GT) - American

Article pubs.acs.org/EF

Fuel-Slurry Integrated Gasifier/Gas Turbine (FSIG/GT) Alternative for Power Generation Applied to Municipal Solid Waste (MSW) Marcio L. de Souza-Santos* and Kevin B. Ceribeli Department of Energy, Faculty of Mechanical Engineering, University of Campinas, Campinas, SP 13083-300, Brazil ABSTRACT: The Fuel-Slurry Integrated Gasifier/Gas Turbine (FSIG/GT) concept for thermoelectric power generation is applied to the case of Municipal Solid Waste (MSW). This alternative allows fuel feeding to a power unit based on gasification using commercially available slurry pumps without the need of usual sequential or cascade feeding systems. It also dispenses with the need of pure oxygensometimes combined with hydrocarbonsto promote ignition of particles in the injected slurry. In the presently proposed process, the fuel slurry is prepared to high content of dry-solid and pumped into a dryer, from which the solid particles are fed into the gasifier. Since both equipment operate under similar pressures, simple rotary valves and Archimedes’ screws might carry the secondary feeding. The gas is cleaned to bring the particle content and size as well alkaline concentration within the acceptable limits for injections into standard gas turbines. Heat-recovering systems are applied to power steam turbine cycles, consequently increasing the overall power-generation efficiency. The present study shows the theoretical feasibility of applying FSIG/GT process to the case of MSW leading to overall efficiencies at similar or even higher values than the achieved by conventional processes based on steam turbines. Additionally, it applies relatively simple bubbling fluidized bed dryers and gasifiers, thus avoiding the use of complex and costly boilers.

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INTRODUCTION The proper disposal and use of Municipal Solid Wastes (MSW) for power generation remains among the most pressing problems of medium to large cities. In most cases, landfills or simple incineration are applied,1 thus with little or no return as useful energy. High moisture content and low average heating value are among the various hurdles to use MSW as fuels for power generation.1−15 Additionally, due to the proximity of urban areas, any process has to conform to low pollutant emission standards. Combining minimization of environmental effects and efficient thermoelectric power generation efficiency is always a challenge.1−15 The relatively high temperatures found in the combustion chambers of conventional processes based on pulverized fuel combustion lead to relatively high NOx concentrations in flue gases.16−18 On the other hand, many technical obstacles have been removed since the introduction of the Coal Integrated Gasification/Gas Turbine (CIG/GT) and its biomass equivalent (BIG/GT) processes.19−27 The gas cleaning to remove particulates and alkaline species to meet acceptable levels for injection into turbines was one of the major hurdles28−31 that has been overcome;32,33 however, another important technical barrier, represented by the difficulty of feeding solid particulate fuels into pressurized vessels, remains. It is well-known that it is not possible to feed particulate solid fuels from atmospheric conditions into pressurized vessels in a single stage without meeting unsurmountable problems. For instance, attempts to do so using feeding screws would compact the particles into high-density blocks that would not disintegrate into smaller particles again inside the reactor. The process can even surpass the maximum torque of the feeding screws leading to mechanical failures. Cascade or sequential feeding systems, composed of two or more levels of pressurized lock hoppers (http://www.google.com/patents/ US20110146153), are the most common alternative to avoid © 2013 American Chemical Society

