Energy Evaluation for Lignite Pyrolysis by Solid Heat Carrier Coupled

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Energy Evaluation for Lignite Pyrolysis by Solid Heat Carrier Coupled with Gasification Qun Yi, Jie Feng,* Bingchuan Lu, Jing Deng, Changlian Yu, and Wenying Li* Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, China, and Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology, Taiyuan 030024, China ABSTRACT: In order to utilize lignite in a clean and highly efficient way, an energy system for lignite pyrolysis by solid heat carrier coupled gasification is proposed in this study. The process is simulated and analyzed by Aspen Plus 11.1 on the basis of experimental data. The energy consumption distribution of the system and the mass ratio of the solid heat carrier to lignite, the most important technological parameters, are revealed. The choice of gasifier has the greatest impact on the energy efficiency of the system. Results show that, with a lignite handling capacity of 41.7 t·h−1, the yields of tar and coal gasified gas are 1.6 and 25.7 t·h−1, respectively, and 17% of the char is burnt to supply energy for the system while the remainder is used in the gasifier. Also, the surface moisture present in lignite and the phenol water from the tar can be utilized as the gasification agent in the coupled process, saving up to 8.9 t·h−1 water and decreasing the handling capacity of phenol water by 2.7 t·h−1, thereby reducing the net volume of polluted water emitted by the system. It is possible for the system as a whole to achieve an energy efficiency of up to 85.8%. The study also shows that the majority of the energy used by the system is consumed during the drying and pyrolysis processes. Exploiting new technology, integrating and optimizing the energy use of the system to reduce energy consumption will be beneficial to improving the overall system performance.

1. INTRODUCTION As an inexpensive and easily obtained fossil energy, coal plays an important role in the world’s energy structure, and most of the world’s electricity is currently generated by coal combustion. The BP statistical review of world energy shows coal to be the fastest growing fossil fuel worldwide, accounting for more than 30% of global energy consumption in 2011, the highest percentage since 1969.1 However, the utilization of coal brings many environmental problems. Air pollution is mainly caused by the combustion of coal, the source of 90% of SO2, 70% of soot, 67% of NOX, and 70% of CO2 found in the air, the presence of which results in acid rain, the greenhouse effect, and ozone depletion.2,3 It is unlikely that the present coaldominated energy structure will change any time soon (before 20504) as drastic changes would have a significant impact on maintaining society’s current rates of development according to the current energy demands. Nowadays, as the utilization of high quality coal resources is not sufficient to meet the ever-growing demand for energy, many countries are investigating the development of technologies to use low-rank coal, such as lignite, which accounts for about 17% of the total reserves of coal resources in the world.5−7 Lignite has for a long time been regarded as a poor quality fuel, its development receiving little attention because of unfavorable characteristics such as high moisture and ash contents, low caloric value, and low ash melting point. Moreover, lignite is unsuitable for long-term storage and longdistance transport due to its high moisture content and tendency to spontaneously combust. As a result, lignite is mainly used for traditional pithead power generation, which is neither environmentally nor economically favorable, and the average energy utilization of a lignite fired power plant, even in ultra supercritical lignite fired power plants, is less than 38%.8 © 2013 American Chemical Society

In order to increase utilization efficiency of rich lignite resources and to reduce resultant environmental problems, the development of more efficient and cleaner ways to use lignite is of great significance. At present, lignite upgrading technology, which involves lignite drying, dewatering, and pyrolysis, as an efficient and clean way for lignite use and has engendered wide interest .9−17 Commercial operation of mechanical thermal expression (MTE), steam fluidized bed drying (SFBD), and K-fuel technology (a registered trademark of Evergreen Energy, Inc. (formerly named KFx)), etc. shows that these upgrading technologies can bring about better performance in the areas of energy, economy, and environment.18−20 However, these technologies have not been widely developed or implemented on an industrial scale due to various technological problems. For instance, HTD (hydrothermal dewatering), MTE, and other upgrading technologies involving the use of gas or steam to dry or supply energy in pyrolysis will inevitably be accompanied by the leaching out of large amounts of organic compounds into an aqueous phase during decomposition reactions, particularly when the treatment is performed under more severe conditions. As a result, wastewater treatment becomes an additional burden of the upgrading process.11,21 In light of this, the use of a solid heat carrier to realize low-rankcoal upgrading has recently come into consideration.22−24 Solid heat carriers manage to avoid the dilution of pyrolytic volatiles and decrease the load of the cooling system and wastewater treatment by making use of sensible heat from char or other Received: December 5, 2012 Revised: July 11, 2013 Published: July 12, 2013 4523

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Figure 1. Design idea for the coupled system.

