Article pubs.acs.org/EF
Investigation of the Initial Stage of Ash Deposition during Oxy-Fuel Combustion in a Bench-Scale Fluidized Bed Combustor with Limestone Addition Hui Wang,* Zhimin Zheng, Shuai Guo, Yongtie Cai, Li Yang, and Shaohua Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, China ABSTRACT: The technology of oxy-fuel combustion in a circulating fluidized bed boiler is one of the advanced technologies for carbon capture and storage; however, operating problems related to ash deposition are worth investigating. When limestone is added as a sorbent during oxy-fuel combustion, excess CaO particles in the fly ash deposit on heating surfaces and react with high concentrations of CO2 in the flue gas. These carbonation reactions lead to structural change and formation of bonded deposits. Therefore, deposition experiments with limestone were carried out in a bench-scale fluidized bed under oxy-fuel combustion conditions to evaluate deposition propensity and compositions of ash deposits formed under different experimental conditions. Effects of the molar ratio Ca/S, probe surface temperature, and combustion atmosphere on deposition behavior were evaluated. Experimental results showed that during the initial stage of deposition, the degree of carbonation increases, with increasing molar ratios of Ca/S; although this is not significant for the total deposition process. Deposits were loose for 1 h, following deposition. Raising the surface temperature of the probe reduced the deposition rate of the fly ash, since this was strongly affected by thermophoresis. Except for elements K, Na, and S, there were no significant changes for other chemical components in the ash deposit under the varied conditions. There were clear differences in the deposition rates of fly ash for oxy-fuel and air combustion cases, which were probably caused by differences in the ash formation mechanism for the case of high O2 concentration, and limestone addition.
1. INTRODUCTION Oxy-fuel combustion (O2/CO2) is one of the advanced technologies in which fuel is burnt in a mixture of air-separated O2 and CO2. One of its significant characteristics is that concentrations of CO2 in flue gas can reach up to 95%, after dehydration. In this way, CO2 can be directly compressed and processed, without any additional separation process. Compared with traditional air combustion, with concentrations of CO2 of about 15% in flue gas, it is a much more efficient way of sequestering CO2. At present, research has focused on developing both pulverized coal (PC) boilers and circulating fluidized bed (CFB) boilers. Compared with oxy-fuel PC combustion,1,2 oxy-fuel combustion in a CFB boiler has particular advantages: e.g., bed temperature can be controlled by adjusting the amount of circulating ash such that higher O2 concentrations can be achieved without risk of drastically increasing the furnace temperature. Other advantageous features include fuel flexibility, an optimum combustion temperature for limestone desulfurization, and low NOx emission owing to low combustion temperature. Hence, it has received worldwide attention.3,4 Fly ash deposition in power plant boilers with conventional air combustion can cause a series of problems, including fouling, slag formation, and corrosion, which plague power plant operations.5,6 Thus, studies of ash deposition during the combustion process are important. At present, few studies look at this issue in oxy-fuel combustion. Fryda et al.7 recently studied ash formation and deposition behavior of coal and coal/ biomass blends under oxy-fuel combustion using a laboratoryscale pulverized coal combustor (drop tube). Their results showed that chemical compositions of fly ash and ash deposits © 2014 American Chemical Society
in 30% O2/70% CO2 combustion conditions were not significantly different from those found in air combustion. However, higher deposition rate and deposition propensity (DP) were observed than in the air-firing case. They concluded that the observed increase in ash deposition under oxy-firing was caused by the changed physical properties of the flue gas and the gas flow field. Besides these factors, the larger particle size distribution, as well as the change in the density and porosity of the ash in oxy-fuel combustion may be contributing factors.8 Yu et al.9 investigated ash and deposit formation during oxy-fuel combustion in a down-fired combustor with a capacity of 100 kW, using three different types of coal. They concluded there were enhanced ash deposition rates based on the thickness of the ash deposit layer. These authors suggested that ash deposition behavior under 32% O2/68% CO2 