such problems. The particulate fuel is fed at the top hopper. From it, the fuel is conducted to a second hopper below through a rotary valve. The pressure in the second hopper is higher than the first above; however, the difference of pressures between the two hoppers is within the capacity of the rotary valve to keep the difference between the two environments. Furthermore, partial devolatilization of the fuel may start due to temperature increases when the pressure is raised in the hopper. If so, tar would be released, causing the particles to stick together, thus preventing them to proceed or drop into the rotary valve. Usually, an inert gas, such as nitrogen, is employed to keep the pressurized atmosphere inside the hoppers. This prevents the onset of pyrolysis and even combustion of the particulate fuel in the hoppers. Then the solid fuel goes through another rotary valve to a third hopper below, which is kept to an even higher pressure than the one above it. The pressure at the final destination, the capacity of rotary valves to keep environments under different pressures without leaks, and the maximum gradual compression applied to inert gas injected into each hopper without provoking fuel devolatilization determine the number of stages or hoppers. The whole procedure consumes expensive inert gases, thus introducing costs to the power generation unit not to mention losses on the overall efficiency of the unit due to power diverted to inert gas compressions and cooling. Additionally, it relies on complex sequential operations, which are prompt to failures mainly due to interruptions of continuous flow of fuel downward to the next hopper and respective rotary feeding valve. Static electricity building-up among particles and entanglements of neighboring particle extremities might Received: September 18, 2013 Revised: November 20, 2013 Published: November 22, 2013 7696

dx.doi.org/10.1021/ef401878v | Energy Fuels 2013, 27, 7696−7713

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Figure 1. Configuration A of the proposed FSIG/GT process. C = Compressor, CB = Combustor, CD = Condenser, CL = Cleaning system, CY = Cyclone, D = Dryer, DF = Dried fuel, FE = Screw feeding, FS = Fuel-slurry pumping, G = Gasifier, GT = Gas turbine, SG = Steam generator, SR = Solid residue, ST = Steam turbine, P = Pump, V = Valve or Splitter.

originate such problems, particularly in the case of fibrous materials commonly present in MSW. Pumping fuel slurries into pressurized vessels has been applied for a long time.19 It greatly simplifies the feeding process and, very likely, decreases the capital, operational, and maintenance costs when compared with methods based on cascade systems of hoppers. On the other hand, the application of slurry feeding to power generation has been confined to the use of boilers19,34−38 because the vaporization of the fuel original moisture, added to the water to prepare the slurry, demands burning substantial fraction of the fuel. This was confirmed by simulation studies conducted during the preliminary phases of the present work. Those showed that direct gasification of fuel slurry would render very low efficiencies. Other processes need to apply pure oxygensometimes combined with hydrocarbonsto promote ignition of particles in the injected slurry.38 The present work presents two alternatives to solve that problem as follows: a) Configuration A, shown in Figure1, where no steam is injected into the gasifier. b) Configuration B, shown in Figure 2, where an intermediary extraction (stream 12) from the main steam turbine cycle. The same basic principles are applied to both configurations. Referring to Configuration A, water is added to the wet MSW to form a high dry solid content slurry (Stream 26), which is pumped by Equipment 17 into the bubbling fluidized bed dryer (D) that operates at around 2 MPa. Gas Stream 28 is used for the drying process. That stream is part of the gas turbine (Equipment 3) exhaust after driving the Rankine

heat-recovering cycle (Equipment 4 to 8). Stream 15 is compressed, leaving Stream 28 at suitable temperatures for the slurry drying process. As shown ahead, detailed simulations are applied to obtain the mass flow of Stream 28 required for slurry drying. Since the dryer (D) and gasifier (G) operate at similar pressures, the dried fuel can be fed into the gasifier using simple rotary valves combined with Archimedes’ screws. Cyclones and dust collectors are used to drop the content as well maximum diameter of particles in Stream 16 at values acceptable for injections into gas turbines (values are described ahead). Then, Stream 16 is cooled at Equipment 11 in order to reach temperatures below the dew points of alkaline species; such procedure decreases their concentrations to values acceptable for injection of Stream 4 into the gas turbine (Equipment 3). The energy recovered from that gas cooling drives another Rankine Cycle composed by Equipment 11 to 15. Since water is required at key gasification reactions. The simple combustion of the fuel provides water to the gasification. However, the heating value of the produced gas might be increased if additional water is available. In view of that, the Configuration B has been studied (Figure 2). There, a partial extraction between Turbines 5 and 7 provides steam (Stream 12) for the gasification process. Both alternatives have been analyzed and compared as described ahead.

