Experimental Study on Combustion of Low Calorific Oil Shale

One is co-combustion with high calorific fuels (such as coal, biomass, and OS) in .... electric heating systems were opened simultaneously to heat bed...
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Experimental Study on Combustion of Low Calorific Oil Shale Semicoke in Fluidized Bed System Yu Yang, Xiaofeng Lu,* Quanhai Wang, Lin Mei, Decai Song, and Yong Hong Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, People’s Republic of China ABSTRACT: Oil shale semicoke produced from the oil shale retorting process causes great hazards to the environment due to its high phenol and polycyclic aromatic hydrocarbons content. A good solution of the current of semicoke is to burn it as feedstock. In the present work, the extremely low calorific oil shale semicoke of 2384 kJ/kg was adopted, and a series of experimental investigations and computational analyses were carried out in bench-scale and pilot-scale bubbling fluidized bed combustors, to understand its combustion characteristics. Results indicated that by adopting effective technique means, the extremely low calorific oil shale semicoke could be directly burnt in the fluidized bed combustor, and the combustion was completely relying on its heat release. The main influence factors of combustion stability were combustion temperature, fluidization air temperature, and burnout rate. However, within the scope of test parameters, its combustion characteristics were mainly affected by dense-bed temperature, fluidization air velocity, and grain size, and less affected by static bed height. Furthermore, its comprehensive utilization system was put forward, and the general configuration and technical data of 35 t/h medium pressure fluidized bed boiler adopting the mixture of fine oil shale and extremely low calorific oil shale semicoke whose calorific value is 2876.2 kJ/kg were also given at the end of this article.

1. INTRODUCTION In China, abundant oil shale (OS) resources are mainly used by retorting for the production of shale oil to alleviate the shortage of petroleum.1−3 According to the relevant literature,4 around 10−30 tons of oil shale semicoke (SC) per ton of shale oil is left after production. However, it is difficult to employ the conventional technique to make a comprehensive utilization of this kind of industrial solid waste whose heat value can be 2400 kJ/kg or even lower. Discarded casually or direct landfill causes great hazards to the environment.5−8 Consequently, it has become a serious issue on how to treat low calorific SC efficiently and in an environmental-friendly way.9−11 Up to the present, some scholars at home and abroad have mainly proposed two kinds of combustion processing approaches to deal with low calorific SC. One is co-combustion with high calorific fuels (such as coal, biomass, and OS) in the circulating fluidized bed combustor, and a series of theoretical analyses and experimental investigations have already been carried out.12−15 But generally speaking, the calorific value of blended fuel is usually required to be more than 5000 kJ/kg. Hence, a large number of high calorific fuels are needed to be consumed, and the cost is relatively high. Another method is direct incineration. The previous experimental result16 has demonstrated that SC whose heat value is more than 4000 kJ/ kg could be directly burnt in the circulating fluidized bed combustor without adding any high calorific fuels, and the combustion could be completely relying on its heat release. However, for extremely low calorific SC of ca. 1600−2400 kJ/ kg, there has been little work in the literature regarding its monocombustion feasibility, combustion characteristics, and engineering application scheme. For the above reasons, a technical scheme that extremely low calorific SC is directly burnt in the fluidized bed combustor was proposed in this article, and it had the characteristics of direct © 2016 American Chemical Society

incineration disposal of SC, as well as the high-efficiency recovery of its combustion heat. In this work, the combustion feasibility of extremely low calorific SC of 2384 kJ/kg as a single fluidized bed fuel was discussed preliminarily, and then, a series of experiments were carried out in bench-scale and pilot-scale bubbling fluidized bed combustors to investigate its combustion stability and economy in detail. Finally, the high-efficiency utilization method of its combustion heat was studied and the design scheme of 35 t/h medium pressure fluidized bed boiler adopting the mixture of fine OS and extremely low calorific SC whose calorific value is 2876.2 kJ/kg was also put forward.