oxy-fuel combustion conditions was largely determined by the lower gas flow rate, higher combustion temperature, and chemical changes to the flue gas. Hence, a number of recent studies indicate that oxy-fired deposition rates are higher than those for air-firing condition. Contrary results were obtained by Li et al.;10 they reported that the smaller Stokes number, and slightly smaller particles developed in 30% O2/70% CO2 combustion, led to a lower ash deposition rate. They attributed their results to the use of a self-sustained coal flame system, with kilogram per hour mass feed rates. Finally, results from our earlier experiments on ash deposition during CFB oxy-fuel combustion showed that DP was higher under oxy-fuel combustion Received: January 26, 2014 Revised: May 7, 2014 Published: May 7, 2014 3623
dx.doi.org/10.1021/ef500254n | Energy Fuels 2014, 28, 3623−3631
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Article
Table 1. Proximate and Ultimate Analyses of the Coal Sample Used in This Study from Jincheng Coal Mine proximate analysis(%)
ultimate analysis(%)
heating value
Mar
Var
Aar
FCar
Car
Har
Nar
Sar
Oar
Qnet,ar(MJ/kg)
4
6.72
30.24
59.04
59.35
2.56
0.72
1.84
1.29
20.48
Table 2. Chemical Composition of the Coal-Derived Ash compd
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
Fe2O3
wt %
0.73
0.17
36.04
44.23
0.23
2.67
1.04
2.99
0.69
5.27
than under air combustion at similar flue gas velocity. This may be because of greater particle size distribution and greater numbers of fine particles of fly ash produced under oxy-fuel conditions.11 When limestone is added to CFB as a sorbent during oxyfuel combustion, carbonation of this fly ash could develop into a serious problem.4,12−15 For the 30% O2 oxy-fuel combustion case, high concentrations of CO2 inhibit the calcinations of limestone, and indirect sulfate reactions only occur when the furnace temperature is higher than 870 °C.16 The reactions are as follows: CaCO3 → CaO + CO2
(1)
CaO + SO2 + 1 2 O2 → CaCO4
(2)
combustion atmosphere on ash deposition behavior were evaluated. We focused on the initial stage of ash deposition for two main reasons. First, it is the initial stage of ash deposition that is important for deposit formation and growth,22,23 which also cause corrosion of tubes.24 Second, even under strong soot-blowing action, the ash blown away mainly consists of the loose ash deposits on the surface, rather than the ash deposits formed during the initial stage; these initial deposits are close to the bottom of the tube and are not likely to be completely blown away owing to their high degree of stickiness.25,26 These initial deposits can easily be transformed into hard bonded deposits through carbonation reactions owing to their long residence times in the tube.
2. EXPERIMENTAL SECTION 2.1. Fuel, Limestone, and Bed Material. Samples of anthracite coal from the Jincheng Coal Mine in China were used in this study; results of proximate and ultimate analyses of this coal are listed in Table 1, while the chemical composition of its ash is given in Table 2.The samples were predried in air before being combusted. The particle sizes of the sieved coal ranged from 0 to 2.36 mm. Limestone from Shou County in China was used as sorbent for capturing SO2, with a particle size range of 0−1.18 mm and average diameter of 564 μm. The particle size distributions (PSDs) of the coal and limestone material are shown in Figure 1. The chemical composition of the limestone is given in Table 3. Quartz sand with a particle size range of 0.18−0.55 mm was used as the bed material. 2.2. Experimental Setup and Operating Conditions for the Fluidized Bed Combustor. Fly ash deposition experiments were conducted in the bench-scale FBC, as shown
The molar ratio of Ca/S is generally 2−3, so a certain amount of excess free CaO will be present in the fly ash (5− 30%).17 When water vapor is present in the flue gas, the following reactions can occur:14 CaO + H 2O → Ca(OH)2
(3)
Ca(OH)2 + CO2 → CaCO3 + H 2O
(4)
