Optical Property Measurements of Mixed Coal Fly Ash and Particulate

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Optical Property Measurements of Mixed Coal Fly Ash and Particulate Carbon Aerosols Likely Emitted During Activated Carbon Injection for Mercury Emissions Control Tian Xia, and Herek L. Clack Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02046 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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Optical Property Measurements of Mixed Coal Fly

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Ash and Particulate Carbon Aerosols Likely Emitted

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During Activated Carbon Injection for Mercury

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Emissions Control

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Tian Xia*, Herek L. Clack

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Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor,

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Michigan, United States

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KEYWORDS: Mercury; Electrostatic Precipitation; Powdered Activated Carbon; Black Carbon; Optical Absorption; Light Scattering.

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ABSTRACT: The most mature technology for controlling mercury emissions from coal

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combustion is the injection into the flue gas of powdered activated carbon (PAC) adsorbents

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having chemically treated surfaces designed to rapidly oxidize and adsorb mercury. However,

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carbonaceous particles are known to have low electrical resistivity, which contributes to their

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poor capture in electrostatic precipitators (ESPs), the most widely used method of particulate

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control for coal-fired power plants worldwide. Thus, the advent of mercury emissions standards

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for power plants has the potential for increased emissions of PAC. Our previous analyses have

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provided estimates of PAC emission rates resulting from PAC injection in the U.S. and

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extrapolated these estimates globally to project their associated climate forcing effect. The

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present work continues our examination by conducting the first comparative measurements of

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optical scattering and absorption of aerosols comprised of varying mixtures of coal combustion

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fly ash and PAC. A partially fluidized bed (FB) containing fly ash-PAC admixtures with varying

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PAC concentrations elutriates aerosol agglomerates. A photo-acoustic extinctiometer (PAX)

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extractively samples from the FB flow, providing measurements of optical absorption and

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scattering coefficients of fly ash (FA) alone and FA-PAC admixtures. Extracted aerosol samples

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from the FB flow provide particulate loading measurements, thermogravimetric analysis (TGA)

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provide estimations of the carbon content of the particulates collected from the FB emission, and

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SEM images of the collected aerosols provide qualitative insight into the aerosols’ size

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distributions and agglomeration state. Soot from an oil lamp flame provides a comparative

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benchmark. The results indicate that the increase of carbonaceous particles in the FB emissions

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can cause a significant linear increase of their mass absorption cross sections (MAC). Thus,

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widespread adoption of activated carbon injection (ACI) in conjunction with ESPs has the

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potential to constitute a new source of light absorbing particle emissions which can absorb light

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efficiently and potentially act like BC in the atmosphere.


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Abstract Art. Measured linear increase of MAC and MSC, linear decrease of SSA of fly ash

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emissions due to PAC addition.

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INTRODUCTION

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Mercury (Hg) is an environmental pollutant of concern due to its tendency to cycle between soil,

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air, and water; its bioaccumulation within the food chain; and its toxic effects on human and

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wildlife health. Coal-fired power plants (CFPPs) are one of the major sources of anthropogenic

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mercury emissions. According to United Nations Environment Programme (UNEP), coal

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combustion accounts for 24% of anthropogenic mercury emissions, which is the second largest

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source.1 In the U.S., about 50% of anthropogenic mercury emission is from electric generating

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units (EGUs), mostly CFPPs.2 In China, coal combustion is responsible for 37% of mercury

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emissions of which only 14% is from CFPPs and the remainder attributable to uncontrolled

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industrial and household burning of coal.3 Anthropogenic mercury emission is regulated by

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several countries in the world. In 2013, the Governing Council of the UNEP agreed on the

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Minamata Convention on Mercury, which is a global treaty that requires countries to reduce

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anthropogenic mercury emissions from all sources. In 2012, the U.S. EPA finalized its Mercury

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and Air Toxics Standard (MATS), which requires existing CFPPs to reduce their mercury

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emissions by 90% starting in 2015. Although the standard was rejected by the U.S. Supreme

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Court in June, 2015 due to insufficient consideration of the costs of the regulation, most electric

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utilities in the U.S. are continuing with their plans to comply with the regulation, including an

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estimated 30 GW of small or old units being retired.4 In China, a revised national emission

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standard for CFPPs, which took effect in 2012, limits mercury emissions from CFPPs to 0.03