2. THEORETICAL SECTION Combustion possibility of extremely low calorific SC as fluidized bed fuel was investigated preliminarily by thermal balance calculations. On the basis of thermal balance theory, some assumptions were implicit in this work: (1) there were not arranged any heating surfaces in the bubbling fluidized bed. (2) The densebed temperature, slag temperature, and furnace outlet temperature were the same. (3) The value of heat loss accounted for 5% of total heat input. (4) Due to its low moisture content, the latent heat of water evaporation was ignored. Consequently, the net heat output of the combustion (Qnet) was as follows. Q net = Q fuel,c + Q fuel + Q air − Q gas − Q ba − Q fa − Q loss

(1)

where Qfuel,c was the fuel combustion released heat (kJ/kg). Qfuel, Qair, Qgas, Qba, and Qfa represented the physical sensible Received: July 29, 2016 Revised: September 9, 2016 Published: September 27, 2016 9882

DOI: 10.1021/acs.energyfuels.6b01870 Energy Fuels 2016, 30, 9882−9890

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Energy & Fuels heat of fuel, fluidization air, exhaust gases, bottom ash, and fly ash (kJ/kg), respectively. Qloss was the heat loss (kJ/kg). If the value of Qnet was equal to or greater than 0, it indicated that the thermal balance was able to be achieved. For extremely low calorific SC, it was theoretically calculated the range of parameters when the combustion achieved thermal balance, through changing values of burnout rate (β), excess air ratio (α), and fluidization air temperature (Tair). The relationship between Qnet and dense-bed temperature under different parameters was plotted in Figure 1. It was concluded that the thermal balance could be achieved at the dense-bed temperature range of 800−1000 °C under a certain parameters, and the main influence factors of combustion stability were combustion temperature, fluidization air temperature, and burnout rate. It was evident that the fluidization air temperature was able to be risen from the heat coming from exhaust gases via the air preheater, and elevating the fluidization air temperature could increase the heat input of dense-phase zone of the furnace, which was a benefit for the steady combustion. Additionally, a higher fluidization air temperature also meant a wider parameter adjusting range.

3. EXPERIMENTAL SECTION 3.1. Fluidized Bed Combustion Experimental Facility Brief. Combustion experiments were carried out in bench-scale and pilotscale bubbling fluidized bed combustion facilities, respectively. In the bench-scale bubbling fluidized bed combustion facility, as illustrated in Figure 2a, the furnace was built by silicon carbide with a rectangle cross-sectional dimension of 150 × 150 mm and 3000 mm in height. The combustion was operated at the positive pressure due to the lack of induced fan, and there was no secondary air. It was designed to burn the fuel whose particle size was below 3 mm. In the pilot-scale bubbling fluidized bed combustion facility, as illustrated in Figure 2b, the rectangle cross-sectional dimension of the furnace was progressively enlarged from 200 × 200 mm to 300 × 300 mm, and totaled 6300 mm in height. The balanced ventilation and staged air were both employed, and it was designed to burn the fuel whose particle size was below 6 mm. The electric heating system was used for ignition and steady combustion, which consists of the air heating system and furnace heating system. The fluidization air could reach 600 °C with the aid of air heating system, and the maximum temperature of bed materials could be heated to about 700 °C. 3.2. Experimental Methods. The fuel was fed into furnace via the screw feeder, which could be quantificationally adjusted by the control motor. The glass rotameter was employed for volumetric air flow measurement. There were several thermocouples (k-type) arranged along the furnace height direction to measure the temperature distribution of the furnace continuously. The temperature and pressure data were constantly collected and monitored by the monitor and control generated system (MCGS). An “ECOM-J2KN” gas analyzer was employed to monitor the online gas concentrations at the outlet of the furnace. 3.3. Fuel Properties. The extremely low calorific SC was sourced from the OS refinery in Fushun, China, and its ultimate and proximate analysis was presented in Table 1. It was observed that it is featured in low carbon content, low volatile matter, low heating value, and high ash content. In addition, it had low moisture content because it was derived from a novel oil extraction technique. This new technique was based on the traditional Fushun type retort, and brought in the concept of whole circulation process, i.e., the fuel gas was employed as the gas heat carrier. Hence, it was directly discharged from the retort in the dry state, and had low moisture content. Table 2 provided particle size distributions of SC fed into benchscale and pilot-scale fluidized bed combustors, which were determined by the way of sieving method. It was observed that particle size distributions of S1, S2, and S3 were respectively 0−3 mm, 0−6 mm

Figure 1. Relationship between Qnet and dense-bed temperature under different conditions: (a) burnout rate, (b) excess air ratio, (c) fluidization air temperature.

(the mass fraction of particles ranging from 3 to 6 mm in size is 10%), and 0−6 mm (the mass fraction of particles ranging from 3 to 6 mm in 9883

DOI: 10.1021/acs.energyfuels.6b01870 Energy Fuels 2016, 30, 9882−9890

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Figure 2. Schematic diagram of thermal-state fluidized bed combustors.

