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A new fluidization–suspension combustion technology for coal water slurry
Chemical Engineering and Processing 49 (2010) 1017–1024
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
A new fluidization–suspension combustion technology for coal water slurry Hui Wang a,∗ , Xiumin Jiang b , Minxiao Zhang a , Yufeng Ma c , Hui Liu a , Shaohua Wu a a
School of Energy Science and Engineering, Harbin Institute of Technology, No. 92, West Da-Zhi Street, Harbin, Heilongjiang 150001, China School of Mechanical Engineering, Shanghai Jiao Tong University, Minhang District, Shanghai 200240, China c Shengli Power Plant, Shandong, Dongying 257087, China b
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 9 June 2010 Accepted 14 July 2010 Available online 21 July 2010 Keywords: Fluidization–suspension CWS Combustion technology Boiler Performance
a b s t r a c t Slagging is a major operating problem in application of the atomization–suspension combustion technology for burning coal water slurry (CWS) fuel in small and low height industrial boilers. The fluidization–suspension combustion is a new alternative for replacement of oil, which is capable of solving the slagging problems. In addition, it can be successfully applied to CWS-fired boilers with capacity smaller than 35 t/h. About 530,000 medium and small scale industrial boilers with low boiler efficiency in China provide the technology a very promising prospect. The principles and contents of CWS fluidization–suspension combustion technology are introduced in detail in this paper. And a new type of 14 MW fluidization–suspension CWS-fired boiler was developed, the performance of which showed that boiler efficiency was 91.53%. Emission of SO2 and NOx was 346.1 mg/m3 and 469.5 mg/m3 , respectively. From the application, the CWS-fired boiler showed good features such as high efficiency, low pollutant emission, good load regulation, good CWS quality adaptability, steady operation and convenient maintenance. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Coal water slurry (CWS) is a coal-based liquid fuel, which can be used as a substitute fuel in oil-fired boilers. Thus, the CWS technology has a great prospect in China to reduce China’s oil consumption. China has approximately 530,000 medium and small scale industrial boilers in operation, with the average capacity of 2.5 t/h and efficiency of only about 65%. Besides the low efficiency, the emissions such as particulates, SO2 and NOx from the small scale boilers are high. A considerable economical benefit and better environmental performance can be achieved if these boilers are retrofitted by efficient and clean coal technologies. The combustion technologies of CWS [1–4] have been studied extensively. The results reveal that the atomization–suspension combustion [5,6] has been successfully applied in large-scale boilers, in spite of the fact that it has a better economical performance to burn pulverized coal directly. But for the industrial boilers with capacities smaller than 35 t/h, most of the previous research was focused on the atomization combustion and seldom achieved high enough efficiency. It was reported that a 2.8 MW grate boiler [7] achieved an efficiency of 80% after retrofitted for burning CWS by pre-combustor and atomization combustion. A 4 t/h grate boiler [8] was retrofitted for firing fine-coal water slurry by multi-stage
∗ Corresponding author. Tel.: +86 451 86412318; fax: +86 451 86412528. E-mail address: wanghui [email protected] (H. Wang).
atomization for CWS feeding. The testing results showed that the maximum furnace temperatures were from 1400 ◦ C to 1500 ◦ C, and the boiler efficiency can be as high as 81%. A new developed 4.2 MW D-type boiler [9] for burning CWS with a colliding-type atomization nozzle achieved boiler efficiency of 88% and slagging occurred in the bottom of the furnace. A retrofitted 7 MW D-type boiler [10] to burn CWS equipped with a versatile burner which can fire both CWS and oil by atomization in a pre-combustor gained combustion efficiency over 96% and boiler efficiency of about 83%. A 10 t/h demonstrative fluidized bed boiler [11] with overbed feeding in specific shape, non-overflow and non-drainage achieved an average combustion efficiency of 95.4% and a boiler efficiency of 77.7%. From all those applications it may be concluded that the CWS fuel burned in small and low height industrial boilers did not get enough residence time to completely burn out, which leads to a low boiler efficiency. Besides, the high temperature in the small furnaces is hard to control, which will lead to local slagging inevitably and risk the operation stability. That is because the slagging problem is a dilemma for the atomization–suspension technology. Slagging happens or not depends on combustion temperature, nozzle aerodynamics and furnace structure. To make the boiler operate stable and reliable, about 1500–1600 ◦ C in the small furnace has to be assured. But the temperature is above the melting point of coal ash, which makes slagging happens easily. In addition, the auxiliary equipments such as atomization, slag-removal made the system complicated and had potential risk of the system. From the operation data of the CWS-fired atomization–suspension boilers
0255-2701/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.07.009
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Fig. 1. Flow chart of CWS fluidization–suspension combustion technology.
retrofitted from the oil-fired ones, only 75% of the design capacity pre-retrofitting can be obtained due to the limitation of furnace outlet temperature. Some small scale boilers fail to burn CWS fuel by atomization technology due to the small furnaces [12,13]. Furthermore, some detailed researches were conducted on the atomization combustion. Manfred et al. [14] reviewed the development of CWS as an alternative boiler fuel through a series tests in 10–20 MW boilers using atomization technology. Miccio et al. [15] carried out an experimental study on the influence of the type of CWS parent fuel and the velocity of injecting air. Rankin et al. [16] described the demonstration work of burning CWS in a boiler designed to burn oil. The Coal Research Establishment of England, Australian CSIRO and Japanese Sumitomo Company carried out combustion tests in fluidized bed boiler to burn CWS [7]. The 10 t/h fluidized bed boiler of the Sumitomo Company started running in 1980 and obtained boiler efficiency of about 65% [7]. Those early tests in small coal-capable front wall and tangentially fired utility boilers showed that two of major problems to be addressed are atomizer durability and poor carbon conversion. As a new type CWS combustion technology, fluidization– suspension is clean and efficient for burning CWS, solving the slagging problem and simplifying the system. In this paper, main principles and design concept of the technology are introduced in detail. Results of the utility tests in a 14 MW boiler are presented.
Fig. 2. Scheme of CWS granulating device.
tion of CWS agglomerates, and intensify the mixing and burnout performance. At the same time, thermal NOx production is effectively controlled and slagging in the furnace is avoided because of the low combustion temperature (850–950 ◦ C). The limestone can be fed with the bed material or mixed with CWS fuel to react with SO2 and reduce the emission. The temperature in the separating device happens to be the optimal temperature of desulfurization, so the objects of high efficiency and low pollution can be simultaneously realized. The technology is characterized by following features: (1) It uses a granulating device to feed the CWS in the shape of drops with diameter of 4–10 mm, and eliminates the need of atomization and filtering systems in atomization–suspension technology. (2) It uses an internal circulating combustion gas–solid separator set inside furnace to separate the large particles and agglomerates leading to a high combustion efficiency. (3) It can solve the slagging problems in atomization–suspension technology. 2.2. Components in fluidization–suspension combustion technology
2. Fluidization–suspension combustion technology of CWS 2.1. Principles of CWS fluidization–suspension combustion technology Fluidization–suspension combustion technology of CWS is a new type technology combining both fluidized bed combustion and suspension combustion. The flow chart [17] of a boiler system using the technology is shown in Fig. 1. The system includes CWS storage and feeding system, ignition system and dust separating system. Comparing with CWS atomization–suspension combustion technology, the airatomization (or steam-atomization), CWS filtering, high-pressure feeding and slag-removal are not needed, so that the total system is simplified and the reliability gets improved. The principles are as follows: high density solid particles are selected to be the bed material. The CWS fuel is delivered to granulating device located on the top (or on the front wall) of the furnace and is fed at about 0.4 MPa onto the bed surface with temperatures of 850–950 ◦ C. The CWS droplets experience drying stage and the volatile matters release and burn [18]. Under the state of fluidization, CWS particles will either be fragmented or form agglomerates [19] in the dense bed. The small particles will be carried upward to the dilute section to finish suspension combustion. The flue gas enters a separating device inside the furnace and the separated fines reenter the bed and burnout. The bed material and large CWS agglomerates carried with the flue gas will be separated and returned to dense section via its feedback channel, which can decrease the loss of bed material, enhance the complete combus-
2.2.1. CWS granulation-feeding system Because of better combustion conditions for CWS in fluidization–suspension combustion boiler, diameter of CWS drops could be a little bigger without the risk of decreasing the combustion performance. Thus, the expensive high-pressure atomization devices in the atomization–suspension technology, which are easy to block [20], are not necessary. To guarantee better fluidization combustion characteristics, the particles formed by CWS drops must meet a specific size distribution. So feeding CWS fuel steadily and reliably into furnace are critical in the fluidization–suspension combustion technology. The granulating device is developed to meet the demands, which is shown in Fig. 2 and composed of cooling air tube, flame detector, CWS inlet tube, dredge hole with seal bolt, sweeping and washing tube, and CWS granulating tube. The sweeping and washing tube (connected with water or compressed air) is used to clean the granulating tube when stop feeding. The dredge hole on the top of the granulating tube is used to clear blocks that cannot be swept or washed, which will be sealed when operating. And the granulating tube is offset placed in the cooling air tube (or a cooling air box) to make the air control CWS flow and avoid solidifying and blocking, since the offset assembled structure can form velocity difference of cooling air in the eccentric annulus, which benefits for granulating the CWS flow into acceptable size distribution. The flame detector is mounted to observe the state of combustion and CWS feeding when operating. The features of the device are as follows: (1) The simple structure with no moving parts can make it work reliably and easy to
H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024
maintain. (2) The CWS feeding stability and uniformity without blocking can be guaranteed, and the landing position and flow flux of CWS could be controlled easily. (3) The low air consumption and air pressure requires no special air source. (4) The formed 4–10 mm CWS particles meet the demands of the fluidization–suspension combustion, in particular the relatively big drop diameter lowers requirements [21] for CWS and need no filtering equipments. (5) It has good flexibility to different fuel types and can be used to feed many kinds of slurry fuels. 2.2.2. Technology of continuously running without slag-removal In the conventional fluidized beds, keeping a relatively steady bed material height during the operation is very important. For the fixed air flow, an excessive thick bed material layer will make fluidization difficult, maybe even worse to form stack and slag, and an extra thin layer will add possibility of the air short-passing the layer and the temperature falling, even lead to extinguish. So the frequent slag-removal is necessary in order to keep a steady bed material layer. But for fluidization–suspension combustion of CWS, it leaves little ash in the bed and the slagging is avoided, therefore it needs no slag-removal in the fluidization–suspension combustion process of CWS. In addition, the dewatered and dried CWS spheres will float and burn on the upper bed material layer because of their lower density comparing with quartz sand. In short, the merits of technology of continuously running without slagremoval are as follows: (1) Losses of incomplete combustion and slag-removal are decreased. (2) The make up requirement of bed material is decreased. In the technology, the ash left in the furnace is of small amount and cannot make up for the bed material loss, so the bed material has to be made up through adding quartz sand. Accordingly, some man-power or feeding device is needed. Without slag-removal, the bed material loss discharged with slag is avoided, then the make up requirement is lowered, so the operating cost and complexity of combustion devices is decreased. 2.2.3. Internal circulating combustion technology in the furnace Because of the wide size distribution of bed material, some fine particles will be carried out from the furnace with the flue gas. If nothing is done, small particles will be carried to convective heat transfer surfaces. This may: (1) increase the mechanical incomplete combustion loss because some fine fuel particles are carried out together, (2) increase the complement quantity of bed material, (3) increase the particle concentration in the flue and speed up the attrition of convective heating surfaces. For these reasons, an internal circulating combustion gas–solid separator is set inside the furnace of the CWS fluidization– suspension combustion boiler. The big particles in the flue will be separated and returned to the furnace to burn again, and in the internal separating device the combustion happens, too. In addition, the overall boiler volume changes little as adding separator in the furnace, so it can be used to retrofit some oil-fired boilers. The device named high temperature internal circulating combined vortex separator [22] is shown in Fig. 3. It is horizontal and composed of several groups of vortex separating cells (two cells form one group). For each group, the first cell is a coarse powder separator, which has one flue entrance and two feedback outlets: the flue entrance is up on one side, one feedback outlet is on the center of the opposite side and the other one on the center of the bottom. The second cell has one flue outlet and a feedback outlet: the flue outlet is up on the opposite side of the flue entrance, the feedback outlet is on the center of the bottom wall. In the group between the first and the second cells, a central passage is formed by an annular baffle plate, which is a plate ring of specific width fixed on the inner circle. The vortex separator is placed at the upper part of the dilute section and connected to the furnace outlet. The flue carrying ash and some bed material particles goes
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Table 1 Design parameters of coal water slurry boiler. No.
