C. Andrew Miller

ON TRIMODAL PARTICLE SIZE DISTRIBUTIONS IN FLY ASH FROM PULVERIZED-COAL COMBUSTION

Combustion-generated fine particles, defined as those with aerodynamic diameters less than 2.5 μm, have come under increased regulatory scrutiny because of suspected links to adverse human health effects. Whereas classical theories regarding coal combustion suggest that mechanisms of ash vaporization and fragmentation lead to bimodal ash particle size distributions (PSDs), this paper presents experimental results supporting other existing hypotheses that three distinct ash modes may be more appropriate. This paper focuses on the existence and generality of a central mode, between approximately 0.7 and 3.0 μm diameter. This central mode is presumably caused by fragmentation mechanisms, but is still important from a health perspective, because a large portion is contained within the 2.5 μm particle size fraction. Presented here are experimental results from two different laboratory combustors and one industrial boiler, all burning pulverized coals. Use of a variety of particle-sampling and size classification methods, including electrical mobility, time-of-flight, and inertial (low-pressure impaction) methods, confirms that the central mode is not an artifact of the particle-sampling and -sizing methods used. Results from the combustion of 10 different coals consistently show that this central mode is significant for both high-and low-rank coals. Size-segregated elemental distributions of calcium, iron, and aluminum provide additional insight into mechanisms of formation of each mode. Field tests show that the central mode can be the major contributor to fine particle emissions leaving an electrostatic precipitator (ESP). The new experimental results presented here are interpreted in the light of complementary existing data and available theories from the literature.

Fine particle emissions from residual fuel oil combustion: Characterization and mechanisms of formation

Abstract : The characteristics of particulate matter (PM) emitted from residual fuel oil combustion in two types of combustion equipment were compared. A small commercial 732 kW rated fire-tube boiler yielded a weakly bimodal particulate size distribution (PSD) with over 99% of the mass contained in a broad coarse mode and only a small fraction of the mass in an accumulation mode consistent with ash vaporization. Bulk samples collected and classified by a cyclone indicate that 30% to 40% of the total particulate emissions were less than 2.5mum aerodynamic diameter (PM2.5). The coarse mode PM was rich in char, indicating relatively poor carbon burnout, although calculated combustion efficiencies exceeded 99%. This characteristic behavior is typical of small fire-tube boilers. Larger, utility-scale units firing residual oil were simulated using an 82 kW rated laboratory-scale refractory-lined combustor. Particulate matter emissions from this unit were in good agreement with published data including published emission factors. These data indicated that the refractory-lined combustor produced less total but more fine particulate emissions, as evident from a single unimodal PSD centered at <0.1 mum diameter. Bulk cyclone segregated samples indicated that here all the PM were smaller than 2.5 mum aerodynamic diameter, and loss on ignition (LOI) measurements suggested almost complete char burnout. These findings are interpreted in the light of possible mechanisms governing the release of organically bound refractory metals and may have particular significance in considering the effects of fuel oil combustion equipment type on the characteristic attributes of the fine PM emitted into the atmosphere and their ensuing health effects.

Air Emission Inventories in North America: A Critical Assessment

Abstract Although emission inventories are the foundation of air quality management and have supported substantial improvements in North American air quality, they have a number of shortcomings that can potentially lead to ineffective air quality management strategies. Major reductions in the largest emissions sources have made accurate inventories of previously minor sources much more important to the understanding and improvement of local air quality. Changes in manufacturing processes, industry types, vehicle technologies, and metropolitan infrastructure are occurring at an increasingly rapid pace, emphasizing the importance of inventories that reflect current conditions. New technologies for measuring source emissions and ambient pollutant concentrations, both at the point of emissions and from remote platforms, are providing novel approaches to collecting data for inventory developers. Advances in information technologies are allowing data to be shared more quickly, more easily, and processed and compared in novel ways that can speed the development of emission inventories. Approaches to improving quantitative measures of inventory uncertainty allow air quality management decisions to take into account the uncertainties associated with emissions estimates, providing more accurate projections of how well alternative strategies may work. This paper discusses applications of these technologies and techniques to improve the accuracy, timeliness, and completeness of emission inventories across North America and outlines a series of eight recommendations aimed at inventory developers and air quality management decision-makers to improve emission inventories and enable them to support effective air quality management decisions for the foreseeable future.

NOx abatement by fuel-lean reburning: Laboratory combustor and pilot-scale package boiler results

The paper discusses two experimental studies related to the abatement of nitrogen oxides (NOx) by fuel-lean reburning. First, systematic tests in a 17-kW down-flow laboratory combustor, in which nitric oxide (NO) in the oxidant was destroyed in long, axial, methane/air diffusion flames, showed that substantial reduction of NO was possible under overall fuel-lean conditions.

Formation of Fine Particles from Residual Oil Combustion: Reducing Nuclei through the Addition of Inorganic Sorbent

The potential use of sorbents to manage emissions of ultrafine metal nuclei from residual oil combustion was investigated by using an 82-kW-rated laboratory-scale refractory-lined combustor. Without sorbent addition, baseline measurements of the fly ash particle size distribution (PSD) and chemical composition indicate that most of the metals contained in the residual oil form ultrafine particles (∼0.1 μm diameter). These results are consistent with particle formation via mechanisms of ash vaporization and subsequent particle nucleation and growth. Equilibrium calculations predict metal vaporization at flame temperatures and were used to define regions above the dew point for the major metal constituents (iron [Fe], nickel [Ni], vanadium [V], and zinc [Zn]) where vapor-phase metal and solidphase sorbents could interact. The addition of dispersed kaolinite powder resulted in an approximate 35% reduction in the ultrafine nuclei as determined by changes to the PSDs as well as the size-dependent chemical composition.