Figure 2. Flow sheet for lignite pyrolysis by solid heat carrier coupled with gasification.

solids, rather than using a gas or steam/water heat carrier. It is an internally, rather than externally, heated system, allowing the solid heat carrier to directly contact the coal, and allowing the coal to be heated uniformly, thus eliminating local overheating. In general, most processes currently burn char to supply energy, and phenol water is treated as wastewater;25−31 this is not economically justified for using energy and water resources. The so-called “two-stage gasification technology”, which is characterized by physically separating coal pyrolysis from char gasification, might be a potential way for solving the abovementioned problems. Coal is first autothermally pyrolyzed in a fluidized-bed reactor, and its products, including pyrolysis gas, tar, and char, are in turn forwarded to a downdraft fixed-bed reactor to implement char gasification and pyrolysis gas upgrading. Between the two reactors, solid heat carriers are circulated to carry the heat from the gasifier to the pyrolyzer. The sensible heat of the circulating particles sustains partially or completely the highly endothermic fuel pyrolysis reactions

occurring inside the pyrolyzer. The technology can effectively improve the heating values of fuel gas, reduce the tar content in the fuel gas, and also avoid secondary pollution, such as phenolcontaining wastewater.32−34 Lignite being rich in volatiles contains high-valued chemical structures such as aromatic rings. These chemical structures are completely converted into simple molecules of H2O, CO2, CO, H2, and hydrocarbons (C1−C3) in the gasification. Thus, dual bed pyrolysis gasification (DBPG) technology was proposed to extract these valuable chemicals prior to gasification or combustion to produce fine chemicals and fuel oils, and only char was sent to the gasifier in DBPG compared with two-stage gasification.29−31,35−37 DBPG technology has the potential to lead to realization of the effects of polygeneration, low emission, high efficiency, good product quality, and wide fuel adaptability, and much research related to DBPG system simulation and analysis has been undertaken to estimate the system performance.37−41 4524

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Table 1. Proximate and Ultimate Analyses of Coal Samples proximate anal., wt % coal char a

ultimate anal., wt %

Mad

Aad

Vad

FCad

C

H

Oa

N

S

9.69 2.47

11.01 21.44

33.60 15.95

45.7 60.14

58.56 65.06

3.51 3.31

15.83 5.98

1.02 0.95

0.38 0.79

By difference.

Table 2. Experimental Operation Conditions of Key Units units drier temp (°C) pressure (MPa) reactor air ratio O2/char (mass/mass, dafb) steam/char (mass/mass, daf) simuln reactor a

130 0.1

pyrolyzer

coolera

550 0.1

80 0.1

gasifier 1500 3 BGL

combustor 750 0.1 1.2

RSTOIC

RYIELD

0.7 0.63 RGIBBS

HEATX

RGIBBS

−1

b

Simulation data. Data on the basis of dry and ash free; operation condition: lignite feeding rate is 41.7 t·h ; water content in lignite varied from 5 to 40 wt %; solid heat carrier size is 300 μm.

a coupled system, the tar, a high-added-value and high-energygrade chemical product, is first separated for further processing or sale, and the highly active char is directly used for gasification to produce syngas. Also, the surface moisture in lignite and the phenol water produced in pyrolysis are combined for use in the gasifier as the gasification agent. The whole process can avoid the necessity of wastewater treatment or briquetting currently required by the single pyrolysis or gasification process, respectively, thereby realizing maximum utilization of water resources and carbon and hydrogen elements, and achieving the goal of comprehensive use and classified refining for lignite. The whole external system displays near-zero emissions, as shown in Figure 1. The flow sheet of lignite pyrolysis by a solid heat carrier coupled with gasification is shown in Figure 2. The process consists of four operation units: drying, pyrolysis, gasification, and combustion. Pyrolysis and gasification are the key units. The function of the drying unit is to avoid the unwanted effects involving large amounts of energy consumption, long residence time, and the difficulty in the recovery of pyrolytic volatiles and tar caused by the high moisture content in the pyrolysis unit. The combustion unit is configured to burn the pyrolytic volatiles recovered during the drying process with the addition of part of the char to regenerate the solid heat carrier so as to meet the energy demands of the drying and pyrolysis units. The lignite is heated to 130 °C by a portion of the solid heat carrier obtained from the pyrolysis unit, and the steam produced in the drying unit is used as the gasifying agent in the gasification unit. The dry coal then enters the pyrolysis unit to produce tar, char, and pyrolytic volatiles. After being cooled, noncondensable gases from the pyrolytic volatiles are sent into the combustion unit to be burnt together with a portion of char to supply energy for the regeneration of the solid heat carrier (quartz sandstone). The rest of char and phenol water from the oil−water separator are sent directly to the gasifier. The recycle path of the solid heat carrier is shown by the red line in Figure 2. The solid heat carrier is heated in the combustion unit, and then is separated out from the ash by a rotary screen. Afterward, the high temperature solid heat carrier is transmitted to the pyrolysis unit by a bucket elevator, and the coal and the high temperature solid heat carrier are thoroughly mixed in the