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PROCESS DEVELOPMENT The conceptual development of the present process involved optimizations of gasification and drying processes as well of the power generation architecture. Since those phases are coupled, many simulation trials are required including revisiting one stage after improvements on the other. 7697


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Figure 2. Configuration B of the proposed FSIG/GT process. C = Compressor, CB = Combustor, CD = Condenser, CL = Cleaning system, CY = Cyclone, D = Dryer, DF = Dried fuel, FE = Screw feeding, FS = Fuel-slurry pumping, G = Gasifier, GT = Gas turbine, SG = Steam generator, SR = Solid residue, ST = Steam turbine, P = Pump, V = Valve or Splitter.

The Comprehensive Simulator of Fluidized and Moving Bed Equipment (CeSFaMB) has been validated39−49 and applied39−57 to various types of equipment, including gasifiers, consuming a wide range of fuels. [CeSFaMBTM is also known as CSFMB (www.csfmb.com).] Therefore, it was used here for optimizing the gasification and drying units. A brief description of the model concept is presented in Appendix A. Details of the mathematical model behind the latest version can be found elsewhere.49 The Exergetic (or second Law) Efficiency could be applied as objective function for dryers and gasifiers. Nonetheless, Cold Efficiency seems more appropriate as objective function for

cases of gasifiers because the produced gas requires cooling in order to condense alkaline compounds before its injection into gas turbines. The Industrial Process and Equipment Simulator (IPES) software has been applied to many previous works34−37,58 particularly to develop and optimize power generation processes. [Apart from published works,34−37,58 IPES has been applied to many R&D projects. A list can be found at www.desouzasantos. info.] Balances of mass and energyaccording to the first and second Law of Thermodynamicsare performed around each equipment or Control Volume. Those provide a matrix with temperature, pressure, and composition of streams involved in the whole process. Once solved, the temperature, pressure, composition, and other physical-chemical properties of each

Table 1. Main Characteristics of the Fuel (MSW) Consumed by the Process property high heating value (dry basis) proximate analysis (wet basis) moisture volatile fixed carbon ash ultimate analysis (dry basis) C H N O S ash

value 22.30 MJ/kg 36.72% 52.64% 6.02% 4.62% 53.00% 7.32% 1.32% 30.96% 0.10% 7.30%

Figure 3. Cold gas efficiencies against rates of air and steam injections into the gasifier. 7698


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stream are printed. In addition, the overall process parameters such as efficienciesare computed.

Table 2. Gasifier Main Characteristics and Operational Conditions at Configurations A and B

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values main input conditions or parameters bed internal diameter (m) bed height (m) freeboard internal diameter (m) freeboard height (m) insulation thickness around the bed and freeboard (mm) number of flutes in the distributor number of orifices per flute diameter of orifices (mm) fuel feeding position (above the distributor) (m) mass flow of feeding fuel (dry) (kg/s) mass flow of injected air (kg/s) temperature of injected air (K) average pressure inside the equipment (MPa) mass flow of injected steam (kg/s) temperature of injected steam (K) main output conditions or parameters mass flow of gas leaving the equipment (kg/s) mass flow of solids discharged from the bed (kg/s) mass flow of solids reaching the top of freeboard (kg/s) fluidization voidage (bed middle) fluidization superficial velocity (bed middle) (m/s) average temperature at the middle of the bed (K) average carbonaceous particle diameter in the bed (mm) average carbonaceous particle diameter at freeboard top (mm) carbon conversion (%) pressure loss at the distributor (kPa) pressure loss in the bed (kPa) TDH-transport disengaging height (m) rate of energy input by fuel to the equipment (MW) total rate of energy input to the equipment (MW) combustion enthalpy of hot gasa (MJ/kg) combustion enthalpy of cold gasb (MJ/kg) rate of energy output by hot gas (MW) rate of energy output by cold gas (MW) hot efficiency (%) cold efficiency (%) exergy flow brought with the dry fuel (MW) exergy flow brought with the injected gas (MW) exergy flow brought with the injected steam (MW) total entering exergy flowc (MW) exergy flow leaving with the gas (MW) total exiting exergyd (MW) ratio between total leaving and entering exergy flows (%) ratio between the exergy leaving with the produced gas and the total entering exergy (%)

