Clean Coal by Oxyacidification H. Gokturk1 Ecoken, San Francisco, USA ABSTRACT vapor 15%, carbon dioxide 10%, oxygen gas (O2) 5% [3]. A comparison of the composition of stack emissions with Coal has become a controversial fuel due to various that of air shows that the fraction of (a) oxygen gas pollutants and carbon dioxide generated during combustion. decreases significantly (from 20% to 5%), (b) carbon Regulated pollutants include nitrogen oxides and sulfur dioxide increases dramatically (from 380 ppm to 10%), and oxides which react with the water vapor in the atmosphere (c) water vapor increases significantly (from several percent to form acid rain. Carbon dioxide also reacts with water to 15%). These changes which are applicable to other molecules to form carbonic acid, which gives rise to ocean hydrocarbon fuels can be attributed to the oxidation of acidification. This research takes advantage of the carbon and hydrogen in the fuel during combustion. Stack acidification tendency of such gaseous pollutants in order to emissions also include the mentioned pollutants in reduce their concentrations. Pollutant molecules in the flue quantities less than 100 ppm [3]. Low concentration of the gas are encouraged to combine with water vapor to form pollutants is due to cleaning processes, such as flue gas droplets of oxyacids which are then separated before the desulfurization and selective catalytic NOx reduction, flue gas is discharged into the air. installed in power plants. Assuming a reduction of ~90% by such cleaning processes, approximate concentration of the Keywords: Clean coal, oxyacid, electron affinity pollutants in the original flue gas is about 1000 ppm (0.1%). One of the adverse impacts of pollutants like NOx and 1 PROBLEM SOx is that these molecules react with the water vapor in the atmosphere to form aerosols of oxyacids which Coal is the most abundant fossil fuel in the world. Coal eventually fall to earth as acid rain. NOx molecules form reserves total about 1 trillion tons which is equivalent to nitrous acid (HNO2) and nitric acid (HNO3). SOx 150 years of production at the current level [1]. Countries molecules form sulfurous acid (H2SO3) and sulfuric acid with the largest reserves include United States (#1), China (H2SO4). Carbon dioxide also reacts with water molecules (#3) and Germany (#6) which are the major industrialized to form carbonic acid (H2CO3). Increasing concentration of economies in America, Asia and Europe, respectively [2]. carbon dioxide in the atmosphere is believed to be the cause Those countries and many others rely heavily on coal of ocean acidification [5]. burning power plants for electricity generation. For + + → example in the US, about half of the electricity is generated H 2 O NO NO2 2HNO2 (1) + → + by using coal due to low cost and plentiful supply. H 2 O 3NO2 NO2 2HNO3 (2) Unfortunately coal has become a controversial fuel due + → H 2 O SO2 H 2 SO3 (3) to various pollutants and carbon dioxide (CO2) generated + → H O SO H SO (4) during combustion [3]. High ranked coals like anthracite or 2 + 3 → 2 4 bituminous coals are reserved for high value applications H 2 O CO2 H 2CO3 (5) such as manufacturing of steel. That leaves low ranked coals like sub-bituminous coal or lignite for power 2 PROPOSED SOLUTION generation. Low rank coals contain less carbon and more impurities like sulfur which gives rise to pollution [2]. One can infer from the past observations that the Regulated pollutants emitted from coal include nitrogen mentioned gaseous pollutants have a natural tendency to oxides (NOx), sulfur oxides (SOx) and mercury. In the US, combine with water to form liquid oxyacids. The approach about 65% of sulfur dioxide emissions (7 Mtons/year), 20% followed in this research to reduce the pollutants in the flue of nitrogen oxide emissions (3 Mtons/year) and 70% of gas is to take advantage of this oxyacidification tendency. mercury emissions (45 tons/year) are attributed to coal Pollutant molecules in the flue gas are encouraged to burning power plants [4]. These percentages could be combine with the water vapor in the flue gas to form higher in developing countries where environmental droplets of oxyacids which are then separated before the regulations are not as well established as in the US. flue gas is discharged into the air. This process can be Typical stack emissions of a coal based power plant euphemistically described as creating acid rain inside the consist of nitrogen gas (N2) about 70% by volume, water power plant. 