Name
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Rated output Circulating water flux Inlet water temperature Outlet water temperature Discharge flue temperature Boiler efficiency Fuel consumption Boiler heating surface Air flux of induced fan Air pressure of induced fan Air flux of blower Air pressure of blower Flux of circulating pump Head of circulating pump
Units
Value
MW kg/h C ◦ C ◦ C % kg/h m2 m3 /h Pa m3 /h Pa m3 /h m
14 266,667 70 115 150 89 3055 652.884 58,000 3300 24,000 14,000 630 50
◦
upward to dilute section from the dense section, and tangentially enter the first cell of the vortex separator at a specific velocity and form a high-speed rotating vortex flow. The solid particles in the flue will be concentrated to the wall and trapped by the feedback outlets under the effect of centrifugal force, and the high temperature flue goes into the second cell via the hollow baffle plate to continue its high-speed vortex flow and separate solid particles further. The separated solid particles trapped by the feedback outlets are returned to dense section to finish circulating combustion and make up for the loss of bed materials. The vortex separator can obtain a separating efficiency of over 90% for particles down to 0.25 mm and has many merits such as low resistance pressure, attrition resistance, small volume, simple structure and easy installation, low requirements of manufacture, big flow flux for flue separating and perfect separating performance, easy to be enlarged and combined according to the capacity of boiler, etc. 2.2.4. High efficiency and low cost technology of desulfurization and reduction of NOx The processes of desulfurization and reduction of NOx can be achieved at low cost through directly adding sorbents such as limestone into furnace or mix them into CWS fuel and staged-air arrangement [23], which is a general feature of CFB, too. For the fluidization–suspension technology, the internal circulating combustion technology can significantly increase the residence time of the desulfurization agent in the furnace and improve its utilization ratio and reduce the operating cost. 3. The 14 MW vertical CWS fluidization–suspension boiler 3.1. Design data of boiler The main design parameters are shown in Table 1. 3.2. Design fuel The design fuel is CWS made of Datong bituminous coal. The detailed analytical data of design CWS and CWS in tests are given in Table 2 and size distribution of coal powder in it is shown in Fig. 4. 3.3. Design principles 3.3.1. Selection of bed material The selection of density and size distribution of inert bed material will directly affect the fluidization quality of CWS fluidization–suspension combustion boiler, further the characteristics of combustion and heat transfer. Considering the features of different density fluidized bed and power consumption of fans, the
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Fig. 3. Structure of high temperature combined vortex separator.
quartz sand with density of 2300 kg/m3 [24,25] is selected. The size distribution of fresh bed material is shown in Table 3 and the mass average diameter is 2.52 mm. 3.3.2. Selection of fluidization air velocity and cross-section heat load Based on the density and size distribution of the bed material and parameters of air, the critical fluidization velocity can be calculated using empirical equation. The actual fluidization velocity used in operating boiler is 6.47 m/s. From the features of CWS fluidization–suspension combustion technology, the designed cross-section heat load is 1106.53 kW/m2 and the volume heat load is 237.96 kW/m3 . 3.3.3. Heat balance and combustion process Combustion process of CWS drops and coal is quite different because CWS needs big latent heat of vaporization and carries out agglomeration combustion. The latent heat of vaporization accounts for more than 47% total ignition heat of the CWS fuel. And the evaporation time is about 0.05–0.1% total residence time in the dense section [26]. The porosity formed by evaporation is 0.59 when the mass fraction of coal in CWS is 50% [2]. When the fed CWS cannot mix with bed material in time, big agglomerates are possible to be formed. Sometimes the burned chars will stick together, too. By different-density combustion technology, the agglomerates
are kept floating and burning on the upper layer in the bed. The agglomerates will break into fragments under the effect of bed material. There were not any clusters found in the dilute section thus far. The CWS fuel can still obtain good combustion efficiency so long as the agglomerate diameter does not exceed 25 mm [27]. After the evaporation finishes, the volatile will start to release and burn immediately to form a flame surrounding the CWS spheres because of the high temperature in the bed material. The combustion processes of volatile and carbon have some overlap. For a single CWS particle, when the combustion of volatile has finished 90%, the carbon has already burned about 10% [26]. When the evaporation and volatile releasing nearly ends, the carbon will form a sphere with certain strength. From the data of a strong coking fine-coal slurry [26], the compressive strength of a 6 mm sphere is about 2 MPa and the strength of the same size sphere of another weak coking coal slurry is about 0.5 MPa. For the core of the CWS sphere, the strength changes little until it is nearly burned out. The burning behavior of CWS spheres in the fluidized bed is a layer-by-layer, inward flaking-off combustion [19]. With the fluidization of bed material and CWS fuel, relatively small particles formed by fragmentation or attrition and small agglomerates are carried into the suspension space to continue combustion. At the same time, the incomplete burned volatile matters and combustible component of particles separated from the combined vortex separator will go up to burn in the suspension space from the dense section, too. 3.3.4. Selection and control of working temperature in the furnace The furnace temperature is an important parameter in boiler designing. It has extremely important influence on the processes of steady and high efficiency combustion, slagging avoidance, desulfurization and removal of NOx . The operating temperature is determined to be 850–950 ◦ C by comprehensive analysis of those influences. 3.3.5. Determination of circulating ratio and separating efficiency Circulating ratio is of great importance for the circulating fluidization mode, which affects combustion, heat transfer and attrition of boiler and fans power a lot. For a CWS fluidization– suspension combustion boiler with the temperature of dense section is 850 ◦ C, a relatively low circulating ratio is selected to maximize the combustion efficiency and minimize the power consumption. The circulating ratio is mainly achieved by the regulation of separator, so the separator is designed carefully to assure a separating efficiency as high as possible, primarily the operating stability and security are guaranteed.
Fig. 4. Size distribution of coal powder in CWS.
Table 2 Proximate and ultimate analyses of design CWS and CWS in tests. Proximate analysis Mar (%) Design CWS CWS in tests
32.9 35.40
Ultimate analysis
Aar (%)
Var (%)
5.64 7.08
30.96 36.94
Qar,net (kJ/kg) 18,877 17,700
Car (%) 50.57 47.43
Har (%)
Oar (%)
Nar (%)
Sar (%)
3.27 2.93
6.13 6.02
0.93 0.80
0.56 0.34
H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024 Table 3 Size distribution of bed material. No.
Aperture of screen (mm)
Average diameter of particle (mm)
Percentage on mass basis (%)
1 2 3 4 5
0.71–1 1–2 2–3 3–4 4–5
0.855 1.5 2.5 3.5 4.5
0.15 12.2 73.2 14.3 0.15
3.3.6. Preventive means of internal attrition in the furnace In circulating fluidized bed boiler, the attrition to internal heating surfaces is a very important problem for the high concentration of fuel or bed material. Measures are taken to consider design parameters, material and local strengthening comprehensively. Attrition-preventive fins and bricks or casting materials were used in the dense section where the flue velocity was very high. Because the attrition of convective heating surface is proportional to the particle concentration in the flue and proportional to 3.6 times of the flue velocity, the flue velocity turns to be more critical, so a relatively low velocity is selected in the design. In addition, the upward flowing flue is opposite to the gravity of particles, which will reduce the influence of the particles and benefit for the attrition prevention. To make the security better, uniform distribution plate and attrition-preventive capping plate are used in the flue segregation area. An air cushion backflow area covered with attrition-resistance casting material is set up in turning chamber of upper furnace to decrease the collision momentum of particles. And in the horizontal internal gas–solid separator, inner lining made of attrition-resistance material is coated to increase resistance of attrition. 3.4. Whole structure of the 14 MW CWS fluidization–suspension boiler A 14 MW CWS fluidization–suspension boiler is used for community heating in winter, whose general arrangement diagram is shown in Fig. 5. And its features are as follows: The vertical boiler has two horizontal drums and its water circulation is combined by controlled circulation and natural circulation. The heat load of the boiler is 14 MW and the pressure of outlet water is 1.0 MPa (gauge pressure). The temperatures of outlet water, inlet water and cold air are respectively 115 ◦ C, 70 ◦ C and 20 ◦ C. The design fuel is CWS. The outside dimensions of the boiler are 7860 mm × 4490 mm × 9660 mm (height × width × depth). The furnace is divided into dense and dilute sections. The height of dense section is 1350 mm with a trapezoid air distributor at the bottom. Some buried heating surface composed of Ф51 mm× 5 mm tubes with attrition-preventive rings are installed in the dense section. One group of the internal high temperature combined vortex separator is installed at the outlet of furnace to achieve the circulating combustion. All the air needed enters the furnace from two ways: one as primary air goes via funnel caps to bottom furnace to keep the bed material fluidizing; and the other one as secondary air goes via ejectors installed at the top of the dense section, entering the dilute section to reinforce burning completely and reduce the releasing of NOx . The CWS fuel is fed from front wall in drop shape into the dense section by the granulating device. The drops are heated by the fluidizing bed material and burn rapidly, then the hot flue carrying some bed material and fragments or agglomerates goes into the separator via flue outlet window. The big particles trapped are fed back to dense section again to achieve a high efficiency circulating combustion. The separated flue goes through the anti-slag tubes in the turning chamber and passes the
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convective tube groups located between the upper and bottom drums, then via the ash separator to the stack. From the heat calculation of the 14 MW fluidization–suspension boiler, the outlet temperature of the furnace is 950 ◦ C, then behind the anti-slag tubes is 869 ◦ C, then 209.5 ◦ C behind the convection tubes, and the exhaust gas temperature is 150 ◦ C. In order to make the boiler structure compact, decrease the radiation heat loss and increase the tightness, the boiler wall is selected to be light-duty type, i.e. from the inner wall, it is made in sequence of firebrick, plates of aluminum silicate fiber and berg meal bricks. A steel layer is used to cover the bricks from outer at last. Some material feeding inlets, manhole doors, peep holes and measuring points of temperature are set up in the front wall. An oil gun is installed in the air box under the distributor plate to achieve ignition under bed when startup. To ensure the safe and economical operation of the boiler, some thermal measuring instruments and automatic controlling devices are equipped to remote control the forced and induce fans and monitor parameters such as feeding water pressure, air box pressure, air box temperature, sub-atmospheric pressure, fluidized bed temperature, exhaust flue temperature, feeding water temperature, flow flux of feeding water, feeding quantity of CWS, outlet water temperature and oxygen concentration in the flue, etc.
