岑寧申

ABSTRACT

The thermal treatment processing of municipal solid waste, used both for volume reduction and energy recovery, is an important element in many integrated solid waste management systems. The purpose of this paper is to introduce the types, features and relative environmental aspects of different forms of thermal treatment including incineration and gasification. It also deals with energy recovery, air emissions and solid residue issues. It deals specifically with issues of site selection, applicability, and economics of this new technology in municipal waste treatment. It also briefly describes the advantages and disadvantages of this technique.

1. INTRODUCTION

Thermal treatment processing of municipal solid waste can be defined as the conversion of solid wastes into gaseous, liquid, and solid conversion products, with the concurrent or subsequent release of heat energy. Thermal treatment processing systems can be categorized on the basis of process temperature, amount of oxygen used and specific wastes catered for but each involves the generation of large quantities of heat which can be recovered as process heat, steam/hot water for district heating or to produce electric power(see Figure 1).

Figure 1: Types of thermal treatment (Rudden, 2000)

While the thermal treatment and incineration are often considered synonymous, it is important to understand that thermal treatment refers to a number of processes, of which incineration in one such process. Other thermal treatment processes include Gasification, Pyrolysis and Liquefaction. Among those four treatment types, there are two broad categories:

· Incineration or combustion either in the form of ‘mass-fired’ of waste or ‘RDF-fired’ where the waste is pre-treated or sorted to produce a ‘refuse derived fuel’ for incineration. The incineration technology is further broken down into grate combustion and fluidised bed combustion. Grate ‘mass-fired’ incineration technology is the basis of modern thermal treatment (Rudden, 2000).

· Pyrolysis/Liquefaction/Gasification. These alternative technologies are at various stages of development from which gasification is the most advanced internationally. None of these technologies however have sufficient track record at a commercial scale.

2. INCINERATION

Incineration is the most common technology used for converting municipal solid waste into energy. It is commonly called basically a controlled burning process (Figure 2). Efficient combustion of any fuel including municipal solid waste depends on careful control of temperature, turbulence and time. The new European Directive on Incineration of Waste (adopted November 2000) requires that products of combustion gases must be kept at a temperature of at least 850^oC for at least 2 seconds in the presence of at least 6% excess oxygen. This is in order to ensure complete combustion and also to minimise the formation of dioxins, which also proves that incineration is therefore a carefully controlled and monitored process and should not be compared with uncontrolled burning of waste.

In incineration process, waste is delivered to the plant, where it is stored in a large enclosed bunker that serves as waste buffer capacity. The bunker area is kept under negative pressure thus preventing odours from escaping the building. Overhead cranes feed the waste to the incineration unit, where it is converted into energy. This usually takes place on a moving grate in the bottom of the combustion chamber.

Figure 2: well-controlled incineration processing system (Rudden, 2000).

Municipal solid waste incineration systems can be designed to operate with two types of solid waste fuel (George, 1993): commingle solid waste (mass-fired) and processed solid waste refuse derived fuel (RDF-fired). Mass-fired incineration systems are the predominant type. In 2002, 68 percent of the operational combustion capacity in the UK was provided by mass-fired units, versus 32 percent by RDF-fired units.

Mass-Fired Incineration systems

In a mass-fired incineration system, minimal processing is given to solid waste before it is placed in the charging hopper of system. The crane operator in charge of loading the charging hopper can manually reject obviously unsuitable items. However, it must be assumed that anything in the solid waste stream may ultimately enter the system, including bulky, oversize noncombustible objects and even potentially hazardous wastes deliberately or inadvertently delivered to the system. For these reasons, the system must be designed to handle these objectionable wastes without damage to equipment or injury to operational personnel.

One of the most critical components of a mass-fired incineration system is the grate system. It serves several functions, including the movement of waste through the system, mixing of the waste, and injection of incineration air. Many variations of grates are possible, based on reciprocating, rocking, or rotating elements.

RDF-Fired Incineration Systems

Refuse-derived fuel (RDF) is produced by a specific process designed to select out the combustible from the non-combustible fraction of mixed municipal solid waste. The RDF consists mainly of the lighter materials in municipal solid waste (paper and plastic), In RDF-fired combustor, RDF is typically burned on a traveling-grate stoker. The grate provides a platform on which the RDF can burn and provides for the introduction of underfire air to promote turbulence and uniform combustion. Best results have been obtained with incineration systems specifically designed for RDF, but some coal-fired have been retrofitted to burn RDF or RDF/coal mixtures successfully.