Config. A 4.0 4.0 4.0 6.0 114

ASSUMPTIONS The main assumptions used in the present work are listed below: 1) The Bubbling Fluidized Bed technique has been chosen for the fuel gasification and drying; however, similar results might be achieved using Circulating Beds, Entrained Flow reactors, or other equivalent processes. Nonetheless, it is important to mention that the Bubbling Fluidization technique allows high flexibility regarding variations in fuel particle sizes, density, and properties;49,59 therefore, it is more suitable to be applied in cases of MSW. Furthermore, usual MSW contains glass and metals that might pass through selective separations and enter the dryer and even the gasifier. That could cause problems if the gasifier operates at temperatures high enough to melt those materials. The bubbling technique is somewhat safe because, in most cases, it operates at relatively low temperaturesusually below 1100 K.49 Additionally, proper distributor designs allow removing relatively heavy particlessuch as small stones, metals, or glassthat would fall on the bed base without interrupting the dryer or gasifier operations.49 2) MSW properties were taken from the work of Gidarakos et al.5 due to its completeness and reproduced in Table 1. The HHV was computed from the ultimate analysis using known relations described elsewhere,49 which agrees with those reported in the literature.9 It is important to notice that those properties vary widely.1−15 Thus, the results achieved by the present study should be reviewed for specific cases. 3) The MSW particle size distribution was set to provide average diameter around 2 mm. That value was reached after preliminary simulations in order to allow good operational conditions for the dryer and gasifier, while not requiring great expenditures in gridding. 4) Cylindrical (typical of fibrous materials) has to be chosen as the basic form of MSW after gridding because cardboard, paper, and food residues usually represent a significant portion of that residue. In any case, variations regarding the dominating particle shape should not compromise the results achieved here. 5) Apparent and real particle densities of moist MSW are assumed to be similar to those of average biomass or as 720 and 1400 kg/m3, respectively. 6) Consumption rate of moist MSW at 28.45 kg/s wet or 18 kg/s (dry basis): Such should be the approximate rate of waste generated by cities with up to 3 million inhabitants.8 Nonetheless, the process can be scaled up or down to consumption rates differing from the assumed here, and such changes should not contradict the main conclusions arrived in the present study. 7) Before injected into the dryer, water is added to the wet MSW in order to form slurry with final dry solid percentage of 44.29%. Commercially available equipment should be able to pump such sort of slurry.60 This has been confirmed by a large piston pump manufacturer [http://www.schwingbioset.com/] that even provides some assurance that slurries with up to 50% dry-solid content can be pumped into high-pressure vessels. Therefore, the value assumed here might be conservative. 8) The average internal pressure of gasifier is set at 2 MPa, and the dryer operates at slighter higher pressure to ensure that the dried solid would be able to be fed into the gasifier using

Config. B 4.0 4.0 4.0 6.0 114

5 × 104 5 × 104 10 10 3.0 3.0 2.0 2.0 18.0 18.0 15.0 15.0 765 765 2.0 2.0 0.0 3.0 ----805 values Config. A ConFig. B 32.06 1.234 0.321

35.12 0.463 1.013

0.680 0.194 966.14 0.967

0.709 0.201 929.81 0.793

0.098

0.080

79.55 0.01 29.93 4.347 376.93 384.28 10.21 9.49 327.49 293.82 85.22 76.46 566.7 7.37 0.00 574.1 322.7 323.5 56.35