On the basis of the above research, in order to develop a method of energy cascade utilization which utilizes the low-rank coal lignite cleanly, efficiently, and with low emissions, lignite pyrolysis by a solid heat carrier coupled with gasification is proposed in this article. The system would make full use of the surface moisture in lignite and the phenol water from tar as the gasification agent, thereby not only avoiding polluted water emissions and high treatment loads, but also negating the need for additional water consumption. Also, tar, as a high-addedvalue chemical product, becomes available for further processing or sale, and the use of char for gasification to produce syngas is a highly efficient and economical way to make use of lignite. With the purpose of revealing the features of energy use and chemical element conversion, and the feasibility and the rationality of the coupled system, some related experiments were performed, and the simulation flow sheet for lignite pyrolysis by a solid heat carrier coupled with gasification was established by Aspen Plus 11.1 on the basis of the experimental data. The energy consumption distribution of the system is revealed, and key parameters and conditions that affect the performance of the process are analyzed. As a result, it becomes relatively easy to identify the potential advantages and disadvantages of this system in terms of energy use, element conversion, and pollutant emissions as compared with single lignite pyrolysis or gasification, and to then optimize the whole production process by improving the technology. These works are not only of theoretical significance for further studying, developing, and optimizing the process, but also significant in the evaluation of practical application.

2. ESTABLISHMENT OF LIGNITE ENERGY UTILIZATION SYSTEM 2.1. Description for the Process of Lignite Utilization. In a single pyrolysis process, the addition of a water treatment system is necessary to deal with the phenol water produced. Also, the surface moisture of lignite gleaned during the drying process is not made full use of. In a single gasification process, only after dewatering and briquetting can lignite undergo gasification, but this results in the production of tar. Compared to the use of lignite being in either of these single processes, in 4525

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Table 3. Yields of Pyrolysis Products (wt %) pyrolysis gases char

tar

water

CH4

CO

H2

CO2

C2H4

C2H6

C3H6

C3H8

68.9

6.2

10.3

2.24

3.3

0.12

7.23

0.51

0.48

0.46

0.27

reaction was expressed as follows: coal→ tar + char + pyrolysis gases. The yields of the pyrolysis products are shown in Table 3. The gasification unit was simulated by the RGIBBS model. In one study, Doherty et al.42 considered the division of the gasification process into three linked processes: pyrolysis, gasification, and combustion. In this study, the gasification process was established by Aspen Plus software. Zhang et al.43 divided the gasification process into two stagesinitial pyrolysis and subsequent char conversionand related research on the gasification reactivity of coal char and carbon was investigated. However, for the coupled process considered in this paper, the gasification process has been divided into only two stages: char gasification and combustion. The function of partial combustion is to supply the heat for the endothermic gasification reactions, and the pyrolysis process is separated out as an independent operation unit in the coupled process. In the RGIBBS model restricted equilibrium specifications can be set to make sure the simulated gasification process more closely resembles actual gasification.37,42,44 Char, as an unconventional component, cannot be handled by Aspen Plus directly, in either chemical or phase equilibrium. Therefore, before the char enters the RGIBBS reactor, it must to be decomposed into its constituent reactants.45,46 The carbon conversion rate was also considered in the simulation process, and the amount of carbon in the ash and slag was controlled by an RSTOIC model. The determination of an appropriate mass ratio between the water, oxygen, and coal is significant in keeping the final temperature of the gasifier at about 1500 °C, matching the temperature of the BGL gasifier and assuring that the effective gas (H2 + CO) and the H/C are within a reasonable range. The combustion unit was simulated by the RGIBBS model, and about 20% excess air was injected into the combustion furnace to ensure full combustion of the char. In the combustion unit, the solid heat carrier was heated to 750 °C. In the simulation process, PR−BM was chosen as the property method, as it is suitable for

