Scholarly article on topic 'Experimental Results for Oxy-fuel Combustion with High Oxygen Concentration in a 1MWth Pilot-scale Circulating Fluidized Bed'

Experimental Results for Oxy-fuel Combustion with High Oxygen Concentration in a 1MWth Pilot-scale Circulating Fluidized Bed Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Haoyu Li, Shiyuan Li, Qiangqiang Ren, Wei Li, Mingxin Xu, et al.

Abstract Combustion experiments with the fuel of Chinese bituminous were conducted in air and O2/recycle flue gas (RFG) atmospheres in a 1MWth pilot-scale oxy-fuel circulating fluidized bed (CFB). The effects of atmospheres on combustion characteristics including temperature profiles, combustion efficiency, and gaseous pollutant emission are investigated. The highest concentration of oxygen in the experiment is increased to 50% per volume. It is shown that CFB combustor under high oxygen concentration condition is very stable and the transition from air to the oxy-fuel firing mode is smooth, presenting little or no operational difficulty, CO2 concentrations in flue gas can reach near 80% on a dry basis. Compared with the air firing mode, the temperature level in the bed is similar, and combustion efficiency is improved under O2/RFG firing mode. Emission in mg/MJ unit such as NO and CO are lower while SO2 are much higher than that under the air firing mode.

Academic research paper on topic "Experimental Results for Oxy-fuel Combustion with High Oxygen Concentration in a 1MWth Pilot-scale Circulating Fluidized Bed"

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Energy Procedía 63 (2014) 362 - 371

GHGT-12

Experimental results for oxy-fuel combustion with high oxygen concentration in a 1MWth pilot-scale circulating fluidized bed

Haoyu Li, Shiyuan Li*, Qiangqiang Ren, Wei Li, Mingxin Xu, Jing zhang Liu, Qinggang

Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Road, Beijing, 100190, China

Abstract

Combustion experiments with the fuel of Chinese bituminous were conducted in air and O2/recycle flue gas (RFG) atmospheres in a 1MWth pilot-scale oxy-fuel circulating fluidized bed (CFB). The effects of atmospheres on combustion characteristics including temperature profiles, combustion efficiency, and gaseous pollutant emission are investigated. The highest concentration of oxygen in the experiment is increased to 50% per volume. It is shown that CFB combustor under high oxygen concentration condition is very stable and the transition from air to the oxy-fuel firing mode is smooth, presenting little or no operational difficulty, CO2 concentrations in flue gas can reach near 80% on a dry basis. Compared with the air firing mode, the temperature level in the bed is similar, and combustion efficiency is improved under O2/RFG firing mode. Emission in mg/MJ unit such as NO and CO are lower while SO2 are much higher than that under the air firing mode.

© 2014TheAuthors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: circulating fluidized bed; oxy-fuel combustion; high oxygen concentration; pollutant emission

1. Introduction

Oxy-fuel combustion is one of the most promising power generation technologies for carbon capture and sequestration due to its burning fuels with the mixture of nearly pure oxygen and CO2 rich recycled flue gas, which will result in a nearly pure steam of CO2 [1]. Compared with pulverized coal for oxy-fuel combustion, circulating

* Corresponding author. Tel.:+86-10-82543055; fax: +86-10-82543119. E-mail address:lishiyuan@iet.cn

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi: 10.1016/j.egypro.2014.11.039

fluidized bed combustion (CFBC) has some unique advantages, such as the possibility of using poor quality fuel including biomass, lower flue gas recycle ratio leading to potentially smaller plants for any given power output, low NOX emissions due to low operating temperature and potential for in bed SO2 capture via addition of sorbent, making it a better choice for addressing CO2 capture in power generation system [2-3].

For the above reasons, CFB oxy-fuel combustion technology has recently attracted further attention from investigators. Foster Wheeler [4] has constructed a 30MWth oxy-CFB in Spain (CIUDEN), which represents an important milestone for the development of oxy-CFB technology, but there are few published results in this pilot-scale facility by now. Alstom [2] carried out extensive work with a 3MWth unit but used only bottled CO2 to simulate recycled flue gas. CanmetENERGY (Canada) [5-6] has started their research with a 100kWth mini-CFBC unit since 2006; further, a retrofitted 0.8 MWth CFBC unit is used for oxy-fuel research, with tests carried out at oxygen concentrations of about 29%. Metso Power also established a 4 MWth oxy-CFB facility in Finland operating at 21-25% oxygen concentration [7]. Czestochowa University of Technology (Poland) [8-9] has done a lot of experiments in an oxy-fuel CFB test rig of 0.1MWth, but mainly operating at O2 and bottled CO2 atmosphere. In China, Southeast University [10-11] has reported a lot of experimental results conducted in a 50kWth CFBC unit at oxygen concentrations of 21-25%. Above all, Most of the mentioned study evaluates the oxy-fuel combustion characteristics when the oxygen concentration is below 40%.

