5. COMBUSTION Combustion is the second most widely used MSW technology, as measured by tons of MSW processed. Steam and/or electricity is generated for use by most of the plants. Two different approaches to MSW combustion can be used: mass burning of the MSW and preparation and combustion of refuse-derived fuels. They differ in the extent of pretreatment of the MSW before combustion and in the design and operation of the furnaces.

The rest of this section is divided into four major subsections. The next section discusses issues common to both mass burning and RDF direct firing: regulations and limitations on cost data. The next two subsections discuss each technology individually. The last subsection identifies data gaps and research needs for both technologies.

Modern combustion facilities date from the late 1970s, after the Clean Air Act required effective pollution control equipment on each plant. This section covers units that have begun operation since the Clean Air Act was passed.

The popularity of combustion as part of MSW management strategies also increased as a result of the perceived energy crises of the late 1970s. Both mass burning and RDF combustion produce energy that can replace consumption of fossil fuels. The magnitude of MSW's potential contribution to the nation's energy supplies is indicated by the following estimate: Conversion of all 180 million tons of U.S. MSW to electricity by direct combustion would supply about 3% of annual U.S. electricity needs(1). This comparison is made only for a sense of proportion.

Both mass burning and RDF can be effectively combined with various other MSW management approaches. Because preprocessing is minimal in most mass burn plants, the opportunity for separation of materials for possible recycling is smaller than it is in an RDF plant, but it is equal to that with land filling. Some materials separation occurs when the ash is processed for magnetic metal recovery. Curbside collection of recyclables can be effectively integrated with either mass burning or RDF. An increasing number of mass burn plants are incorporating mixed waste separation steps in which the MSW is processed for materials recovery before combustion. RDF can be densified into pellets before firing or cofiring, and it can be used as feed for anaerobic digestion, MSW composting, and gasification/pyrolysis.


Pollution Control, New Source Performance Standards for Combustors, and Effects of the Clean Air Act Amendments of 1990

A newly constructed mass burning facility will be required to meet New Source Performance Standards (NSPS) for municipal waste combustors (MWCs) that have been published in the Code of Federal Regulations (CFR, l991a). These requirements specify the maximum emission levels, as shown in Table 5.1, for all facilities that process more than 250 tons per day.

Code of Federal Regulations(CFR, 1991a). These requirements specify the maximum emission levels, as shown in Table 5.1, for all facilities that process more than 250 tons per day.

Guidelines for emission controls for existing facilities are also specified in the Federal Register(FR, 1991a). Because this report covers only new facilities, the guidelines for existing facilities will not be reviewed.

Under Section 129 of the Clean Air Act Amendments passed in 1990, the U.S. Environmental Protection Agency(EPA) will be required by 1993 to issue standards for municipal waste combustion facilities with capacities of 250 tons per day or less, as well as for medical, hospital, and infectious waste incinerators. In 1995, the EPA is required to begin regulating solid waste combustion units burning commercial and industrial waste. Although the EPA currently regulates only particulate emissions from small combustion facilities, a number of states have more restrictive requirements than the federal standards and do not relax those requirements on the basis of size.

Mass burn facilities and RDF facilities use different approaches to control organic emissions. Both approaches effectively destroy a large proportion of organics. including dioxin, in the incoming MSW(Harman, 1991b), and both are acceptable for meeting the regulations outlined above.

In mass burning, combustion conditions are designed to ensure that a very large percentage of the organics will be consumed. That approach reduces the size need for pollution control equipment such as scrubbers and baghouses to control the organic emissions, although these devices are needed to control particulates, metals, and acid gases.

RDF facilities use suspension firing, and combustion gases from RDF contain a somewhat higher concentration of organics, as well as some unburned carbon, along with particulates, metals, and acids. These are removed by pollution control equipment similar to that used for mass burn plants, but larger in size.

Energy Recovery

The subsection on energy presents the results of a life-cycle analysis of energy inputs and outputs over the 20 year time frame used in this study. The basis is 1 ton of MSW at the curb.