Methodology for examining potential technology breakthroughs for mitigating CO2 and application to centralized solar photovoltaics

Aggressive reductions in US greenhouse gas emissions will require radical changes in how society generates and uses energy. Technological breakthroughs will be necessary if we are to make this transition cost effectively. With limited resources, understanding the breakthrough potential of various alternative technology options will be critical. One common approach for comparing technology options is via their relative levelized cost of electricity. This measure does not account for many of the complexities of the landscape in which the technologies compete, however. As an alternative, we describe the use of an energy system model within a nested parametric sensitivity analysis. The approach is applied to examine the breakthrough potential of a specific class of technology, centralized solar photovoltaics (CSPV). We define a “breakthrough” as being a tangible reduction in the system-wide cost of meeting a CO2 mitigation target. As “tangible” is a subjective term, we characterize the relationship between technology cost reductions and system-wide cost reductions for several mitigation targets. The results illustrate the importance of considering contextual factors in evaluating and comparing technologies. For example, the critical role that fuel switching and vehicle electrification play in mitigation scenarios is shown to affect the competition between CSPV and baseload technologies for market share. This breakthrough analysis approach can be applied to other technologies and is expected to be useful in assessing and comparing breakthrough opportunities across the energy system, including both energy production and use.

Potential Adverse Environmental Impacts of Greenhouse Gas Mitigation Strategies

The Fourth Assessment Report released by the Intergovernmental Panel on Climate Change (IPCC) in 2007 was unequivocal in its message that warming of the global climate system is now occurring, and found, with “very high confidence” that it was “very likely” that the observed warming was due to anthropogenic emissions of greenhouse gases (GHGs). To address the problem, the IPCC developed an outline of approaches to reduce GHG emissions to desired levels. The expected changes in technologies and practices needed to mitigate emissions of GHGs will lead to changes in the impacts to the environment associated with energy production and use. Some of these changes will be beneficial, but others will not. This chapter identifies some of the potential environmental impacts (other than the intended mitigation of climate change) of implementing GHG mitigation strategies, but will not attempt to quantify those impacts or their costs. Included are discussions of the impacts of implementing energy efficiency and conservation measures, fuel switching in the power generation sector, nuclear and renewable energy, carbon capture and storage, use of biofuels and natural gas for transportation fuels, and hydrogen and electricity for transportation energy. Environmental impacts addressed include changes in air emissions of nitrogen oxides, sulfur dioxide, and particulate matter; impacts to water quality and quantity; increased mining of coal to meet the power demands of carbon capture systems and of metals to meet demands for vehicle batteries; and impacts to ecosystems associated with biofuel production and siting of other renewable energy systems.

Emissions from the Burning of Vegetative Debris in Air Curtain Destructors

Abstract Although air curtain destructors (ACDs) have been used for quite some time to dispose of vegetative debris, relatively little in-depth testing has been conducted to quantify emissions of pollutants other than CO and particulate matter. As part of an effort to prepare for possible use of ACDs to dispose of the enormous volumes of debris generated by Hurricanes Katrina and Rita, the literature on ACD emissions was reviewed to identify potential environmental issues associated with ACD disposal of construction and demolition (C&D) debris. Although no data have been published on emissions from C&D debris combustion in an ACD, a few studies provided information on emissions from the combustion of vegetative debris. These studies are reviewed, and the results compared with studies of open burning of biomass. Combustion of vegetative debris in ACD units results in significantly lower emissions of particulate matter and CO per unit of mass of debris compared with open pile burning. The available data are not sufficient to make general estimates regarding emissions of organic or metal compounds. The highly transient nature of the ACD combustion process, a minimal degree of operational control, and significant variability in debris properties make accurate prediction of ACD emissions impossible in general. Results of scoping tests conducted in preparation for possible in-depth emissions tests demonstrate the challenges associated with sampling ACD emissions and highlight the transient nature of the process. The environmental impacts of widespread use of ACDs for disposal of vegetative debris and their potential use to reduce the volume of C&D debris in future disaster response scenarios remain a considerable gap in understanding the risks associated with debris disposal options.

Source Emissions in Multipollutant Air Quality Management

This chapter discusses emissions inventories and emission control technologies in the context of a risk-based, results-oriented multipollutant air quality management program. As contemporary emission control technologies often achieve reductions in more than one pollutant, the chapter reviews major emissions reduction in North America as examples of multipollutant reduction strategies for both criteria pollutants and air toxics. The chapter then comments on developments in these technologies that will be required in the future. The chapter reaches four principal conclusions regarding the development of emissions inventories for risk- and results-based air quality management: (1) the need to consider the full range of factors influencing source emissions, from fuels to end-of-pipe control technologies; (2) the need for measurements to determine the direct effects of emission reduction programs; (3) the need for better understanding of the emissions important to determining health and ecosystem effects; and (4) the capability to respond to the rapid changes in generation and end-use technologies that could result from actions taken to mitigate greenhouse gas emissions. The chapter concludes with a series of recommendations for addressing these challenges.

Public Health and Climate Programs at the U.S. Environmental Protection Agency

This chapter explores several key aspects of EPA’s climate change program related to public health: (1) the Agency’s findings regarding the risks posed by greenhouse gases; (2) the federal regulations developed to reduce greenhouse gas emissions; (3) EPA’s efforts on short-lived climate pollutants such as black carbon, methane, and ozone; and (4) the EPA’s intramural and extramural research programs related to climate and public health.