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mg/m3.5

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Among the most mature technologies that have been developed for controlling mercury

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emissions from CFPPs, the injection into the flue gas of powdered activated carbon (PAC)

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adsorbents is attractive for its low capital costs, the small footprint of each installation, and


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smaller ancillary impacts on other pollutants or pollutant control processes as compared to other

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Hg control technologies. Current PAC products for Hg removal have chemically treated surfaces

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designed to rapidly oxidize and adsorb mercury during the seconds of contact time with the flue

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gas after injection and before its removal with fly ash in a downstream particulate control device,

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predominately electrostatic precipitators (ESPs). Detailed descriptions of PAC injection as well

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as other mercury control technologies can be found in other reports.6,7 In 2011, the U.S. EPA

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estimated that a total of 148 GW of coal combustion EGUs would install activated carbon

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injection (ACI) technologies in order to comply with MATS,2 and according to U.S. Energy

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Information Administration’s (EIA) annual report, 106 GW of ACI has been installed in CFPPs

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by 2015.8 There is no particular mercury emissions control technology being installed for CFPPs

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in China, but ACI is believed to be the most promising technology due to the prevalence of ESPs

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in China.9 The drawback of ACI is that the efficiency of PAC removal in ESPs is lower than fly

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ash since the electrical resistivity of PAC (< 104 ohm-cm10,11) is far below the range for optimal

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ESP removal efficiency (108−1012 ohm-cm12,13). As a result, it would be expected that PAC

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would be preferentially emitted from CFPPs during the application of ACI, at a rate that is as yet

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unknown. In early 2000’s, US Dept. of Energy funded a series of pilot-scale testing on ACI

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injection and the darkened particulate filters collected at the ESP outlet at Stanton Unit 1 (a 150-

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MW Foster Wheeler wall-fired boiler that has been operational since 1966 and is equipped with

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three coal feeders and pulverizers and two cold-side ESPs in parallel (SCA = 470 ft2/1000 afcm))

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confirmed the increase of PAC emission.14 Further, sub-micron, black-colored PAC aerosols

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(~4% in volume)15 would be expected to have optical properties similar to submicron black

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carbon (BC, i.e. soot, average size of 0.15-0.40 µm16,17,18) that can form during fossil fuel

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combustion. CFPPs without ACI installed were considered to have little contribution to BC


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emission. Bond et al.19 estimated that only 0.6% of PM emitted from pulverized coal combustors

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were black carbon, and Cao et al.20 reported that power generation only contributed to 0.5% of

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BC emission in China. Clack21 developed estimates indicating that the contributions of CFPPs to

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total U.S. anthropogenic BC emissions (i.e., excluding wildfires) could increase from less than

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6% to 8% in 2020, and from less than 8% to 11% by 2030, due to the emitted PAC. While the

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percent of CFPPs adopting ACI was a primary factor in the variability of the estimates, the

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unknown degree to which ESPs collect PAC less efficiently than fly ash was the principle

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uncertainty.

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The climate warming effect of PAC and atmospheric BC is far less well understood than that of

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CO2. Bond et al. suggested a 7.5 ± 1.2 m2/g mass absorption cross section (MAC) for black

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carbon at 550 nm, and a 0.2-0.3 single scattering albedo.22 The 2013 Intergovernmental Panel on

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Climate Change (IPCC) report estimates the direct forcing effect of BC to be 0.05-0.8 Wm-2, and

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illustrates that BC can have the second largest effect on climate forcing behind CO2.23 It is

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believed that the addition of PAC emissions from CFPPs due to ACI would further increase the

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overall BC radiative forcing unintentionally. According to the model developed by Lin et al,24

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the submicron PAC emissions, if applied globally to CFPP particulate emissions, could cause a

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global warming of 2.10 mW m−2 at the top of atmosphere and a cooling of −2.96 mW m−2 at the

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surface. In addition, comparing the warming effects of emitted CO2 and PAC, each normalized

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by its respective rate of carbon input (as fuel for CO2; as carbon in PAC for ACI), it was

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determined that emitted submicron PAC has eight times larger climate forcing per unit carbon

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input than CO2. These results, however, assume that the emitted PAC has the same optical

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properties as that of BC, while the actual optical properties of PAC are unknown. Furthermore,

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most simulations of BC climate forcing, including that of Lin et al., assume that emitted coal


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combustion fly ash (FA) is entirely mineral and entirely scattering, while in fact, fly ash varies

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widely in color and carbon content, and PAC injection can enhance those variations. In order to

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better understand the actual climate forcing potential of incidentally emitted PAC, it is necessary

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to better understand PAC emission rates (complement of collection rates in particulate control

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devices) and the optical properties of PAC, FA and PAC-FA agglomerates.

