Off-Normal Operations of Six Facilities
Prince Edward Island (Concord Scientific Corporation 1985)
The Prince Edward Island facility consists of three, two-stage incinerators, each rated at about 36 tons of waste per day. The incinerator design uses controlled or “starved”-air combustion (as contrasted with the excess-air operations used at Pittsfield and Westchester discussed later). Municipal solid waste is burned in the primary chamber, where a fraction of the total air needed for complete combustion is provided. The combustible gases enter the secondary chamber, where preheated air is added to complete combustion.
During testing of the primary chamber temperature was maintained at a relatively constant 1292°F —within ±104°F, except for the low-temperature test, where it was maintained at 1250°F. The secondary-chamber temperatures were kept at 840° (1550°F) for two tests, at 1900°F for the high-temperature test, and at 1350°F for the low-temperature test. The percent excess air differed by about 40% between the tests involving normal and low secondary-chamber temperatures. The test data showed a tendency for dioxin concentrations to increase with increasing excessoxygen concentrations, which occurs in conjunction with lower furnace temperature. This relationship was also observed in the Pittsfield data. See Figure B-1.
Conclusions from a comparative study of dioxin emissions vs. operating conditions at Westchester, Pittsfield, and Prince Edward Island were that “test results indicate that levels of dioxins and furans in the flue gas entering a pollution-control device are affected by different plant operating conditions if the conditions deviate sufficiently from normal operations.” This study also indicated that furnace temperature might be a gross indicator of total dioxin and furan formation, and that operating an incinerator at excess oxygen levels below about 5% may cause an increase in dioxin and furan emissions (Visalli 1987).
Pittsfield (Midwest Research Institute 1987)
The Pittsfield, MA facility consists of three 120 ton of waste/day, two-stage, refractorylined incinerators with two waste-heat boilers. Municipal solid waste is burned under excess-air conditions in the primary chamber; hot effluent gases pass into a secondary combustion chamber where any remaining uncombusted gases are burned.
Though no data were collected for startup or shutdown, the Pittsfield study included runs at different temperatures and oxygen levels to show how emissions varied when operating conditions were not optimized (i.e., upset conditions). The data showed a tendency for dioxin and furan emissions to increase with excess oxygen below and above certain levels. In other words, dioxin concentration in the flue gases was at a minimum when excess oxygen was between 9 and 11% in the hot zone (see Figure B-1). Total dioxin rose to over 50 ng/dscm corrected to 7% O2 when excess oxygen was below 5% or above 12%. In fact, when the excess oxygen rose to over 11% the dioxins escalated quickly to over 100 ng/dscm and beyond.
In addition, a clear pattern was found with respect to temperature impacts on dioxin. The optimal temperature range for the Pittsfield facility, measured at the tertiary duct some distance from primary combustion, was roughly between 1500 and 1650°F. At temperature below and above that window dioxins increased. Below 1500°F, dioxins increased dramatically to over 120 ng/dscm at just below 1300°F. The corresponding dioxin concentrations for the two low-temperature runs were much higher than those for all other runs by a factor averaging more than four, and they were statistically different than those emitted under normal operating conditions. The two low furnace temperature runs (1300°F and 1350°F) also produced CO levels that were more than a factor of 10 higher than the rest of the test runs, showing CO as a useful indicator in this.
As important as the level of CO emissions in a medical-waste combustor is, an equally important issue is the averaging time over which these emissions are evaluated. It is important to note, in this regard, that the Pittsfield combustion tests showed that CO levels above 100 ppm were associated with a greater certainty of higher dioxin levels. If new and existing incinerators exceed this 100 ppm level routinely, by virtue of a 4- or 24-hour averaging time, the effect of the MACT regulation would not be to minimize dioxin emissions in these incinerators. The Pittsfield research demonstrates the importance of minimizing the number, intensity, and duration of CO spikes, and thus, of limiting the length of the averaging period for CO. Thus, to minimize the opportunity for formation of products of incomplete combustion, an average limit for CO is needed that would result in a strict limitation on the frequency, intensity, and duration of excursions. For example, New York state requires a onehour averaging time for evaluating CO from medical-waste incinerators and permits typically specify a 100 ppm limitation.
A study managed by the American Society of Mechanical Engineers (ASME 1995) on the relationship between chlorine in waste streams and dioxin emissions, indicates that because combustion control is limited in most batch-mode medical-waste incinerators they “can be expected to emit relatively high PCDD/F levels associated with incomplete combustion.” That finding points to a need to ensure that batch-fed incinerators provide good combustion.
Westchester (New York State Energy Research and Development Authority 1989)
The Peekskill incinerator in Westchester County, NY, consists of three mass-burn waterwall incinerators, each rated at 750 tons/per day. Each has a transverse reciprocating grate made up of modular sections: the drying zone, two burning zones, and two finishing or burnout zones. Rates of underfire air and grate speed can be set for each zone. Overfire air is supplied through nozzles on the front and rear of each furnace.