The carbonation of fly ash was studied by Wang et al.;14 they showed that over the temperature range of 500−800 °C, higher temperatures accelerated the carbonation of CaO, and the high concentrations of CO2 in the flue gas also facilitated these reactions. This work identified the main factors that influence the carbonation of CaO in oxy-fuel combustion. However, taking the process of ash deposition into consideration, the conditions influencing carbonation reactions are likely to be more complicated. Under air-firing conditions, with about 15% CO2 in flue gas, agglomeration and fouling of heating surfaces occur owing to carbonation and sulfation.18−20 Previous research results also indicated that the ash deposition rate largely depended on the content of free CaO in the fly ash.21 Under oxy-fuel firing, with up to 85% CO2 in the flue gas, potential problems related to carbonation reactions are yet to be evaluated, although there is evidence for concern. ASTOM Energy15 confirmed that the phenomenon of carbonation occurred during a pilot-scale test with CFB oxy-fuel combustion. Beisheim et al.12 found that, under oxy-fuel conditions, strong recarbonation reactions of calcined particles on cooled surfaces produced hard and stable carbonate deposits. In this study, combustion and deposition experiments were conducted in a bench-scale fluidized bed combustor (FBC), with a temperature-controlled ash deposit probe to substitute for the heat exchanger surfaces. We investigated the deposition behavior of fly ash under oxy-fuel combustion conditions, with the addition of limestone as a sorbent. The effects of the molar ratio of Ca/S, the surface temperature of the probe, and the
Figure 1. Particle size distributions for the coal and limestone materials used in this study. 3624
dx.doi.org/10.1021/ef500254n | Energy Fuels 2014, 28, 3623−3631
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550 °C. The convective section is long enough to simulate an industrial convection flue gas pass; its temperature is held at 800 °C by two electric heaters, such that the fly ash meets the experimental requirements of this study. Flows of O2 and CO2 into the setup are individually controlled by a mass-flow control system; these gases are subsequently mixed in a static mixer before entering the furnace as fluidized gas. The combustor is equipped with a screw feeder with a steady coal-feed rate of 5− 20 g/min. To preclude the influence of flue gas velocity on fly ash deposition rate conditions, the fluidization velocity was kept constant. Given the hardness of the limestone, too high a Ca/S ratio and coal-feed rate would cause damage to the screw feeder; hence, the feed rate was kept between 7.5 and 13.6 g/ min. To ensure similar O2 concentrations at the outlet, the excess air ratio for different air and oxy-fuel combustion cases was set to1.4 and 1.28, respectively. In all experiments, probe exposure time to the flue gas was 1 h, and the ash deposit thickness was around 2 mm. Conditions used for different experiments are given in Table 4.
Table 3. Chemical Composition of the Limestone Used as Sorbent in This Study compd
CaCO3
MgCO3
Al2O3
Fe2O3
SiO2
others
wt %
91.11
3.02
0.73
0.68
2.5
2.38
in Figure 2a. The main body of the combustor consists of a preheating section, a furnace section (containing both dense phase and dilute phase sections), and a convective section, which is connected to a cyclone and a filter bag. The total height of the combustor is 3500 mm, which includes the preheating section of 385 mm, the dense section of 315 mm, and the dilute phase section of 2800 mm. The inner diameters of the dense section and the dilute section are 51 and 83 mm, respectively; between these sections there is a transition cone section with a slope of 11°. A cyclone is connected to the outlet of the combustor to capture coarse ash; this is followed by a convective section, 850 mm in length with a 56 mm inner diameter, with three sampling ports to measure flue gas, fly ash, and ash deposits, respectively. The setup incorporates a hightemperature filter bag, 150 mm in length with an 80 mm outer diameter, used to collect fly ash. The main body of the combustor is equipped with several electric heaters in different sections. The heater in the preheating section adjusts temperatures to the required furnace temperature. The temperature of the dense phase section is maintained at about 900 °C by an electric heater; while the temperature in the dilute phase section is held around 900 °C by four electric heaters. Gaps between these electric heaters cause deviations of 50−100 °C between the connection position and the central part of the dilute phase section. To reduce heat loss and prevent massive reductions in flue gas temperature, heat insulation materials cover the cyclone and the connection tube to the convective section. The temperature in the cyclone is
Table 4. Experimental Conditions Used in This Study
a
attributes
values
combustion atmosphere combustion temperature flue gas temperature coal feed rate excess air coefficients Ca/S ratios o.d.a and length of probe probe surface temperatures exposure time of probe
air and oxy-fuel (30% O2/70% CO2) ∼900 °C ∼800 °C 7.5−13.6g/min 1.28 and 1.4 0, 1.5, 2.5, 3.5, and 5 22 mm and 50 mm 460, 560, and 700 °C 1h
Note: o.d. = outer diameter.
