on Nitric Oxide Emission and Unburned Carbon in Various Combustion

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Effects of Pretreatment of Coal by CO2 on Nitric Oxide Emission and Unburned Carbon in Various Combustion Environments Benson B. Gathitu† and Wei-Yin Chen* Department of Chemical Engineering, UniVersity of Mississippi, 134 Anderson Hall, UniVersity, Mississippi 38677

Polar solvents are known to swell coal, break hydrogen bonds in the macromolecular structure, and enhance coal liquefaction efficiencies. The effects of the pretreatment of coal using supercritical CO2 on its physical structure and combustion properties have been studied at the bench-scale level. Emphasis has been placed on NO reburning, NO emissions during air-fired and oxy-fired combustion, and loss on ignition (LOI). Pretreatment was found to increase porosity and to significantly alter the fuel nitrogen reaction pathways. Consequently, NO reduction during reburning using bituminous coal increased, and NO emissions during oxidation of lignite decreased. These two benefits were achieved without negative impacts on LOI. 1. Introduction To mitigate climate change due to the effects of the greenhouse gas CO2 emitted by utility boilers, carbon capture and sequestration deep underground after combustion has been suggested. To avoid the cost of compressing N2 along with the CO2, oxy-combustion, where the flue gas is recycled and enriched with purified oxygen, is used instead of air combustion. The vast amounts of CO2 produced represent a valuable resource that is already utilized in economically valuable methods such as enhanced oil and coal bed methane recovery. In the present study, this abundant resource was further utilized by exposing coal to its supercritical state with the goal of modifying the coal’s physical and chemical structure. It is expected that the modified coal will exhibit combustion properties that will enable pollution control and increase energy conversion efficiency. At temperatures as low as 298 K, CO2 is known to induce significant irreversible swelling in coal volume by up to 4.18%, pore volume enlargement of 20-50%, and depression in the glass-transition temperature from 120 to 82 °C.1-3 These effects increase with increasing temperature up to 200 °C and pressure up to 30 atm (subcritical). Separately, it was demonstrated that coal swells in pyridine4 and amines.5 Pretreatment of coal with subcritical steam at 50 atm and 300-370 °C enhances coal dissolution in pyridine.6-8 Moreover, thermal pretreatment of coals in N2 at 150-200 °C for 1 h induces increases in the volatile yield in subsequent pyrolysis.9 All of these low-temperature processes seem to be related to the macromolecular structure of coals. It is believed that coals have three-dimensional, cross-linked, macromolecular configurations containing graphitic crystallites.5,10-12 Hydro-aromatic and aromatic clusters are linked by either covalent or hydrogen bonds. In situ FTIR analysis of treated and untreated coal suggested that hydroxyl groups are the main contributors to these relatively weak hydrogen bonds,9,13 which can be broken by polar solvents or mild heat. Based on solvent swelling data and a modified Flory-Huggins theory, Larsen et al.14 estimated that the number ratio of hydrogen bonds to covalent cross-links in the raw coal is about 5. The temperature where the coal is transformed from a brittle glassy state to a rubbery state is called * To whom correspondence should be addressed. Tel.: (662) 9155651. Fax: (662) 915-7023. E-mail: [email protected]. † Present address: Department of Mechanical Engineering, Jomo Kenyatta University of Agriculture and Technology, P.O. Box 62000, 00200 Nairobi, Kenya.

the glass transition temperature, Tg. Various experiments suggest that, at temperatures above Tg, hydrogen bonds are broken, coal’s fragments become less brittle, its plasticity increases, its volume swells, its pore size increases, the diffusivity of gaseous and liquid components in its solid matrix increases, the solubility of gaseous and liquid components in the solid increases, and its reactivity with fluid components increases. In early studies, Tg for coals was found to vary from 250 to 359 °C,15-17 with a strength of about 5-8 kcal/mol.14 A sudden increase in heat capacity is a characteristic of a glass transition in differential scanning calorimetry (DSC) analysis. It was later found that decomposition of weakly bound water from 30 to 130 °C, more strongly held water and possibly CO2 from 130 to 280 °C, and tightly bound water and low-molecular-weight organics above 280 °C might have masked a glass transition in the temperature range of 110-250 °C.18-21 Rapid cooling was found to help freeze the random nature of the rubbery state of coal. Both DSC and dielectric measurements, as well as multiple scanning of thermally treated samples, supported this conclusion. These works suggest that coal’s macromolecular structure can be weakened at temperatures as low as 250 °C. Above this temperature, coal enters a rubber state, and its reactivity increases. The observed increases in liquefaction and pyrolysis reactivity after pretreatments are likely due to the ruptures of the hydrogen bonds during pretreatment. Indeed, it was found that nucleophilic agents such as tetrahydroquinoline are capable of breaking the hydrogen bonds, catalyzing the tautomerization, weakening the cross-covalent linkages in the carbon structure, swelling the char matrix, and increasing the subsequent liquefaction (dissolution) yields in a hydrogen donor, tetralin.22 The technological implications of these scientific findings have not been fully appreciated in the development of combustion technologies. Indeed, prior studies in the swelling of coal by CO2 at ambient temperature were targeted at correcting the surface area analysis23 and estimating the long-term impact of storing CO2 in coal beds. Nishino24 examined the adsorption of CO2 on 20 different types of coal and found that adsorption at room temperature is related to the carboxylic functional groups. Nishino discovered that, in comparison to other functional groups on coal surfaces, carboxylic groups can be considered the preferential sites for adsorption of H2O vapor and CO2, which is certainly consistent with the discoveries about Tg discussed above. On the basis of adsorption capacity data from Ozdemir25 and Ozdemir et al.26 collected at 22 °C and 4

