WASTE TO ENERGY GENERATION
Acknowledgements
Waste to Energy Power Plant has taken more than a year of researching , writing, calculating, compiling, and editing. I would like to thank the following people for their contributions, dedication, and support, without which this thesis could not have been created.
My family, who have given me unconditional love and support throughout my studies in New York, specifically my brother, George Hadjipavlis who helped me with the collection of the data needed from Cyprus for the completion of this project.
Stanley M. Greenwald, P.E., Chairman of the Department of Environmental Technology, for his guidance, expertise, and support.
Joyce M. Kreatsoulas for her support and help with typing this report.
Cyprus is an island located in the Mediterranean sea, south of Turkey, roughly the size of Connecticut, which currently occupies an estimated 750,000 inhabitants. While visiting this country, one encounters beautiful beaches, palm trees, sunny skies, and, an offensive odor emitted from the island’s dumps. The annual production of Municipal Solid Waste (MSW) in Cyprus is 363,000 tons. In addition to the 750,000 occupants of the island, there are 2 million visiting tourists annually. Certainly, waste is not the tourist's priority, and with respect to the natives, 95% of the population cook and eat at home, thus producing a fairly significant amount of rubbish. The communities of Cyprus presently dispose their garbage in dumps, releasing CH4 into the atmosphere, consequently polluting the environment.
As a tourist, one can simply drive away. However as an environmentalist, the conditions leave a lasting impression. Further, one of the island’s dumps is located next to a water reservoir, and during the winter months, due to the rain season, water carries the pollution into the underground water system. Approximately five years ago human manure was collected by large trucks from each home and disposed into the same dumps, resulting in polluted water levels, which could not be used for drinking.
With the growth of occupants, as well as the growth of the tourist industry, Cyprus waste management has become a major problem. Waste production increases daily, and yet there are no measures taken to prevent pollution or protect the environment. The current waste elimination process in Cyprus is like not having any process at all. There are no existing mandated restrictions or regulations in the country. With the exception of the recently established recycling program, and biological plants developed for each city, there have been no other precautions towards diminishing pollution.
Thus, the goal of this thesis project is to propose an alternative, environmentally safe method of disposing solid waste, while minimizing pollution, and long term cost. However, prior to any pollution prevention alternative being considered, it is imperative that we first identify all environmental acts and regulations that are pertinent to the project. As mentioned above, given that there are no mandated restrictions in Cyprus, and the growing concern regarding environmental safety, this project will attempt to comply with the EPA Guidelines. This project purports that waste to energy is the most cost effective, environmentally sound alternative to employ, specifically when the incineration plant is governed by the EPA Guidelines.
Landfills have rarely been sanitary, and the pollution problems increase as they are filled to capacity. The need for long term management of urban waste disposal problems is now being recognized. Converting refuse to energy through incineration is an effective and environmentally sound alternative presently being adopted by an increasing number of cities. The various advantages of utilizing incineration for the disposal of rubbish is that the MSW is volume and weight reduced, it is immediately distracted, and there is no long term residency required. Air discharges can be controlled; ash residue is usually non-putrescible, sterlie and inert; a small disposal area is required; and cost can be offset by heat recovery/sale of energy. Some of the disadvantages of MSW combustion is that there is high capital cost, not all materials are incinerable, skilled operators are required for boiler operations, some material requires supplemental fuel, and there is often public disapproval.
Other issues of concern regarding WTE are acid gas control, ash management, emissions monitoring, mercury control and NOx control. MSW ash residues are considered to have the potential to more significantly impact environmental quality, due to their compositional characteristics. In Western Europe, WTE facilities with mass burn incinerators equipped with either wet, dry or semi-dry scrubber systems are most common. WTE units that utilize wet scrubbers generally employ electrostatic precipitators (EPS) prior to scrubbing. Units that employ dry or semi-dry scrubbing systems generally utilize fabric filters (FF) for air pollution control (APC).
Examples of the design of a WTE power plant, how it functions, and how to control pollution is described below in detail.