80.19 0.01 21.01 4.465 376.93 387.31 9.37 8.99 328.94 292.57 84.93 75.54 566.7 7.37 4.12 578.2 326.4 327.0 56.56

56.21

56.46

“Hot gas” refers to the temperature, pressure, and composition as found at the exiting point from the gasifier. b“Cold gas” refers to the gas properties if at 298 K, 101.325 kPa, dry and tar free. cSum of exergies brought by gases, liquids, or solids injected or fed into the gasifier. dSum of exergies carried by gases, liquids, or solids leaving the gasifier. a

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Figure 4. Temperature profiles at the gasifier bed region (Configuration A) (EMULS.GAS = gas in the emulsion phase; BUBBLE = gas in the bubble phase; CARBONAC. = solid carbonaceous particles; INERT = inert solid particles or ashes detached from the original solid fuel; AVERAGE = average among all previous phases).

Figure 5. Temperature profiles at the gasifier freeboard region (Configuration A) (GAS = gaseous phase; CARBONAC. = solid carbonaceous particles; INERT = inert solid particles or ashes detached from the original solid fuel; AVERAGE = average among all previous phases).

with the energy involved in the heat exchanging to bring the gas leaving the gasifier to temperatures around 800 K. 10) Turbine and compressor isentropic efficiencies equal to 87%. 11) Pump isentropic efficiencies assumed as 95%. 12) Minimum temperature difference between parallel streams entering or leaving heat exchangers is taken as 10 K. 13) Maximum injection temperature into turbines is set at 1700 K. 14) Due to material limitations, the maximum temperatures of tube walls in heat exchangers were set at 900 K.

commercially available rotary valves combined with Archimedes’ screws. 9) Alkaline species, usually present in combustion and gasification gases, might bring serious problems of combined erosion and corrosion to gas turbine blades.28−31 Those components can be removed by cooling the gas stream to values below their dew points, which fall above 800 K.31 The present work applies that lower limit to ensure proper cleaning of flue gas. Having in mind the very low concentration of alkaline species in the gas stream, it is safe to assume that the energy involved in their condensation is negligible when compared 7700


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Figure 6. Bubble sizes and raising velocities through the gasifier bed (Configuration A).

Figure 7. Concentration profiles of CO, CO2, and O2 throughout the gasifier (Configuration A).

Future works might review those assumptions in order to improve the accuracy of simulations. Moreover, equipment dimensions may be further optimized to achieve lower capital costs and higher efficiencies.

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progressed by studying the influences of air and steam injection rates in the gasifier efficiency. The results for Cold Efficiency are summarized in Figure 3. Due to either slugging-flows or temperatures surpassing ash-softening limits, values of air and steam flow rates above and below the ranges presented here led to either lower efficiency values or difficult operational conditions. As seen, the best result was achieved for airflow around 15 kg/s and no steam injection; therefore, it is suited for Configuration A. This result was possible because enough water was available for important gasification reactions after the oxidation of the hydrogen in the MSW structure.49 The basic gasifier dimensions and operational conditions for that

RESULTS AND DISCUSSION

Gasifier. According to the proposed process configurations (Figures 1 and 2), the gasifier does not receive recycled streams. Thus, it was simpler to start with its optimization. Many variations on the gasifier dimensions, mass flow of injected air, and steam have been tried. Once the basic dimensions that led to good fluidization conditions as well as feasible operations have been achieved, the optimization 7701


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Figure 8. Concentration profiles of CO, CO2, and O2 throughout the gasifier (Configuration A) in logarithmic scale.