Mass burning and RDF combustion are the most efficient MSW management techniques for energy recovery. Comparisons of performance indicate that differences between the two appear to depend more on the particular waste stream than on differences in design.

Limitations on the Cost Data

The subsections on mass burning and RDF preparation and combustion include data on costs of these facilities. Published cost estimates for individual facilities vary over a wide range, and the data are therefore useful only as order-of-magnitude estimates of the possible costs of new combustion facilities. The variations reflect inconsistencies in the sources of the estimates (the analysis used published data only) rather than predictable variations based on the type of technology or the size of the facility. In many cases, the sources of the estimates fail to provide sufficient information to convert the estimate to a consistent basis or to identify the reasons for the differences.

                                             Table 5.1

[Subpart Ea]                                                                         Standard, Converted
                                                                                     to lb/t MSW(a)
    The NSPS apply to MWCs with unit capacities above 250 t/d that combust
residential, commercial and/or institutional discards. Industrial discards
are not covered by the NSPS(b)

Good Combustion Practices
  * Maximum load level demonstrated during dioxin/furan performance test
  * Maximum PM control device inlet temperature no greater than 17C
    hotter than demonstrated during dioxin/furan performance test
  * CO level (averaging time) as follows:
    - Modular starved and excess air MWCs - 50 ppmv (4h)                                                    0.4
    - Mass burn waterwall and refractory MWCs - 100 ppmv (4h)                                               0.8
    - MWCs using fluidized bed combustion - 100 ppmv (4h)                                                   0.8
    - Mass burn rotary waterwall MWCs - 100 ppmv (24h)                                                      0.8
    - RDF stokers - 150 ppmv (24h)                                                                          1.2
    - Coal/RDF mixed fuel-fired MWCs - 150 ppmv (4h)                                                        1.2
  * ASME or State certification for MWC supervisors. Operator training
    and training for other MWC personnel

MWC Organic Emissions (measured as total dioxin/furans)
  * Dioxins/furans(c,d) - 30 ng/dscm                                                                     2.3x10(-7)
MWC Metal Emissions (measured as PM)(a)
  * PM - 34 mg/dscm                                                                                         0.26
  * Opacity - 10 percent (6-minute average)
MWC Acid Gas Emissions (measured as SO(2) and HCI)(a)
  * SO(2) - 80% reduction or 30 ppmv (24h), whichever is less stringent                                     0.54
  * HCI - 95% reduction or 25 ppmv, whichever is less stringent                                             0.25
  * Basis - spray dryer and fabric filter
Nitrogen Oxides Emissions(a)
  * NO(x) - 180 ppmv (24h)                                                                                  2.34
Monitoring Requirements
  * SO(2) - CEMS, 24 h geometric mean.

Source: CFR, 1991(a)

(a) These conversions are not part of the standards. They are approximate because they depend
    on carbon content in the MSW
(b) See glossary for acronyms
(c) All emissions levels are at 7% O(2), dry basis
(d) Dioxins/furans measured as total tetra- through octa-chlorinated dibenzo-p-dioxins and
    dibenzofurans, and not as toxic equivalents

Click here for table in WK1 format.

For example, the finance charge for the capital investment for a given facility would be significantly affected by the interest rate prevalent at the time of project financing, but many sources fail to note that interest rate. Over time, requirements governing the Best Available Control Technologies (BACT), or Best Demonstrated Technology (BDT) tend to become more stringent, and the additional cost of more advanced technologies will increase the capital costs of more recent projects to an unpredictable extent. Moreover, capital investment in general would be affectedly the type and composition of the wastes and the plant site conditions, but many sources fail to provide data on these matters.

Similarly, the operation and maintenance (O&M) costs are affected by site-specific conditions such as labor rates, labor contracts, safety rules, the size of the crew, and so on. Again, information on these factors is rarely provided in the literature.

Mass Burning


Table of Contents