CO2 Combustion with Partial CO2 Removal from

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Scheme of O2/CO2 Combustion with Partial CO2 Removal from Recycled Gas. Part 1: Superlow NO Emission Hao Liu,*,† Ying Yuan,† Xing Yuan,† Hong Yao,‡ Takashi Ando,§ and Ken Okazaki§ †

College of Energy, Soochow University, Suzhou 215006, People’s Republic of China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China § Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan ‡

ABSTRACT: A new scheme of O2/CO2 combustion with partial CO2 removal from recycled gas is proposed. Through experiments and theoretical analysis of the system, the reduction of NO to N2 and emission of NO were investigated for this new scheme. When CO2 is partially removed from the recycled gas, a high gas recirculation ratio is needed to maintain the O2 concentration at the furnace entrance and the stoichiometric O2 amount necessary for the burn-out of coal. It was found that this gas recirculation ratio increased with the CO2 removal ratio from the recycled gas. The gas recirculation ratio could be as high as 98.9%; i.e., only 1.1% of the flue gas was exhausted. The CO2 concentration in the flue gas was above 95%, which made its capture much easier and more economical. The global conversion ratio from fuel N to exhausted NO decreased as the CO2 removal ratio from the recycled gas increased. At a CO2 removal ratio of 15%, the global conversion ratio from fuel N to exhausted NO became as low as 3.95 × 10−3, i.e., down to only 1/85 of that of conventional combustion in air. These results suggested that, with the new scheme of O2/CO2 combustion proposed in this work, superlow NO emission and easy CO2 capture could be realized simultaneously. NO. Kimura et al. 10 conducted experiments on coal combustion with O2/recycled flue gas on a bench-scale test facility with a coal feed rate of 100 kg/h. The experimental results indicated that the NOx conversion ratio from fuel N in O2/recycled flue gas combustion was less than 10%, much lower than the 30% in air-blown combustion. Results from pilot-scale experiments show a 45% increase in the concentration of NOx in the flue gas compared to the air-firing NOx concentrations, resulting from the recycling of NOx in the flue gas back to the combustion chamber and a reduction in the total gas flow. However, the mass of NOx released per energy generated is significantly less for oxy-fuel combustion, around one-third of the total NOx produced by air combustion.1 Emission characteristics of a 0.03 MW oxy-fuel combustor and 0.2 MW liquefied natural gas (LNG) oxy-fuel combustor have been experimentally investigated by Kim et al.11,12 Their experimental results suggest that oxidizer velocity at the oxyfuel combustor could be one of the crucial design parameters to control the NO emission. Emissions of SO2 and NOx during oxy-fuel CFB combustion were investigated by Jia et al.13 in a mini-circulating fluidized-bed combustion reactor, and it was found that emissions, such as CO or NOx, are lower or comparable to those from air firing. In O2/CO2 combustion, the CO2 volume fraction in exhaust gases can be up to 0.95. This CO2-rich atmosphere is likely to alter many features of the process, which remain to be studied in detail. Examples include conversion from fuel N to NO, reduction of recycled NO in the reducing atmosphere in the

1. INTRODUCTION O2/CO2 combustion has been recognized as a promising technology for pulverized coal-fired power plants to control CO2 emissions.1 Essentially different from the conventional flue gas recycling for thermal NO reduction, this process uses pure oxygen instead of air and recycles most (around 80%) of the flue gas but exhausts a small fraction of the total flue gas. The CO2 concentration in the flue gas may be enriched up to 95%, thus facilitating CO2 recovery. Experiments by Croiset and Thambimuthu2 revealed that combustion with recycled flue gas led to lower NOx emission than for once-through combustion in O2/CO2 mixtures, because of the reduction of recycled NO. Nozaki et al.3 conducted experiments on O2/CO2 combustion with low- and medium-volatile bituminous coals and concluded that the recycled NO was rapidly reduced in the combustion zone. Experiments have been carried out by Stadler et al.4 with lignite as well as bituminous coals, showing an overall NOx reduction capability of about 20−50% depending upon the fuel type and stoichiometry at the burner. The experimental data on recycled NO reduction by Hu et al.5−7 indicate that the reduction efficiency increased with the fuel equivalence ratio and recycling ratio. In the fuel-rich region, the reduction efficiency reached as high as 80% at a fuel equivalence ratio of 1.4. Normann et al.8 investigated the possibility of high-temperature reduction of nitrogen oxides (NOx) in O2/CO2 combustion. Combustion characteristics for both air and O 2/CO 2 conditions were numerically investigated by Cao et al.9 and compared to experimental data obtained from a pilot-scale test facility for an Australian sub-bituminous coal. The NO emission under the O2/CO2 condition was predicted to be significantly lower than that in air combustion, even without recycling of © 2012 American Chemical Society