I. A waste to energy power plant: How it works.
Collection trucks enter the site at a computer controlled weight station and are directed to an enclosed tipping pit where the MSW is unloaded directly into the refuse bunker. The overhead refuse crane mixes the waste and transfers it from the bunker and drops into the waste-charging hopper. The charging hopper holds a ready supply of waste for charging the grate system, which has to be constant for a good combustion. At this point the combustion takes place, resulting in remaining ash, which falls on a conveyer, and tips the ash into the trucks to transport it to an ash treatment plant or landfill. The heat generated from the combustion produces steam in the waterwall boiler. This steam is used to produce electricity (usually 10% of that power is used for the operation of the plant). After the gases cool in the boiler, via air pre-heaters, the combustion gases pass through a scrubber for the removal of the acid gases. The exhaust gases then continue through a particulate collection system (bag-house), and the cleansed gases are dispersed into the atmosphere through the stack.
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Different kinds of incinerators were designed over the years for different purposes. In order for the incinerator to work efficiently without producing unacceptable levels of air pollution, it must be a multiple chamber incinerator, and be sealed from the environment so that no gases can escape the combustion chamber before combustion is completed. After various studies were conducted, EPA derived the following rules for good combustion.
In chapter four (4) it will be demonstrated that the higher the combustion temperature and the amount of Excess air in the combustion process, then the higher the amount of NOx production will be. To reduce the amount of NOx the following procedures can be employed.
1. Selective non-catalytic reduction: Homogeneous phase reaction with ammonia in the furnace exhaust between the temperature range of 1700 to 1900 0F.
2.Selective catalytic reduction: Reaction of NOx with ammonia on a metal-metal oxide catalyst downstream of the particulate and acid gas emission control system. The temperature range is 540 to 750 0F.
3.Coke bed catalysis: Reaction of NOx with ammonia over an activated coke catalyst in the temperature range of 210 to 390 0F with simultaneous reduction of dioxin-furan and mercury contaminants. This system consists of a packed bed of the activated coke downstream of the particulate-acid gas emission control system.
4.Flue gas re-circulation: Operation of the combustion zone with flue gas or low excess air.
The use of Ammonia utilizing the reaction 4NO + 4NH3 + O2 à 4N2 + 6H2O can create secondary problems. If there is NH3 that escapes without reacting with NOx, it can react with SO2 and HCl at the stack and can form submicron ammonium salts. As the plume cools, it leads to an opaque white plume whose opacity exceeds the regulatory limits.
Recently studies from Japan proved that use of urea rather than ammonia permits a slightly wider temperature zone of operation. Most significantly, there was not too much ammonia slip outside of the stack, and the reduction of NOx was 50-70%.
Mercury emissions from the incineration process exist in the form of the solid and vapor phases of the metal form Hg and mercuric chloride HgCl2. This results in Hg in the waste stream, 60-70% of which comes from batteries, and the rest from paint, plastics, paper, and fluorescent bulbs.
There are three ways in which one can control the Hg emissions.
For controlling dioxins and furans, EPA suggests Electrostatic Precipitator ESP, Fabric Filter (FF), Lime spray Dryer
Absorption (SD), Good Combustion Practices (GCP), Dry Scrubber Injection (DSI), and (CI). Refer to the diagram below for details.
VII. Particulate and metals Control.
The fabric filter or the bag-house as it is sometimes referred, is a simple device. It is a number of filter bags, which are connected in parallel and trap the particulate from escaping into the atmosphere. It is relatively easy to clean the bag-house either by reverse flow or by mechanical shaking.
The ESP (Electrostatic Precipitator) mechanism is based on the principle of electrostatic attraction. A high negative voltage applied to the discharge electrodes produces a strong electric field between the discharge and collector electrodes. The particles in the gas stream gain a negative charge as they pass through the electrical field. Then there is a grounded electrode, which collects the particles.
The WTE plant has a control room that monitors all emissions. When emissions of SO2, NOX , opacity, CO become too high the proper action must be taken to reduce them. Additionally, the steam pressure for the turbine and the production of electricity is also monitored in this room. The crane operator typically monitors the steam pressure that enters the turbine, and can also raise it by adding more fuel into the incinerator.