A 0.1MWth oxy-CFB combustor at the Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET/CAS) was built in 2010, which can be operated at overall oxygen concentration up to 50% using liquid O2 and CO2 cylinder [12]. Subsequently, that facility was retrofitted for flue gas recycling in 2012. Since then, a lot of fundamental researches on oxy-CFB operating at high O2 concentration have been done in that retrofitted facility [13]. The experimental results are very encouraging. To further study oxy-fuel CFB combustion, a 1MWth pilot scale CFBC test facility has been established at IET/CAS, which can be operated at air and O2/RFG combustion mode with overall O2 concentration between 21% and 50%. This paper describes the pilot oxy-fuel CFBC plant and initial results obtained during the commissioning experiments of the unit.

2. 1MWth pilot-scale oxy-fuel CFBC research facility at IET/CAS

The 1MWth pilot-scale oxy-fuel CFBC research facility is shown schematically in Figure 1, which consists of CFB combustion system, flue gas cooling and cleaning system, dry flue gas recycle system, gas mixing and supplying system, and measurement and control system.

Fig. 1. 1MWth pilot-scale oxy-fuel CFBC research facility exterior view

The 1MWth pilot-scale oxy-fuel CFBC is composed of a refractory riser and cyclone, a dual-exit ash distributor, a loop seal and external heat exchanger. Dual-exit ash distributor is used to distribute the amount of circulating hot ash from the cyclone that enters the loop seal and the external heat exchanger, which is drived by the gas. The net height of the riser is 15m. The diameters of dense and dilute zone are 280mm and 420mm respectively. Eleven retractable bayonet-type water cooling tubes can be inserted into the riser for temperature control, one of which is 10m long, and the other ten are about 1.2m long. The longest one is inserted at the top of the riser; the other ten tubes are obliquely inserted into the riser along the height of 0.65m, 1.24m, 1.83m, 2.42m, 3.01m, 3.60m, 4.19m, 4.78m, 5.37m and 5.96m above the distributor. The CFB combustor is equipped with an oil burner for preheating. High temperature flue gas generated by the oil burner is used to heat the bed materials and ignite the coal. The coal is fed into the combustor by two screw feeders and the feeder position elevation is 0.8m from the bottom distributor. The primary fluidizing gas, secondary gas and material-returning gas are all supplied by gas mixing and supplying system. The secondary gas is staged, which is divided into four layers with each layer having two separate nozzles. The height of each layer is 1.58m, 2.42m, 3.22m, and 4.02m respectively above the distributor.

The flue gas cooling and cleaning system consists of O2 and RFG perheater, four groups of water-cooled tubes and a heat-preserving bag filter. The gas preheater and water-cooled tubes are sleeve tube structure, with flue gas in the inner tube, preheated gas and water in the annular gap between the inner tube and the outer tube. In this test, O2 and RFG are not preheated, air is used to cool the two preheaters, and the heated air is emitted to the atmosphere. The temperature of flue gas after passing through two gas preheaters and four water coolers is below 200 °C. The heat-preserving bag filter is used to remove particles in flue gas and can keep the outlet temperature of flue gas above its dew point. The cooled and dedusted flue gas is separated into two streams, which are recycled flue gas and exhaust flue gas. The exhaust flue gas is vented with the help of an induced draft fan to the atmosphere through a stack.

The dry flue gas recycle system mainly consists of the flue gas condenser, the steam separator and the recirculation roots blower. The recirculation roots blower is capable of delivering up to 700m3/h of recycled flue gas at a discharge pressure of 50kPa. The RFG is extracted after the bag filter, which then passes through the flue gas condenser, the steam separator and the recirculation fan, finally enters the RFG buffer tank.