Figure 9. Concentration profiles of H2O, H2, and CH4 throughout the gasifier (Configuration A).

operation are listed at Table 2. That table also shows the best achieved conditions for the case of Configuration B, when a mixture of air and steam is injected into the gasifier. The table presents only the most important input and output parameters. A more complete description of the variables and results respectively required and provided by the simulator is shown in Appendix B. Since Configuration A led to the best overall results, most of the ahead discussions concentrate on that alternative. Figures 4 and 5 show the temperature profiles of various phases in the bed and freeboard, respectively. The average temperatures throughout the equipment remained within the usually found for bubbling bed operations, i.e., relatively low

and uniform when compared to more conventional processes based on combustion on grades or fuel suspension.16,17,49,59 This should reflect into lower investments in materials, insulation, and control. Figure 6 shows that no large bubbles are produced, therefore, with no risk of a slugging-flow operation. Figure 7 illustrates the average concentrations of CO, CO2, and O2 throughout the gasifier. At the bed region (from 0 to 4 m) the lines represent the average concentrations of each species found in the emulsion and bubbles. Figure 8 presents the same graph in a logarithmic scale to more clearly illustrate the decrease of oxygen concentration at regions near the distributor. 7702


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Figure 10. Concentration profiles of H2S, NH3, and tar throughout the gasifier (Configuration A).

Figure 11. Upward mass flow of particles throughout the gasifier freeboard (Configuration A) (CARBONAC. = solid carbonaceous particles; ABSORBENT = sulfur dioxide absorbent solid particles. Not present in this case; INERT = inert solid particles or ashes detached from the original solid fuel).

Figure 9 shows the evolution of other important fuel gases. It also demonstrates how the concentrations of those gases can only be maintained or significantly increased after severe decreases on the oxygen molar fraction. The surges of fuel gas productions, around 2 m above the distributor, are due to the pyrolysis of feeding fuel. Most of the water forms near the distributor (z = 0) and is derived from the oxidation of hydrogen originally present in the solid fuel. Other variations of concentrations are caused by the combination of several reactions taking place in the gasifier. The complete list of reactions considered by CeSFaMB can be found elsewhere.45,49

Figure 10 illustrates the release of tar near the MSW feeding position and its destruction due to cracking and coking inside the bed. This represents an important characteristic of fluidized beds, which avoids the presence of tar in produced gas. The efficient disentanglement between particles and gases in the freeboard is shown in Figure 11. That and the absence of tar should minimize investments on cyclones, which constitute part of the gas-cleaning process before injection into turbines. Table 3 presents the composition of streams obtained by MSW gasification at both configurations. As expected, the 7703


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heterogeneous reactions do not result solely from the influence of chemical reactions but also due to resistances for the mass-transfers of gases to and from the porous reacting solid particles.45,49 Thus, higher water vapor concentrations in the gas phase do not necessarily lead to greater production of hydrogen by heterogeneous reactions. The simulations have shown that in the present cases, the mass-transfer resistances control the rates of many gas−solid reactions. The main reason for the differences between production of H2 and CO between Configurations A and B rests on the Shift Reaction (CO + H2O = CO2 + H2). The rate profiles of that reaction in both cases can observed by comparing Figures 14 and 15. As seen, more water in the gas phaseas found in the gasification under Configuration Bled to higher rates of the Shift Reaction than in the gasification under Configuration A. Moreover, as CeSFaMB takes into account not only the kinetics of each reaction but also for limitations due to chemical equilibrium, greater concentrations of water forces the equilibrium to the right side of the Shift Reaction. That results into lower concentrations of CO and higher of H2 at Configuration B. There are many other influences in the production and consumption of various chemical species. A detailed account of all reactions considered by the CeSFaMB model as well as the most important iterations during gasification and combustion processes can be found elsewhere.49 Dryer. A simpler approach was used for the dryer optimization because the objective was simply to use as a minimum fraction of the gas turbine exhaust (Stream 28 at Configuration A and Stream 32 at Configuration B) as possible to dry the MSW slurry. Therefore, for each configuration, the optimizations of the dryer operation and the whole process were conducted in parallel. The best results for the dryer operation under Configuration A are listed in Table 4. The results achieved for Configuration B are similar. The temperature profiles of various phases throughout the dryer bed and freeboard are shown in Figures 16 and 17,