pyrolysis reactor by a ribbon mixer. When the pyrolysis reaction is completed (about 30 min), the solid heat carrier is separated out from the char by the rotary screen, and a part of the solid heat carrier is transmitted to the drying unit by an embedded scraper transporter, while the rest of the solid heat carrier is recycled back to the combustion unit to be mixed with the solid heat carrier that had been separated out from the drying unit by the rotary screen. 2.2. Experiment. The proximate and ultimate analyses of coal from HuLunbeier, China, are shown in Table 1. In order to obtain data required in the simulation process, related experiments and product analyses were performed in a fixedbed reactor. The experimental operation conditions of key units are given in Table 2. The yield of each product is shown in Table 3. The proximate and ultimate analyses of char are also listed in Table 1. The composition of coal gasified gas (CGG) is shown in Table 4. Table 4. Composition of Coal Gasified Gas (mol %) compsn

CO

H2

CO2

CH4

others

simuln exptl

54.74 54.32

30.46 29.93

7.13 7.74

5.12 5.48

2.55 2.53

2.3. Theoretical Methods. 2.3.1. Modeling and Simulation. On the basis of the experimental data, the simulation process for lignite pyrolysis by a solid heat carrier coupled with gasification was established (Figure 3). The red line expresses the recycle path of the solid heat carrier, and the dotted line indicates the energy stream. In the simulation process, the drying unit was simulated by the block RSTOIC reactor through inputting the reaction coal (wet) → coal (dry) + 0.055H2O and embedding a calculation block to realize the drying process. The pyrolysis unit was simulated by the RYIELD model, in which the decomposition

Figure 3. Diagram simulation of the coupled system by Aspen Plus. 4526

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Table 5. Yields and Output Temperatures for Fractions fraction temp (°C) yield (wt %)

light oil

phenol oil

naphthalene oil

wash oil

anthracene oil

pitch

≤170 24.47

170−210 17.40

210−230 7.91

230−300 21.94

300−360 11.45

≥360 16.83

any pressure and temperature system.44 This property method uses the Peng−Robinson cubic equation of state with the Boston−Mathias alpha function to calculate all the thermodynamic properties. It is recommended for use in the area of coal chemical industry simulation.47 2.3.2. Assumptions. Assumptions about the system process are as follows: 1. The whole system is a steady process, and heat loss is not considered in the material transportation. 2. The granules of lignite are uniform with no obvious particle diameter gradient. There is no caking as a result of the transportation and reaction processes. 3. The tar products are very complicated in composition, containing more than 10 000 kinds of components, of which only about 500 are identified, so compositions should be simplified when processing the tar, such as the pure substance of C6H6O,48 or the multicomponent mixture of C12H26S, C 15 H 33 N, C 14 H 12 O 2 , and C 10 H 24 O 4 . 49 However, these simplifications may not accurately reflect tar composition. In order to more accurately reflect its composition in this article, tar is divided into six fractional components: light oil, phenol oil, naphthalene oil, wash oil, anthracene oil, and pitch.50 Quality ratios of the six fraction components are obtained by a thermogravimetric analyzer simulating distillation, and the temperature range and the yields of fractions are shown in Table 5. The components of each fraction are very common in structure and have similar boiling points, with the only difference being molecular sizes. According to the literature, the contents of C7H8, C6H6O, C10H8, C12H10, C14H10, and C16H10 are relatively high in the light oil, phenol oil, naphthalene oil, wash oil, anthracene oil, and pitch, respectively, and their physical characteristics are very similar to those of other components in the corresponding fractions.51 Therefore, a mixture of C7H8, C6H6O, C10H8, C12H10, C14H10, and C16H10 compounds was used to represent the real composition of coal tar in the simulation. 4. Potential energy loss was considered in the reaction processes. The energy loss in the drying and pyrolysis units was assumed to be 5% of the supplied energy, the energy loss in the combustion unit was taken to be 5% of the lower heat value (LHV) of the combusted char carried, and the energy loss in the gasification process was taken to be 2% of the LHV of the gasified char owned. The sensible heat recovery ratio was set as 75%. 2.4. Sensitivity Analysis. In this section, effects of the temperature in the pyrolysis unit, the gasifier types, the heat capacity of the solid heat carrier, and the moisture and ash contents of lignite on the performance of the system are investigated. 2.4.1. Temperature of Pyrolysis Unit. The temperature of the pyrolysis unit, as the most important operation parameter, significantly influences the product yield and composition. Tar, with high economic value, is the primary desired product, so a series of experiments were performed to determine an appropriate pyrolysis temperature to extract more tar, as shown in Figure 4. It was observed that the tar yield tended to