4. Results and discussion In order to investigate the overall performance of the boiler, and analyze its technical and economic characteristics, experiments under the rated condition are carried out. Measurements include boiler capacity, boiler efficiency using input–output method and heat loss method, and the releasing characteristics of the flue, etc. The criterion used are standards of The People’s Republic of China: (1)“Test code for industrial boiler” (GB10180-94), (2)“Monitoring and testing method for energy saving of industrial boilers” (GB/T15317-94), (3)“General specification for industrial boilers” (ZB/T10099-1999), (4)“Measurement method of smoke and dust emission from boilers” (GB5468-91) and (5)“The determination of particulates and sampling methods of gaseous pollutants from exhaust gas of stationary source” (GB/T16157-96), etc. The test results show that every technical and economic indicator reaches the expected design requirements and national standards, and the boiler efficiency has reached the international advanced level.
4.1. Efficiency of the CWS fluidization–suspension combustion boiler The objective of tests is to evaluate the overall heat engineering performance of CWS fluidization–suspension combustion boiler. Two tests under rated condition were carried out. The instruments used are shown in Table 4, the fuel used in the tests can be seen in Table 2, the data of heat engineering performance and the boiler efficiency based on input–output method and heat loss method are shown in Tables 5–7, respectively. From the tables, the main characteristics can be seen: (1) Characteristics of boiler capacity. The required boiler capacity is realized and a relatively high boiler efficiency of 91.53% is achieved. The improvement of boiler efficiency is significant [17] comparing with the conventional industrial boilers [8–11], even with boilers retrofitted with atomization–suspension technology [7], which is about 80%. The CWS granulating device is reliable and the operation of the boiler is steady without any slag formed in the furnace [17].
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Fig. 5. 14 MW vertical CWS fluidization–suspension combustion boiler.
Table 4 Measuring instruments used for boiler tests. Measuring items
Instrument
Specification
Heat value of fuel Fuel consumption Carbon content in fly ash Flux of circulating water Inlet water temperature Outlet water temperature Discharge flue temperature Analysis of flue contents Temperature of boiler surface
Jingying automatic calorimeter Platform scale Muffle furnace Ultrasonic flowmeter Standard glass thermometer Standard glass thermometer Combustion efficiency analyser Combustion efficiency analyser Far infrared thermoscope
SDACM3000 – – FBL – – KM9101 KM9101 IRT-1200
Precision 0.01% 0.02 kg 0.1% 0.2% 0.1 ◦ C 0.1 ◦ C 1 ◦C 1% 0.1 ◦ C
Table 5 Data obtained in boiler heat engineering performance test. Test
Capacity (MW)
Heat efficiency of positive balance (%)
Heat efficiency of inverse balance (%)
1 14.067 91.214 88.1698 2 14.231 91.835 88.5464 Average capacity: 14.149 MW; average heat efficiency: 91.53%
(2) Combustion characteristics of CWS. High utilization ratio of CWS has been obtained in the fluidization–suspension combustion boiler. In the two experiments under rated condition, the combustible matter content in the ash are 3.32% and 3.33%, respectively, and the mechanical incomplete combustion loss are 0.4614% and 0.4528%, respectively. The results of so low the combustion losses verified not only the fluidization–suspension combustion in the furnace could obtain high combustion efficiency, but also the internal high
Discharge flue temperature ( ◦ C)
Excess air ratio at flue exit
Content of combustible matters in fly ash (%)
170.0 165.0
1.5417 1.5307
3.32 3.33
temperature vortex separator could improve the combustion conditions of CWS and make it burn completely. The CO concentration in the exhaust flue are 0.0010% and 0.0011%, RO2 (i.e. CO2 and SO2 ) concentration are 11.6% and 11.7%, O2 concentration are 6.9% and 6.8%, and the chemical incomplete combustion loss q3 are 0.0058% and 0.0063%, respectively. The low concentration of O2 and CO and high concentration of RO2 in the exhaust flue shows gaseous combustible matters have obtained sufficient combustion because
Table 6 Data obtained in boiler efficiency test using input–output method. No.
Items
1 2 3 4 5 6 7 8
Circulating water rate of boiler Inlet water temperature of boiler Outlet water temperature of boiler Enthalpy of boiler inlet water Enthalpy of boiler outlet water Capacity of hot water boiler Fuel consumption rate Heat efficiency of positive balance
Units
Rated test 1
Rated test 2
kg/h C C kJ/kg kJ/kg MW kg/h %
379,150 70.1 102.0 293.49 427.05 14.067 3136.64 91.214
378,850 71.4 103.7 298.94 434.17 14.231 3151.83 91.835
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H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024
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Table 7 Data obtained in boiler efficiency test using heat loss method. No.
Items
Units
Rated test 1
Rated test 2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Combustible content in fly ash Ash ratio of fly ash to inlet total ash (mass percentage) Mechanical incomplete combustion heat loss RO2 in discharge flue O2 in discharge flue CO in discharge flue Excess air ratio at discharge flue Theoretical air volume RO2 volume Vapor volume in discharge flue Dry flue volume in discharge flue Discharge flue volume Chemical incomplete combustion heat loss Cold air temperature Discharge flue temperature Dry flue average specific heat at constant pressure in discharge flue Enthalpy of discharge flue Enthalpy of cold air Heat loss of discharge flue Dissipated heat loss Total heat loss Inverse balance efficiency of boiler
% % % % % % – N m3 /kg N m3 /kg N m3 /kg N m3 /kg N m3 /kg % ◦ C ◦ C kJ/(N m3 ◦ C) kJ/kg kJ/kg % % % %
3.32 100 0.4514 11.6 7.6 0.0010 1.5417 4.8038 0.8874 0.8834 7.2910 8.1744 0.0058 10 170.0 1.3314 1888.746 97.715 10.073 1.3 11.8302 88.1698
3.33 100 0.4528 11.7 7.5 0.0011 1.5307 4.8038 0.8874 0.8825 7.2382 8.1207 0.0063 10 165.0 1.3310 1820.752 97.018 9.6945 1.3 11.4536 88.5464
of their fully mixing with oxygen in the furnace of the fluidization–suspension boiler. (3) Heat loss of exhaust flue. In spite of the fact that high water content in CWS increases the quantity of exhaust flue, the boiler can still achieve good combustion and burnout performance of the CWS fuel under relatively low excess oxygen conditions in the furnace. So the excess air ratio needed is relatively small, which means the heat loss of exhaust flue q2 is not very high. The boiler efficiency is about 90%, which is relatively high because of the lower mechanical incomplete combustion loss and exhaust flue heat loss. Along with the better burning stability and slagging performance than that of atomization– suspension CWS boilers [17,28], the fluidization–suspension CWS boiler was proved to be successful. 4.2. Results and analysis of environmental performance of boiler Through the test of environmental performance of boiler, the design and operational quality of sections such as main boiler body, dust removal and desulfurization system are examined to obtain data for further improvement. The test is carried out only one time in the boiler equipped with an ordinary TLS water-bath duster. The results are shown in Table 8 and the analysis is as follows: (1) SO2 . From the “Emission standard of air pollutants for coalburning oil-burning gas-fired boiler (GB13271-2001)”, the national standards for SO2 emission is 1200 mg/m3 in I time period and 900 mg/m3 in II time period. The results of test without adding desulfurizing agents in the desulfurization system show that the SO2 concentration in the flue is only 346.1 mg/m3 , which is much lower than the national standards. The main reason should be the coal used in CWS is a low-sulfur washed coal and its ash is of weak alkaline which has self-desulfurizing effect in the process of low temperature combustion. Accordingly, the SO2 emission of a 2 t/h CWS-fired fluidized bed boiler is 518.21–647.79 mg/m3 [28], 730 mg/m3 for a 4 t/h boiler [29], 352–598 mg/m3 for a 65 t/h boiler [30], 473–503 mg/m3 for a 130 t/h boiler [31], 941–1070 mg/m3 for a 220 t/h boiler [32], 627.4 mg/m3 for a 230 t/h boiler [33]. (2) NOx . The concentration of NOx in the flue is 469.5 mg/m3 , which is a typical result for the technology. The CWS combustion at low temperature guarantees low concentration of thermal
NOx and staged air feeding controls the production of NOx , too. Accordingly, the NOx emission results from above boilers are 425–585 mg/m3 for the 65 t/h boiler [30], 632–721 mg/m3 for the 130 t/h boiler [31], 425–538 mg/m3 for the 220 t/h boiler [32], 495.1 mg/m3 for the 230 t/h boiler [33]. (3) Dust. Even though the dust separator used is of an ordinary TLS type, the dust concentration in the exhaust flue is very low, which is only 123.5 mg/m3 . It is mainly due to the CWS fuel has low ash content, which makes the inlet concentration of separator is only one third of that of a coal-fired boiler. In some other applications with bag-type dust collectors, the dust concentration measured is lower than 30 mg/m3 , which makes the fluidization–suspension combustion boiler very appropriate to be used in regions with stricter environmental requirements. Something has to be mentioned is that the dust concentration after the inducing fan is measured to be higher than that before the fan might be due to measurement error. The dust concentration of a 220 t/h CWS-fired boiler is 140–147 mg/m3 [32], a little higher than the 14 MW boiler. (4) Blackness of flue. Because the concentration of dust and combustible matters is very low, the Ringelmen blackness of the flue is certainly low and the value is 0–1 level.