Received: September 26, 2011 Revised: January 11, 2012 Published: January 12, 2012 829

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combustion region of volatile matter from coal, and interaction between recycled NO and fuel N. Although there are some reports about NO formation and reduction in O2/CO2 combustion,14,15 the detailed mechanism of NO reduction and the factors that contribute to low NO emission are important issues still to be clarified. In this work, a new scheme of O2/CO2 combustion with partial CO2 removal from recycled gas is proposed (Figure 1).

Figure 2. Schematic diagram of the experimental facility. form a laminar flow and facilitates heating of premixed gases. To avoid accumulation of coal particles on the honeycomb, both the mixing chamber and burner were vibrated. A sampling probe of axially adjustable position was used to collect gas samples at various residence times. The sampling probe was cooled by water and kept at a temperature of about 350 K. The sampling probe contained a thermocouple to measure the temperature of the gas. Another thermocouple, protected with ceramic, was placed near the front of the honeycomb structure to confirm ignition. After dust and moisture removal from the flue gas, the concentrations of O2, CO2, CO, CH4, N2, and NO in flue gas were measured using a gas chromatograph and a chemiluminescent NOx detector. To simulate coal combustion by separating the gas-phase reactions and carbon combustion, a flat CH4 flame doped with NH3 for fuel N was formed under the honeycomb and the effects of CO2 and NO concentrations in the recycled gas were examined. Moreover, a small amount of coal was fed into the CH4 flame to test the effect of carbon combustion. Although the major components in coal volatile matter include hydrocarbons, CO, H2, NH3, HCN, and N2, our study used only CH4 and NH3 to facilitate handing and analyses. An initial O2 concentration of 21 vol % was adopted in the experiments to compare to the case of conventional coal combustion in air. To simulate char combustion, an anthracite was adopted. The experimental conditions were summarized in Table 1, in which β is the

Figure 1. Diagram of the new scheme of O2/CO2 combustion proposed in this work.

A mixture of flue gas without CO2 removal and a fraction of flue gas devoid of CO2 is recycled. Thus, the recycled gas has been subjected to partial CO2 removal. Here, CO2 removal can be realized with sorbents, such as activated carbon, limestone, and metal oxides, or through amine-based CO2-capture technology. When CO2 is partially removed from the recycled gas, a high gas recirculation ratio is needed, so that there is enough CO2 to dilute the feeding O2, i.e., to compensate for the removed CO2 and satisfy the initial O2 concentration and O2 amount at the entrance of a combustor needed for combustion. That is why the recycle ratio increases with an increasing CO2 removal ratio. In such a system, the gas recirculation ratio is very high to maintain the O2 concentration at the furnace entrance and the stoichiometric O2 amount necessary for burnout of coal. Although the recycled gas is only partially depleted in CO2, CO2 in the exhausted flue gas can be completely removed if necessary, just as in an existing O2/CO2 combustion system. Our recent work revealed that superlow NO emission is possible for such an O2/CO2 combustion system with a high gas recirculation ratio. Through experiments and theoretical analysis of the system, the NO emission in such a new O2/CO2 combustion system was characterized in this work.

Table 1. Experimental Conditions maximum flame temperature, Tmax (K) initial O2 concentration (vol %) volume ratio CO2/(CO2 + Ar) in recycled gas NO concentration in recycled gas (ppm) replacement ratio for CH4 by coal, β fuel N concentration (N atom mass, wt %)

2. EXPERIMENTS ON NO REDUCTION 2.1. Experimental Facilities and Methods. To avoid the influence of mixing effects and focus purely on the effects of gas concentrations and related phenomena, the experiments on NOx reduction were carried out in an electrically heated one-dimensional, premixed reactor (Figure 2), where the process conditions can be controlled as required. The reaction tube is ceramic, with a 35 mm inner diameter and 200 mm height. The gas concentrations of CH4, O2, CO2, Ar, NO, and NH3 were controlled by flow-meters. Pulverized coal particles were supplied by a vibrating coal feeder. The gas and particles were fed independently into the reactor to simulate combustion conditions corresponding to actual O2/CO2 coal combustion. The maximum flame temperature Tmax was maintained at 1450 K for all experiments by changing the power of the electric heater. To form a flat CH4 flame, a honeycomb structure (30 mm in diameter and 50 mm in length) was placed at the upper edge of the one-dimensional, premixed reactor. The honeycomb also helps to