Figure 4. Effect of pyrolysis temperature on tar yield.

increase with an increase in temperature. However, when the temperature exceeds 550 °C, the tar yield begins to decrease because of secondary cracking caused by the high temperature. As a result, the optimum pyrolysis temperature for this process was determined to be 550 °C. 2.4.2. Gasifier Types. In order to reveal the effect of different gasifier types on system performance, typical fixed-bed, entrained-flow, and fluidized-bed gasifiers as used in TEXACO, SHELL, LURGI, BGL and U-Gas gasifiers are investigated. The gasifying agent consumption, the power consumption for oxygen preparation, the LHV of coal gasified gas, the carbon conversion efficiency of each gasifier, and the energy utilization of the system are shown in Figure 5. The results indicate that the BGL gasifier is the best choice for the coupled system, because, regardless of the energy utilization of the system, this gasifier gave the highest LHV of CGG or the highest carbon conversion efficiency. 2.4.3. Heat Capacity of the Solid Heat Carrier. The energy required by the drying and pyrolysis units is supplied by the solid heat carrier. The heat capacity determines the amount of solid heat carrier, and then leads to different mechanical energy consumptions. Figure 6 shows the mechanical energy consumption and the amount of energy consumed for five different kinds of solid heat carriers. The heat capacity of red mud is the lowest, which leads to the largest mechanical energy consumption. In contrast, a ceramic ball has the largest heat capacity, and also consumes the least amount of energy. Mechanical energy consumption is positively correlated with the amount of solid heat carrier. However, heat capacity is not the only factor to be considered. Ensuring uniform and rapid heat transfer, which is conducive to tar generation, is another key point. Experimental comparison and analysis proved that quartz sandstone is the optimum solid heat carrier, which has the most desirable effect on heat transfer and highest tar yield. 2.4.4. Moisture Content. The moisture content is a negative factor in lignite pyrolysis. The energy consumption of the 4527

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Figure 5. Effect of char gasification with different types of gasifiers on system performance.

while the energy consumption of the pyrolysis unit decreases. Overall, the total energy consumption of the drying and pyrolysis processes increases along with an increase in moisture content, which leads to more char to being burnt, and the energy efficiency of the entire system is decreased. Therefore, decreasing the moisture content is beneficial to the process. 2.4.5. Ash Content. Effects of ash content on energy utilization of the drying unit, the pyrolysis unit, and the system are investigated. The results are shown in Figure 8. The energy

Figure 6. Effect of different solid heat carriers on energy consumption and tar yield.

drying and pyrolysis units is directly related to the moisture content of lignite, which influences the ratio of char to be burnt to supply energy to the pyrolysis unit and negatively affects energy utilization. As shown in Figure 7, the greater the moisture content, the more the drying unit consumes energy,

Figure 8. Effects of ash content on system performance.

consumption of the drying and pyrolysis units is increased along with an increase in ash content, resulting in a decline of the energy utilization of the system. As a result, reducing the ash content before lignite pyrolysis can not only reduce the energy consumption, but also reduce the ash content in the pyrolysis products, thereby reducing the segregation load in the process of production refining, gasification, and tar−dust separation. In summary, based on the energy efficiency of the system as the goal, the sensitivity analysis of the investigated factors (the temperature in the pyrolysis unit, the gasifier types, the heat capacity of the solid heat carrier, and the moisture and ash contents of lignite) shows that the choice of gasifier has the greatest impact on the energy efficiency of the system.

Figure 7. Effects of moisture content on system performance. 4528

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Figure 9. Diagram of energy distribution.