Table 1. Ultimate and Proximate Analysis of Extremely Low Calorific Oil Shale Semicoke ultimate analysis (%) Car 6.29

Har 0.78

heating value (kJ/kg)

Oar 3.72

Nar 0.6

Sar 0.45

Qar,net 2384

proximate analysis (%) FCar 4.16

Var 7.68

Aar 87.16

Mar 1

Table 2. Particle Size Distributions of Extremely Low Calorific Oil Shale Semicoke particle size (mm) (wt%) test apparatus

samples

6−5

5−4

4−3

3−2

2−1

1−0.71

0.71−0.6

0.6−0.355

< 0.355

mean diameter (mm)

bench-scale fluidized bed pilot-scale fluidized bed

S1 S1 S2 S3

0 0 1.89 7.66

0 0 5.28 10.24

0 0 2.83 12.10

9.21 9.21 6.89 4.32

30.57 30.57 10.28 12.48

10.32 10.32 9.98 11.97

13.57 13.57 17.64 12.87

9.50 9.50 13.09 8.53

26.83 26.83 32.12 19.83

0.96 0.96 1.09 1.86

size is 30%). There were no significant property differences among the samples. 3.4. Experimental Procedures. Before ignition, about 7−15 kg bed materials, which are the ash material of SC, were fed into furnace via the screw feeder. Then, the air and furnace electric heating systems were opened simultaneously to heat bed materials. When the bed temperature was heated to approximately 600 °C, a small amount of extremely low calorific SC was fed into furnace for ignition, and the dense-bed temperature would gradually rise with the combustion of SC in the furnace. Then, the furnace electric heating system was switched off step by step, and the fluidization airflow rate and temperature were gradually adjusted to the designed condition. After reaching a steady state condition17 (steady state temperature and flue gas concentrations) and lasting for 2 h, sampling and measuring tests were performed.

4. RESULTS AND DISCUSSION 4.1. Results and Discussion of Bench-Scale Fluidized Bed Combustion Experiments. 4.1.1. Combustion Stability of SC in the Bubbling Fluidized Bed Combustor. The temperature fluctuations in the furnace under a typical working condition are plotted in Figure 3, where r is the relative height

Figure 3. Temperature fluctuations in the furnace.

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(UC) in ashes showed a downward trend with rising dense-bed temperature, which could be explained by the fact that a higher temperature is benefit to improve the chemical reaction rate.18 The combustion efficiency (η) was defined as19 η = 100 − q3 − q4 (2)

of the furnace, i.e., the height of measuring point to the furnace height. As seen from this figure, it was found that SC could maintain a continuous and stable combustion at the dense-bed temperature of 900 °C, and each measuring point temperature was relatively stable with small fluctuations. Figure 4 showed

where q3 and q4 were respectively the heat loss due to unburned gas and unburned carbon. Results showed that the combustion efficiency slightly increased with the rise of dense-bed temperature. During tests, when the dense-bed temperature was risen from 850 to 950 °C, its fluidization air temperature was gradually elevated from 305 to 500 °C. This could be explained that with the rise of dense-bed temperature, the combustion heat, as well as the heat taken away by ashes and exhaust gases increased, but the increased combustion heat could not offset the increased heat loss taken away by ashes and exhaust gases. Therefore, it was required to elevate the fluidization air temperature to stabilize the dense-bed temperature. Comprehensive considering the combustion stability, environmental impact, air preheater manufacture cost, and other factors, 850−900 °C was the optimum temperature range for SC. 4.1.3. Effect of Excess Air Ratio on Combustion Stability of SC. The effect of excess air ratio (α) on combustion characteristics of SC was plotted in Figure 6. It was obvious

Figure 4. Oxygen concentration profile along the furnace height.

the oxygen concentration profile along the furnace height. It was observed that with the increase of furnace height, the oxygen concentration decreased sharply, and then gradually tended to be smooth. As a result of low volatile matter content and no secondary air injection, the combustion of SC was mainly concentrated in the dense-phase zone of the furnace. Additionally, since the cooling area of the combustor was much greater than the combustion area, the temperature of the upper part of the furnace declined rapidly. In conclusion, these phenomena suggested that the combustion share of dilutephase zone was smaller, and the furnace operated in the bubbling fluidized state, meanwhile, the combustion in the furnace could be completely relying on SC’s own heat release. 4.1.2. Effect of Dense-Bed Temperature on Combustion Stability of SC. The effect of dense-bed temperature on combustion characteristics of SC is shown in Figure 5. As seen from Figure 5, it was found that unburned carbon content Figure 6. Effect of excess air ratio on combustion characteristics of SC.