1450 21 0.16−0.8 0−1580 0 and 0.2 0 and 1.22

fuel/replacement ratio for CH4 by coal, defined as

β = (O2 consumption by coal) /(O2 consumption by coal + O2 consumption by CH4)

(1)

To obtain the effect of the CO2 concentration in the recycled gas on CR, experiments were conducted in atmospheres of various CO2 concentrations, at Tmax = 1450 K and fuel N = 1.22 wt % (as N atom mass). To clarify the interaction between fuel N and recycled NO, the 830

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Table 2. Coal Properties proximate analysis (as received, wt %)

ultimate analysis (dry, wt %)

moisture

ash

volatile matter

fixed carbon

C

H

O

N

S

4.6

10.6

41.6

43.2

70.60

5.41

10.98

1.19

0.71

The amount of pure O2 feeding is (λRO2 − REz). Additionally, x and z must satisfy

experiments were conducted at various NO concentrations to clarify their effects on the reduction of recycled NO. 2.2. Experimental Results. The experiments revealed that the conversion ratio from fuel N to NO decreased with an increasing CO2 concentration and increasing NO concentration in the recycled gas.

CR = 0.3167 − 0.0833CCO2 − 25.0CNO,re

⎤ ⎡⎛ GC WC ⎞ + REx⎟(1 − ϕ)⎥ /(λ − 1)R O2 = x /z ⎢⎜ ⎠ ⎦ ⎣⎝ 100 12

(2)

where x/z is the ratio between the fractions of CO2 and O2 in the recycled flue gas and φ is the removal ratio of CO2, defined as the fraction of CO2 removed from the recycled gas (point B in Figure 1) with respect to the flue gas prior to CO2 removal (point A in Figure 1). Solving eqs 5 and 6 gives the oxygen concentration in the recycled flue gas

Moreover, experiments were conducted at CO2/(CO2 + Ar) = 0.48 by volume with no fuel N added to study the reduction of recycled NO in a CH4 flame. We changed the concentration of NO (representing NO in recycled gas). As much as about 60% of recycled NO was reduced to N2, which corresponds to the reduction of recycled NO in the reducing atmosphere of the volatile matter combustion zone in a pulverized coal combustor. The reduction ratio of NO in recycled gas increased with the NO concentration in the recycled gas.

RR = 0.53 + 66.7CNO,re

⎛ ⎞ ⎛ c z = ⎜⎜⎜a + + RE⎟ 1−ϕ ⎠ ⎝ ⎝

(3)

3. SUPERLOW NO EMISSION: NEW SCHEME OF O2/CO2 COMBUSTION 3.1. Proposed New Scheme and Theoretical Analysis of the System. Our proposed new scheme for O2/CO2 combustion is shown in Figure 1. In the current systems for O2/CO2 combustion, flue gas is recycled without CO2 removal, although the flue gas discharged to atmosphere is considered to be removed of CO2. However, in our new scheme, a mixture of flue gas without CO2 removal and a fraction of flue gas depleted of CO2 is recycled. The CO2 removal ratio was defined as the CO2 amount between points B and A. This ratio can be controlled through the flow rate ratio of Q1/Q2, with Q1/Q2 = (CO2 removal ratio)/(1 − CO2 removal ratio). The flue gas recirculation ratio was defined as the gas flow rate ratio between points D and B. In this section, we calculate the effect of the specific conditions in our system (partial removal of CO2 from the recycled gas stream and high gas recirculation ratio) on the performance of the system. If the feed rate of coal is GC (g/s), then the stoichiometric amount of O2 necessary for burn-out of the coal is R O2 =

±

λR O2(100 − CO2) CO2(1 − z)

(7)

where a = GCWC/1200 and c = (λ − 1)RO2. The equations were solved with a trial and error method to derive RE and gas recirculation ratio. The objective is to derive the appropriate gas recirculation ratio, when the coal property, oxygen/fuel stoichiometric ratio, and O2 concentration at the furnace entrance were given and kept constant. First, we supposed a value of RE and calculated z with eq 7. With this z, we calculated RE with eq 5 again and compared it to the supposed value of RE. This process was repeated until their difference was within our tolerance (0.0001). With the gas recirculation ratio derived with this method, not only the oxygen excess but also the O2 concentration at the entrance of the boiler can be kept constant. The NO originating from fuel N (NOf) is then given by