10.1021/ie901069k CCC: $40.75  2009 American Chemical Society Published on Web 10/06/2009

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MPa (subcritical), we found that a CO2 molecule can be adsorbed for every 60 carbons in the coal. The process takes 30 min to reach equilibrium. Although this quantity is not sufficient to capture (fix) all of the carbon from the flue gas, the capacity is expected to be higher under supercritical pressures,27,28 and the equilibrium capacity is already higher than the overall demand for CO2 chemical production. More importantly, our primary goal for this project was to treat raw coal with CO2 to enhance coal’s reactivity particularly in NO reduction and carbon oxidation. An enhancement of the early release of fuel nitrogen will be desirable during NO reburning.29 Coal reburning has a demonstrated 60% NO reduction floor due to the oxidation of char nitrogen to form NO in the burnout zone.30 If pretreated coal can release more nitrogen in the reburning stage in the chemical forms of HCN and NH3, then recent technological developments31 can effectively reduce the HCN, whereas NH3 conversion to NO in the burnout zone is known to be limited. Coal reburning can therefore exceed the 60% reduction floor, and this supercritical CO2 pretreatment technique can be applied to other fuels that have previously been limited because of their char nitrogen conversion to NO in the burnout zone.31 Unburned carbon (UBC) in fly ash is a major concern during pulverized coal combustion. This unburned carbon in fly ash represents wasted energy, and it would therefore be desirable to burn out all the carbon to save on energy costs. Fly ash from coal-fired utility boilers has various uses such as in cement production, embankment construction, and soil stabilization, whereas otherwise, it is disposed in landfills.32 Fly ash used in cement production has to satisfy certain criteria with regard to carbon content, and consequently, high-carbon fly ash is improved using processes such as physical separation and carbon reburning to remove the excess carbon.33,34 For landfilling, excess amounts of carbon cost money to transport from the power plant and also take up expensive space at the landfill. Furthermore, the deployment of low-NOx burners to meet stringent environmental regulations has led to an increase in the amount of carbon in fly ash.35 It has also been reported that fly ash that has a higher carbon content is trapped in the later sections of electrostatic precipitators (ESPs), whereas fly ash with a low carbon content is caught in the earlier sections of ESPs.36 Lower UBC in fly ash would therefore make ESPs more efficient, lowering the energy demand in this unit operation of the boiler and also potentially decreasing the capital costs of ESP installations by reducing size requirements. To remedy this phenomenon of high UBC in fly ash, some have suggested recycling fly ash within the boiler, which has mostly been ineffective.36 In light of the above discussion, a better and different means of reducing the amount of carbon in fly ash is desirable and would result in greater profits for boiler operators, along with a reduction in CO2 emissions per unit of electricity generated. As mentioned earlier, pretreatment of coal by N2 at 150-200 °C for 1 h induces an increase in volatile yield during subsequent pyrolysis and also increases the diffusivity of gaseous and liquid components in the coal’s solid matrix after pretreatment. It therefore seems plausible to expect that pretreatment of coal using CO2 will also cause an increase in reactivity that could remedy high UBC in fly ash. 2. Experimental Section 2.1. Apparatus and Procedure for Supercritical CO2 Pretreatment. The pretreatment reactor (Figure 1) consisted of a 2.54-cm-o.d., 1.75-cm-i.d., and 50-cm-long galvanized steel pipe purchased from a local hardware store. The pipe was

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Figure 1. Experimental setup.