The gas mixing and supplying system consists of primary gas mixer, secondary gas mixer, O2 and RFG buffer tanks. The oxygen is supplied by a liquid oxygen tank. The oxygen is vapourized and brought to the O2 buffer tank at the pressure of 50kPa before entering the facility. The primary and secondary gas mixer has air, O2 and RFG entrances, which are supplied by air roots blower, O2 and RFG buffer tank respectively. So the gas mixer can provide a mixed flow of O2/N2 or O2/RFG with varying oxygen concentrations for different tests. The oxygen level after the two gas mixers is obtained using a Siemens Oxymat 61 which measures oxygen using a paramagnetic alternating pressure method.

Programmable Logic Controlle (PLC) is used in this facility for measurement and control. The data acquisition in this system includes temperature, pressure, flow, and concentration of flue gas component. Temperatures are measured at up to 56 different locations in the facility including flue gas and cooling water side. Temperatures are measured in the furnace at 0.3m, 0.8m, 1m, 1.55m, 2.35m, 3.15m, 4m, 6m, 8m, 10m, 12m and 14.5m above the distributor and at the cyclone, the dual-exit ash distributor, the loop seal, and the external heat exchanger. There are 25 thermal mass flowmeters and 7 turbine flowmeters measuring flow rate of gas and cooling water respectively. The oxygen concentration after the two gas mixers are measured using Siemens Oxymat 61. The O2 concentration in the flue gas is measured using the zirconium O2 analyzer. Fly ash sampling ports are available at flue gas duct before the bag filter. The concentrations of CO2, CO, SO2, NO, and N2O in the flue gas are continuously monitored online by a GASMET DX4000 FTIR analyzer after the bag filter.

3. Commissioning tests

The ultimate and proximate analyses of fuel are listed in Table 1, and the ash composition is shown in Table 2. A typical Chinese bituminous coal, Shenmu (SM) coal was used in the commissioning test. The coal was sieved to a diameter between 1 and 8 mm. Sand with particle diameters between 0 and 1mm was used as the starting bed material. Additional fine sand with diameters between 0.1 and 0.5mm was added during operation process to build up material circulation. No SO2 sorbent was added in this test.

Table 1. Fuel analysis for Shenmu coal.

LHV/MJ-kg"1 ultimate analysis/ wt.% proximate analys is/ wt.%

Qnet ar 24.43 FCar 47.80 Mar A„r 11.80 9.82 f«-30.57 Car 62.94 Har 3.88 Oar 10.18 Sar 0.40 n«- 0.98

Table 2. Composition of the ash for Shenmu coal.

compositon SiÜ2 M2Ü3 Fe2Ü3 CaÜ MgÜ TiÜ2 SÜ3 K2Ü Na2Ü

content (wt %) 21.34 14.55 14.02 36.97 1.59 1.03 5.24 1.98 1.03

3.1. Start-up and oxy-transition behavior

The pilot-scale oxy-CFBC is started in air-firing mode. The ten oblique retractable bayonet-type water cooling tubes were all withdrawn from the riser, and the vertical one which is 10m long was inserted 7m into the riser. The test rig starts with high temperature flue gas generated by the oil burner which preheats the air and the bed material. When the temperature of the lower part of the riser warmed to 500°C, intermittent coal feeding began. As the coal feed rate gradually ramped up, the oil feed rate was reduced until coal feed was completely established, at which point the oil supply was stopped. During this period, secondary gas at lower part was introduced into the riser to establish consistent solids circulation. In air-firing mode, the external heat exchanger was not used. When the combustion and emission remain steady as judged by the free board and cyclone return leg temperature, a 4-hour air combustion test was carried out before the system was transition from air mode to oxy-fuel mode. Fig.2 shows the temperature profile from the beginning of the air-firing start-up to the stable operating condition, in the figure T1

(1.55m) represents the bed temperature of 1.55m above the distributor. The time for the total start-up was nearly 12h, which is attributed to heating of the refractory riser and cyclone.

1 . i Coal feeding

■ Jx^i^ II 1

f / 1 / J fill s 1 I n1 Ml / -T1 (1.55m)

/ \r T2 (4.0m)

J (l1 T3 (8.0m)

I s-r 1 1 - s 1 T4 (12.0m)

10:00 13:00 16:00 19:00 22:00 01:00 04:00 07:00 10:00

Fig. 2. Temperature profile during the air start-up of the facility until reaching stable operating conditions.