2.5. Energy Efficiency. In order to reflect a comprehensive evaluation on the energy use of the system, the energy utilization of the coupled system is defined as follows: η=

energy required in drying and pyrolysis processes is a constant value; in order to keep the temperature of the solid heat carrier from becoming too high, the amount of the solid heat carrier must be increased. Combined with the experimental conditions as shown in Table 2, it can be calculated that the mass ratio of the solid heat carrier to the lignite was 3:1, and about 17.0% of char produced from the pyrolysis unit was burnt to supply the energy for the drying and pyrolysis units. 3.2. Feature of Energy Use. By means of an energy balance calculation for each operation unit, the energy distribution of the system can be derived (see Figure 9). The raw coal (169.6 MW) enters the drying unit to be first dried, utilizing energy supplied by the solid heat carrier (13.5 MW), 12.2 MW of energy is lost to steam, and the energy loss of the drying unit is 0.7 MW. Then, the dry coal (170.2 MW) enters the pyrolysis unit, to which the solid heat carrier supplied 9.9 MW of energy, and energy loss in the pyrolysis process is 0.5 MW. The char is then divided into two parts, 83% of which (120 MW) is sent to the gasifier to produce coal gasified gas (CGG, 105.0 MW), losing 2.9 MW in the gasification process. The remaining 17% of the char (24.6 MW) and the pyrolytic volatiles (15.7 MW) are sent to the combustion unit and burned to provide the necessary energy for the pyrolysis and drying processes. After leaving the combustion unit, the solid heat carrier has an energy of 23.4 MW that is sufficient for the energy demand of the drying and pyrolysis units, and the energy lost in the combustion process is 2.0 MW. The tar, a high-added-value product, has an energy of 16.7 MW. The energy consumption in the drying and pyrolysis processes accounts for 49.3 and 36.1% of the whole energy consumption, respectively. That means developing a special catalyst to decrease energy consumption in the pyrolysis process and realize pyrolysis at a lower temperature while also obtaining a higher tar yield will be beneficial to the improvement of the overall system performance. 3.3. Mechanical Energy Consumption. Lignite pyrolysis using a solid heat carrier can avoid the dilution of pyrolytic volatiles and decrease the load of the cooling system and the wastewater treatment produced when using a gas/steam heat carrier. However, the use of a solid heat carrier increases the separation and transportation processes, leading to additional mechanical energy consumption in the system. As a result, the

Q cgg + Q tar + Q rcgg + Q rfg Q wc + Q O + Q air + Q hs + P 2

(1)

where Qcgg represents the LHV of CGG, Qtar represents LHV of tar, Qrcgg represents recovered sensible heat of the high temperature CGG, Qrfg represents recovered sensible heat of the high temperature flue gas, Qwc represents the LHV of coal, QO2 represents sensible heat of the input oxygen, Qair represents sensible heat of the input air, Qhs represents energy required to heat the water, and P represents mechanical energy consumption of the system. If CGG and the sensible heat of flue gas are used to generate electricity by gas and steam in a combined cycle, the whole coupled system can be seen as a tar and power cogeneration system. Therefore, the energy utilization of the tar and power cogeneration system is defined as follows: η=

Q tar + Q cgga + (Q rcgg + Q rfg)b Q wc + Q O + Q air + Q hs + P 2

(2)

where a represents the gas-fired combined cycle power generation efficiency and b represents the steam turbine power generation efficiency.

3. RESULTS AND DISCUSSION 3.1. Utilization of Solid Heat Carrier. The amount of the solid heat carrier was determined by two factors: (1) the temperature of the solid heat carrier should be heated to 750 °C in the combustion furnace; (2) the energy carried by the solid heat carrier must be sufficient to satisfy the energy demands of the drying and pyrolysis units. According to the equation CP(T1 − T2)M = Q, where CP is the specific heat capacity of the solid heat carrier, (T1 − T2) is the temperature difference for the solid heat carrier (the temperature in the drying unit and that in the pyrolysis unit are 420 and 200 °C, respectively), M expresses the mass flow of the solid heat carrier, and Q is the energy demand for the drying unit and the pyrolysis unit, M can be easily calculated. The 4529

