Municipal solid waste (MSW) combustion plants

Municipal solid waste (MSW) combustion plants

6 Municipal solid waste (MSW) combustion plants L. M. GRILLO, Grillo Engineering Company, USA DOI: 10.1533/9780857096364.2.72 Abstract: Energy was rec...

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6 Municipal solid waste (MSW) combustion plants L. M. GRILLO, Grillo Engineering Company, USA DOI: 10.1533/9780857096364.2.72 Abstract: Energy was recovered from municipal solid waste (MSW) in the United States as early as the 1890s. During the early 1970s, MSW was considered as a fuel and the first ‘modern’ waste to energy facilities were built. Large mass burn facilities were constructed as field-erected grate-fired waterwall furnaces. Smaller facilities were shop-assembled modular systems with combustion taking place on hearths and the energy recovered in waste heat boilers. Refuse-derived fuel (RDF) facilities were developed to prepare a fuel suitable for a boiler designed for that fuel specification. This chapter discusses the mass burn, RDF and modular combustion systems that have been proven in over 30 years of successful operation in the United States. Key words: waste to energy, mass burn, refuse-derived fuel, modular, waste combustion, grate.

6.1

Introduction

Energy has been recovered from municipal solid waste (MSW) in the United States since the 1890s. Those facilities generally provided electricity for in-plant or local use and did not sell excess power to the utilities (Stoller and Niessen, 2009). Most early waste combustors had refractory-lined furnaces and spray chambers to reduce the flue-gas temperature and control particulate emissions. In the late 1960s and 1970s, many combustors were shut down due to public dissent over stack emissions and the passage of the Clean Air Act in 1970. During the early 1970s, with the first of several energy crises, MSW was looked upon as a fuel. The first ‘modern’ waste to energy facilities were built at that time. These were designed as furnaces with integral waterwall or waste heat boilers to recover the energy in the form of steam. Most facilities used the steam to generate electricity. In the late 1970s, several attempts were made to recover recyclable materials from MSW. After recovery, the remaining material was refuse-derived fuel (RDF). Early attempts to use this form of RDF were unsuccessful. Many facilities attempted to co-combust RDF with conventional fossil fuels in power plants. The combustion characteristics of the fuel, the material handling problems and the resulting residue resulted in the utilities refusing to accept the RDF. The recovered materials were also too contaminated to be sold. As a result, these early RDF facilities were unsuccessful. 72 © Woodhead Publishing Limited, 2013

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Later RDF facilities started with the concept of preparing a fuel suitable for firing in a boiler designed specifically for that fuel specification. The materials that were removed, with the exception of ferrous metals, were discarded. These RDF facilities had a great deal of success, and most are still operating after 25 years. Mass burn facilities continued to be constructed through the 1980s and up to the mid-1990s. Larger facilities, generally greater than 400 tons per day, were built as field-erected waterwall furnaces. Smaller facilities, processing less than 400 tons per day, were generally shop-assembled modular systems with combustion taking place on hearths and the energy recovered in waste heat boilers. These units had the advantage of low capital cost, but had lower efficiency than the waterwall boilers. They also did not achieve the high degree of burnout that was typical of waterwall applications. This chapter discusses the mass burn, RDF and modular combustion systems that have been proven through over 30 years of successful operation in the US.

6.2

Principles of combustion

Table 6.1 shows the theoretical amount of air needed to combust the elements contained in waste. The amount of oxygen required for combustion can be determined from the ratio of molecular weights and the following reactions: Carbon: C + O2 = CO2

[6.1]

Atomic weight:

12 + 32 = 44

Ratio:

1 + 2.667 = 3.667

Hydrogen: 2H2 + O2 = 2H2O

[6.2]

Atomic weight:

4 + 32 = 36

Ratio:

1+8=9

Sulfur: S + O2 = SO2

[6.3]

Atomic weight:

32 + 32 = 64

Ratio:

1+1=2

The amount of air required for the above reactions is 4.32 times the weight of oxygen, based on air containing 23.15% oxygen by weight. The remainder of the weight of air is considered to consist of nitrogen only (Velzy and Grillo, 2007a).

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Table 6.1 Stoichiometric air calculations Combustible

Pounds per pound of combustible Required for combustion

Carbon Hydrogen Sulfur

Products of combustion

O2

Natm

Air

CO2

2.67 8.00 1.00

8.87 26.57 3.32

11.54 34.57 4.32

3.67

H2O

SO2

Natm

2.00

8.87 26.57 3.32

9.00

An ultimate analysis of a solid fuel consists of the weight fractions of carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine, moisture and ash. The weight fractions are determined using American Society of Testing and Materials (ASTM) standards for all components except oxygen, which is determined by difference (Velzy and Grillo, 2007b). An assumed ultimate analysis of waste is shown in Table 6.2, which will be used for sample calculations in the following sections. The higher heating value (HHV) is the lower heating value (LHV) plus the latent heat contained in water vapor resulting from combustion (Hammerschlag, et al., 2007). The HHV of waste can be estimated using Dulong’s formula (Schlesinger, 2007): HHV (BTU/lb) = 14 544C + 62 028(H – O/8) + 4050S

[6.4]

where C is the weight fraction of carbon, H is the weight fraction of hydrogen, O is the weight fraction of oxygen and S is the weight fraction of sulfur. For the example calculation, the HHV is calculated to be 5306 BTU/lb using Dulong’s formula. The LHV of the waste is the total quantity of sensible heat released during combustion.