In order to operate at high overall O2 concentration and keep the flow rate of primary and secondary gas constant both in the air firing and oxy-fuel mode, ten oblique retractable bayonet-type water cooling tubes were all inserted to the center of the rise, and the vertical one was all inserted into the riser before the oxy-transition. After this operation, the bed temperature dropped from 850°C to 750°C because of the heat absorption of the cooling water in bayonet-type tubes. When the bed temperature was stable, the oxy-transition began. Firstly, the recirculation roots blower was turned on, the RFG was extracted from the flue gas duct behind the bag filter. The extracted RFG was brought into the primary gas mixer vis a RFG buffer tank after going through the condenser and steam separator. When the RFG entered the gas mixer, the pure O2 was brought into the same gas mixer visa O2 buffer tank at the same time. At this monment, the air flow rate into the gas mixer was decreased till zero. As the flue gas was recycled back to the riser, it absorbed more heat and reduced the dense phase bed temperature due to higher capacity of CO2 and H2O than N2 [11]. Therefore, the O2 concentration in the feed gas and coal feeding rate must be increased. The transition was accomplished by gradually increasing flow rates of pure oxygen and coal feeding rate [13]. In this test, the overall O2 concentration is 50% at oxy-fuel combustion mode, and the overall gases volumes entering the riser in the two atmospheres were the same. Although primary and second gas was switched separately due to supplied by different gas mixer, the transition method of primary and second gas was identical. A 6-hour O2/RFG combustion test was carried out after the system was stable.

Fig.3 shows temperature profile and coal feeding rate during the oxy-transition of the facility. Under oxy-fuel combustion mode, the bed temperature of the dense zone is higher due to higher coal combustion ratio in this zone. So the external heat exchanger was put into operation to control the temperature of the dense zone. As can be seen, the fluidization condition is good and no big temperature fluctuation and agglomeration was observed during the oxy-transition. That is because pure O2 and RFG are mixed uniformly in the gas mixer, there is not partial high O2 concentration in primary and second gas. In addition the large amount of inert bed materials in the CFB riser can weaken the local increase of temperature. So the transition from air mode to oxy-fuel mode in CFB occurs easily and requires less effect than oxy-fuel combustion of PC even under 50% oxygen concentration condition. For both air and oxy-fuel combustion mode, the pilot-scale CFBC is operated at the average bed temperature of 850°C and superficial gas velocity in the riser is about 3-4m/s. The overall O2 concentration during oxy-fuel combustion periods reached 50%.

gtjo итогах!*

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- T1 (1.55m)

- T2 (4.0m)

- T3 (8.0m) T4 (12.0m) Coal feed rate

Fig. 3. Temperature profile and coal feeding rate during oxy-transition of the facility until reaching stable operating conditions.

3.2. Experimental results

Experimental conditions during periods of stable operation under air firing and oxy-fuel firing conditions are given in table3.

Table 3. Experimental conditions.

Shenmu coal

parameter unit air 50% O2/RFG combustion

combustion mode mode

average bed temperature V 850±30 850±21

average cyclone temperature V 820±10 850±10

average loop seal temperature V 800±10 830±10

average external heat exchanger temperature V — 690±10

fuel feeding rate kg/h 70±2.5 175±2.5

thermal power MW 0.43±0.07 1.2±0.07

O2 in primary gas % vol 21 50±2

O2 in secondary gas % vol 21 50±2

overall O2 concentration % vol 21 50±2

flue gas recycle ration % vol 0 45±5

primary gas flow m3/h 380 380

secondary gas flow m3/h 350 350

The average temperature of the riser is 850°C, and the range of bed temperature is 820~880°C. The primary and secondary gas fractions are nearly 0.5 and 0.5 respectively in two tests. During oxy-fuel combustion, oxygen concentrations in the primary gas and the secondary gas are both 50 vol%. The flue gas recirculation ratio is estimated at 45%, which was significantly lower than that for pulverized fuel oxy-firing [14]. It shows that oxy-fuel CFB can run at much higher overall O2 concentration due to its special combustion style. In addition, external heat exchanger is a very effective assembly to control the temperature of the dense zone in high oxygen concentration combustion, which can make the temperature is nearly same as that under the air-firing mode.

The 1MWth pilot-scale oxy-fuel CFBC operates smoothly in both air and oxy-fuel combustion modes. Figure 4 shows temperature profile along the riser at air and oxy-fuel combustion modes. The temperature profiles along the

riser are stable and reasonably uniform. During stable periods of oxy-fuel CFBC with RFG, the average CO2 concentration in the flue gas is near 80%. Figure 5 shows CO2 and O2 concentrations in the flue gas during the O2/RFG combustion mode.