that as α was increased, UC in fly ash increased, which was mainly due to a shorter residence time in the furnace.20 On the contrary, UC in bottom ash decreased, which was attributed to a stronger turbulence, a larger oxygen concentration, and a greater fragmentation degree in the dense-phase zone of the furnace.21 But in summary, its combustion efficiency did not show a significant increase or decrease with increasing fluidization air velocity. During tests, the fluidization air temperature was monotonically elevated from 330 to 600 °C when α was increased from 1.19 to 1.94. The reason was that as the fluidization air velocity was increased, more fuel particles were entrained into the dilute-phase zone of the furnace, thus increasing the combustion share of dilute-phase zone. But its total combustion heat was almost invariant, hence, the combustion heat released in the dense-phase zone was reduced accordingly. On the other hand, the heat taken away by exhaust gases increased with

Figure 5. Effect of dense-bed temperature on combustion characteristics of SC. 9885

DOI: 10.1021/acs.energyfuels.6b01870 Energy Fuels 2016, 30, 9882−9890

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Energy & Fuels increasing fluidization airflow rate. As a consequence, the fluidization air temperature was also required to be elevated to maintain dense-bed temperature stable. After tests, approximately 60−75% of the ash was separated from the furnace as bottom ash, and as expected, the mass fraction of bottom ash gradually decreased with increasing α. 4.1.4. Effect of Static Bed Height on Combustion Stability of SC. The effect of static bed height on combustion characteristics of SC was shown in Figure 7, and it was

Figure 7. Effect of static bed height on combustion characteristics of SC.

observed that there were no obvious changes in UC of ashes and its combustion efficiency when the static bed height was increased from 300 to 500 mm, and the fluidization air temperature was basically kept at about 400 °C. One of the most likely explanations was that SC had high ash content. It is reported that the samples with high ash content are more likely to have the trend to follow a shrinking sphere model.22,23 At the initial combustion stage, the oxygen could easily spread to its external surface and react with surface embedded carbon particles. As the combustion progressed, an ash shell was progressively generated to surround its external surface and make oxygen and heat diffusion difficult. And with the increase of ash layer thickness, its burning rate became slower and slower. At this moment, prolonging the combustion time of SC had little impact on UC of bottom ash. Figure 8 represented SEM images of SC and its ash, and it was found that they were featured in dense surfaces, and the framework of particle was formed through the close integration of mineral microparticles. The X-ray fluorescence result indicated that most of its mineral composition was SiO2. Additionally, after tests, it was also observed that the external surface of SC ash particle was tawny, but its core was still black. In consequence, these phenomena suggested that during the combustion of SC, it formed a shrinking sphere model. On the basis of comprehensive analysis of combustion stability and fan energy consumption, it was advised that the static bed height was maintained at about 300−400 mm. 4.1.5. Relationship Between Combustion Parameters and Sectional Thermal Load. In order to understand the intensity of combustion in a quantitative way, the sectional thermal load of the furnace (qf) was introduced herein, as expressed in eq 3.

Figure 8. SEM images of SC and its ash: (a) SC, (b) SC ash.

qf =

(

Msc × Q ar,net − 3.37 × Mba ×

Cba 100 − Cba

+ M fa ×

C fa 100 − C fa

)×A

ar

1000 × Sf

(3)

where Msc was the mass of SC per second (kg/s). Qar,net was the heat value of SC (kJ/kg). Cba and Cfa represented UC in bottom ash and fly ash (%), respectively. Mba and Mfa were respectively the mass of bottom ash and fly ash per second (kg/ s). Aar was the ash content of SC (%). Sf was the cross-sectional area of the furnace (m2). The following conclusions could be drawn from Figure 9. At dense-bed temperature ranging from 850 to 950 °C, the value of qf was slightly increased from 0.472 to 0.486 MW/m2. 9886

DOI: 10.1021/acs.energyfuels.6b01870 Energy Fuels 2016, 30, 9882−9890

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4.2. Results and Discussion of Pilot-Scale Fluidized Bed Combustion experiments. Figure 10 shows temper-

Figure 10. Temperature profiles along the furnace.

