Combustion modeling and performance evaluation in a full-scale rotary kiln incinerator.

This work summarizes the results of numerical investigations and in situ measurements for turbulent combustion in a full-scale rotary kiln incinerator (RKI). The three-dimensional (3D) governing equations for mass, momentum, energy, and species, together with the kappa - epsilon turbulence model, are formulated and solved using a finite volume method. Volatile gases from solid waste were simulated by gaseous CH4 distributed nonuniformly along the kiln bed. The combustion process was considered to be a two-step stoichiometric reaction for primary air mixed with CH4 gas in the combustion chamber. The mixing-controlled eddy-dissipation model (EDM) was employed to predict the conversion rates of CH4, O2, CO2, and CO. The results of the prediction show that reverse flows occur near the entrance of the first combustion chamber (FCC) and the turning point at the entrance to the second combustion chamber (SCC). Temperature and species are nonuniform and are vertically stratified. Meanwhile, additional mixing in the SCC enhances postflame oxidation. A combustion efficiency of up to 99.96% can be achieved at approximately 150% excess air and 20-30% secondary air. Reasonable agreement is achieved between numerical predictions and in situ measurements.     

Rotary Kiln Incineration III. An Indepth Study—Kiln Exit/Afterburner/Stack Train and Kiln Exit Pattern Factor Measurements During Liquid CCl4 Processing

Temperature and stable species concentration data are presented from various locations within a full-scale rotary kiln incinerator firing natural gas/carbon tetrachloride/air. The data are being collected as part of a cooperative program involving university, industry and government participation. The overall goal of the program is to develop a more sophisticated understanding of and a predictive capability for rotary kiln and afterburner performance as influenced by basic design and operational parameters. Non-uniformities in stable species and temperature exist for this particular kiln, at the kiln exit, under certain operating conditions. Flow perturbations from within the kiln were found to persist into the afterburner, but not into the stack. High destruction and removal efficiencies (DRE’s) were achieved under the operating conditions of these tests through adequate secondary combustion processing.     

On the Occurrence of Transient Puffs in a Rotary Kiln Incinerator Simulator II. Contained Liquid Wastes on Sorbent

The generation of transient puffs resulting from the batch introduction of liquid waste into a 73 kW (250,000 Btu/h) rotary kiln incinerator simulator was investigated. The liquid was added onto a sorbent, enclosed in cylindrical cardboard containers that were introduced into the combustion chamber one at a time. A statistically designed parametric investigation determined the effects of liquid mass, liquid composition, kiln temperature, and kiln rotation speed on the total magnitude and instantaneous intensity of the pollutant puff leaving the kiln. Liquid "wastes" investigated included toluene, methylene chloride, carbon tetrachloride, and No. 5 fuel oil. Transient puffs from these wastes were monitored using on-line measurements for FID-measurable hydrocarbons, CO, and integrated particulate filter loadings, although the suitability of any one of these indicators depended on the chemical nature of the waste involved.

Results demonstrate that puffs formed during transient conditions are generated easily, even with small quantities of wastes and with the kiln operating at 100 percent excess air. High kiln temperatures and increased kiln rotation speeds exacerbated the generation of puffs, due to increased liquid evaporation rates. Transient puffs may contain hazardous products of incomplete combustion (PICs) even though adequate destruction and removal efficiencies are achieved. Mixtures of chlorinated and nonchlorinated principal organic hazardous constituents (POHCs) in the waste can lead to the formation of more diverse chlorinated compounds than are formed from a single chlorinated POHC such as carbon tetrachloride alone.

This paper is the second of a series. Its companion paper, Part I, which has been published previously,¹ is concerned with solid plastic wastes.     