900850800-

O 7500 ' ^ 7002 ' S 650 -

0 600 -550500450400-

- T1 (0.3m)

- T2 (0.8m)

- T3 (1.0m)

- T4 (1.55m) T5 (2.35m)

- T6 (3.15m) T7 (4.0m) T8 (6.0m) T9 (8.0m) T10 (10.0m) T11 (12.0m) T12 (14.50m)

800750700650600550500450400

T1 (0.3m)

T2 (0.8m)

T3 (1.0m)

T4 (1.55m)

-T5 (2.35m)

T6 (3.15m)

T7 (4.0m)

T8 (6.0m)

T9 (8.0m)

T10 (10.0m)

T11 (12.0m)

T12 (14.50m)

(a) Air combustion mode. (b) oxy-fuel combustion mode.

Fig. 4. CFBC temperature profile along the riser at air and oxy-fuel combustion modes.

03:00 04:00 05:00 06:00 07:00 08:00 09:00

Fig. 5. CO2 and O2 concentrations in the flue gas during the O2/RFG combustion mode.

As can be seen in Fig. 4, although the thermal input power of the facility is much higher under O2/RFG combustion mode, a relatively small increase in the temperature level is noticed. It mainly results from the recirculation solids are used as a heat moderating agent and more heat was absorbed in the riser, when the external heat exchanger and bayonet-type water cooling tubes are put into operation. The temperature increases first and then decreases as the height increases above the distributor for both combustion mode. Figure 6 shows average bed temperature profile along the riser at air and oxy-fuel combustion modes. As can be seen in Fig.6, under O2/RFG combustion mode, the temperature is much lower in entire dense zone due to the introduction of the cool primary gas and circulating solids from external heat exchanger, and the highest temperature along the riser is at the position where the secondary gas is introduced. The combustion rate of coal is high and more heat is released when O2 is sufficient at the high O2 concentration condition, so the temperature increases rapidly at the introduction of secondary gas. At air combustion mode the highest temperature is at the upper part of the position where the secondary gas is introduced. As the height increases, the unburnt carbon content in the riser decreases, the heat

release the top

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from char combustion becomes mild, and the heat absorption of vertical bayonet-type water cooling tube at is intense, so the temperature at the upper part is a little lower than that at the lower part of the riser.

—air combustion —•— oxy-fuel combustion

temperature, 0C

Fig. 6. Average bed temperature profile along the riser at air and oxy-fuel combustion modes.

Table 4 shows the gaseous pollutant emissions, carbon content of fly ash and the combustion efficiency under two combustion modes. All data for gas pollutant emissions are reported as time-averaged values on a dry basis. The combustion efficiency (n) is defined as: n=100-q3-q4; the combustible gases heat loss (q3) and unburnt carbon heat loss (q4) are calculated based on the GB10184-88 and ASME PTC4-1998 [10].

Table 4. Pollutant emissions, carbon content of fly ash and the combustion efficiency under two combustion modes

Shenmu coal

parameter unit air combustion mode 50% O2/RFG combustion mode

CO ppm 134.41±21 108.06±25

CO mg/MJ 47.12±7.38 27.75±6.42

NO ppm 227.41±28 245.51±35

NO mg/MJ 85.32±10.51 67.35±9.61

Fuel N to NO conversion % 21.43±2.64 16.91±2.41

SO2 ppm 78.41±23 410.14±44

SO2 mg/MJ 62.81±18.23 239.96±25.74

Fuel S to SO2 conversion % 3.94±1.14 14.93±1.61

Carbon content in flyash % 21.74±1.35 13.33±1.26

Combustion efficiency % 71.86±1.24 82.79±1.16

As can be seen in table 4, under oxy-fuel combustion coal combustion is excellent, low CO concentrations in flue gas are indications of good combustion performance. Furthermore, the CO concentrations in mg/MJ unit are much lower than that achieved by air combustion. Since bed temperature at upper part of the dilute zone has a powerful effect on CO concentration in CFBC [5], as can be seen in Fig. 6, higher average dilute zone temperature in the oxy-fuel combustion are most likely responsible for the observed CO decrease. In addition high CO2 concentration in the riser under oxy-fuel CFB combustion apparently has no effect on CO concentration at 850 °C. The carbon content of fly ash in two cases is relatively higher. The main reason is that the lower combustion temperature may result in low combustion rate. The carbon content of fly ash is higher in air combustion mode. In the experiment, the superficial

gas velocities are nearly the same due to the same overall gases volumes entering the riser in the two cases. So the difference in carbon content of fly ash is because coal char combustion rate is higher at high overall O2 concentration.

