Article pubs.acs.org/EF
Development of Low-Cost Functional Adsorbents for Control of Mercury (Hg) Emissions from Coal Combustion Chenggong Sun,* Colin E. Snape,* and Hao Liu Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom ABSTRACT: A range of environmentally advantageous low-cost carbon materials, including pulverized fuel ashes (PFA) and chars derived from different waste streams, were examined as the potential sorbents for use in mercury capture by sorbent injection. The test results demonstrated large variations in mercury retention among the fly ash samples examined, with those generated from burning bituminous coals having the best performance in general. By and large, the mercury retention capacity of fly ash appears to be mainly determined by its unburned carbon content and, to some extent, also by the chemical and morphological properties of its mineral matter. For the char samples examined, it was found that those from the gasification of paper waste materials displayed the best performance. Without any further treatment, the wastepaper-derived char materials outperformed the benchmark Norit-Darco FGD carbon by a factor of 2, in terms of mercury retention capacities. Although steam activation and KMnO4 impregnation can both be used to improve the performance in mercury retention of the samples, the onestep KMnO4 impregnation was found to be much more effective, particularly for the PFA samples where steam activation could lead to lower rather than higher mercury capacities for the fly ashes. It should be noted, however, the presence of moisture at significant levels was found to be required for the KMnO4-impregnated sorbent samples to perform efficiently in mercury capture due to the chemistry involved.
1. INTRODUCTION Mercury has long been recognized as a potential health and environmental hazard; therefore, the emissions and depositions of mercury in the environment have become a significant global environmental concern, as highlighted in an IEA report1 and the Global Mercury Assessment by the United Nations Environment Programme.2 There is a wide variety of anthropogenic mercury emission sources; however, apart from the gold mining industry, coal combustion for heat and power generation represents the single largest source of unintentional mercury emissions, accounting for 24% of mercury emissions at a global scale.2 During combustion, coal-contained mercury is released into the combustion flue gas in three different forms: elemental mercury (Hg0), oxidized mercury present in variable chemical forms (Hg2+) and the mercury associated with particulate matter or soot (HgP). While the oxidized and particulate-bound mercury species can be easily removed with existing pollution control installations, a majority of Hg0, which is mostly in its gaseous phase, because of its high volatility, is often emitted into the atmosphere. As a result, development of viable technologies for the effective control of Hg0 emission from coal-fired power plants has become the focus of many investigations over recent years.1,2 A combination of electrostatic precipitation (ESP), selective catalytic reduction (SCR), and wet flue gas desulfurization (FGD) has been found to be effective in removing appreciable levels of elemental mercury (Hg0) (ca. >80%) from the flue gas of pulverized coal combustion, thanks to the mercury oxidation, which occurs as a co-benefit of SCR installation.3−5 However, without this combination essentially with SCR, mercury-specific control measures must be introduced to facilitate high levels of Hg0 removal. Otherwise, a majority of elemental mercury (Hg0) will © 2013 American Chemical Society
evade capture and remain predominantly in its gaseous form, even at stack temperatures.1 Two approaches can be used to effectively remove Hg0. One is to inject a strong mercury oxidizer, such as halogens6,7 and ozone,8 into the system to transform Hg0 to Hg2+, which can then be removed by wet scrubber (e.g., wet FGD). Another method is the injection of effective sorbents to capture Hg0 and with the spent sorbent materials to be removed by existing facilities for particulate control. Candidate sorbent materials for injection include supported precious metals,9,10 noncarbon inorganic materials11,12 and, in particular, active carbons.3−5,13−18 The mechanisms involved in capturing mercury with these potential sorbents vary from physical and chemical adsorption to amalgamation where supported noble metals are used. The injection of active carbons usually upstream of a particulate control device has been regarded as being the most suitable technology for mercury emissions control particularly in the case of pulverized combustion for power generation. In several laboratory and industrial tests at various scales, the efficacy of different active carbon materials and the effect of flue gas conditions, including both temperatures and chemical compositions, have been examined.3−5,13−18 These tests at various scales have demonstrated that, because of the low concentrations of mercury in utility flue gas streams and masstransfer limitations, as a result of short residence times (ca. 1−2 s) of injected carbons in the duct system, hefty carbon to mercury (C/Hg0) mass ratios varying from 3000 to 20 000 may have to be used in conjunction with the requirement of using Received: December 3, 2012 Revised: June 7, 2013 Published: June 14, 2013 3875
dx.doi.org/10.1021/ef3019782 | Energy Fuels 2013, 27, 3875−3882
Energy & Fuels
Article
Figure 1. Schematic diagram of the sorbent screening rig used for mercury adsorption.