Table 8 Comprehensive analysis table of air pollutant emission from boiler. No.
Items
Units
Value
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Flue temperature Dynamic pressure of flue Total pressure of flue Flow velocity of flue Flux of flue Moisture of flue Excess air ratio Dust concentration before inducing Dust concentration after inducing SO2 concentration NOx concentration CO concentration Oxygen content in flue Releasing rate of dust Releasing rate of SO2 Ringelmen blackness of flue
◦
86 80 −2000 13.3 39,780 6.5 2.23 99.69 (6% O2 ) 123.5 (6% O2 ) 346.1 (6% O2 ) 469.5 (6% O2 ) 25 (6% O2 ) 11.6 3.699 13.768 0–1
C Pa Pa m/s m3 /h % – mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 % kg/h kg/h Level
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In brief, the CWS fluidization–suspension combustion boiler has good environmental characteristics and the emission is lower than national standards for type II regions. The boilers will obtain better performance if the desulfurizing agents such as limestone are added. 4.3. Application of CWS fluidization–suspension combustion boiler Since 2002, the combustion technology of fluidization– suspension is used in more than 30 boilers and of more than 10 types. The capacity is concentrated in the range of 2.8–35 t/h, and one 75 t/h utility boiler was retrofitted to 60 t/h successfully. In Shengli Oil Field of Shandong province, there are more than 20 boilers in operation using the CWS fluidization–suspension combustion technology. The total capacity is 400 t/h and the biggest one is 60 t/h. In the single year of 2002, the burned CWS was 65,400 t, and 24 million Yuan was saved. In 4 years’ operation since 2002, the burned CWS was 297,000 t, and 135,000 t of oil was saved. The boilers were very successful which burns steadily and is easy to be operated. Among the 530,000 industrial boilers, about 76% of them could be considered to be retrofitted using the CWS fluidization–suspension combustion technology and it gives the technology a promising prospect. In addition, the internal vortex separating device used in the combustion technology is of great importance in achieving the performance, but it has no application in boilers bigger than 35 t/h, so careful design is needed to scale-up CWS-fired fluidization–suspension boilers. 5. Conclusion Using the fluidization–suspension combustion technology, the CWS fuel can be burned in the small and low height furnaces with high efficiency and good environmental performance. It solved the problems appeared in the combustion of CWS in boilers smaller than 35 t/h such as burning stability and slagging in local sections. An application of 14 MW boiler showed that it obtained good heat engineering performance and environmental performance for washed-coal CWS. The boiler efficiency is 91.53%, emission of SO2 and NOx are 346.1 mg/m3 and 469.5 mg/m3 , respectively. The boiler using the technology showed good features such as high efficiency, low pollutant emission, good load regulation, good CWS quality adaptability, steady operation and convenient maintenance. The combustion technology of CWS has been used successfully in more than 20 boilers and it will have a good prospect. References [1] E.T. Mchale, R.S. Scheffee, N.P. Rossmeissi, Combustion of coal/water slurry, Combustion and Flame 45 (1982) 121–135. [2] K.F. Cen, X.Y. Cao, M.J. Ni, et al., Experimental study on combustion of coal water slurry in fluidized beds, Journal of Engineering Thermophysics 4 (1983) 177–182. [3] R.K. Manfred, Coal–water slurry: a status report, Energy 11 (1986) 1157– 1162. [4] G. Papachristodoulou, O. Trass, Coal slurry fuel technology, Canadian Journal of Chemical Engineering 65 (1987) 177–201. [5] Z.X. Hu, F.M. Liu, G.H. Gong, Design of a 200 MW coal water slurry firing boiler, Power Engineering 26 (2006) 369–374.
[6] L. Wang, X. Zhao, X.Y. Cao, Z.Y. Huang, Heat transfer and emission characteristics tests of a 220 t/h oil-fired utility boiler retrofitted for firing coal water slurry, Journal of Engineering for Thermal Energy & Power 17 (2002) 589–591. [7] Y.G. Xie, C.M. Zhang, F.Y. Wang, X. Zhao, The combustion test and analysis of a 2.8 MW hot-water travelling-grate boiler retrofitted for firing coal–water slurry, Journal of Engineering for Thermal Energy & Power 19 (2004), pp. 309–311, 328. [8] Z.Y. Huang, W.G. Weng, J.Z. Liu, et al., Combustion experimental research of a 4 t/h fine-coal water slurry fired industrial boiler, Clean Coal Technology 5 (1999), pp. 36-39, 50. [9] Z.G. Zhang, Q. Tang, W.B. Zhu, Z.D. Cao, Development of 4.2 MW coal–water mixture (CWM) fired hot-water boiler, Industrial Boiler 3 (2004) 29–31. [10] X. Zhao, X.Y. Cao, Z.Y. Huang, et al., The coal–water fuel combustion test on the 7 MW oil-fired hot-water boiler, Clean Coal Technology 3 (1997) 38–41. [11] G.Q. Huang, M.J. Ni, J.H. Yang, et al., A 10 t/h demonstration boiler buring washery tailings, Journal of Engineering Thermophysics 8 (1987) 374–377. [12] X.P. Huang, Discussion on using coal–water slurry instead of fuel oil in boiler, Petrochemical Design 21 (2004) 65–67. [13] B.C. Zhao, L.J. Zhu, B.Q. Gu, The application for industrial heating boilers of a modification technology involving the conversion from oil firing to coal water mixture firing, Journal of Engineering for Thermal Energy & Power 19 (2004) 634–637. [14] R.K. Manfred, T.C. Derbidge, R. Perkins, Program of coal–water slurry development, Energy Progress 4 (1984) 85–88. [15] M. Miccio, U. Arena, L. Massimilla, et al., Combustion of fuel–water slurries injected in a fluidized bed, AIChE Journal 35 (1989) 2040–2042. [16] D.M. Rankin, H. Whaley, P.J. Read, D.J. Burnett, Performance of a compact oildesigned utility boiler when firing coal–water fuel, Journal of Engineering for Gas Turbines and Power, Transactions of the ASME 112 (1990) 28–30. [17] Y.F. Ma, Y.K. Qin, X.M. Jiang, Q.K. Wan, Application of water coal slurry fluidization–suspension combustion technology at Shengli Oil Field, Journal of Engineering for Thermal Energy & Power 21 (2006) 644–647. [18] H. Wang, X.M. Jiang, J.G. Liu, C.Q. Zhang, Analysis of the combustion parameter of coal water slurry at various heating velocities, Chemical Engineering 34 (2006) 25–28. [19] H. Wang, X.M. Jiang, J.G. Liu, W.G. Lin, Experiment and grey relational analysis of CWS spheres combustion in a fluidized bed, Energy & Fuels 21 (2007) 1924–1930. [20] J.Z. Liu, J.H. Zhou, Z.Y. Huang, et al., Study on technology of coal water slurry in a 65 t/h oil-fired boiler, Journal of China Coal Society 30 (2005) 773–777. [21] K. Sato, K. Shoji, K. Okiura, I. Akiyama, A. Baba, Effect of coal particle and spray droplet sizes on combustion characteristics of coal water mixtures, Powder Technology 54 (1988) 127–135. [22] X.M. Jiang, C. Yan, Z.J. Hao, Performance test of high temperature combined vortex separator in furnace of circulating fluidized bed boiler, Chemical Engineering 29 (2001) 37–40. [23] D.G. Ji, Z.N. Wang, X.H. Fu, X.W. Wang, Release characteristics of fuel nitrogen during coal water slurry suspension combustion, Journal of China University of Mining & Technology 35 (2006) 389–392. [24] H. Wang, X.M. Jiang, J.G. Liu, D.Q. Yuan, Attrition experiment and gray relational analysis of quartzite particles as medium material in fluidized bed, Journal of Chemical Industry and Engineering 57 (2006) 1133–1137. [25] J.G. Liu, X.M. Jiang, H. Wang, et al., Thermal behavior and surface microstructure of quartzite particles, Journal of Chemical Industry and Engineering 58 (2007) 765–770. [26] K.F. Cen, Q. Yao, X.Y. Cao, et al., Theory and Application of Combustion, Flow, Heat Transfer, Gasification of Coal Slurry, Press of Zhejiang University, Hangzhou, 1997. [27] M.J. Ni, G.Q. Huang, X.Y. Cao, et al., Effects of operation conditions to the operation process of the fluidized bed combustor burning washing coal sludge, Journal of Zhejiang University (Engineering Science) 19 (1985) 153–159. [28] H.H. Zhan, C.S. Dai, X.F. Zhang, D.W. Zhang, Coal water mixture applied to 2 t/h steam boiler, Coal Science and Technology 30 (2002) 5–6. [29] H.M. Zhang, J. Wang, Controlling sulfur dioxide pollution from industrial coalfired boiler by clean combustion technology of coal water mixture, Electric Power Environmental Protection 21 (2005) 35–38. [30] J.Z. Liu, J.H. Zhou, Z.Y. Huang, W.J. Yang, J. Cheng, K.F. Cen, Study of technology of coal–water slurry in a 65 t/h oil-fired boiler, Journal of China Coal Society 30 (2005) 773–777. [31] K. Yuan, The designing and operating of 130 t/h boiler of slime CWS, Industrial Boiler 1 (2008) 19–23. [32] C.M. Zhang, J.Z. Liu, J.H. Zhou, Z.Y. Huang, Z.J. Zhou, K.F. Cen, Technology of retrofitting designed 220 t/h oil-fired boiler into coal–water-slurry fired one and application thereof, Thermal Power Generation 5 (2006) 30–33. [33] D.Q. Sun, New oil substitute-coal water slurry application in a 230 t/h utility boiler, Conservation and Environmental Protection 1 (2001) 28–30.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
A new fluidization–suspension combustion technology for coal water slurry Hui Wang a,∗ , Xiumin Jiang b , Minxiao Zhang a , Yufeng Ma c , Hui Liu a , Shaohua Wu a a
School of Energy Science and Engineering, Harbin Institute of Technology, No. 92, West Da-Zhi Street, Harbin, Heilongjiang 150001, China School of Mechanical Engineering, Shanghai Jiao Tong University, Minhang District, Shanghai 200240, China c Shengli Power Plant, Shandong, Dongying 257087, China b
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 9 June 2010 Accepted 14 July 2010 Available online 21 July 2010 Keywords: Fluidization–suspension CWS Combustion technology Boiler Performance
a b s t r a c t Slagging is a major operating problem in application of the atomization–suspension combustion technology for burning coal water slurry (CWS) fuel in small and low height industrial boilers. The fluidization–suspension combustion is a new alternative for replacement of oil, which is capable of solving the slagging problems. In addition, it can be successfully applied to CWS-fired boilers with capacity smaller than 35 t/h. About 530,000 medium and small scale industrial boilers with low boiler efficiency in China provide the technology a very promising prospect. The principles and contents of CWS fluidization–suspension combustion technology are introduced in detail in this paper. And a new type of 14 MW fluidization–suspension CWS-fired boiler was developed, the performance of which showed that boiler efficiency was 91.53%. Emission of SO2 and NOx was 346.1 mg/m3 and 469.5 mg/m3 , respectively. From the application, the CWS-fired boiler showed good features such as high efficiency, low pollutant emission, good load regulation, good CWS quality adaptability, steady operation and convenient maintenance. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Coal water slurry (CWS) is a coal-based liquid fuel, which can be used as a substitute fuel in oil-fired boilers. Thus, the CWS technology has a great prospect in China to reduce China’s oil consumption. China has approximately 530,000 medium and small scale industrial boilers in operation, with the average capacity of 2.5 t/h and efficiency of only about 65%. Besides the low efficiency, the emissions such as particulates, SO2 and NOx from the small scale boilers are high. A considerable economical benefit and better environmental performance can be achieved if these boilers are retrofitted by efficient and clean coal technologies. The combustion technologies of CWS [1–4] have been studied extensively. The results reveal that the atomization–suspension combustion [5,6] has been successfully applied in large-scale boilers, in spite of the fact that it has a better economical performance to burn pulverized coal directly. But for the industrial boilers with capacities smaller than 35 t/h, most of the previous research was focused on the atomization combustion and seldom achieved high enough efficiency. It was reported that a 2.8 MW grate boiler [7] achieved an efficiency of 80% after retrofitted for burning CWS by pre-combustor and atomization combustion. A 4 t/h grate boiler [8] was retrofitted for firing fine-coal water slurry by multi-stage
∗ Corresponding author. Tel.: +86 451 86412318; fax: +86 451 86412528. E-mail address: wanghui [email protected] (H. Wang).
