Doing Double Duty

Energy and the environment are two inseparable issues of critical importance to the citizens of the United States and indeed the world. Continued population growth will require increased energy production but will also result in increased waste generation. Can the latter be beneficially used to aid the former?

Municipal Waste as a Fuel Source

Waste-to-energy conversion is a process that has been around for a number of decades worldwide. At least 102 waste-to-energy plants operate in 31 states throughout the United States. These plants consume 14 percent (97,000 tons per day) of the trash generated nationwide and produce more than 2,800 megawatts of electricity.1

The fuel composition of municipal refuse is highly variable in both size and energy content. Waste-to-energy plants are either of the mass burning type or the refuse-derived-fuel (RDF) version. RDF preparation is a multi-step process, in which recyclable materials including glass, paper and some plastics are removed from the waste by hand. Aluminum may be extracted either by hand or with an eddy-current separator, and ferrous material is removed magnetically. The remaining material is shredded into two-inch pieces or smaller. Fuel preparation for RDF plants is a double-edged sword in that recycling is environmentally friendly but significantly increases labor and equipment costs. In fact, the price received for many recycled materials may not be enough to recover the costs associated with separation processes. Mass burn units require no up-front separation, and ferrous materials can be removed from the ash stream after the material has been burned.


Waste-to-energy plants are either of the mass burning type or the refuse-derived-fuel (RDF) version.

Combustion Techniques

Until recently, the two dominant combustor types for waste-to-energy plants were the traveling-grate stoker combustor for RDF and the reciprocating-grate combustor for mass burning. RDF-fired, traveling-grate boilers are derivatives of the coal stoker boilers that have been around for more than 60 years. Typically, a rotating paddle device with air ports pneumatically propels the fuel onto the rear portion of the stoker. Combustion takes place as the fuel is carried along the grate. Ash drops off the end of the grate to a quench tank, and the cooled material is removed by a drag-chain conveyor. Combustion air flows from underneath the grate to the furnace. Proper fuel/air mixing is critical to maximize combustion efficiency and minimize production of carbon monoxide (CO) and potentially toxic hydrocarbons, including dioxins and furans. Poor combustion will also increase unburned carbon levels in the ash.

The majority of waste-to-energy plants in the United States are of the mass burn design. Reciprocating grates make use of hydraulic rams to feed the waste directly onto a series of moving bars. The hydraulically operated bars move back and forth and transport the waste into the combustion zone. As with traveling-grate stokers, combustion air flows from underneath the unit to the furnace, and proper design and operation are necessary to establish thorough fuel/air mixing.

The steam generator portion of many large (greater than 200 ton-per-day) waste-to-energy units resembles conventional boilers with vertical waterwall tubes and additional heat transfer surfaces located in the backpass of the boiler. Refractory lining of the lower portion of the furnace is common to prevent corrosion from acidic gases. Smaller units are generally not based on the waterwall design. Instead, combustion occurs in a refractory-lined chamber that is coupled via an exhaust duct to a heat recovery steam generator (HRSG). In either type of unit, combustion temperatures must be maintained below the ash fusion (melting) temperature to prevent slagging on the combustor walls.

This article outlines a new mass burn waste-to-energy system that improves upon previous technologies, especially with regard to fuel firing and emissions control. The process combines aspects of fluidized bed combustion, heat recovery steam generation, acid gas scrubbing and fabric filter particulate removal into one unit.

The process starts with direct refuse feed to the combustor. No fuel preparation is necessary, although certainly removal of recyclable items is an option. The fuel flows to an inclined grate, which the company (Barlow Projects, Inc., Ft. Collins, Colo.) refers to as their "AirealTM combustion system." The grate is not a stoker and has no moving parts. The unique feature is a series of air discharge ports along the length of the grate, from which sequenced pulses of air augment gravity to keep fuel flowing as it travels from top to bottom. The periodic pulses improve fuel/air mixing and combustion, but are regulated to prevent fuel from being prematurely blown to the bottom of the grate. The pulsing system adds an additional fuel flow control method previously not available for solid fuel combustion.

The grate is divided into four distinct zones from top to bottom. All are fed with a combination of fresh air and recirculated flue gas in approximately equal proportions. The first section is the "dry-out" zone. The upward flowing air plus heat from the furnace dry the fuel and initiate combustion. Main combustion occurs in the next two zones, while the final section is the ash-cooling zone. At the end of the grate a rotating drum removes the ash and discharges it into a water-filled pit where a moving drag chain conveys the ash away from the unit. A magnetic ferrous-removal cylinder located downstream of this conveyor removes iron-based residue from the ash. The rotating drum controls the rate of fuel migration along the grate surface.

An important aspect of the system is flue gas recirculation (FGR) to the four combustion zones. FGR is a well-established technique for moderating combustion temperatures in many types of boilers, but in this system the ratio of FGR to fresh air can be regulated in each of the four zones via operator or automatic adjustment of valves on the four air feed streams.

A positive feature of this system is reduced maintenance due to the fact that the grate is a solid frame and has no moving parts to fail. Moving grate stokers are notoriously maintenance intensive. Also, the pulse air system resides outside of the combustion chamber, which allows for maintenance even during normal operation.