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system mainly occurs in the mixing and separation processes, accounting for 1.1 MW, while the material transportation consumes 0.1 MW. The total mechanical energy consumption is 1.2 MW. According to the above results, as the total energy consumption is approximately 27.4 MW, the mechanical energy consumption only accounts for 4.4% of the total energy consumption, implying that the use of a solid heat carrier results in low energy consumption. 3.4. Energy Efficiency. The energy and mass balances are shown in Table 7. It can be seen that, at a handling capacity of 41.7 t·h−1, the yields of tar and CGG were 1.6 and 25.7 t·h−1, respectively. The phenol water (2.7 t·h−1) produced by the system and a part of the surface moisture in lignite (16.0 t·h−1) were combined for use as a gasification agent in the gasifier, so the water discharged from the system was only 9.8 t·h−1, the clean water produced by the drying unit. As can be seen in Table 8, with the same coal input (41.7 t·h−1), the coupled

estimate of mechanical energy consumption is of great importance to the further development and application of the system. The mechanical energy consumption for the system includes the following processes: the mixing and separation of the lignite and solid heat carrier, the mixing of dry coal and the solid heat carrier, the separation of char and the solid heat carrier, the separation of the solid heat carrier and ash, and also material transportation, which is primarily the recycling of the solid heat carrier, and coal and char transportation. In the production process, spiral mixers are used for mixing and rotary screens are used for separation. Bucket elevators and scraper conveyers are used for material transportation. The mechanical energy consumption for material mixing, separation, and transmission in different processes is shown in Table 6.52 It is apparent that the energy consumption of the Table 6. Mechanical Energy Consumption of the System handling capacity (t·h−1)

type energy consumption for material mixing and separation dry coal mixed with solid heat carrier row coal mixed with solid heat carrier separation for char and solid heat carrier separation for ash and solid heat carrier separation for dry coal and solid heat carrier energy consumption for material transportation bucket elevators coal delivery (raw coal) from drying unit to pyrolysis unit (dry coal) from pyrolysis unit to gasification unit (char) from pyrolysis unit to combustion unit(char) from pyrolysis unit to drying unit (solid heat carrier) from pyrolysis unit to combustion unit (solid heat carrier) from drying unit to combustion unit (solid heat carrier) total

power (MW)

energy consumption (MW)

150.6

0.538

0.538

149.2

0.538

0.538

142.7

0.075

0.075

125.7

0.075

0.075

133.1

0.075

0.075

Table 8. Comparison of Technology Index between the Coupled System and Single System pyrolysis (t·h−1)

gasification (t·h−1)

pyrolysis coupled with gasification (t·h−1)

raw coal water oxygen air heat (MW) mechanical energy (MW)

41.7 0 0 79.2 0 1.2

41.7 6.9 10.4 0 17.1 1.5

41.7 0 5 79.2 2.8 1.21

tar char CGG flue gas CO2 emissions (t·t−1 coal) wastewater emissions (t·h−1) energy utilization (%)

1.6 21.3 0 85.2 0.31

0 0 27.7 0 0.13

1.6 0 25.7 85.2 0.42

5.21

12.22

3.49

86.5

83.6

85.8

input

output

41.7 25.6

0.011 0.011

0.026 0.011 0.011

14.7

0.0075

0.0075

0.004

0.004

107.5

0.022

0.022

17.5

0.011

0.011

107.5

0.022

0.022

3.01

system showed the least amount of wastewater emissions, only 3.59 t·h−1, while the wastewater emissions of the single processes of pyrolysis and gasification were 5.21 and 12.22 t·h−1, respectively. Also, CO2 emissions (0.42 t·t−1 coal) of the coupled system showed a decrease of 0.02 t·t−1 coal compared with the total emission (0.44) of the single pyrolysis (0.31 t·t−1

1.2

Table 7. Mass Balance and Energy Balance for the System mass balance (t·h−1) item