Figure 2. Experimental setup: (a) Schematic of the bench-scale fluidized bed combustor and ash deposit sampling system; (b) ash deposit sampling system; (c) temperature control curve for the probe (probe surface temperature, 560 °C; exposure time, 1 h). 3625
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2.3. Ash Sampling and Analysis. Ash deposits were collected with an ash deposition probe, 50 mm in length with a 22 mm outer diameter. There is a hole near the surface of the probe, 10 mm in depth and 1 mm in diameter, whereas a Ktype thermocouple was mounted to test the surface temperature of the probe. The surface temperature of the probe can be set within the range of 200−700 °C, with a deviation of 2 °C. The ash deposit sampling system is shown in Figure 2b. Since sampling time was 1 h, the influence of unsteady experimental conditions was avoided by adding a telescopic sleeve to protect the probe. The sleeve, 100 mm in length with a 38 mm outer diameter, could be moved by pushing or pulling a ring. When combustion conditions reached a stable level, the protective cover was removed, and the probe was directly exposed to the flue gas; the preset probe surface temperature was reached rapidly. The control temperature curve of the probe, as a function of time, is shown in Figure 2c. The coarse ash collected in the cyclone behind the furnace exit, the fine ash collected in the high-temperature filter bag, and the fly ash passing through the probe were all analyzed in this study. The flue gas was analyzed by Fourier transform infrared spectroscopy (FTIR), so concentrations of O2, CO, CO2, SO2, NOx, and H2O were measured online to ensure the stability of experiments. Each experimental condition was repeated at least 3 times to ensure the accuracy of the deposition rate of the fly ash. The three ash deposit samples with smallest deviations were selected for analysis, using inductively coupled plasma− atomic emission spectrometry (ICP-AES). The extent of the carbonation reaction in ash deposits was determined using thermal gravimetric analysis (TGA), and the micromorphology of ash deposits was observed with a scanning electron microscope (SEM). 2.4. Data Processing. Two parameters were used to quantify fly ash deposition: deposition rate (DR) and DP, as subsequently defined. Fluidized gas velocity was kept constant to eliminate its effect on ash deposition. Thus, under similar fluidized gas velocity, different Ca/S ratios will result in different amounts of ash. To take into account this effect, formulas 5 and 6 were used for all experimental conditions. DR =
Figure 3. Deposition propensities for different Ca/S ratios with probe surface temperature of 560 °C under oxy-fuel combustion conditions.
coarse particles, containing Ca compounds, remain in the bed material at high Ca/S ratio and low superficial gas velocity (∼0.5m/s), while fine particles from the coal ash and limestone escape from the furnace to the convective section and are deposited on the air-cooled surface of the probe. According to conventional deposition models,27,28 viscosity can be used to determine whether a fly ash particle will deposit or rebound when it impacts on the heating surface. The lower the viscosity of the particle, the more likely it is to adhere to the surface of the ash deposit. Thus, our bulk ash was close to dry ash for very high viscosity at 800 °C. However, fine particles (10 μm) deposited on the cooled surface of the probe by inertial impaction, but fine particles (