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In this paper, we aim to provide the first comparative measurements of optical scattering and

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absorption of aerosols comprised of varying mixtures of coal combustion fly ash and PAC, in an

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effort to better constrain the potential climate forcing effects of emitted PAC resulting from ACI

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used to reduce mercury emissions. Soot is also collected and analyzed to provide a black carbon

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standard against whose optical properties the fly ash and PAC mixtures could be compared. A

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photoacoustic extinctiometer (PAX, Droplet Measurement Technologies, Boulder, CO) is used

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to optically characterize both the combustion soot and the effluent from a partially fluidized bed

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(FB) of prepared mixtures of PAC and FA samples obtained from the hoppers of power plant

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ESPs. Extracted aerosol samples from the FB effluent provide the particulate mass loading

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measurements. Thermogravimetric analysis (TGA) is applied to evaluate the carbon content of

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the mixed aerosols in the FB effluent.

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MATERIALS AND METHODS

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Experimental Setup

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For this experiment, fly ash samples were collected from the first three rows of hoppers of a

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utility ESP at DTE’s Belle River (BR) Power Plant in Michigan. At the time of fly ash

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collection, the Belle River station burned a low-sulfur, western sub-bituminous coal. Because of

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the cohesive properties of the finer ash taken from the second and third rows, only fly ashes from


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the first row of hoppers were used here, with the result being that these ash samples are a coarser

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fraction of the overall size distribution of ash collected in the ESP. Unburned carbon content was

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determined from loss on ignition (LOI) measurements conducted by DTE Central Chemical Lab.

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Norit Darco FGD used in previous studies of fly ash-activated carbon behavior11,15,25 was the

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powdered activated carbon (PAC) product added to the fly ash to form the particulate admixture,

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mixed on a mass per mass basis. The PAC particles had a lognormal size distribution, with a

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Sauter mean diameter of 5.9 µm and a long tail in the low end.15 It should be noted that PAC

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samples used in this research had relatively large size distributions as compared to freshly

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emitted BC, which may affect the particles’ optical properties. Bond et al.22 and Martins et al.26

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have shown that the volume and/or mass absorption cross sections would decrease with

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increasing particle size for particles are larger than 0.1 µm. Particle’s scattering property is

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governed by the ratio of the particle size to the light wavelength, and generally for particles

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larger than 1 µm, the scattering coefficient would decrease with increasing particle size.27

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Admixtures consisted of BR FA samples amended with 0%, 0.5%, 1% and 2% (by mass) PAC.

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The two constituents were added to PTFE bags and the bags were tumbled inside the barrel of a

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rock tumbler (Thumler's Tumbler, Model B), for 10 minutes to ensure thorough mixing and

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uniform distribution of the PAC in the fly ash.

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During measurement of optical properties, the FA-PAC aerosols were elutriated from the

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partially fluidized bed. Before fluidization, an empty fluidization vessel was loaded with 1,000

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grams of each prepared admixture. Facility-supplied dried (10% RH) compressed air entered the

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vessel from below at five liters per minute (LPM) to partially fluidize the loaded admixture, a

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fraction of which was continuously elutriated, from which real-time optical characterization and

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aerosol integrated average mass loading determinations were made. Two sampling probes were


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installed in the top of the fluidized bed: one was connected to the PAX which sampled a portion

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of the elutriate (1 LPM) and took readings of scattering coefficients and absorption coefficients

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simultaneously; the other probe was connected to a particulate filter (filter cases: Savillex, Model

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No. 401-21-47-10-21-2; Polycarbonate filter: Whatman, 131135012E) used to determine the

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average aerosol mass loading of the elutriate which was weighed before and after collecting

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samples using an analytical balance. A rotameter (Omega, Model No. FL3840-ST) connected

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downstream of the PM filter indicated the air flow rate through the filter. The air flow rate

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needed to establish partial fluidization of the admixture varies with particle size (i.e., changing

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fluidization regime, based on Geldart classification28), and in all cases exceeds the PAX

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sampling flow rate. As a result, most of the elutriated flow from the fluidized bed was directed to

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an auxiliary PM filter and only 1LPM flow was directed into PAX. This auxiliary filter was not

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used for mass loading measurements but rather to maintain the same exhaust flow rate, and thus

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the same fluidization air flow rate and fluidized bed behavior, inside the vessel.