Because of the higher energy content of RDF compared to unprocessed municipal solid waste, RDF incineration systems can be physically smaller than comparatively rated mass-fired systems. An RDF-fired system can also be controlled more effectively than a mass-fired system because of the more homogeneous nature of RDF, allowing for better incineration control and better performance of air pollution control device.

3.GASIFICATION

Gasification is the general term used to describe the process of partial combustion in which a fuel is deliberately combusted with less than stoichiometric air (George, 1993). Gasification like pyrolysis is a thermal treatment method, which can be applied to transform organic waste into combustible gas and residues (Figure 3). Essentially, the process involves partial combustion of a carbonaceous fuel to generate a combustible fuel gas rich in carbon monoxide, hydrogen, and some saturated hydrocarbons, principally methane. The combustible fuel gas can then be combusted in an internal combustion engine, gas turbine, or boiler under excess-air conditions.

Figure 3: Schematic flowchart of gasification processing system (Rudden, 2000)

Gasification takes place typically at temperatures of 800-1,100^oC (while oxygen blown entrained flow gasification may reach 1,400-2,000^oC) depending on the calorific value of the waste. The process includes a number of chemical reactions and results in the formation of a combustible gas with traces of tar. Ash (approximately 25-30% by weight) may be vitrified due to the high temperatures involved and is separated out as solid residue which may have a high potential for recycling.

Pre-treatment equipment is normally needed for screening and homogenising of the waste and sometimes also for drying, sorting and pelletising. Common wastes to be treated by this method are impregnated wood waste, wood chips and carefully sorted fractions of household waste preferably in the form of Refuse Derived Fuel pellets

4. PYROLYSIS/LIQUEFACTION

Pyrolysis is the thermal processing of waste in the complete absence of oxygen (George, 1993). Unfortunately, there is quite a bit of confusion in the literature, and many so-called pyrolysis systems are actually gasification systems. Both pyrolysis and gasification systems are used to convert solid waste into gaseous, liquid, and solid fuels. The principal difference between the two systems is that pyrolysis systems use an external source of heat to drive the endothermic pyrolysis reactions in an oxygen-absence environment, whereas gasification systems are self-sustaining and use air or oxygen for the partial combustion of solid waste.

Figure 4: Schematic flowchart of pyrolysis processing system (Rudden, 2000)

Pyrolysis is a thermal pre-treatment method, which may be employed to transform organic waste into a gas, liquid and a char fraction (Figure 4). Coarsely-shredded waste is converted in an enclosed reactor which is normally operated at or below atmospheric pressure and in the absence of oxygen. Waste within the pyrolysis chamber is transformed by a cracking process into hydrocarbons (combustible gases and oils/tar) at temperatures of 500-800^oC. A solid residue containing carbon, ash, glass and metals (up to 40% by weight) is all that remains of the waste and this may be burned separately after it is screened for metals etc.

Pyrolysis of waste is not a stand-alone process, but is generally followed by a gasification or combustion step and in some cases extraction of pyrolytic oil when it may be referred to as ‘liquefaction’.

The three major component fractions resulting from the pyrolysis process are the following:

· A gas stream, containing primarily hydrogen, methane, carbon monoxide, carbon dioxide, and various other gases, depending on the organic characteristics of the material being pyrolyzed.

· A liquid fraction, consisting of a tar or oil stream containing acetic acid, acetone, methanol, and complex oxygenated hydrocarbons. With additional processing, the liquid fraction can be used as a synthetic fuel oil as a substitute for conventional fuel oil.

· A char, consisting of almost pure carbon plus any inert material originally present in the solid waste.

5. EMISSIONS & FLUE GAS CLEANING

The operation of thermal systems produces several impacts on the environment, including gaseous and particulate emissions. Although gaseous emissions from pyrolysis, gasification and incineration of waste (or indeed any other fuel) are unavoidable, their capacity to impact upon the environment can be effectively controlled. The proper design of control systems for these emissions is a critical part of the design of a thermal processing system.

This environmental control system requires the Environment Protection Agency (EPA) to identify pollutants of specific importance. Scientific data are collected on the relationship between various concentrations of air pollutants and their adverse effect on humans and the environment. Then the emission limits standard should be set and therefore modern thermal waste treatment facilities do not represent a serious risk to human health or the environment. In order to achieve this level of cleanliness, specialised flue gas cleaning equipment is required but before describing this equipment, it is useful to first examine the basic composition of flue gases.