ature distributions of the furnace when burning S1, S2, and S3 in the pilot-scale fluidized bed combustor. Results indicated that S1, S2, and S3 could be steadily burnt at the dense-bed temperature of approximately 850 °C through adjusting the fuel feeding rate, and the furnace outlet oxygen concentrations were 11.02, 5.63, and 3.84%, respectively. This behavior was mainly related to combustion characteristic differences among samples.24 The proportion by weight of large particles in S1 was the lowest, and its average burnout rate was the highest. On the contrary, the proportion by weight of large particles in S3 was the highest, and its average burnout rate was the lowest. Thus, under the same fluidization airflow rate and temperature, when adopting S1, the lowest fuel feeding rate was able to guarantee the steady combustion of the furnace and its furnace outlet oxygen concentration was the highest. While for S3, its fuel feeding rate was the highest and the furnace outlet oxygen concentration was the lowest. Figure 11 summarizes UC distribution characteristics in different particle size of bottom ashes. Obviously, it was found that UC in bottom ash increased with increasing particle size. The reason was that the specific surface area was increased as the size of the fuel particles was decreasing, which was a benefit for the combustion. Consequently, with increasing particle size, it took longer time to completely burn and its burnout rate was lower, which was consistent with Liu’s results.25

5. GENERAL DESIGN SCHEME OF OIL SHALE SEMICOKE FLUIDIZED BED BOILER Based on the above analysis, it was demonstrated that extremely low calorific SC could be directly burnt in the fluidized bed combustor, and its optimal utilization scheme was to use the generated high temperature and pressure stream to drive steam turbine to generate electricity.26 A general design scheme of extremely low calorific oil shale semicoke fluidized bed boiler was related as follows. The whole structure is shown in Figure 12. Along the flow direction of the hot gas, the boiler in turn included a furnace with a membrane waterwall, two cyclones, a high-temperature superheater, a low-temperature superheater, inclined convection tubes, a high-temperature air preheater, a low-temperature

Figure 9. Sectional thermal load of the furnace under different conditions: (a) dense-bed temperature, (b) excess air ratio, (c) static bed height.

However, within the scope of excess air ratios and static bed heights, the value of qf was basically maintained at about 0.48 MW/m2. 9887

DOI: 10.1021/acs.energyfuels.6b01870 Energy Fuels 2016, 30, 9882−9890

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low calorific value and huge consumption, the sectional dimension of the distributor plate was relatively large. To avoid slag-bonding and nonuniform absorption heat of bed materials, an ignition system below the plate and a wind box with a membrane waterwall were employed.27 In addition, the external bed was also adopted to prolong the combustion time of circulating ash separated by cyclones dramatically, hence, according to actual combustion states of the furnace and the external bed, the circulating ash could be directionally controlled to guarantee the steady combustion of the furnace. In order to improve boiler efficiency, heat taken by a large amount of hot slag released from the furnace must be retrieved as soon as possible. In view of high ash content of extremely low calorific oil shale semicoke, the composite fluidized bed bottom ash cooler28 was adopted, which consists of a separation chamber and three cooling chambers. Along the flow direction of ash particles, there were evaporating heating surfaces and a high-temperature economizer respectively arranged in the cooling chamber I and II. The recycled flue gas was used as the fluidization medium of separation chamber and cooling chamber I and II, to avoid the reburning of slags, and the normal temperature air was used as the fluidization medium of cooling chamber III. On the other hand, the heated gas was returned back to the tail flue from the return air inlet. Gas passing through the ash separator was then divided into two paths. One was introduced into the ash cooler as the fluidization medium with the aid of gas recirculating fan. Another was sequentially entered into the subsequent flue gas treatment equipment. After being heated in economizers, feedwater was fed into the stream drum, and then it was separated out into four circulating paths. The first path was stream drum-evaporating heating surfaces-stream drum. The second path was stream drum-the side lower header-the side upper header of the boilerstream drum. The third path was stream drum-the back lower header of the boiler-wind box-the front and back waterwall-the back upper header of the boiler-stream drum. The forth path was stream drum-immersed tube heating surfaces-inclined convection tubes-stream drum. In addition, the temperature of superheated stream was adjusted by a spray-type desuperheater lying in the outlet of the superheaters. In summary, the most advantage of this proposed system was the high-efficiency recovery of physical sensible heat of hightemperature ashes, and the heat loss due to sensible heat in bottom ash was reduced obviously. Currently, the prevailing method of shale oil produced in China is still using Fushun-type retorts, resulting in a portion of fine oil shale particles (size