Table 6.2 Ultimate analysis of waste Component Carbon Hydrogen Oxygen Nitrogen Sulfur Chlorine Ash Moisture Total

Percentage by weight 30.0 4.0 20.0 0.5 0.3 0.5 21.0 23.7 100.0

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The LHV is calculated as follows (Velzy and Grillo, 2007b): LHV = HHV – [WH2O + (9 × WH)] × 1050

[6.5]

where WH2O is the weight fraction of moisture in the fuel and WH is the weight fraction of hydrogen in the fuel. For the example calculation, the LHV is calculated to be 4679 BTU/lb. The stoichiometric quantity of air is that which is needed to completely oxidize all of the combustible matter in the waste. Excess air is the amount of additional air that is injected to ensure complete burnout. The stoichiometric quantity of air can be calculated from the ultimate analysis of the waste. Table 6.3 shows the sample theoretical air calculation for the ultimate analysis contained in Table 6.2. The theoretical stoichiometric air for the sample calculation is 3.991 pounds of air per pound of waste. This does not account for any excess air. A facility designed for 100% excess air would require 7.982 pounds of air per pound of waste. Table 6.4 shows the products of combustion for the sample calculation. A total of 4.828 pounds of flue gas would be generated per pound of waste combusted, with no excess air. At 100% excess air, the flue gas is 8.871 pounds of flue gas per pound of waste. To check the calculation, the sum of the products of combustion should equal the theoretical air plus moisture in the combustion air plus the weight fraction of carbon, hydrogen, oxygen, nitrogen, sulfur and moisture from the ultimate analysis. In this case for stoichiometric conditions, 4.828 = 3.991 + 0.052 + 0.30 + 0.040 + 0.20 + 0.005 + 0.003 + 0.237 and for 100% excess air, 8.871 = 7.982 + 0.104 + 0.30 + 0.040 + 0.20 + 0.005 + 0.003 + 0.237.

Table 6.3 Sample theoretical air calculation Substance

Weight fraction

Carbon 0.300 Hydrogen 0.040 Sulfur 0.003 Total Less oxygen in the fuel Theoretical air required

Oxygen required Theoretical for combustion, oxygen, lb/lb lb/lb of element of element a

Theoretical dry air, lb/lb of elementb

2.67 8.00 1.00

3.460 1.382 0.013 4.855 (0.864) 3.991

0.801 0.320 0.003 1.124 (0.200) 0.924

Notes: a b

Weight fraction of the substance times the oxygen required for combustion. Theoretical oxygen times 4.32.

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Table 6.4 Sample products of combustion calculation Substance

lb/lb of element

Pounds of product at 0% excess air

Pounds of product at 100% excess air

Carbon dioxide Moisture from hydrogen Oxygen Nitrogen Sulfur dioxide Moisture from fuel Moisture from air Total moisture

3.67 9.00

1.101a 0.360a

1.101 0.360

2.00 1.00

0.000 3.072b 0.006a 0.237a 0.052c 0.649

0.924 6.139 0.006 0.237 0.104 c 0.701

4.828

8.871

Total Notes: a

Weight fraction of substance times lb/lb of element. Theoretical dry air times 0.7686 plus weight fraction nitrogen in fuel. c Specific humidity of air at 80°F and 60% relative humidity gives 0.013 pounds of moisture per pound of dry air. b

The maximum temperature occurs at the point when stoichiometric air is completely combusted and there is no excess air. The temperature developed during combustion can be calculated using the following equation (Velzy and Grillo, 2007b): Tcomb = Ta + LHV / [(WW × CpW) + (WFG – WW) × CpFG]

[6.6]

where Tcomb is the combustion temperature, Ta is the ambient temperature, LHV is the lower heating value of the waste, WW is the weight of water in the flue gas, CpW is the heat capacity of moisture in the flue gas (= 0.55), WFG is the total weight of the flue gas and CpFG is the heat capacity of dry flue gas (= 0.28). The maximum temperature (at stoichiometric conditions) is calculated to be 3124°F. The combustion temperature at 100% excess air is calculated to be 2105°F. Excess air contributes to boiler efficiency loss, and should be kept to a minimum to maximize boiler efficiency.

6.3

Mass burn waterwall combustion systems

Mass burn waterwall facilities combust unprocessed waste that is fed directly into the furnace. These facilities generally process 400 tons per day of MSW or more. There is no presorting or materials recovery prior to combustion. The material is introduced into the furnace where it passes through drying, combustion and burnout stages before the residue is discharged. Combustion occurs in the furnace and the flue gas passes through an integral waterwall boiler, superheater, generation bank and economizer, producing steam.

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Some technologies also incorporate an air heater to preheat combustion air. Flue gases then pass through the air pollution control devices.

6.3.1 Waste storage and handling Tipping areas are enclosed to prevent odors and litter from escaping the building. Combustion air is drawn from the tipping and storage area to induce a negative draft in the building to control the release of these materials further. Using this air for combustion also destroys any odors that are present when the air is exposed to the high temperature. The waste should be handled on a first in/first out basis to reduce the potential for odor and decomposition. Approximately four days of storage is necessary to ensure the availability of fuel over long weekends and during periods when deliveries are low. Waste can be stored in a pit or on a floor. Most large facilities use a pit to store the waste. The pit is long, narrow and typically 30 to 40 feet deep, and runs in front of all of the units in the facility. The waste is dumped into the pit and mixed and stacked using an overhead crane that spans the width of the pit. The crane also retrieves the waste and feeds it to the charging hoppers. Depending on the depth of the pit, the waste can have an average density of 15 to 25 lb/cf. The size of the pit is determined by the throughput of the facility and the ability to store about four days of waste. The available storage is calculated using the pit depth from the bottom of the pit to the tipping floor plus waste that is stacked above the tipping floor level. The available volume above the tipping floor is calculated from the edge of the tipping floor to the back wall of the pit, assuming the waste is stacked at a 45° angle. During operation, a trench is dug by the crane along the front of the pit along the tipping floor. This trench allows trucks to dump into the pit when the pit is relatively full. The waste that is dug from the trench is stacked against the rear wall. Although the angle of repose of MSW is approximately 45° when dropped onto a pile, waste in a pit can be handled and stacked so it forms a vertical wall. As time passes, the waste compacts in the pit and must be ‘fluffed’ by the crane before being fed to the charging hoppers. The fluffing breaks up clumps in the waste and allows for better combustion in the furnace. The waste is dug up by the grapple and released slowly above the waste in the pit, allowing it to loosen as it drops. It is then picked up again by the crane and delivered to the charging hoppers. When excavation for construction of a pit is not possible due to high groundwater or other geologic conditions, a tipping floor may be used instead of a pit for storage. Operation of a tipping floor is more labor intensive, since the waste must be moved, stacked, retrieved and fed using front-end loaders. When a tipping floor is used, the density of the waste should be assumed to be 12 to 15 lb/cf when calculating the storage area. The density is lower because the loaders can only

