Article pubs.acs.org/IECR
ReEngineered Feedstocks for Pulverized Coal Combustion Emissions Control Cheng Zhu,†,‡ Sheng Chu,† Geoffrey A. Tompsett,† Jin Yang,† T. J. Mountziaris,† and Paul J. Dauenhauer*,†,‡ †
Department of Chemical Engineering, University of Massachusetts−Amherst, 159 Goessmann Laboratory, 686 North Pleasant Street, Amherst, Massachusetts 01003, United States ‡ Department of Chemical Engineering and Materials Science, University of Minnesota, 432 Amundson Hall, 421 Washington Avenue, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: New coal reaction technology called ReEngineered Feedstock (ReEF), consisting of post-recycled paper and plastics, was evaluated for combustion emissions control when cofiring with pulverized coal. Experiments were conducted with four types of ReEF in a 2 in. diameter laboratory-scale fluidized bed combustor system heated to 1200, 1400, and 1600 °C. Flue gas emission was continuously monitored with an online infrared multigas analyzer and mass spectrometer. The results indicate that co-firing ReEF with coal provides SO2 emission reduction in flue gas up to 85% and moderate decrease in NO emissions, as well as higher carbon conversion than pure coal combustion. ReEF, slag and fly ash solids were were analyzed by X-ray diffraction; identification of sulfates in the product ash conclusively supports the mechanism of in situ sulfur capture.
1.0. INTRODUCTION Coal combustion remains and is predicted to be an important process for electricity production in the United States and many countries around the world for the foreseeable future.1 The United States produces approximately 7 quadrillion BTUs of electrical power per year from coal, constituting 42% of all electrical energy being produced in the country in 2011.2 However, coal power plants produce gases such as SOx, NOx, and HCl, which can lead to environmental problems including formation of acid rain. Power stations that use coal boilers exhibit typical stack flue gas concentrations before scrubbing of 200−2000 ppm of SO2, 50−100 ppm of CO, and 800 ppm of NOx,3 which are orders of magnitude higher than permitted by regulation.4,5 Increasingly protective regulations, such as the Clean Air Act, mandate the reduction of emissions from coal combustion facilities. For instance, to comply with the Acid Rain Program in New England, Merrimack Station has completed its Clean Air Project on Mar. 30, 2012. This plant has reduced its mercury and sulfur dioxide emissions by more than 95%6 and is currently one of the cleanest coal-fired power plants in the United States. Significant investment is required downstream of the coalfired boiler to remove SOx and NOx pollutants to regulated levels. Although numerous technologies exist for emissions cleaning, such as wet scrubbing of the exhaust gas, augmentation of new chemical processing equipment to existing power plants introduces significant capital costs, and sometimes increases water consumption and decreases energy efficiency. A transformational solution aims to develop an energy-rich coal coreactant from postrecycled materials, which can be utilized within existing coal combustion facilities as both a fuel substitute and a sorbent of harmful process emissions. Co-combustion of coal and CO2-neutral fuel such as refusederived fuels (RDF) is one method of reducing CO2 emissions © 2014 American Chemical Society
during energy production while simultaneously decreasing other undesirable emissions such as SOx and NOx. RDF is produced from municipal solid waste (MSW) through a process of screening, size reduction, separation, and drying.7 The heating value of RDF is comparable to other biomass such as straw and wood, which allows for the possibility of being injected into a pulverized coal boiler for energy generation.8 Co-firing of coal with RDF has been investigated previously by others.9−20 In general, undesired emissions such as SOx are reduced by cofiring RDF due to its relatively low sulfur content compared to coal. Sorbent addition, blended with coal combustion, is another well-known technique for the reduction of SOx, NOx, mercury, and other pollutants.21−26 Traditional sorbent materials used for desulfurization are typically alkali compounds, including lime (CaO) or hydrated lime (Ca(OH)2), calcium carbonate (CaCO3), soda ash (Na2CO3), caustic soda (NaOH), and others. Lime is the most common sorbent used during combustion forming the stable calcium sulfate (CaSO4) in oxidizing conditions and calcium sulfide (CaS) under reducing (low oxygen) conditions.23 CaO + SO2 +