Rotary Kiln Incineration IV. An Indepth Study— Kiln Exit,Transition and Afterburner Sampling During Liquid CCl4 Processing

Detailed temperature and stable species concentration data are presented from the kiln exit, transition section and afterburner of a full-scale incinerator facility firing natural gas/carbon tetrachloride/air. The data are collected as part of a cooperative program involving university, industry and government participation. The overall goal of the program is to develop an understanding and predictive capability for rotary kiln and afterburner performance as influenced by basic design and operational parameters. The data demonstrate that nonuniformities in stable species and temperature exist, under certain operating conditions, at the kiln exit in the vertical direction only. Measurements from the transition section indicate that non-uniformities may exist within this region under certain operating conditions. Flow perturbations from within the kiln can persist into the afterburner, although the degree of nonuniformity is substantially reduced compared to either the kiln or transition sections. High destruction and removal efficiencies were achieved under all operating conditions of these tests through a combination of kiln and secondary combustion processing.     

Screening Approach for Principal Organic Hazardous Constituents and Products of Incomplete Combustion

The body of information presented in this paper is directed to those individuals concerned with developing or implementing screening strategies for characterizing organic emissions from incinerators and other combustion sources. The need to characterize hazardous waste incinerator emissions for multiple organic compounds has been steadily increasing for several years. The regulatory approach makes use of a type of indicator compound procedure that concentrates on principal organic hazardous constituents (POHCs). In addition to continuing interest in POHCs, interest has been growing in the types and concentrations of products of incomplete combustion (PICs). Sampling and analysis methods have been developed previously for approximately 225 of the more important POHCs and PICs. These methods may be used as components of a cost-effective screening protocol aimed at maximum characterization of emissions, whether the project budget is large or small. This paper contains a discussion of fundamental principles of several kinds of screening strategies and recommends an approach suitable for incinerators and other combustion sources. The concept of a risk-driven analysis strategy is introduced and illustrated with a simplified example.     

A Life-Cycle Inventory Model of Municipal Solid Waste Combustion

ABSTRACT

Evaluation of alternate strategies for municipal solid waste (MSW) management requires models to calculate environmental emissions as a function of both waste quantity and composition. A methodology to calculate waste component-specific emissions associated with MSW combustion is presented here. The methodology considers emissions at a combustion facility as well as those avoided at an electrical energy facility because of energy recovered from waste combustion. Emission factors, in units of kg pollutant per metric ton MSW entering the combustion facility, are calculated for CO₂-biomass, CO₂-fossil, SO_x , HCl, NO_x , dioxins/furans, PM, CO, and 11 metals. Water emissions associated with electrical energy offsets are also considered. Reductions in environmental emissions for a 500-metric-ton-per-day combustion facility that recovers energy are calculated.     

Remote sensing-based estimates of annual and seasonal emissions from crop residue burning in the contiguous United States

Crop residue burning is an extensive agricultural practice in the contiguous United States (CONUS). This analysis presents the results of a remote sensing-based study of crop residue burning emissions in the CONUS for the time period 2003-2007 for the atmospheric species of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), nitrogen dioxide (NO2, sulfur dioxide (SO2), PM2.5 (particulate matter [PM] < or = 2.5 microm in aerodynamic diameter), and PM10 (PM < or = 10 microm in aerodynamic diameter). Cropland burned area and associated crop types were derived from Moderate Resolution Imaging Spectroradiometer (MODIS) products. Emission factors, fuel load, and combustion completeness estimates were derived from the scientific literature, governmental reports, and expert knowledge. Emissions were calculated using the bottom-up approach in which emissions are the product of burned area, fuel load, and combustion completeness for each specific crop type. On average, annual crop residue burning in the CONUS emitted 6.1 Tg of CO2, 8.9 Gg of CH4, 232.4 Gg of CO, 10.6 Gg of NO2, 4.4 Gg of SO2, 20.9 Gg of PM2.5, and 28.5 Gg of PM10. These emissions remained fairly consistent, with an average interannual variability of crop residue burning emissions of +/- 10%. The states with the highest emissions were Arkansas, California, Florida, Idaho, Texas, and Washington. Most emissions were clustered in the southeastern United States, the Great Plains, and the Pacific Northwest. Air quality and carbon emissions were concentrated in the spring, summer, and fall, with an exception because of winter harvesting of sugarcane in Florida, Louisiana, and Texas. Sugarcane, wheat, and rice residues accounted for approximately 70% of all crop residue burning and associated emissions. Estimates of CO and CH4 from agricultural waste burning by the U.S. Environmental Protection Agency were 73 and 78% higher than the CO and CH4 emission estimates from this analysis, respectively. This analysis also showed that crop residue burning emissions are a minor source of CH4 emissions (< 1%) compared with the CH4 emissions from other agricultural sources, specifically enteric fermentation, manure management, and rice cultivation.     