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stack the waste to about 20 feet high. The sides of the piles should be assumed to have a 45° angle when calculating the storage area. Some facilities use a bulldozer or similar equipment to densify the waste periodically to increase storage. Densifying the waste increases the energy and labor required to operate the facility. This procedure is usually employed when additional storage is needed because a unit is out of service and waste quantities are high. It is not the recommended normal operating mode when using a tipping floor for storage. When a tipping floor is used, the trucks dump on the floor and a front-end loader pushes the waste into the storage pile. The loader retrieves waste from a different area to feed to the furnaces. Again, the oldest waste is recovered first to reduce odor and decomposition of the waste. The loaders feed the waste onto a conveyor, which elevates it to the charging hopper. The conveyors are usually steel apron pan conveyors because of the potential damage from impact of the waste onto the conveyor.

6.3.2 Feeding All of the systems use a charging hopper and chute that accepts the waste and feeds it to the furnace. The charging hopper is designed with a slope of at least 45° and has steel plates on which the waste is dumped. The waste is deposited on the slope of the hopper, where it slides into the charging chute toward the boiler. Feeding waste directly over the charging chute often causes jams in the chute. The waste is held back on the slope by the waste in the chute. As the waste is pushed into the furnace from the bottom of the chute, the waste in the chute falls by gravity, allowing the waste in the hopper to slide slowly into the top of the chute. When the hopper is nearly empty, the crane deposits another load onto the hopper slope. The waste in the charging chute forms an air seal to the boiler. The charging chute usually has a cut-off gate to prevent back fires and maintain an air seal during start-up and shutdown. The lower part of the charging chute is frequently water cooled to protect it from the high temperature in the furnace. At the bottom of the charging chute is a hydraulic ram feeder that pushes the waste into the furnace and onto the grates. There is usually one ram feeder for each grate section. The rams may operate in parallel (all stroking together) or in alternating mode.

6.3.3 Grates The basic difference between the various technologies is the means by which the waste is transported through the furnace during the combustion process. The primary purpose of the grate is to convey the waste automatically from the ram feeder to the residue discharge point. A second purpose is to tumble the waste to

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ensure that all of the waste is exposed to the combustion air and that complete combustion occurs. Grates can use a reciprocating motion, rotary drum, rollers or other means to move the waste through the furnace. Rocking grates were used in older combustors. However, they have not been used in facilities in the past 20 years. Reciprocating grate The reciprocating grate is the most common grate used in mass burn waterwall facilities. This type of grate uses a step action with alternating stationary and moving grates to push the waste through the furnace (see Fig. 6.1). As the waste is pushed over a stationary grate, it tumbles, exposing unburned particles to the combustion air and allowing good burnout. Grate manufacturers generally have a standard length of grate with the same number of steps, air zones, etc. The width of the grate determines the throughput of the unit. The width of the grate can be increased to a maximum size, then additional grate sections are added. Thus, facilities with high throughputs have wide grates with several independent grate runs. Each grate run would be fed by a dedicated ram feeder, and contain separate underfire air compartments and grate drives. This allows for independent control of the various grate runs for better combustion control. The size of the grate is determined by the grate heat release rate, which should be between 250 000 and 300 000 BTU/sf/hr (Velzy and Grillo, 2007b). Underfire air is injected into the hoppers below the grate. Air penetrates through the grate to keep the grate cool and promote combustion. The grates have between three and five zones from the inlet point to the discharge. At each zone change,

6.1 Reciprocating grate.

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there is a drop of one to two feet to promote additional tumbling. The waste passes through drying, combustion and burnout zones. Most of the air is injected into the zones where active combustion is taking place, which begins about one-quarter of the way down the grate. Little air is fed into the final burnout zone. At the end of the grate, the residue falls off the last step into the residue disposal system. Most systems use either a water-filled ash discharger or water-filled trough to quench the residue and maintain an air seal with the boiler. Residue is then transported by conveyor to the disposal system. System suppliers that provide traditional reciprocating grate technology include Von Roll (Wheelabrator) (air-cooled or water-cooled grate) (Owens and Sczcepkowski, 2010), Foster Wheeler, Steinmuller, Detroit Stoker and Keppel Seghers (air-cooled or water-cooled grate). Reverse reciprocating grate The Martin grate (see Fig. 6.2) uses a reverse acting reciprocating grate. Alternating stationary and moving grates are on a 26° downward angle and push the waste

6.2 Reverse reciprocating grate.

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upward, causing it to flip over the grate and tumble downward through the furnace. This motion provides a good deal of tumbling and mixing of the waste, exposing unburned material for good burnout. Three grate drive zones move the waste through the drying, combustion and burnout zones of the furnace. Each grate drive can be controlled individually so the combustion may be controlled within the burning zones. Underfire air is supplied through five individually-controlled air zones, providing the proper amount of air for the drying, combustion and burnout stages. Roller grate Figure 6.3 shows a cross section of the Deutsche Babcock roller grate. It consists of five or six cylinders, depending on the throughput, on a 30° downward angle that transport the waste through the furnace. The drums are made of cast iron grate sections that rotate at three to six revolutions per hour, causing a tumbling action on the grate. As the burning waste moves down the slope, the rotating speed of each successive roller is reduced to keep the fuel bed thickness approximately uniform. Combustion air, which can be controlled in each combustion zone, enters the interior of each drum from both ends and flows through the many small gaps in between the interlocking grate bars (Hickman, 2003). Rotary combustor Figure 6.4 shows a cross section of an O’Connor rotary combustor. The combustor is a cylinder approximately 40 feet long, approximately 14 feet in diameter and on