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For the optical characterization of combustion-generated soot, an oil lamp (TIKI® Molded Glass

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Table Torch, Model No. 114109, burning TIKI® Brand Citronella Torch Fuel) was positioned at

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the bottom of a brick chimney built with four equally spaced openings around its periphery to

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promote a swirling updraft to stabilize the flame. A sampling probe connected to a vacuum pump

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sampled from the plume of the sooting flame at the top of the chimney at a rate of 12 LPM, a

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flow which was divided into two streams: one (1 LPM) directed to the PAX and the other (11

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LPM) directed to the PM filter for aerosol mass loading determination.

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Measurement Processes


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In this experiment, the duration of the PAX measurement cycle was set to 500 seconds. In each

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measurement cycle, the first 70 seconds were devoted to automated flushing the PAX

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measurement chamber with clean air and zero all sensors. The remaining 430 seconds were

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devoted to taking optical property measurements (referred to as the measurement stage).

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For each new FA-PAC admixture, the fluidized bed was allowed to freely and continuously

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elutriate for at least 90 minutes before any measurements were taken to reduce aerosol mass

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loading so that the measured absorption and scattering coefficients were within the measurement

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range of PAX (30% carbon content should be due to carbon content fluctuations in the emission

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from the FB, where active fluidization was promoted by the addition of PAC.

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Figure 6. Correlation of the obtained MSC and MAC with particles’ carbon content.

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In addition to the MAC and MSC, the SSAs of the emissions for every two seconds’ PAX data

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points were calculated, and the final SSA of the emissions from each FB with a specific bed FA-

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PAC mixing ratio were calculated by averaging the three groups of data points. The correlation

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of the calculated SSAs with the emissions’ carbon contents is presented in Figure 7 with a data

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point representing the soot measurement result assuming a 100% carbon content. From the

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figure, it can be found that the SSA of the FA-PAC admixture particles decreases linearly with

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the increase of carbon content with a slope of -0.82. When the carbon content reaches 100%, the

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SSA obtained from the trend line is 0.13, which is about half of the measured soot SSA. This

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may be because of the assertive assumption that soot contains 100% carbon. Soot is a mixture of

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carbon and various organic materials, and to further examine the optical propriety of soot

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generated in this research, its chemical composition should be studied.


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Figure 7. Correlation of the obtained SSA with particles’ carbon content.

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Comparison with Other Research

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In this section, the optical properties obtained in last section are compared with relevant

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measurement results from other research. The comparison is only focused on MAC and SSA

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results since the two properties are the most commonly used ones by models to predict aerosol

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climate forcing.

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The MAC of soot, or black carbon (BC), has been measured and reported by many researchers,

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while the results vary significantly due to the vague definition of black carbon. Before any

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comparison, it should be noted that the aerosol light absorption property is dependent on the

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wavelength of light. The equation below is a commonly used one to characterize the spectral

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dependence of MAC using the absorption Ångstrom exponent (Åabs):

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MAC=Kλ- Åabs

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where K is a constant and λ is the wavelength of light.31 Bond29 suggested Åabs=1 in regions

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where externally mixed BC dominates absorption, while it is stressed that the assumption is only

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valid over the light wavelength throughout the visible spectrum.22 The research conducted by


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Kirchstetter32 measuring light attenuation by soot generated from motor vehicles throughout the

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light wavelength from 330nm to 1000nm proved that the assumption of Åabs=1 is valid over a

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larger wavelength range, which is supported by the research of Russell et al.33 In this research the

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wavelength used by PAX is 870nm and it is assumed that Åabs=1 for all samples, which means

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that the obtained MACs depend inversely on light wavelength.