atomization for CWS feeding. The testing results showed that the maximum furnace temperatures were from 1400 ◦ C to 1500 ◦ C, and the boiler efficiency can be as high as 81%. A new developed 4.2 MW D-type boiler [9] for burning CWS with a colliding-type atomization nozzle achieved boiler efficiency of 88% and slagging occurred in the bottom of the furnace. A retrofitted 7 MW D-type boiler [10] to burn CWS equipped with a versatile burner which can fire both CWS and oil by atomization in a pre-combustor gained combustion efficiency over 96% and boiler efficiency of about 83%. A 10 t/h demonstrative fluidized bed boiler [11] with overbed feeding in specific shape, non-overflow and non-drainage achieved an average combustion efficiency of 95.4% and a boiler efficiency of 77.7%. From all those applications it may be concluded that the CWS fuel burned in small and low height industrial boilers did not get enough residence time to completely burn out, which leads to a low boiler efficiency. Besides, the high temperature in the small furnaces is hard to control, which will lead to local slagging inevitably and risk the operation stability. That is because the slagging problem is a dilemma for the atomization–suspension technology. Slagging happens or not depends on combustion temperature, nozzle aerodynamics and furnace structure. To make the boiler operate stable and reliable, about 1500–1600 ◦ C in the small furnace has to be assured. But the temperature is above the melting point of coal ash, which makes slagging happens easily. In addition, the auxiliary equipments such as atomization, slag-removal made the system complicated and had potential risk of the system. From the operation data of the CWS-fired atomization–suspension boilers
0255-2701/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.07.009
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Fig. 1. Flow chart of CWS fluidization–suspension combustion technology.
retrofitted from the oil-fired ones, only 75% of the design capacity pre-retrofitting can be obtained due to the limitation of furnace outlet temperature. Some small scale boilers fail to burn CWS fuel by atomization technology due to the small furnaces [12,13]. Furthermore, some detailed researches were conducted on the atomization combustion. Manfred et al. [14] reviewed the development of CWS as an alternative boiler fuel through a series tests in 10–20 MW boilers using atomization technology. Miccio et al. [15] carried out an experimental study on the influence of the type of CWS parent fuel and the velocity of injecting air. Rankin et al. [16] described the demonstration work of burning CWS in a boiler designed to burn oil. The Coal Research Establishment of England, Australian CSIRO and Japanese Sumitomo Company carried out combustion tests in fluidized bed boiler to burn CWS [7]. The 10 t/h fluidized bed boiler of the Sumitomo Company started running in 1980 and obtained boiler efficiency of about 65% [7]. Those early tests in small coal-capable front wall and tangentially fired utility boilers showed that two of major problems to be addressed are atomizer durability and poor carbon conversion. As a new type CWS combustion technology, fluidization– suspension is clean and efficient for burning CWS, solving the slagging problem and simplifying the system. In this paper, main principles and design concept of the technology are introduced in detail. Results of the utility tests in a 14 MW boiler are presented.
Fig. 2. Scheme of CWS granulating device.
tion of CWS agglomerates, and intensify the mixing and burnout performance. At the same time, thermal NOx production is effectively controlled and slagging in the furnace is avoided because of the low combustion temperature (850–950 ◦ C). The limestone can be fed with the bed material or mixed with CWS fuel to react with SO2 and reduce the emission. The temperature in the separating device happens to be the optimal temperature of desulfurization, so the objects of high efficiency and low pollution can be simultaneously realized. The technology is characterized by following features: (1) It uses a granulating device to feed the CWS in the shape of drops with diameter of 4–10 mm, and eliminates the need of atomization and filtering systems in atomization–suspension technology. (2) It uses an internal circulating combustion gas–solid separator set inside furnace to separate the large particles and agglomerates leading to a high combustion efficiency. (3) It can solve the slagging problems in atomization–suspension technology. 2.2. Components in fluidization–suspension combustion technology
2. Fluidization–suspension combustion technology of CWS 2.1. Principles of CWS fluidization–suspension combustion technology Fluidization–suspension combustion technology of CWS is a new type technology combining both fluidized bed combustion and suspension combustion. The flow chart [17] of a boiler system using the technology is shown in Fig. 1. The system includes CWS storage and feeding system, ignition system and dust separating system. Comparing with CWS atomization–suspension combustion technology, the airatomization (or steam-atomization), CWS filtering, high-pressure feeding and slag-removal are not needed, so that the total system is simplified and the reliability gets improved. The principles are as follows: high density solid particles are selected to be the bed material. The CWS fuel is delivered to granulating device located on the top (or on the front wall) of the furnace and is fed at about 0.4 MPa onto the bed surface with temperatures of 850–950 ◦ C. The CWS droplets experience drying stage and the volatile matters release and burn [18]. Under the state of fluidization, CWS particles will either be fragmented or form agglomerates [19] in the dense bed. The small particles will be carried upward to the dilute section to finish suspension combustion. The flue gas enters a separating device inside the furnace and the separated fines reenter the bed and burnout. The bed material and large CWS agglomerates carried with the flue gas will be separated and returned to dense section via its feedback channel, which can decrease the loss of bed material, enhance the complete combus-
2.2.1. CWS granulation-feeding system Because of better combustion conditions for CWS in fluidization–suspension combustion boiler, diameter of CWS drops could be a little bigger without the risk of decreasing the combustion performance. Thus, the expensive high-pressure atomization devices in the atomization–suspension technology, which are easy to block [20], are not necessary. To guarantee better fluidization combustion characteristics, the particles formed by CWS drops must meet a specific size distribution. So feeding CWS fuel steadily and reliably into furnace are critical in the fluidization–suspension combustion technology. The granulating device is developed to meet the demands, which is shown in Fig. 2 and composed of cooling air tube, flame detector, CWS inlet tube, dredge hole with seal bolt, sweeping and washing tube, and CWS granulating tube. The sweeping and washing tube (connected with water or compressed air) is used to clean the granulating tube when stop feeding. The dredge hole on the top of the granulating tube is used to clear blocks that cannot be swept or washed, which will be sealed when operating. And the granulating tube is offset placed in the cooling air tube (or a cooling air box) to make the air control CWS flow and avoid solidifying and blocking, since the offset assembled structure can form velocity difference of cooling air in the eccentric annulus, which benefits for granulating the CWS flow into acceptable size distribution. The flame detector is mounted to observe the state of combustion and CWS feeding when operating. The features of the device are as follows: (1) The simple structure with no moving parts can make it work reliably and easy to
H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024
maintain. (2) The CWS feeding stability and uniformity without blocking can be guaranteed, and the landing position and flow flux of CWS could be controlled easily. (3) The low air consumption and air pressure requires no special air source. (4) The formed 4–10 mm CWS particles meet the demands of the fluidization–suspension combustion, in particular the relatively big drop diameter lowers requirements [21] for CWS and need no filtering equipments. (5) It has good flexibility to different fuel types and can be used to feed many kinds of slurry fuels. 2.2.2. Technology of continuously running without slag-removal In the conventional fluidized beds, keeping a relatively steady bed material height during the operation is very important. For the fixed air flow, an excessive thick bed material layer will make fluidization difficult, maybe even worse to form stack and slag, and an extra thin layer will add possibility of the air short-passing the layer and the temperature falling, even lead to extinguish. So the frequent slag-removal is necessary in order to keep a steady bed material layer. But for fluidization–suspension combustion of CWS, it leaves little ash in the bed and the slagging is avoided, therefore it needs no slag-removal in the fluidization–suspension combustion process of CWS. In addition, the dewatered and dried CWS spheres will float and burn on the upper bed material layer because of their lower density comparing with quartz sand. In short, the merits of technology of continuously running without slagremoval are as follows: (1) Losses of incomplete combustion and slag-removal are decreased. (2) The make up requirement of bed material is decreased. In the technology, the ash left in the furnace is of small amount and cannot make up for the bed material loss, so the bed material has to be made up through adding quartz sand. Accordingly, some man-power or feeding device is needed. Without slag-removal, the bed material loss discharged with slag is avoided, then the make up requirement is lowered, so the operating cost and complexity of combustion devices is decreased. 2.2.3. Internal circulating combustion technology in the furnace Because of the wide size distribution of bed material, some fine particles will be carried out from the furnace with the flue gas. If nothing is done, small particles will be carried to convective heat transfer surfaces. This may: (1) increase the mechanical incomplete combustion loss because some fine fuel particles are carried out together, (2) increase the complement quantity of bed material, (3) increase the particle concentration in the flue and speed up the attrition of convective heating surfaces. For these reasons, an internal circulating combustion gas–solid separator is set inside the furnace of the CWS fluidization– suspension combustion boiler. The big particles in the flue will be separated and returned to the furnace to burn again, and in the internal separating device the combustion happens, too. In addition, the overall boiler volume changes little as adding separator in the furnace, so it can be used to retrofit some oil-fired boilers. The device named high temperature internal circulating combined vortex separator [22] is shown in Fig. 3. It is horizontal and composed of several groups of vortex separating cells (two cells form one group). For each group, the first cell is a coarse powder separator, which has one flue entrance and two feedback outlets: the flue entrance is up on one side, one feedback outlet is on the center of the opposite side and the other one on the center of the bottom. The second cell has one flue outlet and a feedback outlet: the flue outlet is up on the opposite side of the flue entrance, the feedback outlet is on the center of the bottom wall. In the group between the first and the second cells, a central passage is formed by an annular baffle plate, which is a plate ring of specific width fixed on the inner circle. The vortex separator is placed at the upper part of the dilute section and connected to the furnace outlet. The flue carrying ash and some bed material particles goes
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Table 1 Design parameters of coal water slurry boiler. No.