A study conducted in the 1980s includes two test runs in which dioxin emissions were recorded during cold starts, as well as several under more-normal operating conditions. The study was not intended to examine cold starts in great detail. In fact, the report excluded the cold-start runs from most of its analyses because analysis of variance (ANOVA) results showed that dioxin emissions during cold starts were statistically different (higher) than those emitted under normal operating conditions at a significance level of 0.0001 for both CDD and CDF. Run 4 was a normal cold-start condition where the auxiliary gas burner was used to get the furnace up to normal-operating temperature, the garbage was ignited, and the gas was turned off. This test sample was taken over a 65 minute period once the furnace was at “elevated temperature ”. Run 14 was research-oriented, in an attempt to determine if adding more natural gas than usual would lower emissions during cold starts. The report stated that the purpose of the cold-start tests was to observe the effect on CDD and CDF emissions of feeding refuse to a furnace that was not at thermal equilibrium. These tests reflect continually changing operating conditions (non-steady state).
The quantities of dioxins and furans generated during startup are striking. “The testing results show that the average CDD and CDF concentrations measured at the superheater exit during the first cold starts are two to three times higher than the average of the other 12 runs that were at steady operating conditions. For the electrostatic precipitator (ESP) inlet, the increase in dioxins from normal operations to normal cold start is between 18 and 51 times, and these increase further to between 40 and 96 times at the ESP outlet.” The temperature at the superheater exit was considerably less during the normal cold start (892°F) than the other normal runs (995°F – 1150°F, with most over 1100°F). Temperature was not recorded at the superheater for run 14.
Specifically, the normal cold start generated 124 ng/dscm of CDD @7% O2 at the superheater outlet as compared with a range of 11.4 to 84.3 ng/dscm for the other runs. However, at the ESP inlet, the dioxins measured were 7226 ng/dscm for the first cold start vs. a range of 43.8 to 209 ng/dscm for the other runs. This precipitous rise shows that secondary dioxin formation was taking place near the ESP inlet due to temperatures conducive to such information and the presence of dioxin precursors and catalysts. The secondary dioxin generation for the cold start run occurred at a rate between one and two orders of magnitude higher than for the other runs.
Why should the dioxin generation rate increase faster under cold-start conditions than under steady-state conditions? There are two variables that might account
for this. The temperature at the ESP inlet for the normal cold start run was 383°F vs. 434 to 472°F for the others. Because research by Stieglitz and Vogg (see Chapter 3) indicated the optimal temperature for secondary dioxin formation to be between 430°F and 750°F, peaking at 570°F, it would seem that the normal runs would have greater secondary dioxin formation if temperature were the only variable. The variable that likely distinguishes the normal cold start run from the others, and explains the result observed, is the furnace temperature, which is decidedly lower for the cold start. Because dioxin precursors are created in the furnace at the highest rate in the few hundred degrees below optimal furnace temperatures, the lower furnace temperature during cold start is likely to have caused a higher generation rate for dioxin precursors and a lower rate of destruction. A greater generation rate during the cold start run of dioxin precursors (e.g., chlorobenzenes and chlorophenols), some of which require higher destruction temperatures (e.g., 1800°F, vs. 1300°F for dioxins), would seem to be the cause of the tremendously higher amount of dioxins in the flue gas further downstream.
Carbon Monoxide (CO) was also measured for the various runs at Westchester. The mean CO concentration during the normal cold start was 180 ppmv at the superheater exit, not an astoundingly high figure considering the quantity of dioxins formed. The second, modified cold start had a mean of 114 ppm CO, and was characterized by dioxin emissions nearly as high as the first, normal cold start. By comparison, the CO levels for the other 12 runs ranged from 6 ppm to 57 ppm at the superheater exit. (While the NYSERDA report pointed out that the mean CO level does not adequately characterize the range of CO experienced during the one hour test, it is nonetheless of great interest because EPA standards for existing municipal solid-waste incinerators specify averaging times of 4 hours for compliance with the CO emissions standard for four types of municipal solid-waste incinerators and 24 hours for compliance by four other types of municipal solid-waste incinerators. The CO standard for municipal solid-waste incinerators for all plant types is 40 ppm, but over a 12-hour rolling average. Further, only one of the eight municipal solid-waste incinerator plant types is required to meet a 50 ppmv CO standard for existing plants. Averaging times are similar for new plants, and the range of CO emissions permitted is 50 to 150 ppmv, with only one of eight plant types being required to meet 50 ppmv.