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Bond and Bergstrom22 suggested a value of 7.5±1.2 m2/g (550nm wavelength) for the MAC of

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fresh light-absorbing carbon, which is over two times that of the acquired soot MAC (3.4 m2/g

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adjusted to 550nm) in this research. Other photoacoustic measurements of BC aerosols emitted

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from diesel engines or acetylene smoke also suggested much larger MAC values.34,35,36 One

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reason of the low MAC value obtained in this research may be the low temperature and short

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height of the oil lamp flame. Combustion generated carbon is a type of amorphous carbon which

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contains both sp3 and sp2 bonds, and the islands of sp2-bonded (graphitic) clusters control the

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light-absorbing properties of the particle.37 The transformation of carbon with primarily sp3

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bonds to that with sp2 bonds is called graphitization, which is a common process that occurs to

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carbonaceous particles passing through the flame and is positively correlated to the flame

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temperature and particles’ residence time in flame. The flame generated by an oil lamp in this

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research was only about 10 cm tall, which is expected to have a relatively low flame temperature

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compared with diesel engines and acetylene flames, and emit carbonaceous particles quickly into

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the surrounding environment.38,39 As a result, fewer sp2 bonds were formed in the sampled soot

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and therefore the measured MAC was low.

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The soot SSA obtained in this research is 0.29, which is within the 0.2-0.3 value range suggested

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by Bond and Bergstrom22. The value of SSA depends one both light wavelength and particle size

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distributions, so further comparison of the obtained SSA with other measurement results is not


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representative and applicable. The measurements also report a SSA value of 0.98 for BR fly ash,

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which is comparable with the sand duct SSA value reported previously by other researchers.

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Tanre et al.40 reported a 0.96-0.97 value for the SSA of sand dust measured using 870nm

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wavelength light, and Kim et al.41 reported a SSA of 0.94-0.95 over a wide wavelength range.

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These consistent results are reasonable since the major chemical component of FA and sand dust

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are both SiO2. Because of this similarity, the emission of FA from CFPPs is often assumed to be

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entirely mineral in climate simulations of BC climate forcing and BC emissions

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inventories.19,20,42

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Estimation of PAC emissions from CFPPs

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According to the discussion in the last section, the emission of FA from CFPPs is often assumed

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to be entirely mineral, while the measurement results presented in Figure 6 show that the

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increase of carbon content of the particles emitted from the power plants may cause a significant

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linear increase of MAC of the emissions. It is expected that the installation of ACI technologies

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has the potential to increase the carbon content of CFPPs’ emissions, and in this section the

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carbon content (as the PAC mass fraction of the total particles emitted) of the emission from an

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ESP is examined as a function of the ESP’s FA removal efficiency.

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Figure 8 shows the calculation results of the emission carbon content ([PAC]out) as a function of

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ESP’s FA collection efficiency (ηFA). In the calculation it is assumed that 0.5% PAC by mass

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was mixed with the FA in the flue gas upstream of the ESP, and the ESP’s PAC collection

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efficiency (ηPAC) either had a constant differential with ηFA (ηFA-ηPAC=const.), or was a constant

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fraction of ηFA (ηPAC/ηFA=const.). From the figure it can be found that for both ηPAC assumptions,

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as ηFA approaches 100%, there will be significant carbon content increases (up to 60%) in the


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emission. Modern ESPs generally have ηFA larger than 95% or even larger than 99%,43 so it is

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reasonable to claim that the installation of ACI technologies will significantly increase the

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carbon content in the CFPPs’ emissions. If PAC and FA particles do not agglomerate as

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discussed in Section “Physical characterization of FB emissions”, the increased carbon content

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will be emitted as individual carbonaceous particles, which makes CFPPs a new source of BC

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emissions. This new BC source is a large-scale point source, generally operated under steady

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state conditions yielding relatively steady BC emissions, which is quite different from most of

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the conventional BC sources that are believed to be either small and numerous, or large but

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episodic.29 Due to CFPPs’ nature of relatively steady emission, BC emission should be

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monitored by CFPPs with ACI installed and the international BC emission inventories for CFPPs

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should be established for the emission control and simulation of climate forcing associated with

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BC emissions from CFPPs, in a way that cannot be done with other BC sources.