6.3 Roller grate.

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6.4 Cross section of a rotary combustor (O’Connor, 1984).

a 6° downward angle. The actual length and diameter vary depending on the throughput. The cylinder has a series of tubes and membranes running the length of the combustor. The membranes have holes to allow combustion air to enter the cylinder. The tubes and membranes are formed into a cylinder in which the MSW combusts, and are an integral part of the boiler, containing hot water and steam. The tubes of the combustor are connected at each end by a ring header, which is larger in diameter than the tubes. MSW is pushed into the combustor by ram feeders. Heated combustion air is injected through the holes in the membrane. The combustor has four combustion air zones, and air entering below the MSW (underfire air) and above the MSW (overfire air) can be controlled to ensure good combustion. The combustor rotates at about six revolutions per hour. As the combustor rotates, MSW tumbles and moves toward the discharge end of the combustor. Combustion continues until the waste reaches the end of the combustor and falls onto an afterburning grate to complete the burnout.

6.3.4 Combustion air Combustion air is injected into a furnace as underfire air (below the grate) and overfire air (into the flame above the grate). Generally, 50% to 70% of the total air is underfire air and the remaining portion is overfire air. Most mass burn furnaces operate with between 50% and 100% excess air. Combustion air and flue-gas handling equipment should be sized for 100% excess air.

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Combustion air is drawn from the waste storage area for two reasons. First, it maintains a negative draft in the tipping area, preventing litter and odor from escaping the building. It also destroys the odor by exposing it to the high temperatures in the furnace. Underfire air is frequently preheated using steam coil air heaters. Heated air assists with drying the waste and allows for more stable operation. Using heated air slightly reduces the throughput of the unit because of the heat added to the furnace in the air. Underfire air is directed by a forced draft fan that blows the air into a plenum below the grates. Most furnace designs have multiple air plenums along the length of the grate. This allows the operator to control the amount of combustion air as the waste passes through the drying, combustion and burnout stages. The underfire air enters the furnace through slots or spaces between the grate components. Underfire air passes through the fuel bed, providing air for drying and combustion. Overfire air is generally not heated. It may be drawn from either a common air duct from the receiving and storage area or from a separate source, such as the upper level of the boiler house. Overfire air is injected through a series of nozzles, usually located on the back and/or front wall of the furnace above the grate. The air is injected at a higher pressure than underfire air, and often requires a booster fan if the source of the air is the same as underfire air. Overfire air is injected into the active flame zone to provide additional air to complete burnout of the volatile gases, which are produced by heating the waste. It provides the turbulence needed to completely mix the flue gas to ensure good combustion.

6.3.5 Boilers Boiler designs for mass burn facilities generally include a radiant pass above the grate with integral waterwall tubes. The superheater generally is the next component with two to four sections. Most facilities have a desuperheater between the final two stages of the superheater to control steam temperature. The superheater is followed by an evaporator section and bare tube economizer. Some facilities also have a bare tube air heater to preheat the combustion air following the economizer. The radiant section consists of vertical tubes, nominally two inches in diameter, with a continuously welded membrane between the tubes. This creates an airtight enclosure within which combustion takes place. The tubes are connected by headers along the grate and the roof that connect to the steam drum. The lower section of the tubes, along the grate line, frequently have a refractory coating to protect against abrasion from the fuel as it moves along the grate. The tubes and membranes above the refractory, generally to the top of the radiant section, are overlaid with Inconel. The Inconel protects the tubes from corrosion and erosion by the flue gases. Early mass burn installations did not have this protective overlay and deteriorated rapidly. It is imperative that some coating be used in the radiant section of the boiler to protect the tubes.

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At the top of the radiant section, the flue gases turn and pass through a superheater. Here, saturated steam enters the superheater from the steam drum and rises in temperature. Most boilers have two to four superheater sections. At the inlet of the final stage, a desuperheater is used to maintain the desired outlet temperature by spraying boiler feedwater into the steam. Superheaters are generally either shielded or overlaid in part with Inconel to protect against corrosion and erosion. After passing through the superheater, the flue gas passes through an evaporator section or generation bank. This may consist of an upper steam drum and lower mud drum connected with a series of tubes. Alternatively, additional waterwall passes provide for water evaporation without the drums. In this case, a series of headers connects the various boiler sections. The evaporator section adds sufficient energy to convert the saturated water to saturated steam. The economizer follows the evaporator section of the boiler. Boiler feedwater is pumped at high pressure into the economizer. The economizer raises the temperature of the water up to the saturation point before it goes into the evaporator section. Economizer tubes in waste combustion facilities have bare tubes (no fins) to reduce fouling due to the presence of particulate matter. Some facilities incorporate an air heater as the final heat trap. This raises the combustion air temperature to as much as 500°F. The air heater tubes usually do not have fins due to the presence of particulate matter. Combustion air flows through the tubes and flue gas flows outside the tubes.

6.3.6 Residue After combustion is complete, the burned out residue is discharged from the grate into a water-filled discharger. The water cools the residue and maintains a seal to prevent tramp air from entering the boiler. Two types of residue dischargers are commonly used. The first is an extractor, which is a water-filled container below the end of the grate. The extractor has a ram that extends periodically to force the residue up an inclined chute where it is partially dewatered. When the chute is full of residue, each stroke of the ram forces some residue out of the extractor onto a conveyor that removes it for storage. The second type of discharger is a water-filled trough that contains a drag conveyor. The residue is quenched by the water and removed by the drag conveyor. In small units with narrow grates, the trough may be oriented in the same direction as the grate. Each unit would have a dedicated conveyor for removing the residue. In larger units, with wide grates, the trough usually runs perpendicular to the grates and collects residue from several units. This orientation would have a redundant conveyor system, where the residue may be discharged into either of two conveyor systems through a bifurcated chute. The drag conveyors move slowly and dewater the residue as the conveyor inclines upward out of the water for discharge to another conveyor to take the residue to storage. The extractor type

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of residue discharger results in better dewatering, resulting in lower moisture in the residue. In addition to residue that falls off the grate, some small particles of residue fall through the grate. These siftings, or riddlings, are collected and conveyed to the extractor or trough and removed with the bottom ash.