Name
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Rated output Circulating water flux Inlet water temperature Outlet water temperature Discharge flue temperature Boiler efficiency Fuel consumption Boiler heating surface Air flux of induced fan Air pressure of induced fan Air flux of blower Air pressure of blower Flux of circulating pump Head of circulating pump
Units
Value
MW kg/h C ◦ C ◦ C % kg/h m2 m3 /h Pa m3 /h Pa m3 /h m
14 266,667 70 115 150 89 3055 652.884 58,000 3300 24,000 14,000 630 50
◦
upward to dilute section from the dense section, and tangentially enter the first cell of the vortex separator at a specific velocity and form a high-speed rotating vortex flow. The solid particles in the flue will be concentrated to the wall and trapped by the feedback outlets under the effect of centrifugal force, and the high temperature flue goes into the second cell via the hollow baffle plate to continue its high-speed vortex flow and separate solid particles further. The separated solid particles trapped by the feedback outlets are returned to dense section to finish circulating combustion and make up for the loss of bed materials. The vortex separator can obtain a separating efficiency of over 90% for particles down to 0.25 mm and has many merits such as low resistance pressure, attrition resistance, small volume, simple structure and easy installation, low requirements of manufacture, big flow flux for flue separating and perfect separating performance, easy to be enlarged and combined according to the capacity of boiler, etc. 2.2.4. High efficiency and low cost technology of desulfurization and reduction of NOx The processes of desulfurization and reduction of NOx can be achieved at low cost through directly adding sorbents such as limestone into furnace or mix them into CWS fuel and staged-air arrangement [23], which is a general feature of CFB, too. For the fluidization–suspension technology, the internal circulating combustion technology can significantly increase the residence time of the desulfurization agent in the furnace and improve its utilization ratio and reduce the operating cost. 3. The 14 MW vertical CWS fluidization–suspension boiler 3.1. Design data of boiler The main design parameters are shown in Table 1. 3.2. Design fuel The design fuel is CWS made of Datong bituminous coal. The detailed analytical data of design CWS and CWS in tests are given in Table 2 and size distribution of coal powder in it is shown in Fig. 4. 3.3. Design principles 3.3.1. Selection of bed material The selection of density and size distribution of inert bed material will directly affect the fluidization quality of CWS fluidization–suspension combustion boiler, further the characteristics of combustion and heat transfer. Considering the features of different density fluidized bed and power consumption of fans, the
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Fig. 3. Structure of high temperature combined vortex separator.
quartz sand with density of 2300 kg/m3 [24,25] is selected. The size distribution of fresh bed material is shown in Table 3 and the mass average diameter is 2.52 mm. 3.3.2. Selection of fluidization air velocity and cross-section heat load Based on the density and size distribution of the bed material and parameters of air, the critical fluidization velocity can be calculated using empirical equation. The actual fluidization velocity used in operating boiler is 6.47 m/s. From the features of CWS fluidization–suspension combustion technology, the designed cross-section heat load is 1106.53 kW/m2 and the volume heat load is 237.96 kW/m3 . 3.3.3. Heat balance and combustion process Combustion process of CWS drops and coal is quite different because CWS needs big latent heat of vaporization and carries out agglomeration combustion. The latent heat of vaporization accounts for more than 47% total ignition heat of the CWS fuel. And the evaporation time is about 0.05–0.1% total residence time in the dense section [26]. The porosity formed by evaporation is 0.59 when the mass fraction of coal in CWS is 50% [2]. When the fed CWS cannot mix with bed material in time, big agglomerates are possible to be formed. Sometimes the burned chars will stick together, too. By different-density combustion technology, the agglomerates
are kept floating and burning on the upper layer in the bed. The agglomerates will break into fragments under the effect of bed material. There were not any clusters found in the dilute section thus far. The CWS fuel can still obtain good combustion efficiency so long as the agglomerate diameter does not exceed 25 mm [27]. After the evaporation finishes, the volatile will start to release and burn immediately to form a flame surrounding the CWS spheres because of the high temperature in the bed material. The combustion processes of volatile and carbon have some overlap. For a single CWS particle, when the combustion of volatile has finished 90%, the carbon has already burned about 10% [26]. When the evaporation and volatile releasing nearly ends, the carbon will form a sphere with certain strength. From the data of a strong coking fine-coal slurry [26], the compressive strength of a 6 mm sphere is about 2 MPa and the strength of the same size sphere of another weak coking coal slurry is about 0.5 MPa. For the core of the CWS sphere, the strength changes little until it is nearly burned out. The burning behavior of CWS spheres in the fluidized bed is a layer-by-layer, inward flaking-off combustion [19]. With the fluidization of bed material and CWS fuel, relatively small particles formed by fragmentation or attrition and small agglomerates are carried into the suspension space to continue combustion. At the same time, the incomplete burned volatile matters and combustible component of particles separated from the combined vortex separator will go up to burn in the suspension space from the dense section, too. 3.3.4. Selection and control of working temperature in the furnace The furnace temperature is an important parameter in boiler designing. It has extremely important influence on the processes of steady and high efficiency combustion, slagging avoidance, desulfurization and removal of NOx . The operating temperature is determined to be 850–950 ◦ C by comprehensive analysis of those influences. 3.3.5. Determination of circulating ratio and separating efficiency Circulating ratio is of great importance for the circulating fluidization mode, which affects combustion, heat transfer and attrition of boiler and fans power a lot. For a CWS fluidization– suspension combustion boiler with the temperature of dense section is 850 ◦ C, a relatively low circulating ratio is selected to maximize the combustion efficiency and minimize the power consumption. The circulating ratio is mainly achieved by the regulation of separator, so the separator is designed carefully to assure a separating efficiency as high as possible, primarily the operating stability and security are guaranteed.
Fig. 4. Size distribution of coal powder in CWS.
Table 2 Proximate and ultimate analyses of design CWS and CWS in tests. Proximate analysis Mar (%) Design CWS CWS in tests
32.9 35.40
Ultimate analysis
Aar (%)
Var (%)
5.64 7.08
30.96 36.94
Qar,net (kJ/kg) 18,877 17,700
Car (%) 50.57 47.43
Har (%)
Oar (%)
Nar (%)
Sar (%)
3.27 2.93
6.13 6.02
0.93 0.80
0.56 0.34
H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024 Table 3 Size distribution of bed material. No.
Aperture of screen (mm)
Average diameter of particle (mm)
Percentage on mass basis (%)
1 2 3 4 5
0.71–1 1–2 2–3 3–4 4–5
0.855 1.5 2.5 3.5 4.5
0.15 12.2 73.2 14.3 0.15
3.3.6. Preventive means of internal attrition in the furnace In circulating fluidized bed boiler, the attrition to internal heating surfaces is a very important problem for the high concentration of fuel or bed material. Measures are taken to consider design parameters, material and local strengthening comprehensively. Attrition-preventive fins and bricks or casting materials were used in the dense section where the flue velocity was very high. Because the attrition of convective heating surface is proportional to the particle concentration in the flue and proportional to 3.6 times of the flue velocity, the flue velocity turns to be more critical, so a relatively low velocity is selected in the design. In addition, the upward flowing flue is opposite to the gravity of particles, which will reduce the influence of the particles and benefit for the attrition prevention. To make the security better, uniform distribution plate and attrition-preventive capping plate are used in the flue segregation area. An air cushion backflow area covered with attrition-resistance casting material is set up in turning chamber of upper furnace to decrease the collision momentum of particles. And in the horizontal internal gas–solid separator, inner lining made of attrition-resistance material is coated to increase resistance of attrition. 3.4. Whole structure of the 14 MW CWS fluidization–suspension boiler A 14 MW CWS fluidization–suspension boiler is used for community heating in winter, whose general arrangement diagram is shown in Fig. 5. And its features are as follows: The vertical boiler has two horizontal drums and its water circulation is combined by controlled circulation and natural circulation. The heat load of the boiler is 14 MW and the pressure of outlet water is 1.0 MPa (gauge pressure). The temperatures of outlet water, inlet water and cold air are respectively 115 ◦ C, 70 ◦ C and 20 ◦ C. The design fuel is CWS. The outside dimensions of the boiler are 7860 mm × 4490 mm × 9660 mm (height × width × depth). The furnace is divided into dense and dilute sections. The height of dense section is 1350 mm with a trapezoid air distributor at the bottom. Some buried heating surface composed of Ф51 mm× 5 mm tubes with attrition-preventive rings are installed in the dense section. One group of the internal high temperature combined vortex separator is installed at the outlet of furnace to achieve the circulating combustion. All the air needed enters the furnace from two ways: one as primary air goes via funnel caps to bottom furnace to keep the bed material fluidizing; and the other one as secondary air goes via ejectors installed at the top of the dense section, entering the dilute section to reinforce burning completely and reduce the releasing of NOx . The CWS fuel is fed from front wall in drop shape into the dense section by the granulating device. The drops are heated by the fluidizing bed material and burn rapidly, then the hot flue carrying some bed material and fragments or agglomerates goes into the separator via flue outlet window. The big particles trapped are fed back to dense section again to achieve a high efficiency circulating combustion. The separated flue goes through the anti-slag tubes in the turning chamber and passes the
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convective tube groups located between the upper and bottom drums, then via the ash separator to the stack. From the heat calculation of the 14 MW fluidization–suspension boiler, the outlet temperature of the furnace is 950 ◦ C, then behind the anti-slag tubes is 869 ◦ C, then 209.5 ◦ C behind the convection tubes, and the exhaust gas temperature is 150 ◦ C. In order to make the boiler structure compact, decrease the radiation heat loss and increase the tightness, the boiler wall is selected to be light-duty type, i.e. from the inner wall, it is made in sequence of firebrick, plates of aluminum silicate fiber and berg meal bricks. A steel layer is used to cover the bricks from outer at last. Some material feeding inlets, manhole doors, peep holes and measuring points of temperature are set up in the front wall. An oil gun is installed in the air box under the distributor plate to achieve ignition under bed when startup. To ensure the safe and economical operation of the boiler, some thermal measuring instruments and automatic controlling devices are equipped to remote control the forced and induce fans and monitor parameters such as feeding water pressure, air box pressure, air box temperature, sub-atmospheric pressure, fluidized bed temperature, exhaust flue temperature, feeding water temperature, flow flux of feeding water, feeding quantity of CWS, outlet water temperature and oxygen concentration in the flue, etc.