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(a)

(b)

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Figure 8. Calculated ESP emission carbon contents as function of ηFA assuming (a) ηFA-

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ηPAC=const. or (b) ηPAC/ηFA=const. (Colored)

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CONCLUSIONS AND LIMITATIONS

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This research measured Babs, Bscat and mass loadings of combustion-generated soot as well as

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FA/FA-PAC aerosols emitted from a fluidized bed, and calculated the corresponding MSC,

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MAC and SSA of soot, FA aerosols and FA-PAC admixture aerosols. For the relatively large

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PAC and FA particles sampled in the experiment, the SEM images indicate agglomeration in the

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flue gas is unlikely. Both MSC and MAC of FA-PAC admixture aerosols would increase linearly

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with the increase of particles’ carbon content, while the increase of carbonaceous particles in the

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FB emissions had a more significant effects on the MAC of the particles in the emissions, and

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the SSA would decrease linearly with the carbon content increase. The acquired MAC value of

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soot is relatively low compared with other reported values, which may be due to the small size of

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the oil lamp flame, and the acquired SSA values for both soot and FA are comparable with other

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relevant reports. The measurements and calculations illustrated that the installation of ACI in

464

CFPPs would increase the MAC of the emissions from the power plants and make the CFPPs a

465

new source of light absorbing particle emissions. Although large in average size, the emitted

466

PAC particles, especially the submicron fraction, can absorb light efficiently and potentially act

467

like BC in the atmosphere. Further studies should be conducted to quantify the intensity of this

468

new emission and examine its potential climate forcing effects.

469

Several limitations have been observed during the research. The measurements conducted for FB

470

emissions only sampled relatively large particles since the smaller particles may have been

471

exhausted during the free elutriation of the FB before any measurements, and the size

472

distributions of the sampled particles were not fully examined. In future a dilution chamber

473

should be built downstream of the FB to capture and dilute the small particles emitted by the FB

474

during the early stage of fluidization for PAX measurements, and scanning mobility particle sizer


25

Energy & Fuels

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Page 26 of 33

475

spectrometer (SMPS) should be utilized to further examine the size distribution of sampled

476

particles. Last but not least, since most CFPPs use industrial opacity sensors to monitor their PM

477

emissions, the light extinction measured by PAX and industrial opacity sensors should be

478

compared to examine the effectiveness of opacity sensors on detecting carbon emissions from

479

CFPPs.

480 481

ASSOCIATED CONTENT

482

Supporting Information. One two-page file containing one figure showing the FA-PAC

483

admixtures with different PAC mixing ratio.

484

.

485

AUTHOR INFORMATION

486

Corresponding Author

487

*1913 McIntyre Street, Ann Arbor, MI, USA 48105-2420

488

*Phone: (+1)7346807921. E-mail: [email protected]; [email protected].

489

Author Contributions

490

The manuscript was written through contributions of all authors. All authors have given approval

491

to the final version of the manuscript.

492

Funding Sources

493

No funding was received for this research project.

494

Notes

495

The authors declare no competing financial interest.


26

Page 27 of 33

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Energy & Fuels

496

ACKNOWLEDGMENT

497

The authors thank Prof. James Hower, Dr. Kevin Henke (Univ. of Kentucky) and Tony Bassi

498

(DTE Energy) and the engineers at E.W. Brown and Trimble County electric generating stations

499

of Louisville Gas & Electric and Kentucky Utilities Co. for providing plant access and assisting

500

with fly ash collection. The authors also thank Cristina Koss, Alexander Mattia and Akosua

501

Miller for helping obtain the measurements, to Prof. Andre Boehman and Chenxi Sun (Univ. of

502

Michigan) for conducting the TGA measurements, and to Gavin McMeeking and John Walker at

503

DMT for their technical support in operating the PAX. No funding was received for this research

504

project.

505

ABBREVIATIONS

506

ACI, activated carbon injection; BC, black carbon; CFPPs, Coal-fired power plants; EIA, Energy

507

Information Administration; EGUs, electric generating units; EPA, Environmental Protection

508

Agency; ESP, electrostatic precipitator; FA, fly ash; FB, fluidized bed; IPCC, Intergovernmental

509

Panel on Climate Change; LOI, loss on ignition; LPM, liter per minute; MAC, mass absorption

510

cross sections; MATS, Mercury and Air Toxics Standard; MSC, scattering cross section; PAC,

511

powdered activated carbon; PAX, photo-acoustic extinctiometer; SEM, scanning electron

512

microscopy; SMPS, scanning mobility particle sizer spectrometer; SSA, single scattering albedo;

513

TGA, thermogravimetric analysis; UNEP, United Nations Environment Programme.

514


27

Energy & Fuels

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Optical Property Measurements of Mixed Coal Fly Ash and Particulate

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