NO concentration in ppm unit in the flue gas is higher in oxy-fuel combustion than that in air combustion because of the higher coal feeding rate (about two times). But in mg/MJ unit, the NO emission is lower in oxy-fuel combustion than that in air combustion. The results are consistent with the former results of oxy-CFBC [5-6, 11, 13]. The main reason is that under O2/RFG combustion mode the recycled NO in flue gas is reduced by volatile and char from the coal in the bed by homogeneous and heterogeneous reactions, which is dominant and contributes to amount to 80% of the total reduction [15]. Another reason is that the replacement of N2 with CO2 reduces the fuel nitrogen conversion ratio [15]. In the literature the conversion ration from the fuel-N to exhausted NO is reduced more than two-third or three-fourth of that with air combustion at O2/RFG pulverized coal combustion mode [14-16]. However, the reduction ratio is less than one-tenth in this oxy-CFBC, which is possibly because of the low combustion temperature and less recycled NO.

For the accumulation of SO2 in the riser due to flue gas recirculation, SO2 concentration in the flue gas increases significantly under oxy-fuel combustion mode compared to air combustion mode. Previous experiences in the field of CFB under air combustion mode indicate that without limestone addition conversion ratios of combustible sulfur to SO2 are relatively high [6]. However, in this test that conversion ratio is low under air combustion mode. It is suspected that S is bonded with Ca and Mg contained in fuel-ash (Table 2). Similar behavior between sulfur and fuel-ash components in the CFB environment is also reported by other authors [8]. In oxy-fuel combustion mode, CO2 concentration greatly increases to such an extent that at typical CFB operating conditions (~850 °C), and SO2 concentration in flue gas is also high due to flue gas recirculation, so the desulfurization of fuel-ash is weakened under this combustion mode [17]. A report by de Diego et al.[18-19] shows that, for oxy-fuel CFB combustion, the optimum temperature to achieve the higher sulfur retention was 900-925 °C, whereas operations using enriched air required optimum combustion temperatures of 850-870°C.

4. Conclusions

Oxy-fuel combustion experiments employing high oxygen concentrations with dry flue gas recirculation were successfully carried out in a 1MWth pilot-scale oxy-fuel CFBC unit at the IET/CAS. This test facility can be operated at air and total oxygen concentrations in the range of 21-50% with different oxygen level in primary and secondary gas. This paper presets initial commissioning experimental results under air and 50% O2/RFG combustion mode. The effects of atmosphere on combustion characteristics including temperature distribution profiles, combustion efficiency, and gaseous pollutant emission are investigated.

The results show that the operation of oxy-fuel CFBC at high overall O2 concentration was stable and reliable, and is able to produce flue gas highly concentrated in CO2. The transition from air combustion to oxy-fuel combustion occurs easily and smoothly. Under 50% O2/RFG combustion mode, with external heat exchanger and bayonet-type water cooling tubes to cool the return solids, the recirculation flue gas ratio is much lower compared to pulverized coal oxy-fuel units.

In 1MWth pilot-scale oxy-fuel CFBC, the temperature level in the bed and the overall gases volumes entering the riser are similar both in the air and 50% O2/RFG combustion mode. Under 50% O2/RFG combustion mode, gaseous pollutant emissions in mg/MJ unit such as CO and NO are lower than that achieved by air combustion. Furthermore, the carbon content of fly ash is relatively low. However, SO2 concentration significantly increases with levels up to three times higher compared to air combustion mode, which may produce considerably higher corrosion concerns, so more work is needed to address the issue of sulfur capture in oxy-fuel CFBC.

Acknowledgements

The authors gratefully acknowledge the support by the "Strategic Priority Research Program" Demonstration of Key Technologies for Clean and Efficient Utilization of Low-rank Coal, Grant XDA07030200) and the External Cooperation Program of the Bureau of International Cooperation (BIC), Chinese Academy of Sciences (GrantGJHZ201301).

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