Research and development to improve naval shipboard waste management using a compact closed-loop-controlled waste incinerator

Research has been conducted into the application of forced acoustics for enhancing the performance of a pyrolyzed waste afterburner configured as a dump combustor. Subscale studies showed that acoustic forcing of an air jet entering a dump chamber could trigger the formation of coherent vortices generated by entrainment of ambient gases. Subsequent studies showed that combustible gases could be introduced into the coherent vortices, and with additional modulation this configuration would lead to an enhanced combustion rate with low emissions of pollutants. The acoustically forced burner concept was scaled up to practical levels and tested as an afterburner on a commercial waste incinerator operating in pyrolysis mode. Results show that the afterburner can promote both compactness, due to the rapid combustion rate, and low pollutant emissions resulting from enhanced mixing prior to combustion.     

Speciated Hydrocarbon and Carbon Monoxide Emissions from an Internal Combustion Engine Operating on Methyl Tertiary Butyl Ether-Containing Fuels

ABSTRACT

In the present work, engine and tailpipe (after a three-way catalytic converter) emissions from an internal combustion engine operating on two oxygenated blend fuels [containing 2 and 11% weight/weight (w/w) methyl tertiary butyl ether (MTBE)] and on a nonoxygenated base fuel were characterized. The engine (OPEL 1.6 L) was operated under various conditions, in the range of 0-20 HP. Total unburned hydrocarbons, carbon monoxide, methane, hexane, ethylene, acetaldehyde, acetone, 2-propanol, benzene, toluene, 1,3-butadiene, acetic acid, and MTBE were measured at each engine operating condition. As concerns the total HC emissions, the use of MTBE was beneficial from 1.90 to 3.81 HP, which were by far the most polluting conditions. Moreover, CO emissions in tailpipe exhaust were decreased in the whole operation range with increasing MTBE in the fuel.

The greatest advantage of MTBE addition to gasoline was the decrease in ethylene, acetaldehyde, benzene, toluene, and acetic acid emissions in engine exhaust, especially when MTBE content in the fuel was increased to 11% w/w. In tailpipe exhaust, the catalyst operation diminished the observed differences. Ethylene, methane,and acetaldehyde were the main compounds present in exhaust gases. Ethylene was easily oxidized over the catalyst,while acetaldehyde and methane were quite resistant to oxidation.     

Effects of combustion and operating conditions on PCDD/PCDF emissions from power boilers burning salt-laden wood waste

This paper discusses the effects of combustion conditions on PCDD/PCDF emissions from pulp and paper power boilers burning salt-laden wood waste. We found no correlation between PCDD/PCDF emissions and carbon monoxide emissions. A good correlation was, however, observed between PCDD/PCDF emissions and the concentration of stack polynuclear aromatic hydrocarbons (PAHs) in the absence of TDF addition. Thus, poor combustion conditions responsible for the formation of products of incomplete combustion (PICs), such as PAHs and PCDD/PCDF precursors, increase PCDD/PCDF emissions. PAH concentrations increased with higher boiler load and/or low oxygen concentrations at the boiler exit, probably because of lower available residence times and insufficient excess air. Our findings are consistent with the current understanding that high ash carbon content generally favours heterogeneous reactions leading to either de novo synthesis of PCDD/PCDFs or their direct formation from precursors. We also found that, in grate-fired boilers, a linear increase in the grate/lower furnace temperature produces an exponential decrease in PCDD/PCDF emissions. Although the extent of this effect appears to be mill-specific, particularly at low temperatures, the results indicate that increasing the combustion temperature may decrease PCDD/PCDF emissions. It must be noted, however, that there are other variables, such as elevated ESP and stack temperatures, a high hog salt content, the presence of large amounts of PICs and a high Cl/S ratio, which contribute to higher PCDD/PCDFs emissions. Therefore, higher combustion temperatures, by themselves, will not necessarily result in low PCDD/PCDFs emissions.     