4. Results and discussion In order to investigate the overall performance of the boiler, and analyze its technical and economic characteristics, experiments under the rated condition are carried out. Measurements include boiler capacity, boiler efficiency using input–output method and heat loss method, and the releasing characteristics of the flue, etc. The criterion used are standards of The People’s Republic of China: (1)“Test code for industrial boiler” (GB10180-94), (2)“Monitoring and testing method for energy saving of industrial boilers” (GB/T15317-94), (3)“General specification for industrial boilers” (ZB/T10099-1999), (4)“Measurement method of smoke and dust emission from boilers” (GB5468-91) and (5)“The determination of particulates and sampling methods of gaseous pollutants from exhaust gas of stationary source” (GB/T16157-96), etc. The test results show that every technical and economic indicator reaches the expected design requirements and national standards, and the boiler efficiency has reached the international advanced level.
4.1. Efficiency of the CWS fluidization–suspension combustion boiler The objective of tests is to evaluate the overall heat engineering performance of CWS fluidization–suspension combustion boiler. Two tests under rated condition were carried out. The instruments used are shown in Table 4, the fuel used in the tests can be seen in Table 2, the data of heat engineering performance and the boiler efficiency based on input–output method and heat loss method are shown in Tables 5–7, respectively. From the tables, the main characteristics can be seen: (1) Characteristics of boiler capacity. The required boiler capacity is realized and a relatively high boiler efficiency of 91.53% is achieved. The improvement of boiler efficiency is significant [17] comparing with the conventional industrial boilers [8–11], even with boilers retrofitted with atomization–suspension technology [7], which is about 80%. The CWS granulating device is reliable and the operation of the boiler is steady without any slag formed in the furnace [17].
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Fig. 5. 14 MW vertical CWS fluidization–suspension combustion boiler.
Table 4 Measuring instruments used for boiler tests. Measuring items
Instrument
Specification
Heat value of fuel Fuel consumption Carbon content in fly ash Flux of circulating water Inlet water temperature Outlet water temperature Discharge flue temperature Analysis of flue contents Temperature of boiler surface
Jingying automatic calorimeter Platform scale Muffle furnace Ultrasonic flowmeter Standard glass thermometer Standard glass thermometer Combustion efficiency analyser Combustion efficiency analyser Far infrared thermoscope
SDACM3000 – – FBL – – KM9101 KM9101 IRT-1200
Precision 0.01% 0.02 kg 0.1% 0.2% 0.1 ◦ C 0.1 ◦ C 1 ◦C 1% 0.1 ◦ C
Table 5 Data obtained in boiler heat engineering performance test. Test
Capacity (MW)
Heat efficiency of positive balance (%)
Heat efficiency of inverse balance (%)
1 14.067 91.214 88.1698 2 14.231 91.835 88.5464 Average capacity: 14.149 MW; average heat efficiency: 91.53%
(2) Combustion characteristics of CWS. High utilization ratio of CWS has been obtained in the fluidization–suspension combustion boiler. In the two experiments under rated condition, the combustible matter content in the ash are 3.32% and 3.33%, respectively, and the mechanical incomplete combustion loss are 0.4614% and 0.4528%, respectively. The results of so low the combustion losses verified not only the fluidization–suspension combustion in the furnace could obtain high combustion efficiency, but also the internal high
Discharge flue temperature ( ◦ C)
Excess air ratio at flue exit
Content of combustible matters in fly ash (%)
170.0 165.0
1.5417 1.5307
3.32 3.33
temperature vortex separator could improve the combustion conditions of CWS and make it burn completely. The CO concentration in the exhaust flue are 0.0010% and 0.0011%, RO2 (i.e. CO2 and SO2 ) concentration are 11.6% and 11.7%, O2 concentration are 6.9% and 6.8%, and the chemical incomplete combustion loss q3 are 0.0058% and 0.0063%, respectively. The low concentration of O2 and CO and high concentration of RO2 in the exhaust flue shows gaseous combustible matters have obtained sufficient combustion because
Table 6 Data obtained in boiler efficiency test using input–output method. No.
Items
1 2 3 4 5 6 7 8
Circulating water rate of boiler Inlet water temperature of boiler Outlet water temperature of boiler Enthalpy of boiler inlet water Enthalpy of boiler outlet water Capacity of hot water boiler Fuel consumption rate Heat efficiency of positive balance
Units
Rated test 1
Rated test 2
kg/h C C kJ/kg kJ/kg MW kg/h %
379,150 70.1 102.0 293.49 427.05 14.067 3136.64 91.214
378,850 71.4 103.7 298.94 434.17 14.231 3151.83 91.835
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H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024
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Table 7 Data obtained in boiler efficiency test using heat loss method. No.
Items
Units
Rated test 1
Rated test 2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Combustible content in fly ash Ash ratio of fly ash to inlet total ash (mass percentage) Mechanical incomplete combustion heat loss RO2 in discharge flue O2 in discharge flue CO in discharge flue Excess air ratio at discharge flue Theoretical air volume RO2 volume Vapor volume in discharge flue Dry flue volume in discharge flue Discharge flue volume Chemical incomplete combustion heat loss Cold air temperature Discharge flue temperature Dry flue average specific heat at constant pressure in discharge flue Enthalpy of discharge flue Enthalpy of cold air Heat loss of discharge flue Dissipated heat loss Total heat loss Inverse balance efficiency of boiler
% % % % % % – N m3 /kg N m3 /kg N m3 /kg N m3 /kg N m3 /kg % ◦ C ◦ C kJ/(N m3 ◦ C) kJ/kg kJ/kg % % % %
3.32 100 0.4514 11.6 7.6 0.0010 1.5417 4.8038 0.8874 0.8834 7.2910 8.1744 0.0058 10 170.0 1.3314 1888.746 97.715 10.073 1.3 11.8302 88.1698
3.33 100 0.4528 11.7 7.5 0.0011 1.5307 4.8038 0.8874 0.8825 7.2382 8.1207 0.0063 10 165.0 1.3310 1820.752 97.018 9.6945 1.3 11.4536 88.5464
of their fully mixing with oxygen in the furnace of the fluidization–suspension boiler. (3) Heat loss of exhaust flue. In spite of the fact that high water content in CWS increases the quantity of exhaust flue, the boiler can still achieve good combustion and burnout performance of the CWS fuel under relatively low excess oxygen conditions in the furnace. So the excess air ratio needed is relatively small, which means the heat loss of exhaust flue q2 is not very high. The boiler efficiency is about 90%, which is relatively high because of the lower mechanical incomplete combustion loss and exhaust flue heat loss. Along with the better burning stability and slagging performance than that of atomization– suspension CWS boilers [17,28], the fluidization–suspension CWS boiler was proved to be successful. 4.2. Results and analysis of environmental performance of boiler Through the test of environmental performance of boiler, the design and operational quality of sections such as main boiler body, dust removal and desulfurization system are examined to obtain data for further improvement. The test is carried out only one time in the boiler equipped with an ordinary TLS water-bath duster. The results are shown in Table 8 and the analysis is as follows: (1) SO2 . From the “Emission standard of air pollutants for coalburning oil-burning gas-fired boiler (GB13271-2001)”, the national standards for SO2 emission is 1200 mg/m3 in I time period and 900 mg/m3 in II time period. The results of test without adding desulfurizing agents in the desulfurization system show that the SO2 concentration in the flue is only 346.1 mg/m3 , which is much lower than the national standards. The main reason should be the coal used in CWS is a low-sulfur washed coal and its ash is of weak alkaline which has self-desulfurizing effect in the process of low temperature combustion. Accordingly, the SO2 emission of a 2 t/h CWS-fired fluidized bed boiler is 518.21–647.79 mg/m3 [28], 730 mg/m3 for a 4 t/h boiler [29], 352–598 mg/m3 for a 65 t/h boiler [30], 473–503 mg/m3 for a 130 t/h boiler [31], 941–1070 mg/m3 for a 220 t/h boiler [32], 627.4 mg/m3 for a 230 t/h boiler [33]. (2) NOx . The concentration of NOx in the flue is 469.5 mg/m3 , which is a typical result for the technology. The CWS combustion at low temperature guarantees low concentration of thermal
NOx and staged air feeding controls the production of NOx , too. Accordingly, the NOx emission results from above boilers are 425–585 mg/m3 for the 65 t/h boiler [30], 632–721 mg/m3 for the 130 t/h boiler [31], 425–538 mg/m3 for the 220 t/h boiler [32], 495.1 mg/m3 for the 230 t/h boiler [33]. (3) Dust. Even though the dust separator used is of an ordinary TLS type, the dust concentration in the exhaust flue is very low, which is only 123.5 mg/m3 . It is mainly due to the CWS fuel has low ash content, which makes the inlet concentration of separator is only one third of that of a coal-fired boiler. In some other applications with bag-type dust collectors, the dust concentration measured is lower than 30 mg/m3 , which makes the fluidization–suspension combustion boiler very appropriate to be used in regions with stricter environmental requirements. Something has to be mentioned is that the dust concentration after the inducing fan is measured to be higher than that before the fan might be due to measurement error. The dust concentration of a 220 t/h CWS-fired boiler is 140–147 mg/m3 [32], a little higher than the 14 MW boiler. (4) Blackness of flue. Because the concentration of dust and combustible matters is very low, the Ringelmen blackness of the flue is certainly low and the value is 0–1 level.