extremely fine carbon particle sizes (average of ca. 5 μm) in order to achieve high levels of Hg0 removal (>90%). While the required high C/Hg0 ratios in this technology can significantly increase the cost of mercury emissions reduction, the resultant increases in the carbon contents of fly ash, even only by 1 wt %, are also concerned with the saleability of fly ash as a cement additive, although standards do allow for ∼6% carbon in fly ashes.19 To improve the cost efficiency of the carbon-injection-based mercury control technology, either a more-functional carbon sorbent that can enable a lower C/Hg0 ratio, or a lower-cost sorbent, or ideally a combination of both is required. The use of sulfur-, bromine-, and iodine-modified active carbons can effectively reduce C/Hg0 mass ratios,20−23 because of their higher capacities and faster reaction kinetics than general commercial carbons (e.g., the benchmark Norit FGD carbon). However, the use of these halogenated activated carbons has generally proven to be cost-inhibitive. There have been considerable efforts focusing on other nonsophisticated but effective carbon surface modifications, which can effectively improve the efficiency of mercury removal provided that the mercury is oxidized prior to adsorption or the carbon surface itself can catalyze Hg0 oxidation.3,24,25 Oxidative capture of mercury is well documented in aqueous solution in analytical chemistry literature.26,27 Permanganate in acidic solution is generally used for trapping Hg0 while MnO2 has been employed for collecting mercury from air, but this serves only as an analytical methodology. Granite et al.28 have compared a range of adsorbents for Hg capture, including aluminasupported manganese dioxide (MnO2), which gave rise to mercury retention capacities as high as those of iodine- or sulfur-modified activated carbons. In the present study, a variety of low-cost carbon or carbon precursor materials, including tire pyrolysis chars, as well as combustion and gasification residues, were investigated as the replacement materials for commercial activated carbon materials, with a particular interest in identifying potential nonsophisticated but effective means to enhance their performance in mercury uptake.
2. EXPERIMENTAL SECTION 2.1. Samples. Samples examined in this investigation included both the pulverized fuel ash (PFA) samples from different power plants and a range of chars generated from the pyrolysis or gasification of different waste feedstocks, such as scrap tires, waste wood, and paper materials. A total of 17 PFA samples were collected from PF combustion plants, 13 of which were obtained from different power plants; the other four samples were obtained from combustion test facilities, including the NOx reduction test facility (NRTF) at Doosan Babcock Energy and the fluidized-bed combustor (FCB) at ECN Petten. It is believed that all these fly ash samples were collected from the electrostatic precipitators (ESP) installed in these combustion facilities. While most of these PFA samples were used as received, the four original fly ash samples received from the Instituto Nacional del Carbon (denoted as CTA, CTL, CTES, and CTSR) were further subjected to size fractionations to yield their fractions with higher contents of unburned carbon. Of the 17 fly ash samples, one was from burning sub-bituminous coals (CTES), one was from a coal blend containing significant amount of anthracite, and two were from burning a bituminous coal blended with meat and bone meal (MBM) at varying ratios. Apart from the environmentally advantageous carbon materials, commercially available carbons were also tested under the same conditions in order to provide the benchmarks against which the selection of low-cost carbon materials could be assessed. The commercial carbons tested include the activated carbon for gas adsorption obtained from Fisher Scientific, a wood-based and chemically activated carbon from PICA Carbon and the benchmark Norit-Darco’s FGD carbon. 2.2. Sample Analyses. The BET surface areas of the PFA and char samples were measured in N2 at 77 K, using a Micromeritics Model ASAP-2000 analyzer. Before the measurements, all samples were first dried and degassed at 105 °C for 3 h. A DSC-TGA SDT Q600 (TA Instruments) was used to measure the loss on ignition values (LOI) of PFA samples at 805 °C in air. To measure the LOI value, each individual sample was first heated in a flow of N2 (100 mL min−1) at 105 °C for 30 min to remove its moisture. With the gas flow switched from N2 to air, the sample was then heated up to 805 °C at a rate of 20 °C min−1 and held at this temperature for 60 min. The weight loss calculated on a dry basis was taken as the LOI value for each sample. 2.3. Sample Treatments. Unless specified, all samples were ground and sieved to the particle size range from 75 μm to 250 μm and dried at 105 °C before use. For selected carbon samples, modifications by steam activation and/or chemical doping were 3876
dx.doi.org/10.1021/ef3019782 | Energy Fuels 2013, 27, 3875−3882
Energy & Fuels
Article
Table 1. Benchmark Tests on Commercial Activated Carbons carbon sample activated carbon for gas adsorption (Fisher Scientific, 75−250 μm) Picazine (75−250 μm) Norit-Darco FGD (98%