threaded and capped at both ends. These end caps were fitted with stainless steel fittings to form inlet and outlet ports. The inlet and outlet ports were connected to high-temperature-highpressure trunnion ball valves (supplied by Swagelok) through 0.635-cm-o.d. tubing with a wall thickness of 0.089 cm. A vacuum and a high-pressure gauge were connected to the reactor assembly, with each gauge separated from the main reactor body by the trunnion ball valves. This allowed for the high-pressure gauge to be isolated from the system during the vacuum cycle and for the vacuum gauge to be isolated from the system during the high-pressure cycle. An analysis of the pressure ratings of the parts used to make the reactor indicated that the weakest component was the 0.635-cm-o.d. stainless steel tubing with a wall thickness of 0.089 cm. According to the Swagelok Tubing Data Catalog,37 at an elevated temperature of 204 °C, these tubes can handle pressures up to 4896 psig. During our experiments, we carefully operated below this temperature and pressure. Future plans include the addition of an appropriate pressure-relief valve. The caps at both ends of the steel tube were sealed using Teflon tape and pressure leak tested several times using helium at a pressure of 1400 psig by means of a pressure gauge and Snoop liquid leak detector to ensure that the seals could hold. The trunnion ball valves were tested by the manufacturer using N2 at 1000 psi to ensure that they leak less than 0.1 standard cm3/min. In-house, we observed a small leak using a sensitive gauge connected through a trunnion ball valve to the hot CO2-

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filled pressurized reactor. This leak was quantified and found to be much less than the manufacturer’s specification. We also determined that, over a 48-h period, which was the typical pretreatment duration, less than 0.1% of the CO2 present in the filled reactor was lost through valve leakages and, therefore, that the trunnion ball valves were adequate for this work. Because of this valve leakage, the vacuum gauge, which had a range of between 30 inHg vacuum and 30 psig, was detachable via a Swagelok quick-connect fitting to avoid damaging it. The coal was placed in pouches fabricated from stainless steel wire mesh cloth (width of opening ) 10 µm) and placed appropriately within the length of the galvanized steel tube so that, after assembly of the reactor and placement within the furnace, the pouches would be within the 30-cm-long heated section of a Lindberg model 55035 electrically heated furnace. A short stainless steel tube was placed in the lower section of the reactor to ensure that the pouches did not drop beneath this heated section. Those sections of the reactor assembly not enclosed within the furnace (transfer lines) were wrapped with heating tape and heated to the furnace temperature. The temperature of the transfer lines was monitored using a thermocouple. To avoid both damaging the high-pressure gauge and incurring errors in pressure measurements due to heating of the high-pressure gauge during pretreatments, a flexible armored capillary was used to connect the gauge to the hot reactor, thereby isolating the gauge from the heat. After two coal-filled pouches containing a total of 30 g of coal had been loaded, the tube was sealed, and the reactor assembly was evacuated to 30 inHg while the furnace and the transfer lines were heated to 26 °C. The reactor was then filled with instrument-grade (99.99% purity) CO2 delivered from a siphon-tube-fitted CO2 gas cylinder. The cylinder was wrapped with a domestic electrically heated blanket to raise its temperature to 34 °C, which increased the cylinder pressure and allowed the reactor to be filled with a fluid of greater density than would have been the case if the tank had been at room temperature. This greater density allowed for higher pressures to be achieved at a given pretreatment temperature than would have been achieved if the reactor had been filled using a cylinder at room temperature. Before the reactor was isolated from the CO2 gas cylinder, adequate time was allowed for the pressure to stabilize within the reactor (about 10 min), and then the furnace was turned on to heat the reactor to the desired temperature. With the reactor loaded with 30 g of coal and saturated liquid CO2 at 26 °C, the approximate solvent-to-coal ratio (by mass) was 2.81 for asreceived bituminous coal and 2.92 for dried bituminous coal; for lignite, the values were 2.59 and 2.26, respectively. The differences in these solvent-to-coal ratios are due to the different mass densities of the coals as measured using the water displacement method. A pretreatment duration of 48 h was chosen based on a rough extrapolation of the time it took to achieve steady state in a study conducted by Reucroft and Sethuraman.2 At the end of the pretreatment duration, the CO2 was released over an approximate duration of 10 min. This rate of release was adequate to allow the gas to be taken up by the fume hood and also to avoid movement of the reactor body, which could break the delicate lining of the heating element on the furnace. The reactor was cooled to ambient before removal of the coal from the reactor. For the samples pretreated at pressures beyond the operating limit of the reactor fabricated in-house, the pretreatment was performed on a Supercritical Fluid Technologies model FCF