input

coal air O2 CGG tar water flue gas ash total

41.7 79.2 5.0

125.9

energy balance (MW) output

25.7 1.6 9.8 85.2 3.6 125.9

item

input

coal O2 air heat CGG tar flue gas water

169.6 0.2 0.6 2.8

173.2 4530

output

125.8 16.7 16.2 7.5 166.2

unit

loss

drier pyrolysis gasifier combustion cooling

0.7 0.5 2.9 2.0 0.9

7.0

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coal) and single gasification (0.13 t·t−1 coal). The gasification agent (steam) in the coupled system was internally supplied and there was no need for additional water to be added to the system, as is the case with the single gasification process, which needs an addition of 6.9 t·h−1 from an external source. Also, for lignite gasification, a large amount of mechanical energy was consumed in the briquetting process (1.5 MW), which was 0.3 MW higher than that of both the single pyrolysis process and the coupled system. Most importantly, the coupled system still maintains a high energy utilization of 85.8%, which is 2.2% higher than that of single gasification and only 0.7% lower than that of single pyrolysis. Utilizing formula 1 and the data presented in Tables 7 and 8, the energy utilization of the whole system can be calculated as 85.8%. The gas turbine combined cycle power generation efficiency is over 55%, while the steam turbine power generation efficiency is only approximately 35−38%.53,54 Provided that the CGG is used as a fuel gas for generating power by a gas and steam combined cycle and has an efficiency of 55%, and that the sensible heat of CGG and the flue gas used for generating power by steam turbines has an efficiency of 35%, the system could potentially generate 67.4 MW of electricity. When considering the energy of the tar produced (1.6 t·h−1), utilizing formula 2, the total energy utilization of the tar and power cogeneration system could reach as much as 48.3%, as outlined in Figure 9.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21076136, 51276120), the National High Technology Research and Development Program 863 (2011AA05A202, 2011AA05A204), and the Program for Changjiang Scholars (2009). The authors thank Ms. Sarah Enslow for English proofreading and editing.

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NOMENCLATURE

Abbreviations

CP = specific heat capacity of solid heat carrier, J·kg −1·K −1 Hi = enthalpy of stream i, MJ·h−1 mi = mass of stream i, t P = mechanical energy consumption of system, MJ·h−1 Q = energy demand for drying unit and pyrolysis unit, MJ·h−1 Qair = sensible heat of input air, MJ·h−1 Qhs = energy required to heat water, MJ·h−1 Qj = heat of unit j exchanged with environment QO2 = sensible heat of input oxygen, MJ·h−1 Qrcgg = recovered sensible heat of high temperature coal gasified gas, MJ·h−1 Qrfg = recovered sensible heat of high temperature flue gas, MJ·h−1 Qtar = low heat value of tar, MJ·h−1 Qwc = low heat value of coal, MJ·h−1 T1 = higher temperature of solid heat carrier, °C T2 = lower temperature of solid heat carrier, °C W = work input or output from system, MJ·h−1

4. CONCLUSIONS Lignite pyrolysis by a solid heat carrier coupled with gasification is a highly efficient and clean way of using lignite, making full use of the surface moisture in lignite and phenol water from the tar as the gasification agent, which not only avoids the issues of wastewater emissions and its subsequent treatment, but also conserves water in general. Furthermore, the tar, a byproduct of the system which possesses high added value as a chemical product on its own, can be utilized for further processing or sale, and the use of char being used for gasification is highly efficient and economical. The whole system realizes the graded refining of lignite and also achieves the aim of making full use of the system’s internal resources, overall resulting in near-zero emissions. However, the energy consumption of the drying and pyrolysis units accounts for nearly 85% of the whole energy consumption, which directly results in a large amount of solid heat carrier utilization and a high ratio of char being combusted to supply the energy required by pyrolysis and accordingly increasing the mechanical energy consumption. Therefore, developing catalysts to be loaded on the solid heat carrier in order to decrease energy consumption during the pyrolysis process and achieving pyrolysis at a lower temperature with a higher tar yield, while integrating and optimizing the energy use of the coupled system to reduce energy consumption in the drying process, is of great benefit to improving the overall system performance, which is the key research direction in the next phase.

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Article

Greek Symbols

υ = hoisting speed, m·s−1 η1 = transmission efficiency of the retarder η2 = gear transmission efficiency

Acronyms

■

Aspen = Advanced System for Process Engineering DBPG = dual bed pyrolysis gasification CGG = coal gasified gas HTD = hydrothermal dewatering LHV = low heat value MTE = mechanical thermal expression RGIBBS = equilibrium reactor RSTOIC = stoichiometric reactor RYIELD = yield reactor SFBD = steam fluidized bed drying PR−BM = Peng−Robinson cubic equation of state with the Boston−Mathias alpha function

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 351 6018957. Fax: +86 351 6018453. E-mail: [email protected] (J.F.); [email protected] (W.L.). Notes

The authors declare no competing financial interest. 4531

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