6.3.7 Air pollution control After the flue gas leaves the boiler, it passes through a series of air pollution control devices to remove acidic gases, particulates, oxides of nitrogen, mercury, heavy metals and other pollutants. This equipment is discussed in detail in Chapter 11.

6.4

Refuse-derived fuel (RDF) combustion systems

Unlike mass burn combustion systems, which process waste directly with no presorting, RDF systems produce a more uniform material prior to combustion. The fuel processing may be done at a remote facility or at a facility adjacent to the combustion plant. Co-locating the processing and combustion facilities eliminates the additional expense of transporting the RDF.

6.4.1 Waste storage and handling RDF facilities must have provision for storing both raw waste and the prepared fuel. Roughly two days of raw waste storage and four days of RDF storage are usually provided. RDF facilities generally use a tipping floor instead of a pit to store both unprocessed and processed waste. Raw waste and RDF are stored in separate buildings and pushed into piles using front-end loaders. The loaders also recover the waste and feed it to the processing lines and feed the RDF to the boiler feed lines. As with mass burn facilities, the tipping and RDF storage areas are enclosed to prevent odor and litter from escaping the building. In most cases, combustion air is drawn from the areas to induce a negative draft in the building to control the release of these materials further.

6.4.2 RDF preparation All RDF facilities include shredding and ferrous metal removal, and some incorporate additional screening steps to remove grit or use eddy current separation for non-ferrous metal recovery. Primary shredding may be accomplished with a flail mill, or bag breaker, to coarse shred the waste to a nominal 12 inch particle size and expose it to the downstream equipment. Ferrous metal separation can be accomplished with a drum or belt magnet. Further classification can be achieved

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using a trommel, disc or other type of screening equipment to remove grit and non-combustible material and sort the waste by size. Small particles of waste that are suitably sized for RDF bypasses further equipment, while material with a larger particle size goes on to a second stage of shredding. The desired final particle size is generally two to six inches. Details of the types of equipment used for RDF production are included in Chapter 5. Some RDF facilities, called ‘shred and burn’, are of a simpler design. They include a single stage of primary shredding followed by ferrous metal removal. There is no further classification of the waste, and the resulting RDF is appropriately sized for the boiler.

6.4.3 Feeding RDF is recovered from storage by front-end loaders, which push the fuel onto apron conveyors. Apron conveyors are used because they can withstand the impact of the weight of the fuel falling onto them and they can elevate the fuel at a relatively steep angle. A series of belt conveyors and diverters are used after the apron conveyors to deliver the fuel to the front of the boiler. One boiler feed chute per approximately five feet of boiler width are needed. Two types of boiler feed systems are used: storage bin and direct feed by conveyor. The storage bin method consists of a bin that is the width of the boiler, which holds about a 15 minute supply of fuel. Screw augers on the floor of the bin remove the fuel and meter it into the individual feed chutes that lead to the boiler. The direct feed method provides an oversupply of RDF to the boiler face. Each boiler feed chute has a small bin that holds about a 15 minute supply of fuel. The bins are constantly overfed and the excess fuel is returned to RDF storage. Each bin has a dedicated ram feeder and drag conveyor to remove the RDF and meter it to the boiler feed chute (Gittinger and Arvan, 1998).

6.4.4 Combustion systems RDF is most commonly combusted on a spreader-stoker traveling grate (see Fig. 6.5), similar to the type used for coal combustion. The fuel flows by gravity down the feed chute at the front of the boiler and is blown to the back of the grate through an air swept spout. Much of the combustion of light materials occurs above the grate. Heavier materials that require more time for combustion fall onto the grate. The grate moves slowly from the back to the front, discharging the burned out residue into a water quench trough at the front of the boiler. The boiler design is similar to that of mass burn systems. Underfire combustion air enters through a plenum beneath the grate, and passes through the fuel bed. Overfire air is injected above the grate to complete combustion of the volatile gases. Since the fuel is more homogeneous than unprocessed waste, the combustion process can be accomplished with a lower amount of excess air. RDF facilities

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6.5 Spreader- stoker traveling grate (Russell and Roberts, 1984).

usually operate with between 30% and 50% excess air. This improves the efficiency of the process, and a greater amount of energy can be extracted from the fuel. Another type of combustor that has been used with RDF with limited success is the fluidized bed. Fluidized beds are reactors that have a sand or similar medium and jets of air that are injected vertically from the bottom of the bed. The air jets cause the sand to become fluidized. Waste is injected into the hot sand bed, where combustion occurs. The turbulent mixing of solids provides a high degree of heat transfer. Once heated, the medium retains its heat, causing a stable combustion process. Flue gases rise upward and out of the reactor, then pass through a boiler for heat recovery. Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). A sulfur-absorbing chemical, such as limestone or dolomite, may be added to the bed. The mixing action of the fluidized bed brings the flue gases into contact with the sorbent (REI, 2005). Even with the ability to reduce sulfur

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emissions, additional scrubbing equipment is necessary in waste to energy facilities.

6.5

Modular combustion systems

Modular combustion systems are generally constructed in a shop and shipped to the field for erection. The capacity of each unit is generally about 150 tons per day or less. Modular units are similar to mass burn facilities in that they combust unprocessed waste that is fed directly into the furnace without presorting or materials recovery. Primary combustion occurs in a refractory-lined furnace, and the flue gas passes into a secondary combustion chamber where combustion of the flue gas is completed. Flue gas is then directed through a waste heat boiler. The waste heat boiler includes a superheater, generation bank and economizer sections. Flue gases then pass through the air pollution control devices.