Quebec City (Stieglitz and Vogg 1987)
The Quebec City mass burn incinerator includes four incinerators/boilers rated at 227 tonnes of waste/per day each. Each of those has a vibrating feeder-hopper, drying/burning/burnout grates, refractory-lined lower burning zone, and waterwalled upper burning zone.
The second of two studies of this incinerator conducted by Environment Canada compared the combustor performance at a variety of operating conditions: low-, medium- and high-load; percent excess air; furnace temperature; and primary/secondary air ratio. Some of these conditions were characterized as “very poor” (where primary/secondary air ratio was 90/10 and excess air was considered “high” (115%)), and “poor” (where furnace temperature was 1562 °F, excess air was very high at 130%, and primary/secondary air ratio was 60/40). Three other combinations, under low-, design-, and high-load, were considered to be “good” operations.
Dioxin and furan emissions were measured for each of these test combinations, and statistical analysis of the data showed a fairly strong correlation between high excess air levels and dioxin/furan. See Figure B-1.
In addition, the best single parameter correlation (r2 = 0.876) was a comparison of uncontrolled particulate matter entering the ESP versus dioxin/furan in the stack. Two other variables with extremely good single parameter fits were flue gas flow concentrations rate (r2 = 0.771) and primary air flow rate (r2 = 0.723). Notice that the rate of increase in dioxin and furan becomes exponential at around 123% excess air, indicating a move towards upset conditions. Data for Pittsfield and PEI test are also shown. It was also found that load has an effect on dioxin. This effect is shown in Figure B-2.
Oswego (Radian Corporation 1990)
The Oswego facility consists of four, two-stage mass-burn units each of which are rated at 50 tons of waste/day. Batch loads are fed to the primary chamber and moved through the chamber along the stepped bottom by air-cooled transfer rams on a cycle of approximately seven to eight minutes. Combustible gases and entrained particles exit the primary chamber to the secondary chamber where the flue gas is mixed with preheated secondary air to complete combustion of unburned gases and particulate matter.
The Oswego study compared dioxin and furan generation in groups of three runs under each of the following four conditions:
The low furnace temperature condition had a negative effect on dioxin emissions, increasing dioxin emissions over the normal temperature condition by about a factor of six at the secondary chamber exit and also at the ESP Inlet. At the secondary chamber exit total dioxlns ranged from 67.7 to 110.1 ng/dscm @ 7%O2 (averaging 84.5) at the secondary chamber exit At the ESP inlet the range was 255.1 to 349.8 ng/dscm @7%O2 (averaging 289.9). As compared with the normal operating condition (mid-range), averaging 13.6 ng/dscm @ 7%O2 at the secondary chamber exit, and 53.4 ng/dscm @ 7%O2 at the ESP inlet, the aforementioned means for the dioxin emissions for the low temperature runs were found to be statistically different and significantly correlated. Using one-way ANOVA, the chance that the dioxin means for the four conditions measured at the secondary chamber exit are not really different is 0.0049 or half of one percent. The chance that the dioxin means for the conditions measured at the ESP inlet are not really different is even lower at 0.0001. Thus, it can be stated definitively that the lower furnace temperature tested here is associated with a six-fold increase in dioxins.
Dioxin emissions were also correlated with CO emissions. Because CO measurements were taken continuously, different assumptions could be made about representation of CO as single value representations (SVRs). The correlation between total dioxins and CO at the secondary chamber exit that was most pronounced was at the 90th percentile CO value, where r = 0.921, a nearly perfect positive correlation. In general, the SVRs representing extreme values of CO (i.e., 90th, 95th and 99th percentile) correlated most frequently with the dioxin and furan levels measured, indicating that it is frequency and duration of the highest values of CO that best predict changes in dioxin and furan concentrations in the flue gas. All relationships were significant at the 0.05 level. These data highlight the importance of avoiding both CO spikes and poor combustion efficiency, and accurately recording the amount of time during which elevated CO occurs—not simply averaging CO over long periods of time to mask such excursions.
Hartford (EPA 1994)
The Mid-Connecticut facility is a refuse-derived fuel facility (RDF) and consists of three RDF-fired, spreader-stoker boilers that process a total of 2,000 tons/day of municipal solid waste. Four pneumatic distributors spread the RDF across the width of the combustion grate. Ten underfire air zones allow the operator to optimize combustion and to respond quickly to “piling” situations by manual adjustment of underfire air dampers. The overfire air system is equipped with four tangential assemblies iocated in the furnace corners; each assembly includes three levels that are separately controlled. Preheated combustion air enters the furnace forming a vortex, providing longer residence times for the combustion gases.
A major goal of the project was to determine generation of trace organics and metals in the furnace under different process operating conditions, not under “upset” conditions, per se. Steam flow rate (an indicator of load) and combustion air flow rates were the primary independent variables defining operating conditions as “good,” “poor,” “very poor.” However, as compared with the other studies discussed in this appendix, the variation in combustion conditions was smaller, because this study was focused on a more-efficient range of operation than these other studies, Dioxins, furans, CO, total hydrocarbons, PCB, cholorobenzenes, chlorophenols, and PAHs were measured.