Flue Gas Components

·Nitrogen Oxides (NOx). The two most important nitrogen oxides are NO (nitric oxide) and NO₂ (nitrogen dioxide), collectively referred to as NOx. There are two primary sources of NOx in combustion. Thermal NOx is formed by reactions between nitrogen and oxygen in the air used for combustion. Fuel NOx is formed by reactions between oxygen and organic nitrogen in the fuel. Nitrogen oxides are precursors to the formation of ozone (O₃) and the photochemistry oxidants known as smog. Nitrogen oxides also contribute to the formation of nitrate aerosols (liquid droplets), which can cause acid fog and rain.

·Sulfur Dioxide (SO₂). Sulfur dioxide is formed by the combustion of fuels containing sulfur. Sulfur dioxide is an eye, nose, and throat irritant. In high concentrations, it can cause illness or death to persons. Sulfur dioxide is related to the production of acid rain and snow, which affect lakes, rivers, and forests.

·Carbon Monoxide (CO). Carbon monoxide, formed during the combustion of carbonaceous materials when insufficient oxygen is present, reacts with the hemoglobin in the bloodstream to form HbCO, which can cause the lack of oxygen in human body.

·Particulate Matter (PM). Particulate matter is formed during combustion by several processes, including incomplete combustion of fuel and the physical entrainment of noncombustibles. Particulate emissions cause visibility reductions and health effects.

·Metals. Municipal solid waste is a heterogeneous mixture. Many relatively innocuous items contain metallic elements. Small quantities of heavy metals which are contained in the waste become vaporised at the high temperatures and therefore enter the flue gas stream. As the flue gases cool on their way through the plant, heavy metals condense and adhere to the particulate matter which is entrained in the gas flow.

·Acid Gases. The combustion of wastes containing fluorine and chlorine leads to the generation of the acid gases hydrogen fluoride (HF) and hydrogen chloride (HCl).

Flue Gas Cleaning Technology

Flue gas cleaning is an integral part of all thermal waste treatment facilities. Flue gases produced by thermal waste treatment need comprehensive cleaning before they can be released to atmosphere. Figure 5 displays a typical schematic flowchart of flue gas cleaning process.

Figure 5: schematic flowchart of flue gas cleaning process (Rudden, 2000)

Flue gas treatment is generally based around the following basic steps, some processes differing from others depending on whether best available or best available not entailing excessive cost technology is used.

(1) Addition of ammonia to combustion chamber. Measured quantities of ammonia (NH3) are added to the combustion chamber of a waste combustion facility as an effective secondary means of controlling emissions of oxides of Nitrogen (NOx). Control of NOx emissions is affected mainly by controlling the air/fuel ratio within the combustion chamber.

(2) Cooling. By cooling the flue gases quickly, formation of dioxins/furans is greatly reduced. Dioxins/furans are formed in the flue gases of a thermal treatment plant especially within the temperature range of 250-400^oC.

(3) Acid neutralization. Acid neutralization is required to deal with HCL, HF and SO2 flue gas components. These corrosive elements are dealt with as mentioned above by the addition of basic (as opposed to acidic) reagents to the flue gases.

(4) Addition of activated carbon. Dioxins and furans are captured from the flue gases by the addition of activated carbon. Heavy metals and dioxins/furans tend to adhere to dust particles hence the addition of this material (in finely powdered form) which is effective at trapping these toxic substances. In a similar way heavy metals such as mercury can be removed from the flue gases with activated carbon.

(5) Filtration. Filtration is an important step in flue gas cleaning. Apart from the need to remove dust particles from the flue gases because of their abrasive effect on plant internals, particulate matter must be effectively removed because it becomes loaded with harmful elements including heavy metals, dioxins and furans.

6. RESIDUAL BY-PRODUCTS

Several solid residuals are produced by thermal treatment processing, including bottom ash, fly ash, and scrubber product (George, 1993). Management of these solid residuals is an essential part of the design and operation of a thermal treatment system.

Bottom Ash

The unburned and no burnable portion of municipal solid waste is known as bottom ash. In a mass-fired facility, bottom ash can contains considerable amounts of metals and glass as well as unburned organics. Less metal and glass occur in the bottom ash from RDF-fired facilities, because most of this material has already been removed from the waste stream. Bottom ash from most municipal solid waste combustion systems is landfilled without processing. It is possible to recover metals and other materials from bottom ash by magnetic separation and screening. The limiting factor is finding a market for materials. Bottom ash can also been used as an alternative material for road base construction.

