the concept will be studied in the numerical laboratory where CFD simulation will be used to evaluate the technology and to determine the potential process improvements. The final improvement of electricity production has to be determined in the coming test period on a full scale installation, which is currently being planned.

This paper focuses on a Computational Fluid Dynamics (CFD) application to Energy-from-Waste (EfW) boiler systems, which are much smaller than utility boilers, but typically have more complicated design and performance issues due to the nature of their fuel: municipal solid waste (MSW). The majority of the commercially available CFD software packages have impressive capabilities in homogeneous flow modeling, which make them very suitable for gas fired boilers and with some reasonable simplifications to oil and pulverized coal fired boilers. But this is not the case for EfW boilers, especially mass-burn technology, where MSW combustion on the grate is the “heart” of the process. These boilers have two interacting phases: MSW burning on the grate and combustion products with entrained fly ash particles above the grate. This challenge has been recognized from the very first applications of CFD modeling in the EfW industry. This paper describes several approaches to numeric modeling of MSW boilers. In the most successful cases, two different models have been built: one for the grate combustion and another for the homogeneous gas flow, with back-and-forth iterations between these two models. Such an approach has given Covanta Energy (Covanta) a good start in its CFD modeling program. A number of models have been built, tested and validated, resulting in several successful project executions. However, some serious limitations have been found in this approach as explained in this paper. Recognizing these deficiencies, Covanta has recently made a new significant step in its CFD development program by creating an innovative, fully integrated CFD model that comprises solid fuel combustion on the grate, gaseous phase interactions above the grate, and the rest of the boiler. While this integrated model and its validation are still in progress, Covanta is moving forward on its application to on-going projects.

It is generally believed that incineration systems based on combustion grates are old fashioned and it is widely understood that there are no new developments in that area. This image is incorrect; it is proven technology – but it is still improving! The waste-to-energy sector has undergone a rapid technological development over the latest 10 years. Lately, new technologies have been introduced, such as Advanced Combustion Control (ACC), the use of Inconel ® protection in boilers, boiler cleaning, water cooled combustion grates and wear zones. Moreover, the presentation will show how modern engineering tools such as CFD (Computational Fluid Dynamics software) are used for designing the best furnace and boiler configurations. These new developments are illustrated through a case study of a new class of waste fired power plant burning 20 tonnes of waste per hour. This paper will present some of the test results from operating the plant, including the development of a new advanced control system.

The TURBOSORP ® -process is a dry flue gas cleaning process to remove certain pollutants like SO 2 , HCl, Hg, heavy metals, dioxins and furans. The main principle of this process is to bring flue gas in an intensive contact with Ca(OH) 2 , open hearth furnace coke, water and recirculated material in the Turboreactor. The Turboreactor operates as circulating fluidized bed in the manner of fast fluidisation. The gas/solid mixture leaves the Turboreactor at the top and the solids are separated in a fabric filter from the flue gas. More than 99% of the separated solids are recirculated to the Turboreactor and the rest leaves the process as product. Due to the high sorbent recirculation percentage a high sorbent utilization and low stoechiometric rates are reached in the TURBOSORP ® -process. Due to the fact to have plants in operation for the spray absorption and for the TURBOSORP ® process, a comparison definitely showed advantages for the TURBOSORP ® process. Experiences of the plant start up of a TURBOSORP ® plant in Poland concerning optimisation in pressure loss and hydrodynamics of the Turboreactor using CFD-Simulation are presented. Results concerning mercury and dioxin separation in our Turbosorp ® pilot plant after the refuse incinerator MV Spittelau, Vienna, are discussed.

American Ref-Fuel Company (ARC) spends millions of dollars each year on corrosion related costs in the boilers. The corrosion is caused by chloride salts in the slag that deposit on the boiler tubes, coupled with the high temperatures of flue gas going through the boiler. Corrosion rates are known to be very sensitive to the flue gas temperature and velocity, surface temperature and heat flux through the slag, oxygen in flue gas distribution, etc. These parameters are primarily determined by the firing rate of the boiler, and they are also affected by combustion control and air distribution in the boiler. Some design parameters, such as surface area of refractory, tile, and inconel overlay, also affect the flue gas temperature throughout the boiler, and thereby impact corrosion.

Essex County Resource Recovery Facility (one of American Ref-Fuel Company’s six operating plants) has processing MSW capacity of approximately 2700 TPD and about 60% of this waste comes from NY City. Therefore, availability of the Essex plant boilers is very important not only for the company’s financial performance, it is also critical for the overall garbage disposal situation in the NYC Metropolitan area. One of the main factors affecting plant availability is boiler unscheduled downtime. The most recent data show that approximately 85% of Essex boilers unscheduled downtime is caused by tube failures, the majority of which occur in the superheater tubes. These tube failures are almost exclusively caused by fireside tube metal wastage driven by complicated mechanisms of corrosion in combination with local erosion. The corrosion is caused by chloride salts in the slag that deposits on the boiler tubes, coupled with high temperatures of flue gas going through the boiler. Corrosion rates are known to be very sensitive to flue gas temperature, tube metal temperature, heat flux, flow distribution. Erosion is typically caused by high velocities and flyash particle loading and trajectories. Extensive research revealed that in addition to this typical to WTE boiler corrosion/erosion mechanism, Essex boiler superheater tubes experienced a unique problem, resulting in tube overheating, accelerated wastage, and ultimate failure. In order to address this problem a modification plan was developed, which comprised several redesign options. A specially developed Three-dimensional Computational Fluid Dynamics (3-D CFD) model was utilized for comprehensive technical evaluation of the considered design options and for predicted performance simulations of the selected design at different operating conditions. The economical analysis, conducted in conjunction with the superheater redesign, provided financial justification for this project. The project has been recently executed, and field data collection is still in progress. Some preliminary data analyses have been performed. They have shown that the boiler performance after superheater modification is very close to the predicted target simulated by the CFD model. The plant and the company are already measuring financial benefits as a result of this project, the initial phase of which is presented in this paper.

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.