⎛ WN ⎞ ⎟ NOf = CR × GC⎜ ⎝ 14 × 100 ⎠

(mol/s)

(8)

The recycled NO remaining in the flue gas after reduction in the furnace is

(4)

where WC, WH, WS, and WO represent the carbon, hydrogen, sulfur, and oxygen contents of coal (wt %), respectively. Next, the fractions of CO2 and O2 in the recycled flue gas (x and z, respectively) will be evaluated. To satisfy the requirement of the oxygen concentration (CO2) at the entrance of the furnace prior to combustion, the flux of recycled gas is derived as

RE =

⎞ ⎞2 ⎛ c c ⎟ + RE⎟ − 4RE ⎜a + 1−ϕ 1−ϕ⎟ ⎠ ⎝ ⎠

/2RE

GC ⎛ WC W W ⎞ W ⎜ + H + S − O⎟ ⎝ 100 12 4 32 32 ⎠

(mol/s)

(6)

NOr = (1 − RR)RECNO × 10−6 (mol/s) (9) Because NO in the flue gas is the sum of NO originating from fuel N and recycled NO remaining in the flue gas (NOf + NOr), the NO concentration in the flue gas is W

CR × GC N + (1 − RR)RECNO × 10−6 1400 a + RE(1 − z)(1 − ϕ) + c × 106 = CNO

(5)

where RE is the flux of recycled gas (mol/s), CO2 is the oxygen concentration at the entrance of the furnace prior to combustion, and λ is the oxygen/fuel stoichiometric ratio.

(10)

From a practical point of view, the amount of fuel N exhausted from the system as NO is important. For the whole system, the global conversion ratio from fuel N to exhausted NO is defined 831

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CO2 removal ratio increased from 0 to 15%. This result comes from the enrichment of O2 because of the removal of CO2 . Figure 5 shows the NO concentrations considering only the effect of gas recirculation and neglecting the reduction of

as

CR sys = (N atom number of NO in exhaust gas) /(N atom number of fuel N)

(11)

Using our experimental results (eqs 2 and 3) for the coal described in Table 2 and the theoretical analysis of the system described in this section, our new scheme of combustion can now be evaluated and parameters such as the NO concentration and global conversion ratio of fuel N to NO can be obtained. In this work, the oxygen/fuel stoichiometric ratio = 1.2 was used. In the following results, Figures 4−8 are the results by combining the experimental results and theoretical consideration. From our experiments, the fundamental reaction kinetics were obtained. On the other hand, the effect of the recycled system was considered from our system approach, i.e., theoretical consideration. 3.2. Superlow NO Emission from the New Scheme of O2/CO2 Combustion. Figure 3 shows the change in the gas

Figure 5. NO concentration considering only the effect of gas recirculation while neglecting the reduction of recycled NO in the furnace.

recycled NO in the furnace. The NO concentration increased with the CO2 removal ratio, most significantly between CO2 removal ratio values of 10 and 15%. This is because the gas recirculation ratio increased with the CO2 removal ratio and consequently enriched NO in the recycled gas. This high NO concentration provides ideal conditions for NO reduction in the furnace. Figure 6 shows the NO concentration when

Figure 3. Change of the gas recirculation ratio with the CO2 removal ratio.

recirculation ratio with the CO2 removal ratio. A CO2 removal ratio of 0% corresponds to the existing scheme for O2/CO2 combustion. The gas recirculation ratio increased with the CO2 removal ratio. In particular, at a CO2 removal ratio of 15%, the gas recirculation ratio could be as high as 98.9%, which means that only 1.1% of the flue gas was exhausted. Figure 4 presents the CO2 concentrations in the exhausted gas at various CO2 removal ratios. For all CO2 removal ratios

Figure 6. NO concentration considering both the effect of gas recirculation and the reduction of recycled NO in the furnace.

considering both the effect of gas recirculation and the reduction of recycled NO in the furnace. As expected, including the reduction of recycled NO in the furnace results in a drastic decrease in the NO concentration, as seen in Figure 6 compared to Figure 5. The global conversion ratio from fuel N to exhausted NO for the new system is shown in Figure 7. It was found that the

Figure 4. CO2 concentration in the exhausted gas at various CO2 removal ratios.

investigated, the CO2 concentration was above 0.95 (or 95%). This high concentration of CO2 should make its removal much easier and more economical. Moreover, it was revealed that the O2 concentration in the flue gas increased a little, changing from 3.59 to 4.2%, when the