Like some of the conventional combustors mentioned earlier, current AirealTM combustors are refractory lined, and the hot flue gas is taken to a heat recovery steam generator (HRSG) for steam production. It is critical that slagging be prevented in the HRSG, so a small portion of flue gas is recirculated to the combustor exhaust duct to lower gas temperatures. Fifteen hundred degrees Fahrenheit is a good rule-of-thumb guideline for the maximum allowable inlet gas temperature to the HRSG to ensure that slag formation does not occur in the boiler. The lower temperature also minimizes the potential for chloride corrosion in the superheater and convection sections of the HRSG. The standard HRSG design for these systems is a single-drum boiler at one operating pressure. A typical main steam temperature and pressure might be 650 degrees Fahrenheit and 500 pounds per square inch gage (psig), respectively. For a 100 ton-per-day unit, this can generate sufficient steam to power a 2.5 megawatt turbine.


A positive feature of this system is reduced maintenance due to the fact that the grate is solid frame and has no moving parts to fail.

What About Environmental Issues?

The variable composition of refuse and the subsequent wide-ranging combustion byproducts generate a number of potential air pollutants. While the good combustion dynamics of the process tend to minimize carbon monoxide and hydrocarbon formation, many other pollutants are present. These may include sulfur dioxide (SO2), nitrogen oxides (NOX), acid gases including hydrogen chloride, heavy metals including cadmium, lead and mercury and particulates. Waste-to-energy boilers, like their fossil-fired counterparts, are subject to U.S. Environmental Protection Agency (EPA) regulations. Table 1 lists the EPA emissions requirements for new small waste combustors.

Several techniques are used to minimize pollutant discharge. First is the use of overfire air (OFA) for NOX control. The basic principle involves splitting the combustion air stream, such that the prime combustion zone receives slightly less than the required amount of air for complete combustion. Although the resulting chemistry is complex, the overall process creates reduced carbon species that strip oxygen from NOX to generate elemental nitrogen (N2). The remaining air needed for combustion is added to the flue gas downstream of the primary combustion zone. In the system, the OFA ports are located in the combustor outlet duct just ahead of the FGR ports. While flue gas recirculation to the combustor exhaust serves the primary purpose of lowering HRSG inlet temperatures, the process also helps reduce NOX emissions.

Acid gas removal takes place in a dry scrubber immediately following the HRSG. Hydrated lime (Ca(OH)2) particles are injected into the flue gas to react with acid constituents. These acid gases consist primarily of SO2 but also include HCl. The scrubber design includes a venturi tower to promote gas-reagent contact and mixing. An important aspect of the system design is that boiler exit gas temperatures are significantly lower than conventional designs, thus allowing higher boiler efficiencies (typically four to five percent higher). The reacted products, along with other particulates generated during combustion, collect in a fabric filter (baghouse) particulate removal system located downstream of the scrubber.

Fabric filters are preferable to electrostatic precipitators (ESPs) because of better collection efficiencies for all types of particulates, including particles less than 10 microns in diameter. This is especially important for heavy metal control. Heavy metals, most notably mercury, exit the combustor in both an oxidized and elemental state. Oxidized metals often scrub out of solution with the acid gases, but the elemental forms pass through the system. Unit design includes activated carbon injection with the lime to adsorb elemental heavy metals and reduce dioxin/furan emissions. Electric utility researchers are studying activated carbon injection closely, as this technique may be the only practical method to reduce mercury emissions by 90 percent, as is proposed in future EPA guidelines.

Results and Future Issues

A number of these units are already in commercial operation. Unit size ranges from 50 to more than 100 tons per day of feed. A 90 percent reduction in waste volume is common, and where iron products are recycled from the ash, the reduction may reach 95 percent.

Combustion ash is typically disposed in landfills that comply with federal Resource Conservation Recovery Act (RCRA) Subtitle D regulations as a non-hazardous waste. Another advantage is that these units, in whole or in part, may be retrofitted at existing sites to serve as a replacement for older, less reliable systems. Emissions from retrofitted units easily meet New Source Performance Standards (NSPS) regulations for this category of combustor. An added advantage stems from staffing requirements. Some plants operate with only two persons per shift.

Conclusion

For years the global population has been faced with growing concerns regarding both new energy sources and waste disposal. Landfill areas are becoming scarce. In fact, approximately 90 percent of U.S. landfills operating since 1970 have been closed due to RCRA requirements. Waste-to-energy power production, with dedication to environmental issues, is a practical method to address both concerns. New technologies have emerged that allow efficient energy production with tighter environmental control.

Table 1

Proposed EPA Emissions Limits for Small Waste Combustors (SWC)

Pollutant

New Class II SWC

PCDD/PCDF

13ng/dscm

Cadmium

0.020 mg/dscm

Lead

0.20 mg/dscm

Mercury

0.080 mg/dscm or

85% reduction

Opacity

10%

Particulates

24 mg/dscm

Hydrogen Chloride

25 ppm or

95% reduction

NOx

500 ppm

SO2

30 ppm or

80% reduction

CO

100 ppm

References

  • "Waste-To-Energy Industry Fact Sheet"; published by the Integrated Waste Service Association, August 14, 2000, Washington, DC.



  • This article appeared in the July/August 2002 issue of Environmental Protection, Vol. 13, No. 7, page 38.

    This article originally appeared in the 07/01/2002 issue of Environmental Protection.

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