When the SO2 or and NOX , content is too high it implies that either the scrubber needs more lime, the combustion temperature is too high, or there is too much excess air in the incinerator. When the CO content becomes too high, it suggests that the combustion temperature is too low, or that there is not enough air in the incinerator. Usually this happens when the MSW is very wet and brings down the temperature in the incinerator. Sometimes the injection of fuels with high heating values is necessary in order to correct this problem. Opacity is also called visual thickness. It is relevant to the particle concentration, particle size carried out through the flue gases, the particle optical properties, the solar illumination angle and the moisture content of the plume.
Bottom ash is the unburned and non-burnable portion of MSW. In a mass-fired facility, bottom ash can contain considerable amounts of metals and glass as well as unburned organics. A well-operating MSW combustor should be able to achieve a 95-99% Ash Burnout Index. ABI is calculated using the following formula.
ABI= {1-[a-b]/a} * 100%.
Where a= original weight of ash sample
b= weight of ash sample after firing in muffle furnace.
Fly ash is the ash that is produced from the combustion and it is filtered in the bag house or separated in the ESP. The better the air pollution control of the WTE plant is the more fly ash is collected. Fly ash can be moistened and mixed with bottom ash prior to disposal. Ash residues from combustion of MSW have been used as roadway fill and subbase for parking lots, stabilized road base, concrete masonry block, Portland cement concrete, and to cover landfills.
The geographical location of Cyprus is such where there is no significant problem of air pollution, since the island doesn't have any large mountainous terrain to hinder the circulation of air. If a WTE plant is built and monitored correctly, all forms of emissions, hence air pollution, can be controlled. Typically the WTE plants use the combustion air from the air where the MSW is stored. Consequently the storeroom is always under negative pressure, there is always flow inwards, so no odors escape the plant. As aforementioned, the way Cyprus disposes its MSW today in dumps contaminates its limited drinking water and creates a foul odor.
Ash is a byproduct of the process and another environmental concern. If ash tests hazardous it must be treated prior to disposal. Truthfully, if nothing hazardous is dumped into the MSW mainstream, then the ash will not be hazardous. The positive aspects of ash are the 90% reduction by volume of the MSW when it is combusted, and, as stated above, the fact that it can be reused. Finally, the impact of the increase in traffic that the WTE plant will create given that the WTE will be located at a farther distance than the existing dumps, this requiring the trucks carrying the waste to travel more miles. Nonetheless, the traffic system of the island recently improved its roadways, via adding lanes and constructing alternative routes, and can therefore compensate for the increase in traffic and congestion.
CONCLUSIONS
The goal of this project was to propose an alternative, environmentally safe method of disposing solid waste, while minimizing pollution and long-term cost. Further it sought out to compare the amount of emissions of MSW versus fuel #5, and to determine the amount of excess air needed to supply the incinerator for minimum pollution and maximum energy recovery. Finally, it examined the current method of disposing MSW taking the environmental and economical factors into consideration.
Given that the current findings revealed that when MSW is incinerated between 1700-1900 oF with 90-100% excess air, which produces 1.5 times more emissions than burning fuel #5, it is environmentally acceptable to have a WTE plant. Even though more emissions are less favorable, overall environmentally and economically WTE plants are more appealing for the following reasons.
The problems created by dumping far out weight those resulting from the air pollution created by burning MSW. As discussed earlier dumps use up acreage, emit a fowl odor, contaminate underground water and cost more in the long run to treat and clean. Burning MSW not only diminishes unwanted waste but it recycles into energy. Economically speaking WTE plants are also more appealing than burning fuel #5. The present economic analysis revealed that a WTE plant has 11-23 years payback period that means in the long run the republic of Cyprus will have saved money in fuel.
In closing it should be noted that these findings are very close to existing power plants that are currently operating efficiently today in the US, which operate at 90-100% excess air in the range of 1700-1900 oF for optimum performance. Since these results are realistic the present study achieved that which it sought out to accomplish.
NOTE : ALL THE INVESTIGATIVE PROCEDURE AND THE CALCULATIONS WILL NOT BE PUBLISHED
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