6.5.1 Waste storage and handling With few exceptions, modular facilities use a tipping floor and front-end loaders to store and move the waste. As with larger facilities, tipping areas are enclosed to prevent odor and litter from escaping the building and combustion air is drawn from the tipping and storage area. Approximately four days of storage is also provided.

6.5.2 Feeding Modular systems are batch fed by front-end loaders, which push the waste into a hopper in front of the combustion chamber. After the hopper is loaded, a door is closed over it to prevent flue gas from escaping as the waste is fed in. An isolation door separating the charging hopper from the furnace is raised and a hydraulic ram pushes the waste into the first stage of the combustion chamber. The ram retracts, the isolation door closes and the hopper door opens in preparation for the next load. Loads are charged about every ten minutes.

6.5.3 Combustion chamber Most modular facilities burn the waste on refractory-lined hearths. A series of three to seven step hearths usually make up the primary combustion chamber. The hearths have about a one foot drop off the end to allow the waste to tumble as it proceeds through the furnace. A hydraulic ram under each hearth pushes the waste through the furnace. The last ram strokes first, providing space for the material falling from the hearth above. After the ram extends and retracts, the next hearth ram strokes. This continues until all of the rams have stroked. After the last ram

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has stroked at the inlet to the furnace, another hopper full of waste is charged, pushing the waste ahead of it onto the second hearth. When the last ram strokes, it pushes the residue off the hearth into a water-filled trough. The water serves to quench the ash and maintain an air seal for the combustion chamber. The residue is dragged up an incline where it is partially dewatered prior to disposal.

6.5.4 Combustion air Underfire combustion air is injected into the furnace through nozzles in the hearth or in the front face of the hearth rams. Overfire air is injected through nozzles in the roof. Because the waste is combusted on hearths and does not receive the high degree of agitation as with grate systems, a higher amount of excess air is needed to ensure good combustion. Excess air is generally in the 100% to 150% range. Also, burnout is not as good as with modular systems due to the limited agitation. Air is also injected into the secondary combustion chamber, where the products of primary combustion are completely burned out. Some modular designs operate in a ‘starved air’ mode, where sub stoichiometric air conditions are maintained in the primary combustion chamber. This causes gasification of the waste, producing combustible flue gases. The flue gas is burned with excess air in a secondary combustion chamber. Some supplementary fuel such as natural gas may be added to enhance the combustion of the gases.

6.5.5 Boiler The boiler design for modular facilities is different from that of mass burn facilities in that the boilers are not integral waterwall boilers, but are waste heat boilers that are prefabricated in sections in a shop and delivered to the site for assembly. Boiler sections consisting of evaporators, superheaters and economizers are shop assembled and shipped as components to the field for erection. This reduces construction costs substantially. After leaving the boiler, flue gases enter air pollution control devices similar to those used in mass burn facilities.

6.5.6 Manufacturers Enercon Systems Combustors produced by Enercon consist of a two-stage excess air combustion process (see Fig. 6.6). Underfire air is injected through nozzles in the hearth rams. A system of ‘push rods’ driven by the ash transfer rams provides cleaning of these holes with every ram stroke (Clark, 1996). Dual-fuel burners, accepting gas or oil, are located in the primary chambers and are used for initial ignition of the refuse.

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6.6 Enercon Systems MSW combustor (RFG: recirculated flue gas).

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These are turned off after the fire is established (Clark, 1982). Overfire air is provided by nozzles in the roof of the primary chamber. Flue gas from the primary combustion chamber enters a secondary combustion chamber where secondary air is added to complete combustion. Fossil fuel burners are installed at the end of the secondary combustion chamber to maintain constant temperature of the flue gas entering the waste heat boiler. However, these burners are rarely used (Clark, 1982). Recirculated flue gas is used for underfire air and for furnace cooling and combustion gas cooling prior to entering the waste heat boiler. Using recirculated flue gas for furnace cooling reduces the quantity of fresh air needed, thereby increasing thermal efficiency and minimizing thermal NOx formation (Clark, 1996). ConsuTech (Consumat) Consumat combustors use a starved air process with two chambers: the primary (or lower) chamber and the secondary (or upper) chamber. Combustion in each chamber is controlled independently to ensure efficient waste processing in the lower chamber and complete combustion of flue gases in the upper chamber. The units comprise a refractory-lined combustion chamber (no grate is required) into which waste is loaded. Air is supplied to this chamber at a rate less than that required for complete combustion. The waste is initially ignited by an auxiliary burner and undergoes essentially a gasification/pyrolysis process. For normal wastes, the reaction will proceed without the need for additional fuel. Waste decomposes under quiescent conditions, therefore carry-over of particulate matter is minimized (Consumat, 2004). The partial combustion products pass into an afterburning secondary chamber, which is mounted immediately above the main combustion chamber. The gases are mixed with additional air at an increased temperature to ensure successful burnout of particulate matter to eliminate smoke. This chamber is designed to retain the combustion gases at 1800°F with a retention time based on waste composition and regulatory guidelines. Combustion air is supplied to the lower and upper chambers independently. The rate of air supply to both chambers is automatically controlled to provide the correct combustion conditions. The lower chamber operates at low interior gas velocities under controlled temperature conditions. The amount of heat released from the burning waste is controlled by limiting the air added into the lower chamber to less than what is required for complete combustion of the waste. Underfire air is introduced into the lower chamber through air holes located under the waste. Sufficient heat is released to keep the waste burning for partial combustion. The combustion gases then pass into the upper chamber through a turbulent mixing zone where ignition takes place and additional air is provided to complete

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the oxidation reactions. Oxidation of the combustible products is completed in the upper chamber (Consumat, 2004). Laurent Bouillet A Laurent Bouillet combustor is different from other modular systems. The combustors have a refractory-lined drum that rotates for 210° then reverses. The waste tumbles through the drum where combustion occurs. Flue gases exit the combustor and enter a tranquilization chamber where combustion is completed. Flue gases then pass through the boiler and air pollution control devices (MMWAC, 2010).