Directions for Combustion Engine Aerosol Measurement in the 21st Century

Abstract

The Coordinating Research Council convened two Real-Time PM Measurement Workshops in December 2008 and March 2009 to take an intensive look at the current status and future directions of combustion aerosol measurement. The purpose was to examine the implications of parallel rapid developments over the past decade in ambient aerosol science, engine aftertreatment technology, and aerosol measurement methodology, which provide bene?ts and challenges to the stakeholders in air quality management. The workshops were organized into sessions targeting key issues in ambient and source combustion particulate matter (PM). These include (1) metrics to characterize and quantify PM, (2) the need to reconcile ambient and source measurements, (3) the role of atmospheric transformations on modeling emissions and exposures, (4) the impact of sampling conditions on PM measurement, and (5) the potential bene?ts of novel PM instrumentation. This paper distills the material presented by subject experts and the insights derived from the in-depth discussions that formed the core of each session. The paper’s objectives are to identify areas of consensus that allow wider practical application of the past decade’s advances in combustion aerosol measurement to improve emissions and air quality modeling, develop emissions reduction strategies, and to recommend directions for progress on issues in which uncertainties remain.     

Speciated Hydrocarbon Emissions from the Combustion of Single Component Fuels. I. Effect of Fuel Structure

Speciated hydrocarbon emissions data have been collected for six single-component fuels run in a laboratory pulse flame combustor (PFC). The six fuels include n-heptane, isooctane (2, 2, 4-trimethylpentane), cyclohexane, 1-hexene, toluene, and methyl-t-butyl ether (MTBE: an oxygenated fuel extender). Combustion of non-aromatic fuels in the PFC (at a fuel/air equivalence ratio of Φ = 1.02) produced low levels of unburned fuel and high yields of methane and olefins (> 75 percent combined) irrespective of the molecular structure of the fuel. In contrast, hydrocarbon emissions from toluene combustion in the PFC were comprised predominantly of unburned fuel (72 percent). With the PFC, low levels of 1, 3-butadiene (0.3-1.8 percent) were observed from all the fuels except MTBE, for which no measurable level (<0.2 percent) was detected; low levels of benzene were observed from isooctane, heptane, and 1-hexene, but significant levels (9 percent) from cyclohexane and toluene. No measurable amount of benzene (< 0.2 percent) was observed in the MTBE exhaust.

For isooctane and toluene the speciated hydrocarbon emissions from a spark-ignited (SI) single-cylinder engine were also determined. HC emissions from the SI engine contained the same species as observed from the PFC, although the relative composition was different. For the non-aromatic fuel isooctane, unburned fuel represented a larger fraction (50 percent) of the HC emissions when run in the engine. HC emissions from toluene combustion in the engine were similar to those from the PFC.     