1 O2 → CaSO4 2
oxidizing conditions (1)
CaO + H 2S → CaS + H 2O
reducing conditions
(2)
Another sorbent, sodium bicarbonate (NaHCO3), has been utilized for flue gas sorption including SO2, HCl, and HF. Received: Revised: Accepted: Published: 17919
July 9, 2014 October 6, 2014 October 22, 2014 November 6, 2014 dx.doi.org/10.1021/ie502711r | Ind. Eng. Chem. Res. 2014, 53, 17919−17928
Industrial & Engineering Chemistry Research
Article
Table 1. Elemental Analysis of Coal and ReEF proximate analysis (wt %) sample coal ReEF ReEF ReEF ReEF a
SLa MLb SBc Tronad
ultimate analysis (wt %)
heat of combustion
moisture
volatile matter
fixed carbon
ash
H
C
N
O
S
Btu/lb
kcal/kg
2 1.24 2.39 7.79 9.52
33.13 59.67 49.9 55 52.32
57.2 5.33 15.13 9.51 9.51
7.7 33.76 32.58 27.7 33.63
5.4 5.08 5.24 4.93 5.04
75.3 33.76 28.99 36.46 34.88
1.8 0.06 0.05 0.086 0.067
8 26.11 33.03 30.82 26.33
1.7 0.04 0.039 0.041 0.051
13607 5787 5546 6018 5538
7564.50 3217.15 3083.17 3345.57 3078.73
Sorbacal limehigh surface area hydrated lime. bMississippi limestandard hydrate lime. cSodium bicarbonate. dPremilled Trona.
portion of sulfur can be retained throughout the entire burning period. In this study, we investigate the effects of cofeeding sorbentcontaining ReEngineered Feedstock (ReEF) materials with coal on the sequestration of pollutants in flue gas, such as SO2 and NO. ReEF materials are composed of nonrecyclable fibers and plastics combined with sorbents including hydrated lime, sodium bicarbonate, and Trona. Integration of waste materials with these sorbents has led to a new clean coal reaction technology called ReEngineered Feedstock (ReEF), developed by Accordant Energy, LLC (formerly known as ReCommunity Energy, LLC). ReEF34−36 is designed to be physically and chemically compatible to coal utilized in a pulverized coal combustor, such that it can be directly coreacted with existing coal feeding/handling infrastructure in combustion facilities. Utilization of RDF can replace up to 30% of coal with post recycled materials,18 which otherwise would be landfilled. The study aims to evaluate the effectiveness of ReEF as an innovative and efficient reactant cofeed for coal combustion and emission reduction. This technology has the dual potential to impact the economics and environmental footprint of both waste management and coal-fired power plants.
NaHCO3 decomposes at high temperatures to form Na2CO3, which has high specific surface area and reacts with HCl and SO2 to produce NaCl and Na2SO4.27−29 The following reactions take place at high temperatures for the neutralization of SO2. 2NaHCO3 + heat → Na 2CO3 + CO2 + H 2O Na 2CO3 + SO2 +
1 O2 → Na 2SO4 + CO2 2
(3)
(4)
It is reported that NaHCO3 sorbent achieves a high degree of desulfurization, but it is not recommended for high temperature applications. Hydrated lime desulfurization efficiency in dry conditions is in the range of 30−40%.27 Although the addition of inorganic sorbents to control the emission of toxic trace elements from coal combustion has previously been reported in laboratory, pilot, or commercialscale fluidized bed combustors,22,24,25 the effectiveness of these technologies has proven limited. The furnace sorbent injection process is cheap to install but can be expensive to operate, because inefficient utilization of sorbents and high furnace temperatures makes the desulfurization product unstable.23,25,26 For instance, the conventional sulfation product CaSO4 thermally decomposed with high flame temperature above 1200 °C, regardless of combustor type (industrial grate furnaces (IGF) or pulverized coal fired boilers (PCFB)). Cheng et al.22 observed that sorbent absorptive capacity is related to both physical qualities of the absorbents (type, amount, and particle size) and the combustion temperature. For example, smaller particles have greater surface area and more internal pores, which can enhance the reaction between trace element-containing vapors and absorbents. Desulfurization efficiency at 1100 °C, measured by Cheng et al.,22 was only 22.1% for CaO of 125−300 μm, whereas sulfur removal could reach 35.1% for CaO of