Table 3. Composition of the Gas Exiting the Gasifiers at Configurations A and B molar percentages chemical species

Config. A

Config. B

H2 H2O H2S NH3 NO NO2 N2 N2 O O2 SO2 CO CO2 HCN CH4 C2H4 C2H6 C3H6 C3H8 C6H6 tar

26.5051 4.0931 0.0308 0.9716 0.0000 0.0000 27.1793 0.0000 0.0000 0.0050 28.6827 7.1187 0.0367 5.0437 0.1509 0.1173 0.0056 0.0053 0.0542 0.0000

28.4857 8.5121 0.0279 0.8976 0.0000 0.0000 24.5476 0.0000 0.0000 0.0045 20.7938 11.5177 0.0098 4.9022 0.1363 0.1059 0.0050 0.0048 0.0489 0.0000

steam injection at the gasifier operating with air and steam (Configuration B) led to a higher concentration of hydrogen in the produced gas than from the option using just air (Configuration A). On the other hand, the reverse is observed in the case of carbon monoxide. As shown in Figures 12 and 13, that happened despite relatively similar rates of the main reaction between the carbonaceous solid and water, or C + H2O = CO + H2. [This is just a simplistic representation. The heterogeneous reactions involve other important chemical species in the solid fuels, such as H, N, O, and S. More details can be found elsewhere.45,49] This is understandable since the rates of

Figure 12. Rates of important heterogeneous reactions in the emulsion phase of gasifier operating at Configuration A. 7704


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Figure 13. Rates of important heterogeneous reactions in the emulsion phase of gasifier operating at Configuration B.

Figure 14. Rates of important homogeneous reactions in the emulsion phase of the gasifier operating at Configuration A.

respectively. As seen, the temperature of gas leaving the dryer is relatively low, thus minimizing the energy losses from the process. This is also shown by the relatively low loss of exergy (near 7%) carried by the gas leaving the dryer (Table 4). Process. The present results are the product of a first round of optimizations. Tables 5 and 6 list the mass flows and properties of each stream under Configurations A and B, respectively. The main overall results achieved for both configurations are summarized in Table 7. It should be noticed that heat exchangers 4 and 11 (Figure 1) should operate with parallel flows to avoid tube wall temperatures

above 900 K. A similar procedure would be required if Configuration B (Figure 2) is adopted. Configuration A should be preferred due its slightly higher efficiency in relation to Configuration B. The small difference in Gasifier Cold Efficiency achieved by operation under Configuration A (shown in Table 2) is not decisive. The main reason for the difference rests on the fact that Configuration B requires an intermediate turbine extraction (Stream 12, Figure 2), which diverts steam to gasification that otherwise could be used to generate more power. Additionally, the former alternative should provide savings in capital costs by avoiding the additional equipment and controlling systems demanded by the intermediate steam turbine extraction. 7705


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Figure 15. Rates of important homogeneous reactions in the emulsion phase of gasifier operating at Configuration B.

Both Configurations A and B led to overall efficiencies that matches or slightly surpasses the reported overall 30% (based on the first Law of Thermodynamics) achieved in typical power plants based on MSW.61 It also matches efficiency values around 33% arrived at recent studies.37 Moreover, processes with the present configurations operate with dryers and gasifiers without internals. Those are simpler and very likely less costly than boilers. Future economic analysis might confirm that.