Comparison of Air Pollutant Emissions from Vaporizing and Air Atomizing Waste Oil Heaters

Information presented in this paper is directed to individuals concerned with emissions from combustion of waste crankcase oil for space heating. Studies were performed to characterize gaseous and particulate emissions and vaporizing pot solid residues resulting from the combustion of waste crankcase oil. Two types of waste oil burners were tested. One was a vaporizing oil burner rated at 35.2 kW (120,000 Btu/h heat input), and the other was an air atomizing oil burner rated at 73.3 kW (250,000 Btu/h heat input). Except for NO_X and SO_X, gaseous emissions were similar to those from conventional distillate oil combustion. NO_X and SO_X emissions were higher due to higher fuel nitrogen and sulfur concentrations. Waste oil from automotive use showed higher inorganic levels than crankcase oil used for truck engine lubrication. Both burner types discharged high levels of metallic species, but the air atomizing unit had much higher stack emission levels than did the vaporizing pot system. Also, particulate loading levels were an order of magnitude higher for the air atomizing burner than for the vaporizing pot burner when firing the waste oils. However, the vaporizing pot burner generated a waste residue containing the majority of its elemental emissions. Elements which exceeded threshold limit values for one or both heaters were cadmium, chromium, cobalt, copper, iron, lead, nickel, phosphorus, and zinc. However, the nickel and much of chromium appeared to be a sampling artifact caused by the stainless steel sampling system.     

Consultant Guide/1989

The European Economic Community (EEC) has proposed strict limits on emissions of dioxins and furans from hazardous waste incinerators. The proposed limit is 0.1 ng/Nm³, expressed as the 2,3,7,8 TCDD toxic equivalent of 17 specific dioxin and furan congeners. These limits will potentially redefine technology selection and design for combustion, energy recovery, and air pollution control. The EPA has a different approach for controlling emissions of products of incomplete combustion (PICs) and reformation products such as dioxins and furans. Rather than limiting these contaminants individually and quantitatively, EPA proposes controlling them by assuring good combustion as measured by stack emissions of carbon monoxide (CO) and total hydrocarbons (THC).

Dioxins and furans are combustion by-products and emission control relies mainly on control of the combustion process. These compounds can also be reformed from certain precursor compounds and elements in lower temperature regions of the system downstream of the combustion process. Air pollution control technologies have demonstrated the ability to remove dioxins and furans as contaminants on fine particulate.

This paper will discuss the two regulatory approaches, the mechanisms for the formation and reformation of dioxins and furans, and the technologies available to control emissions.     

Prediction of Dioxin/Furan Incinerator Emissions Using Low-Molecular-Weight Volatile Products of Incomplete Combustion

ABSTRACT

Emissions of polychlorinated dibenzo-p-dioxins and poly-chlorinated dibenzofurans (PCDDs/Fs) from incinerators and other stationary combustion sources are of environmental concern because of the toxicity of certain PCDD/F congeners. Measurement of trace levels of PCDDs/Fs in combustor emissions is not a trivial matter. Development of one or more simple, easy-to-measure, reliable indicators of stack PCDD/F concentrations not only would enable incinerator operators to economically optimize system performance with respect to PCDD/F emissions, but could also provide a potential technique for demonstrating compliance status on a more frequent basis. This paper focuses on one approach to empirically estimate PCDD/F emissions using easy-to-measure volatile organic C₂ chlorinated alk-ene precursors coupled with flue gas cleaning parameters. Three data sets from pilot-scale incineration experiments were examined for correlations between C₂ chlorinated alk-enes and PCDDs/Fs. Each data set contained one or more C₂ chloroalkenes that were able to account for a statistically significant fraction of the variance in PCDD/F emissions. Variations in the vinyl chloride concentrations were able to account for the variations in the PCDD/F concentrations strongly in two of the three data sets and weakly in one of the data sets.     