150 instrument. When 43 g of as-received coal was placed in the reactor’s 80-mL sample cell and the reactor was filled with saturated liquid CO2 at 26 °C, the solvent-to-coal ratio (by mass) was 1.3. 2.2. Surface Area and Pore Volume Analysis. The gas sorption technique, using liquid N2 at 77 K and CO2 at 298 K as the adsorbates, was the methodology used to quantify surface area and pore volume. The N2 isotherms were reduced using the Brauner-Emmett-Teller (BET) equation, and the CO2 isotherms were reduced using the Dubinin-Radushkevic (D-R) equation. The process is described in detail by Gathitu et al.38 in a concurrent study on the effects of coal pretreatment by CO2 on surface area and pore structure. 2.3. Apparatus and Procedure for Simulated Reburning and Burnout. A two-stage drop-tube reactor that allowed for addition of reactant gas between the two stages was the approach used in this study. The ceramic reactor tubes were enclosed in electrically heated furnaces that could heat to 1700 and 1100 °C. The apparatus is described in detail by Su et al.31 in a previous study on efficient and cost-effective reburning by natural gas substitutes. The simulated flue gas for reburning using Illinois #6 bituminous coal in the primary stage had a composition of 15.8% CO2, 1.84% O2, 0.05% NO, 6.31% H2O, and the balance He. That for reburning using Beulah lignite in the primary stage had a composition of 16.6% CO2, 1.81% O2, 0.05% NO, 6.56% H2O, and the balance He. 2.4. Apparatus and Procedure for Oxidation Tests of Raw and Pretreated Coals. The experimental apparatus used was the same one as used by Su et al.31 in a previous study. For oxy-combustion with recycle tests, the apparatus was modified by adding a desiccator and then a vacuum pump at the reactor exit to draw flue gas and then recycle a major portion of it back into the reactor after addition of oxygen. A dilution system was added in the downstream section of the reactor to measure CO2 concentrations beyond the 20% limit of our analyzer. This dilution system also served to add volume to the flue gas that exited the recycle loop during oxy-combustion with recycle tests, as this alone was not sufficient to meet the needs of both the NO and CO2/CO analyzers arranged in a parallel flow pattern. The dilution system added helium to the stream leaving the reactor and passed the diluted stream through a mixing chamber (1-L stainless steel bottle) before disposing excess sample and feeding the necessary volume into the gas analyzers. Residence time, temperature, and stoichiometric ratio were optimized so that the carbon burnout, as determined by loss on ignition of the solid combustion residue, of the dried raw coals during air-fired combustion (21% O2 in He) was similar to that found in actual boilers. The optimal conditions were a volumetric flow rate of 1 L/min (residence time of approximately 0.4 s.) for both coals, a furnace temperature of 1450 °C for bituminous coal and 1250 °C for lignite, and a stoichiometric ratio of 1.2 for the bituminous coal and 1.1 for the lignite. The lower furnace was set at 200 °C to avoid condensation of water on the inner wall of the reactor tube, which would cause the particles of char to adhere to the wet wall, thus distorting our carbon mass balance analysis and loss on ignition. Both singlepass oxy-combustion and oxy-combustion with recycle were conducted using 30% O2 in CO2. For the oxy-combustion with recycle tests, appropriate amounts of recycled flue gas and O2 were mixed and fed into the reactor to ensure that the residence time and stoichiometric ratio were similar to those of singlepass air-fired and oxy-combustion tests for each coal.

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Table 1. Ultimate Analysis of the Samples (wt %) samplea

loss on drying

C

H

O

N

S

mineral matter

H/C atomic ratio

O/C atomic ratio

mass change after pretreatment (%)

Illinois #6 bituminous coal (as-received) Illinois #6 bituminous coal (dried) D80B48@2000psig (dried) A80B48@8100psig (as-received) A130B48@3075psig (as-received) Beulah lignite (as-received) Beulah lignite (dried) D80L48@2600psig (dried) A130L48@3300psig (as-received)

2.30 2.30 1.44 1.23 1.85 11 11 7.09 10.36

71.26 72.9 70.4 69.57 70.13 58.57 65.01 59.28 57.61

4.79 4.9 4.59 4.7 4.72 3.88 4.31 3.99 4.73

6.45 6.6 12.24 11.02 12.52 16.38 18.18 29.64 31.36

1.47 1.5 0.97 1.44 1.43 0.88 0.98 0.8 0.84

2.74 2.8 2.67 2.63 2.49 0.74 0.82 0.95 0.97

10.56 10.80 9.73 9.40 9.71 6.01 10.78 9.44 8.18

0.81 0.81 0.78 0.81 0.81 0.79 0.80 0.81 0.99

0.06 0.06 0.12 0.11 0.12 0.16 0.18 0.29 0.30

NA NA 7 -4 -3 NA NA 10 -10

a Sample naming convention: D or A indicates dried or as-received, respectively; temperature of pretreatment (°C); B or L indicates bituminous coal or lignite, respectively; duration of pretreatment (h); pressure of pretreatment (psig). For example, D80B48@2000psig stands for dried bituminous coal pretreated at 80 °C and 2000 psig for 48 h.

Table 2. Mineral Analysis of Coal (% of Mineral) sample

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

TiO2

MnO

Fe2O3

Illinois No. 6 bituminous coal A130B48@3075psig Beulah lignite A130L48@3300psig

0.51 0.51 4.60 3.59

1.02 0.95 7.00 7.50

20.49 19.91 9.97 10.20

51.65 51.05 22.62 17.80

0.07