6.6

Advantages and limitations

The types of combustors discussed above have all been proven in commercial operation for over 25 years. While many systems are similar within each classification of combustor, each type has advantages and limitations, as discussed below.

6.6.1 Efficiency Boiler efficiency can be calculated using ASME Performance Test Codes (PTC). PTC 34 is used for waste combustors with energy recovery using the boiler as a calorimeter. Using this method, the heat outputs, losses and credits must be calculated. Heat credits correct the efficiency to standard ambient conditions, such as air temperature. When comparing the efficiency of different systems, the heat credits do not matter, since they would be substantially the same for any system. Heat losses have the most significant impact on efficiency. Many parameters that contribute to heat loss, such as moisture and hydrogen in the fuel, are independent of the type of technology. Others, such as loss due to carbon monoxide in the flue gas, are small and can be ignored when comparing systems. The most significant losses that can be attributed to the type of technology are the dry gas loss, water-from-fuel loss and loss due to unburned carbon in the residue. The dry gas loss is dependent on the flue-gas exit temperature from the boiler and the amount of excess air. The water-from-fuel loss is also dependent on the flue-gas exit temperature. The unburned carbon in the residue is related to how complete combustion is. The three main factors in determining efficiency are therefore flue-gas exit temperature, excess air and burnout, all of which are dependent on the technology. The flue-gas exit temperature is related to boiler design rather than the combustor. A well-designed boiler can achieve low exit gas temperatures for any of the technologies.

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Excess air and burnout are the two main contributors to heat loss that are strictly combustor related. They are also related to each other. Insufficient excess air will increase the amount of unburned carbon. Less excess air will improve efficiency, while the resulting higher amount of unburned carbon will reduce efficiency. A careful balance of excess air and burnout must be made to optimize efficiency in any system. Of the three types of technologies discussed, modular combustors use the greatest amount of excess air and have the worst burnout, resulting in the lowest efficiencies, generally around 55% to 65%. Mass burn uses less excess air and has better burnout, and results in higher efficiencies, around 68% to 72%. RDF burned on a spreader-stoker traveling grate uses the lowest amount of excess air and also results in the best burnout, with efficiencies around 72% to 75%. The reason for the lower excess air and better burnout is that the fuel has been shredded and blown into the furnace, resulting in good mixing of the fuel and combustion air. Mass burn and modular facilities fire unprocessed waste. Items that tend to go through the furnace in clumps, such as telephone books, are not completely exposed to the combustion air, resulting in poorer burnout.

6.6.2 Cost From 1995 to 2007, no new waste to energy facilities were constructed in the US. There were, however, additions to existing facilities and the complete overhaul of systems. Any cost data presented would be dated. In light of the changes in air pollution control based on current regulations, cost data from older facilities is not considered to be reflective of today’s costs. In general, modular combustion facilities are the least costly to construct on a dollars per installed ton of capacity. This is because many of the components are fabricated in the shop, which is a lower cost method of construction. RDF facilities are the most expensive plants to construct on a per ton basis. This is due to the additional need to process the waste prior to combustion. The important factor when evaluating costs is the net cost. This is the amortized cost of construction, plus annual operating and maintenance costs, minus the revenues. The differences in the cost of construction discussed above are offset in both cases by the lower thermal efficiency of the modular units and the higher efficiency of the RDF units. This affects the revenue stream and brings the three types of facilities close in terms of bottom line costs.

6.7

New developments

6.7.1

Seghers PRISM

The Keppel Seghers PRISM technology is a means of increasing combustion and boiler performance, based on the installation of a prism-shaped body in the first

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6.7 Keppel Seghers PRISM combustor.

empty boiler pass (see Fig. 6.7). The prism optimizes secondary air injection, leading to more homogeneous flue-gas conditions. This leads to increased performance and solves some of the corrosion problems that occur in the radiant and superheater sections. The flue-gas temperature measurements in the passages next to the prism regulate the secondary air injection and control the air distribution under the grate (Keppel Seghers, 2007). The flue-gas flow is divided into two partial flows, A and B, prior to entering the radiant pass of the boiler. This division is achieved by means of a membrane-wall construction, in the shape of a prism, which is water cooled and integrated with the natural circulation system of the boiler and protected with a refractory lining (Perilleux and Eeraerts, 2002). Secondary air is injected into the divided flue-gas streams, A and B, through multiple secondary air nozzles. The secondary air injection nozzles are installed in the boiler front and rear walls, and on both sides of the prism. This results in complete burnout of the flue gases just above the prism. Since the prism is located in a turbulent and high-temperature zone, its membrane walls are water cooled and protected with a ceramic coating. The water cooling is integrated into the natural circulation system of the boiler (Perilleux and Eeraerts, 2002). The secondary air nozzles of the prism contain an on-line cleaning system to prevent slag build-up on the nozzles. This is achieved by periodically blowing low-pressure steam into the air nozzles. The fast expansion of the water in the steam removes any slag deposits on the secondary air nozzles during operation (Perilleux and Eeraerts, 2002).

6.7.2 Recirculated flue gas Some of the underfire air that is used in furnaces is needed to penetrate the fuel bed. Using more air than is necessary for combustion raises the excess air and results in lower boiler efficiency. Flue-gas recirculation is the process where a slipstream of flue gas is returned to the furnace as underfire air. This provides the

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air needed to penetrate the fuel bed while improving the efficiency of the boiler since part of the combustion air is replaced by flue gas that has a lower oxygen content. This process has been incorporated into several operating facilities with good results.