Figure 7. Conversion ratio of fuel N to exhausted NO. 832

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enabling reduction to N2 in the reducing atmosphere of the coal-burning region in the furnace; i.e., using a high gas recirculation ratio in the system maximizes the effect of NO enrichment and reduction in the furnace. This new scheme is no more expensive than existing O2/CO2 combustion schemes, because both types of schemes require CO2-capture facilities. Although the new system is a little more complicated, its merit is obvious. Work to further clarify the features of this scheme is needed.

global conversion ratio from fuel N to exhausted NO decreased as the CO2 removal ratio increased. At a CO2 removal ratio of 15%, the global conversion ratio from fuel N to exhausted NO reached the very low value of 3.95 × 10−3. Figure 8 shows the

5. CONCLUSION A new scheme of O2/CO2 combustion with partial CO2 removal from recycled gas was proposed. Through experiments and a theoretical analysis of the system, the reduction and emission of NO were investigated for this new scheme. The following conclusions were reached: The gas recirculation ratio increased with the CO2 removal ratio from the recycled gas. The gas recirculation ratio could be as high as 98.9%; i.e., only 1.1% of the flue gas was exhausted. The CO2 concentration in the exhausted gas was above 95%, making its capture much easier and more economical. The global conversion ratio from fuel N to exhausted NO decreased as the CO2 removal ratio from the recycled gas increased. At a CO2 removal ratio of 15%, the global conversion ratio from fuel N to exhausted NO decreased to a very low value of 3.95 × 10−3; i.e., NO emission could be as low as 1/85 of that of conventional air combustion. These results suggested that, using the new scheme of O2/CO2 combustion proposed in this work, superlow NO emission and easy CO2 capture could be realized simultaneously.

Figure 8. Ratio of CRsys/CR (CRsys refers to the global conversion ratio of fuel N to exhausted NO in O2/CO2, and CR refers to the conversion ratio of fuel N to exhausted NO in conventional air combustion).

ratio of CRsys/CR (CRsys refers to the global conversion ratio of fuel N to exhausted NO in O2/CO2, and CR refers to the conversion ratio of fuel N to exhausted NO in conventional air combustion). Obviously, the new scheme proposed in this work yields a global conversion ratio of fuel N to exhausted NO much lower than that for conventional air combustion. At a CO2 removal ratio of 15%, the NO emission can be as low as 1 /85 of that of conventional air combustion. These results suggest that, with this new scheme, superlow NO emission can be realized. Figure 9 shows a mass flow diagram for N in the new scheme of O2/CO2 combustion at a CO2 removal ratio of 15%. A flux of 0.0119Gc (g/s) fuel N enters the furnace, and 0.0042Gc (g/ s) of the N atom is recycled. However, only 0.000 05Gc (g/s) of the N atom (NO) is exhausted into the atmosphere.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-512-6787-0271. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (50936001), the Foundation of State Key Laboratory of Coal Combustion (China), and the National Key Scientific Instruments and Equipment Funding (2011YQ120039).

4. DISCUSSION The new scheme of O2/CO2 combustion proposed by us is more promising than the existing O2/CO2 combustion systems, because it can simultaneously realize superlow NO emission and easy CO2 capture. This is attributed to the high gas recirculation ratio needed to maintain the O2 concentration at the furnace entrance and the stoichiometric O2 amount necessary for burn-out of coal. In a system with a high gas recirculation ratio, the NO concentration is drastically enriched,

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NOMENCLATURE CCO2 = CO2 concentration

Figure 9. Mass flow diagram of N for the new scheme of the O2/CO2 combustion system. 833

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CNO = NO concentration in the exhausted gas CNO,re = NO concentration in the recycled gas CO2 = O2 concentration CR = conversion ratio from fuel N to NO CRsys = global conversion ratio from fuel N to exhausted NO GC = feed rate of coal (g/s) NOf = NO originating from fuel N (mol/s) NOr = recycled NO remaining in the flue gas after reduction in the furnace (mol/s) RE = flux of recycled gas (mol/s) RO2 = stoichiometric amount of O2 necessary for burn-out of the coal (mol/s) RR = reduction ratio of NO in recycled gas W = mass fraction of some element in coal (%) x and z = fractions of CO2 and O2 in the recycled flue gas, respectively λ = oxygen/fuel stoichiometric ratio φ = removal ratio of CO2 Subscripts

C, H, O, N, and S = carbon, hydrogen, oxygen, nitrogen, and sulfur

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