Fly Ash

As the efficiency of air pollution control systems increases, greater proportions of particulates, or fly ash, are removed from the flue gases. Particulate removal efficiencies exceeding 99 percent are common with fabric filter systems. The resulting fly ash is another solid residual, which must be managed.

Because fly ash is composed of the micron and submicron particulates that have been collected by the air pollution control system, it must be handled very carefully to avoid fugitive dust emissions, which may be harmful to workers and the surrounding environment. Fly ash should be removed from collection devices with pneumatic conveyors and transported in closed containers to an acceptable disposal site. When permitted by local regulations, fly ash can also be moistened and mixed with bottom ash prior to disposal.

Scrubber Product

Scrubber product is the sludge produced by a wet scrubber used for SO₂ and acid gas cleanup. Scrubber product consists of the calcium and sodium sulfate salts formed in the scrubbing reaction as well as trace organics and heavy metals. Management of scrubber product includes dewatering, to reduce volume, and subsequent disposal of the sludge as a solid residue and the supernatant as a wastewater.

7.ENERGY RECOVERY

By its very nature Municipal Solid Waste is a heterogeneous substance composed of a range of materials (Han kohl, 1999). Figure 6 illustrates a typical composition for MSW in the United Kingdom, which could be expected from developed nations and Figure 7 illustrates a typical composition of SRF (solid recovery fuel).

Figure 6: Indicative municipal solid waste composition (Han kohl, 1999)

Figure 7: Solid recovery fuel composition (Han kohl, 1999)

Thus it can be seen that energy recovery from thermal waste treatment plants is possible because of the waste itself contains large amounts of thermal energy ready to be liberated either by combustion or by synthesis gas production followed by combustion. High pressure steam is used by practically all fuel fired electricity generating stations in order to produce electrical power. In this regard, thermal treatment plants are no different and will typically raise steam at a pressure of 40bar and a temperature of 400^oC (Rudden, 2000). This high pressure steam has a high value as it carries the potential to do much work. However, this resource must be used nearby and straight away as it cannot be stored.

Electricity is the most valuable form of energy the plant can produce as it may be used in so many different ways. It also comes with the great advantage of being easy to supply to customers via a supply network. However, the net electrical efficiency of a waste combustion facility with energy recovery is only about 20%. This may seem low, but standard steam based power stations have electrical efficiencies of about 30-35% (Rudden, 2000). The lower efficiencies associated with a waste combustion facility are mainly due to increased internal power use (extra handling, processing and flue gas cleaning). Such low efficiencies are still acceptable because of the usefulness of the product i.e. electricity.

In addition to the electrical output from a waste-to-energy plant, there is also the potential to recover ‘waste heat’ from the generation process. Where the possibility exists, the waste heat from such a plant may be up-graded for use by consumers of low-grade heat. The cost of this exercise will be reduced electrical output, but the benefit will be a higher overall plant thermal efficiency due to a higher overall energy recovery rate of 50-60% (Rudden, 2000). This is the Combined Heat and Power (CHP) option which can be very attractive in the proper circumstances i.e. where heat load and power load can be balanced effectively. Combined heat and power production is also feasible with gas engine type installations.

Waste heat in excess of demand e.g. in summer must be dissipated using a nearby water course for example or with air-cooled coils. Waste heat may also be used to dry incoming waste such as sewage sludge or in cases where the moisture content of the input waste is high. The practicality of doing this will of course depend on the individual plant.

8.SITE SELECTION

The identification of a potentially suitable site for a proposed thermal plant must have regard to a number of criteria, not just physical and environmental but also social and economic. It must be carried out in accordance with best international practice.

The primary aims of the site selection process could be considered to be as follows:

• To minimise environmental impacts.
• To maximise acceptability of the project by the local community.
• To minimise the cost of the development.

The criteria for the identification of a suitable site for a proposed thermal treatment plant will depend on the potential impacts both real and, to a lesser extent, perceived which arise from such a facility. It is necessary that the assessment of these potential impacts and the subsequent development of the site criteria be seen as logical, transparent and above all defensible in the public arena. It is proposed therefore that a staged process be carried out which progressively excludes unsuitable areas before identifying and shortlisting potentially suitable sites based on agreed criteria. The procedure to be adopted is as follows:

Step 1: Define the study area by reference to the special limits of waste collection and transportation. Define the site requirements for the project proposed including the size of site and services required.