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

ABSTRACT

Particulate matter (PM) emissions from stationary combustion sources burning coal, fuel oil, biomass, and waste, and PM from internal combustion (IC) engines burning gasoline and diesel, are a significant source of primary particles smaller than 2.5 μm (PM_2.5) in urban areas. Combustion-generated particles are generally smaller than geologically produced dust and have unique chemical composition and morphology. The fundamental processes affecting formation of combustion PM and the emission characteristics of important applications are reviewed. Particles containing transition metals, ultrafine particles, and soot are emphasized because these types of particles have been studied extensively, and their emissions are controlled by the fuel composition and the oxidant-tem-perature-mixing history from the flame to the stack. There is a need for better integration of the combustion, air pollution control, atmospheric chemistry, and inhalation health research communities. Epidemiology has demonstrated that susceptible individuals are being harmed by ambient PM. Particle surface area, number of ultrafine particles, bioavailable transition metals, polycyclic aromatic hydrocarbons (PAH), and other particle-bound organic compounds are suspected to be more important than particle mass in determining the effects of air pollution. Time- and size-resolved PM measurements are needed for testing mechanistic toxicological hypotheses, for characterizing the relationship between combustion operating conditions and transient emissions, and for source apportionment studies to develop air quality plans. Citations are provided to more specialized reviews, and the concluding comments make suggestions for further research.     

Experimental Evaluation of Waste Tires Utilization in Cement Kilns

ABSTRACT

The present work outlines the main results of a full-scale study conducted on the utilization of waste tires as auxiliary fuel in cement production. Experimental tests were conducted for determining the influence of shredded tires on combustion conditions, emissions produced, and the characteristics of clinker obtained, for feeding ratios over 35% in terms of total heat input. The addition of tire chips did not lead to any appreciable modification in either the whole process or the quality of clinker produced; gaseous emissions were mostly unaffected, with significant improvements related to the reductions obtained in nitrogen and sulfur oxides concentrations. Experimental findings from tests conducted with tire chips exposed to kiln combustion flue gases compare favorably with the typical burnout times derived from theoretical approaches. These experimental data and calculations to estimate particle trajectories beyond the injection point, through proper theoretical analysis of the kinetic behavior, result in important indications for the shredding operation and for optimum injection modes.     

Field Testing and Computer Modeling of an Oxygen Combustion System at the EPA Mobile Incinerator

The LINDE® Oxygen Combustion System has been demonstrated successfully at the EPA Denney Farm site as part of the modified EPA mobile incinerator. This paper describes the field testing results and computer modeling of the LINDE system. The oxygen system enables the EPA unit to incinerate dioxin and PCB contaminated soil at a consistent rate of 4000 lb/h—200 percent of the original maximum capacity. The pure oxygen combustion system improved the thermal efficiency of the incinerator by over 60 percent and reduced the flue gas volume dramatically. Therefore, the dust carryover problem was mitigated. The destruction and removal efficiencies of hazardous wastes exceeded EPA requirements.

The design of the proprietary burner allows the use of up to 100 percent oxygen in place of air for incineration with improvements over conventional oxy-fuel burners. As a result, the temperature distributions in the rotary kiln are uniform and NO_x emissions are low.

The oxygen combustion system, controlled by a programmable controller, provided much better response and flexibility than conventional air based systems. The system generated a stable flame and responsed well to the transient conditions of the rotary kiln. Kiln puff occurrence was virtually eliminated in the operation of the mobile incinerator.

A computer model of the incinerator was developed and used for the process design of the LINDE system. The model predicted the test results reasonably well. This model can be a useful tool in the design and operation of rotary kiln incineration systems.     

Speciated Hydrocarbon Emissions from Small Utility Engines

ABSTRACT

Partially speciated hydrocarbon (HC) emissions data from several small utility engines, as measured by a Fourier Transform Infrared analyzer, are presented. The engines considered have nominal horsepower ratings between 3.7 and 9.3 kW. Both side-valve and overhead-valve engines are studied, and four different fuels are used in the engines. The results indicate that the small HCs present in the exhaust tend to be in the form of either methane or unsatur-ated HCs. Other small alkanes, such as ethane and propane, are present in only relatively small concentrations. In terms of ozone formation potential, the HCs in the form of methane will lead to little ozone, but the distribution of the C₂ and C₃ species is not ideal from an ozone reduction standpoint. It is also found that the presence of oxygen in the fuels appears to lead to somewhat more complete combustion, although the effects are not large. Finally, the overhead-valve engines appear to have lower HC emissions than side-valve engines, which is primarily due to higher operating A/F ratios and the engine geometry.