1 Email: [email protected] NSTI-Nanotech 2012, www.nsti.org, ISBN 978-1-4665-6276-9 Vol. 3, 2012 753 The cleaning process is illustrated in Fig. 1. Given a flue moments which are important to understand how ions and gas generated by burning coal or other fuel, a charging neutral molecules interact with each other. Flue gas device provides electrons to those molecules which have molecules which have dipolar charge distributions include affinity to accept extra electrons. Molecules which acquire NO, NO2, SO, SO2, H2O, HgO and those with quadrupolar electrons and become ions are allowed to float through the charge distributions include N2, O2, CO2, and SO3. flue gas without losing the charged state so that they can Generally interaction of an ion with a dipole is stronger electrically attract other pollutants and water molecules to than that with a quadrupole. The strength of the interaction form droplets. Pollutants and water molecules brought into between an ion and other molecules in the flue gas can be close proximity by the attraction of the ion react to form quantified by calculating the interaction energy. Interaction oxyacids. Charged droplets containing the oxyacids are between pairs of various molecules and ions has been separated from the rest of the flue gas by manipulating investigated using first principle quantum mechanical them with electric and/or magnetic forces. calculations [7]. Results indicate that interaction energy ranges from 0.5 eV to 0.9 eV for dipolar molecules like 2.1 Electron Affinity of the Pollutants NOx, SOx and H2O {Table 2}. These interaction energies are large enough for the ion to attract and capture dipolar Ability of a molecule to accept an extra electron can be pollutants and water molecules even at the relatively quantified by calculating its electron affinity (Eea) which is elevated temperature of the flue gas. defined as the energy required to detach an electron from its singly charged negative ion [6]. Eea can be calculated from Minus Ion Molecule IE (eV) the quantum mechanical energy of the molecule in the NO2 SO 0.6 neutral state (En) and with an extra electron (Ei). NO2 SO2 0.9 = − NO2 H2O 0.8 E ea E n E i (6) SO H2O 0.7 SO2 H2O 0.8 Most of the molecules have their lowest energy in the SO2 NO 0.5 neutral state, therefore Eea is generally negative. For SO2 NO2 0.5 example, energy of CO2 minus ion is higher than that of neutral CO2 by about 0.9 eV. If CO2 is bombarded with Table 2: Interaction energy (IE) between an ion and dipolar energetic electrons, an electron with more than 0.9 eV molecules as calculated by first principle methods. kinetic energy can get temporarily attached to the molecule. A small number of molecules exhibit positive electron The interaction energy for a quadrupole like N2 is 0.07 affinity. For example energy of O2 minus ion is about 0.6 eV which is comparable to the thermal energy of the flue eV lower than that of neutral O2. Such a molecule accepts gas. Another quadrupole of interest is CO2 which has an an extra electron readily and becomes a relatively stable electron distribution where carbon is slightly positive, negative ion. An investigation of the electron affinity of 0.74Qe and oxygens are slightly negative, -0.37Qe (Qe: molecules typically encountered in the flue gas of a coal unit charge). When CO2 is in the vicinity of a negative ion, plant indicates that many of the pollutants including NO2, carbon gets attracted to the ion, whereas oxygens get SO, SO2, SO3, HgO have Eea>0 {Table 1}. Hence these repelled by the ion which bends the molecule from a linear molecules have the potential to serve as ionic scavengers to an angular shape (Fig. 2). The shape change creates a which can attract and capture other pollutants. small dipole which yields an interaction energy of 0.3 eV. Although this energy is smaller than those of the other Formula Molecule Eea (eV) dipoles mentioned above, it is large enough to enhance the NO Nitrogen monoxide -0.9 absorption of CO2 into the droplets {Table 2}. NO2 Nitrogen dioxide 2.3 SO Sulfur monoxide 1.3 Minus Ion Molecule IE (eV) 2 2 SO2 Sulfur dioxide 1.6 NO N <0.1 2 2 SO3 Sulfur trioxide 1.2 NO CO 0.4 HgO Mercury oxide 2.4 SO N2 <0.1 2 CO2 Carbon dioxide -0.9 SO CO 0.3 SO2 N2 <0.1 Table 1: Electron affinity (Eea) of the pollutants obtained SO2 CO2 0.3 by first principle quantum mechanical calculations [7]. Table 3: Interaction energy (IE) between an ion & quadru- 2.2 Interaction Energy polar molecules as calculated by first principle methods. Electron distribution of a molecule provides information One can conclude from this analysis that the ions about dipole, quadrupole and higher order electronic created within the flue gas would attract dipolar pollutants 754 NSTI-Nanotech 2012, www.nsti.org, ISBN 978-1-4665-6276-9 Vol. 3, 2012 and water molecules to form droplets. CO2 which exists in 5) These droplets are separated from the rest of the large quantities within the flue gas will also be absorbed flue gas by manipulating them with electric and/or into these droplets.