6.7.3 Tube metallurgy and coatings Corrosive conditions in waste to energy boilers produce rapid wastage rates of traditional boiler tube materials. It is not unusual to see corrosion rates in the range of 1 to 3 mm/y on carbon steel boiler tubes and occasionally corrosion occurs at even higher rates (Paul et al., 2004). Municipal waste contains various constituents and impurities that induce corrosion attacks on boiler tubing. Among the leading contributors are chlorine, sulfur, zinc, lead, sodium and potassium. During combustion, various metallic chlorides and sulfates as well as HCl and SO2 are formed and then deposited on the cooler surfaces, such as the waterwall and superheater tubes. Based on the low melting points and high vapor pressures of many metallic chlorides, severe corrosion will occur when carbon and low alloy steels are in contact with these chloride salts. Additionally, flue-gas streams can contain HCl gas, which also can cause a chloride attack on steel (Joiner and Lai, 1999). There are four principles for selecting the right metals for boilers:

• • • •

Reducing the iron content in the boiler tube material will reduce iron chloride formation. Replacing iron with nickel. Nickel chlorides have much lower vapor pressures hence better resistance to chloride attack. Relying on an alloy that will create a corrosion protective scale. Chromium is excellent in the formation of a chromium oxide scale even in lower temperature ranges. Utilizing molybdenum because of its resistance to high-temperature chloride attacks at temperatures up to 1100°F (Joiner and Lai, 1999).

Many materials have been evaluated in refuse-to-energy boilers. The most successful of these materials include Alloys 625, 50, 59, 825 and 45TM. The

Table 6.5 Chemistry of common alloys used in waste to energy boilers Alloy

No.

Ni %

Cr %

Mo %

Fe %

Other %

FM625 FM59 FM50 825 45TM

N06625 N06059 N06650 N08825 N06045

63 59 53 42 46

22 23 19.5 21.5 27

9 16 11 3 –

<1 1 14 30.4 23

3.4 Nb

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chemistry of these alloys is given in Table 6.5. Alloys 625, 50 and 59 are applied as a weld overlay onto carbon steel boiler tubes and Alloys 825 and 45TM are used as solid tubing only for superheater applications (Paul et al., 2004).

6.8

Sources of further information

Materials and Energy Recovery Division, American Society of Mechanical Engineers, divisions.asme.org/MER. North American Waste to energy Conference (NAWTEC, annual), www. nawtec.org. Solid Waste Association of North America (SWANA), www.swana.org. Waste to energy Research and Technology Council (WTERT), www.seas. columbia.edu/earth/wtert.

6.9

References

Clark, L (1982), Case history of a 240 ton/day refuse to energy project: Vicon, Crane & Co., Pittsfield, Massachusetts, in Proceedings of the 10th National Waste Processing Conference, New York, ASME, pp. 1–9. Clark, L (1996), Case history of a 240 ton/day resource recovery project: Part II, in Proceedings of the 17th Biennial Waste Processing Conference, New York, ASME, pp. 235–248. Consumat (2004), Consumat controlled-air incinerator, available from www.consutech. com/inciner.htm, accessed 19 January 2011. Gittinger, J and Arvan, W (1998), Considerations for the design of RDF-fired refuse boilers, presented to Power-gen Europe 1998, Milan. Hammerschlag, R, et al. (2007), Energy storage, transmission, and distribution, in Kreith, F and Goswami, D (eds), Handbook of Energy Efficiency and Renewable Energy, Boca Raton, CRC Press, pp. 18-1–18-33. Hickman, L (2003), American Alchemy: the History of Solid Waste Management in the United States, Santa Barbara, CA, Forester Press. Joiner, D and Lai, G (1999), Economic impacts and solutions for waste to energy boiler corrosion management, in North American Waste to energy Conference (NAWTEC 7), New York, ASME, pp. 221–223. Keppel Seghers (2007), Keppel Seghers PRISM – for waste to energy installations, available from www.kepcorp.com/en, accessed 21 January 2011. MMWAC (2010), Mid-Maine Waste Action Corporation, Auburn, ME, available from www.eskerridge.com/MMWAC%20Brochure%20Package.pdf, accessed 19 January 2011. O’Connor, C (1984), The Sumner County mass burning experience, in Proceedings of the 11th Biennial National Waste Processing Conference, New York, ASME, pp. 301–319. Owens, E and Sczcepkowski, J (2010), Advancements in grate cooling technology, in 18th North American Waste to Energy Conference, New York, ASME, pp. 1–3. Paul, L, Clark, G, Eckhardt, M and Hoberg, B (2004), Experience with weld overlay and solid alloy tubing material in waste to energy plants, in 12th North American Waste to Energy Conference, New York, ASME, pp. 111–119.

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Perilleux, M and Eeraerts D (2002), Retrofit of WTE boiler: Case study on Bonn plant, in 10th North American Waste to Energy Conference, New York, ASME, pp. 25–33. REI (2005), Fluidized bed combustion, Renewable Energy Institute, available from www. fluidizedbedcombustion.com, accessed 24 January 2011. Russell, S and Roberts, J (1984), Oxides of nitrogen: Formation and control in resource recovery facilities, in Proceedings of the 11th Biennial National Waste Processing Conference, New York, ASME, pp. 417–423. Schlesinger, MD (2007), Fuels and Furnaces, in Avallone, E, Baumeister, T, and Sadegh, A, Marks’ Standard Handbook for Mechanical Engineers, 11th edition, New York, McGraw Hill, p. 7–6. Stoller, P and Niessen, W (2009), Lessons learned from the 1970s experiments in solid waste conversion technologies, 17th Annual North American Waste to energy Conference, NAWTEC17-2348, 1. Velzy, C and Grillo, L (2007a), Fuels and furnaces, in Avallone, E, Baumeister, T, and Sadegh, A, Marks’ Standard Handbook for Mechanical Engineers, 11th edition, New York, McGraw Hill, pp. 7-48–7-53. Velzy, C and Grillo, L (2007b), Waste to energy combustion, in Kreith, F and Goswami, D, Handbook of Energy Efficiency and Renewable Energy, Boca Raton, CRC Press, pp. 24-1–24-42.

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