Step 2: Define the site selection and exclusionary criteria with relevant interested parties.

Step 3: Identify areas for potential sites by application of agreed objectives and any exclusionary criteria.

Step 4: Sieve and screen the sites identified in Step 3 to identify possible sites against agreed criteria.

Step 5: Compare the shortlisted sites using agreed criteria

9. ECONOMICS

MSW thermal treatment is an advanced waste treatment technology which is costly to implement, operate, and maintain. Normally, MSW thermal treatment furnaces are designed with a capacity limit of about 20 to 30 metric tons/h. The recommendation is 10 to 20 metric tons/h. It is recommended to divide the total plant capacity into two or more identical incineration lines, thus improving the plant’s flexibility and availability—for example, when one line is closed for maintenance.

The investments in an MSW plant depend to a great extent on the required form of energy output (Han kohl, 1999). The least expensive plants are those equipped with hot water boilers only. Production of steam and electricity makes the investments in mechanical plant and civil works much higher (about 40 percent). The operating costs are also higher for electricity producing facilities.

Figures 8 and 9 indicate estimated investments and net operating costs as a function of the annual amount of waste processed at power generating plants. The figures indicate a significant scale of economy with respect to investment as well as net treatment costs.

Figure 8: Estimated Cost of Incineration Plant (Han kohl, 1999)

Figure 9: Estimated Net Treatment Cost (Han kohl, 1999)

10. APPLICABILITY OF THERMAL TREATMENT

MSW thermal treatment projects are immediately applicable only if the following overall criteria are fulfilled.

· A mature and well-functioning waste management system has been in place for a number of years.

· Solid waste is disposed of at controlled and well operated landfills.

· The supply of combustible waste will be stable and amount to at least 50,000 metric tons/year.

· The lower calorific value must on average be at least 7 MJ/kg, and must never fall below 6 MJ/kg in any season (Han kohl, 1999).

· The community is willing to absorb the increased treatment cost through management charges, tipping fees, and tax-based subsidies.

· Skilled staff can be recruited and maintained.

· The planning environment of the community is stable enough to allow a planning horizon of 15 years or more.

11. ADVANTAGES AND DISADVANTAGES

The advantages and disadvantages of thermal treatment could be illustrated in the following chart (Philp, 1996):

Advantages Disadvantages

1) The process is assumed to produce a high degree of contaminant of wastes.

2) The additives used can be relatively inexpensive.

3) Can be performed without close contact between workers and waste materials.

4) Can be located close to the center of gravity of waste generation, thus reducing the cost of waste transportation.

5) Reducing the original volume of combustibles by 80 to 95 percent.

6) Organic materials can be decomposed into simpler carbon molecules such as CO₂ and CH₄.

(1)　 Research method.

(2)　 Some constituents (especially metals) can be vaporized and lose before they are bind with the molten silica if high-temperature processes are used.

(3)　 The process is energy-intensive. The waste silicate charge must be heated (often up to 1350c) for melting and fusion.

(4)　 Specialized equipment and trained personnel are required for this type of operation.

(5)　 No experience with organics, potential to create dioxin and other hazards.

In any event, thermal treatment is a better environmental option than landfill for wastes which cannot be recycled particularly in terms of global warming potential. This is principally because landfills emit methane gas while thermal treatment emits carbon dioxide. Methane, even if recovered to some degree is twenty times more powerful as a greenhouse gas than carbon dioxide.

12. CONCLUSION

Thermal treatment offers a means of managing municipal solid waste, thereby reducing landfill requirements, and recovering the energy present in the materials being burned. Thermal treatment technology has evolved dramatically during the past 15 years with the introduction of new system designs. Each modification of these systems has the potential to influence the physical and chemical nature of the residue streams. An appreciation of the differences in technology and how these differences can affect residue quality is necessary to developing an understanding of municipal solid waste incinerator residues and the options available for managing them in a sound environmental manner.

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Dr Niranjan Patel, 2003, MUNICIPAL SOLID WASTE AND ITS ROLE IN SUSTAINABILITY, IEA Bioenergy.

George, 1993, INTEGTATED SOLID WASTE MANAGEMENT, University of California.

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