Multiple-regression analysis was used to study the effect of various continuously monitored emission and process parameters on dioxin emissions (prediction models) and the effect of various combustion control measures on dioxin emissions (control models). The best prediction model showed that CO, NOx, moisture in the flue gas at the spray dryer inlet, and furnace temperature explained 93% of the variation in uncontrolled dioxin emissions, with CO explaining 79% by itself. The best control model showed that RDF moisture, rear-wall overfire air, underfire air flow, and total air explained 67% of the variation in uncontrolled dioxin emissions.
Because CO was found to be such a strong predictor of dioxin emissions, the relationship was explored further. It was found that the percent of time the CO level was over 400 ppm was quite strongly correlated with the amount of uncontrolled dioxins generated, particularly when examining only those runs where there was poor combustion. The authors of the report indicated that “Poor combustion implies that greater amounts of organic material escape the combustor unburned. In the correlation between CO and PCDD/PCDF, use of only the poor combustion tests would improve R2 from 0.70 to 0.95.” The correlation between total hydrocarbons and dioxin/furan improved from a R2 value of 0.68 to 0.97 for the poor combustion tests. This indicates that for the poor combustion tests, 95% of the change in dioxin/furan values are explained by the change in CO emissions. This is consistent with the theory that, during periods of incomplete combustion, the amount of organic matter leaving the furnace strongly influences the formation of PCDD/PCDF. Thus, these data indicate that CO is an important surrogate for dioxin, and that allowing longer averaging times for CO levels for compliance with standards will more likely result in higher dioxin/furan emissions because, under these conditions, more CO spikes can occur without exceeding standards.
The furnace temperature was at 1,789°F for “poor” conditions and at 1,920°F for “good” combustion conditions under high load. The resulting dioxin emissions at the spray dryer inlet were 317 ng/Sm3 for the poor combustion conditions vs. 67 ng/Sm3 @ 12%CO2 for the good combustion conditions, a factor of almost five. The same relationship was true for furans, total hydrocarbons, PAHs, chlorobenzenes and chlorophenols. CO was 397 ppm for “poor” combustion and 116 ppm for “good” combustion under high load. Under intermediate load, the underfire/overfire ratio was 923 under “good” conditions and 1.632 under “very poor” conditions. This resulted in a ten-fold increase in CO from 93 to 903 ppm. At this foad total hydrocarbons increased from 2.5 to 52.4, PAH from 7,330 to 112,000, chlorophenols from 14,300 to 114,000, chlorobenzenes from 6,050 to 15,800, dioxins from 228 to 580, and furans from 579 to 1,280, all in units of ng/Sm3 @CO2.
ASME (The American Society of Mechanical Engineers). 1995. The Relationship Between Chlorine in Waste Streams and Dioxin Emissions from Waste Combustor Stacks, CRTD Vol. 36 Fairfield, N.J.: ASME Press.
Concord Scientific Corporation. 1985. National Incinerator Testing and Evaluation Program: Two-Stage Combustion (Prince Edward Island). Environment Canada Report EPS 3/UP/1 Volumes I-IV.
EPA (U.S. Environmental Protection Agency). 1994. National Incinerator Testing and Evaluation Program: The Environmental Characterization of Refuse-derived Fuel (RDF) Combustion Technology: Mid-Connecticut Facility, Hartford, Connecticut, Summary Report. Environment Canada Report EPS 3/UP/7 and U.S. Environmental Protection Agency Report EPA-600/R-94-140, December.
Midwest Research Institute. 1987. Results of the Combustion and Emissions Research Project at the Vicon Incinerator Facility in Pittsfield, Massachusetts. Final Report 87-16. Prepared for New York State Energy Research and Development Authority by Midwest Research Institute.
New York State Energy Research and Development Authority. 1989. Combustion and Emissions Testing at the Westchester County Solid Waste Incinerator, Volume I, Final Report. Report 89-4. New York State Energy Research and Development Authority.
Radian Corporation. 1990. Results from the Analysis of MSW Incinerator Testing at Oswego County, New York, Volume I, Final Report. Prepared for the New York State Energy Research and Development Authority and the New York State Department of Environmental Conservation by Radian Corporation. Energy Authority Report 90-10.
Stieglitz, L. and H. Vogg. 1987. New Aspects of PCDD/PCDF Formation in Incineration Processes. Preliminary Proceedings, Municipal Waste Incineration, October 1-2, 1987, Montreal, Quebec.
Visalli, J.R. 1987. A comparison of dioxin, furan and combustion gas data from test programs at three MSW incinerators. JAPCA 37(12): 1451-1463.