Table 8 Comprehensive analysis table of air pollutant emission from boiler. No.
Items
Units
Value
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Flue temperature Dynamic pressure of flue Total pressure of flue Flow velocity of flue Flux of flue Moisture of flue Excess air ratio Dust concentration before inducing Dust concentration after inducing SO2 concentration NOx concentration CO concentration Oxygen content in flue Releasing rate of dust Releasing rate of SO2 Ringelmen blackness of flue
◦
86 80 −2000 13.3 39,780 6.5 2.23 99.69 (6% O2 ) 123.5 (6% O2 ) 346.1 (6% O2 ) 469.5 (6% O2 ) 25 (6% O2 ) 11.6 3.699 13.768 0–1
C Pa Pa m/s m3 /h % – mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 % kg/h kg/h Level
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In brief, the CWS fluidization–suspension combustion boiler has good environmental characteristics and the emission is lower than national standards for type II regions. The boilers will obtain better performance if the desulfurizing agents such as limestone are added. 4.3. Application of CWS fluidization–suspension combustion boiler Since 2002, the combustion technology of fluidization– suspension is used in more than 30 boilers and of more than 10 types. The capacity is concentrated in the range of 2.8–35 t/h, and one 75 t/h utility boiler was retrofitted to 60 t/h successfully. In Shengli Oil Field of Shandong province, there are more than 20 boilers in operation using the CWS fluidization–suspension combustion technology. The total capacity is 400 t/h and the biggest one is 60 t/h. In the single year of 2002, the burned CWS was 65,400 t, and 24 million Yuan was saved. In 4 years’ operation since 2002, the burned CWS was 297,000 t, and 135,000 t of oil was saved. The boilers were very successful which burns steadily and is easy to be operated. Among the 530,000 industrial boilers, about 76% of them could be considered to be retrofitted using the CWS fluidization–suspension combustion technology and it gives the technology a promising prospect. In addition, the internal vortex separating device used in the combustion technology is of great importance in achieving the performance, but it has no application in boilers bigger than 35 t/h, so careful design is needed to scale-up CWS-fired fluidization–suspension boilers. 5. Conclusion Using the fluidization–suspension combustion technology, the CWS fuel can be burned in the small and low height furnaces with high efficiency and good environmental performance. It solved the problems appeared in the combustion of CWS in boilers smaller than 35 t/h such as burning stability and slagging in local sections. An application of 14 MW boiler showed that it obtained good heat engineering performance and environmental performance for washed-coal CWS. The boiler efficiency is 91.53%, emission of SO2 and NOx are 346.1 mg/m3 and 469.5 mg/m3 , respectively. The boiler using the technology showed good features such as high efficiency, low pollutant emission, good load regulation, good CWS quality adaptability, steady operation and convenient maintenance. The combustion technology of CWS has been used successfully in more than 20 boilers and it will have a good prospect. References [1] E.T. Mchale, R.S. Scheffee, N.P. Rossmeissi, Combustion of coal/water slurry, Combustion and Flame 45 (1982) 121–135. [2] K.F. Cen, X.Y. Cao, M.J. Ni, et al., Experimental study on combustion of coal water slurry in fluidized beds, Journal of Engineering Thermophysics 4 (1983) 177–182. [3] R.K. Manfred, Coal–water slurry: a status report, Energy 11 (1986) 1157– 1162. [4] G. Papachristodoulou, O. Trass, Coal slurry fuel technology, Canadian Journal of Chemical Engineering 65 (1987) 177–201. [5] Z.X. Hu, F.M. Liu, G.H. Gong, Design of a 200 MW coal water slurry firing boiler, Power Engineering 26 (2006) 369–374.
[6] L. Wang, X. Zhao, X.Y. Cao, Z.Y. Huang, Heat transfer and emission characteristics tests of a 220 t/h oil-fired utility boiler retrofitted for firing coal water slurry, Journal of Engineering for Thermal Energy & Power 17 (2002) 589–591. [7] Y.G. Xie, C.M. Zhang, F.Y. Wang, X. Zhao, The combustion test and analysis of a 2.8 MW hot-water travelling-grate boiler retrofitted for firing coal–water slurry, Journal of Engineering for Thermal Energy & Power 19 (2004), pp. 309–311, 328. [8] Z.Y. Huang, W.G. Weng, J.Z. Liu, et al., Combustion experimental research of a 4 t/h fine-coal water slurry fired industrial boiler, Clean Coal Technology 5 (1999), pp. 36-39, 50. [9] Z.G. Zhang, Q. Tang, W.B. Zhu, Z.D. Cao, Development of 4.2 MW coal–water mixture (CWM) fired hot-water boiler, Industrial Boiler 3 (2004) 29–31. [10] X. Zhao, X.Y. Cao, Z.Y. Huang, et al., The coal–water fuel combustion test on the 7 MW oil-fired hot-water boiler, Clean Coal Technology 3 (1997) 38–41. [11] G.Q. Huang, M.J. Ni, J.H. Yang, et al., A 10 t/h demonstration boiler buring washery tailings, Journal of Engineering Thermophysics 8 (1987) 374–377. [12] X.P. Huang, Discussion on using coal–water slurry instead of fuel oil in boiler, Petrochemical Design 21 (2004) 65–67. [13] B.C. Zhao, L.J. Zhu, B.Q. Gu, The application for industrial heating boilers of a modification technology involving the conversion from oil firing to coal water mixture firing, Journal of Engineering for Thermal Energy & Power 19 (2004) 634–637. [14] R.K. Manfred, T.C. Derbidge, R. Perkins, Program of coal–water slurry development, Energy Progress 4 (1984) 85–88. [15] M. Miccio, U. Arena, L. Massimilla, et al., Combustion of fuel–water slurries injected in a fluidized bed, AIChE Journal 35 (1989) 2040–2042. [16] D.M. Rankin, H. Whaley, P.J. Read, D.J. Burnett, Performance of a compact oildesigned utility boiler when firing coal–water fuel, Journal of Engineering for Gas Turbines and Power, Transactions of the ASME 112 (1990) 28–30. [17] Y.F. Ma, Y.K. Qin, X.M. Jiang, Q.K. Wan, Application of water coal slurry fluidization–suspension combustion technology at Shengli Oil Field, Journal of Engineering for Thermal Energy & Power 21 (2006) 644–647. [18] H. Wang, X.M. Jiang, J.G. Liu, C.Q. Zhang, Analysis of the combustion parameter of coal water slurry at various heating velocities, Chemical Engineering 34 (2006) 25–28. [19] H. Wang, X.M. Jiang, J.G. Liu, W.G. Lin, Experiment and grey relational analysis of CWS spheres combustion in a fluidized bed, Energy & Fuels 21 (2007) 1924–1930. [20] J.Z. Liu, J.H. Zhou, Z.Y. Huang, et al., Study on technology of coal water slurry in a 65 t/h oil-fired boiler, Journal of China Coal Society 30 (2005) 773–777. [21] K. Sato, K. Shoji, K. Okiura, I. Akiyama, A. Baba, Effect of coal particle and spray droplet sizes on combustion characteristics of coal water mixtures, Powder Technology 54 (1988) 127–135. [22] X.M. Jiang, C. Yan, Z.J. Hao, Performance test of high temperature combined vortex separator in furnace of circulating fluidized bed boiler, Chemical Engineering 29 (2001) 37–40. [23] D.G. Ji, Z.N. Wang, X.H. Fu, X.W. Wang, Release characteristics of fuel nitrogen during coal water slurry suspension combustion, Journal of China University of Mining & Technology 35 (2006) 389–392. [24] H. Wang, X.M. Jiang, J.G. Liu, D.Q. Yuan, Attrition experiment and gray relational analysis of quartzite particles as medium material in fluidized bed, Journal of Chemical Industry and Engineering 57 (2006) 1133–1137. [25] J.G. Liu, X.M. Jiang, H. Wang, et al., Thermal behavior and surface microstructure of quartzite particles, Journal of Chemical Industry and Engineering 58 (2007) 765–770. [26] K.F. Cen, Q. Yao, X.Y. Cao, et al., Theory and Application of Combustion, Flow, Heat Transfer, Gasification of Coal Slurry, Press of Zhejiang University, Hangzhou, 1997. [27] M.J. Ni, G.Q. Huang, X.Y. Cao, et al., Effects of operation conditions to the operation process of the fluidized bed combustor burning washing coal sludge, Journal of Zhejiang University (Engineering Science) 19 (1985) 153–159. [28] H.H. Zhan, C.S. Dai, X.F. Zhang, D.W. Zhang, Coal water mixture applied to 2 t/h steam boiler, Coal Science and Technology 30 (2002) 5–6. [29] H.M. Zhang, J. Wang, Controlling sulfur dioxide pollution from industrial coalfired boiler by clean combustion technology of coal water mixture, Electric Power Environmental Protection 21 (2005) 35–38. [30] J.Z. Liu, J.H. Zhou, Z.Y. Huang, W.J. Yang, J. Cheng, K.F. Cen, Study of technology of coal–water slurry in a 65 t/h oil-fired boiler, Journal of China Coal Society 30 (2005) 773–777. [31] K. Yuan, The designing and operating of 130 t/h boiler of slime CWS, Industrial Boiler 1 (2008) 19–23. [32] C.M. Zhang, J.Z. Liu, J.H. Zhou, Z.Y. Huang, Z.J. Zhou, K.F. Cen, Technology of retrofitting designed 220 t/h oil-fired boiler into coal–water-slurry fired one and application thereof, Thermal Power Generation 5 (2006) 30–33. [33] D.Q. Sun, New oil substitute-coal water slurry application in a 230 t/h utility boiler, Conservation and Environmental Protection 1 (2001) 28–30.