Table 4. Dryer Main Characteristics and Operational Conditions (Configuration A) input condition or parameter

value

bed internal diameter bed height freeboard internal diameter freeboard height insulation thickness around the bed and freeboard number of flutes in the distributor number of orifices per flute diameter of orifices slurry feeding position (above the distributor) mass flow of feeding fuel (36.72% wet) mass flow of water added to form the fuel slurry percentage of dry fuel in the slurry mass flow of injected gas (stream 28) temperature of injected gas (stream 28) average pressure inside the equipment output condition or parameter

4.0 m 3.0 m 8.0 m 10.0 m 114 mm 5 × 104 10 3.0 mm 0.5 m 28.45 kg/s 12.19 kg/s 44.30% 55.0 kg/s 930.0 K 2.01 MPa value

mass flow of gas leaving the equipment concentration of water in the leaving solid fluidization voidage (bed middle) fluidization superficial velocity (bed middle) mixing index in the bed tar flow at the top of the freeboard pressure loss at the distributor pressure loss in the bed exergy flow brought with the slurry exergy flow brought with the injected gas total entering exergy flowa exergy flow leaving with the gas total exiting exergyb ratio between leaving and entering exergy flows ratio between the exergy leaving with the produced gas and the total entering exergy

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CONCLUSIONS The new power generation processhere called Fuel Slurry Integrated Gasification/Gas Turbine or FSIG/GTis applied to the case of MSW. Two basic configurations have been studied, and the respective power efficiencies compared. Notwithstanding close results, the configuration based on gasification using just air led to higher overall efficiency than the other using air and steam as gasifying agents. This is because the steam for gasification should be extracted from one of the coupled Rankine cycles, thus preventing it from generating more power. Despite the low heating value of MSW, the study also shows the possibility of achieving overall efficiencies nearing 35%. That matches and even surpasses the present value of 30% [based on the first Law of Thermodynamics] obtained in more traditional processes61 as well as other recently studied alternatives that led to efficiencies around 33%.37 However, different from processes based on boilers, the present alternatives apply much simpler central units such as fluidized bed dryers and gasifiers, without any internals. Consequently, it is very likely that the configurations shown here would result in savings of capital costs. FSIG/GT is also advantageous when compared with other alternatives using pressurized gasification19−27 because it allows feeding particulate fuel as slurry, thus simplifying that operation when compared with traditional cascade systems. Additionally, it dispenses the need of pure oxygen and hydrocarbons required to ignite the fuel particles when injected as slurry.38

77.65 kg/s 0.0% 0.826 0.443 m/s 1.000 0.000 kg/s 0.08 kPa 2.23 kPa 578.7 MW 38.25 MW 616.9 MW 43.08 MW 390.1 MW 63.30% 6.98%

a

Sum of exergies brought by gases, liquids, or solids injected or fed into the dryer. bSum of exergies carried by gases, liquids, or solids leaving the dryer. 7706


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Figure 16. Temperature profiles at the dryer bed region (Configuration A) (EMULS.GAS = gas in the emulsion phase; BUBBLE = gas in the bubble phase; CARBONAC. = solid carbonaceous particles; INERT = inert solid particles or ashes detached from the original solid fuel; AVERAGE = average among all previous phases).

Figure 17. Temperature profiles at the dryer freeboard region (Configuration A) (GAS = gaseous phase; CARBONAC. = solid carbonaceous particles; AVERAGE = average among all previous phases).

2) The equipment is separated in two main regions: dense region (or bed in cases of bubbling condition) and lean region (or freeboard in bubbling processes). 3) The dense or bed region is divided in two main phases: bubble and emulsion. 4) There are three possible solid phases: fuel, inert, and sulfur absorbent such as limestone, dolomite, or mixture of those. Ash, eventually detached from the spent fuel, would constitute part of the inert solid phase. 5) The emulsion is composed by solid particles and percolating gas.

The present preliminary study would be followed by others including economic evaluations. Additionally, further investigations would explore the limits of solid content in the slurry. That might lead to improvements on the overall efficiency.

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APPENDIX A

Figure 18 presents a simplified scheme of the model behind CeSFaMB. The basic assumptions and computational strategy can be summarized as follows: 1) The unit operates in steady-state regime. 7707


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Table 5. Description of Conditions at Each Stream of the Proposed Process (Configuration A) stream

fluid nature

temp (K)

pressure (kPa)

mass flow (kg/s)

enthalpy (kJ/kg)d

entropy (kJ/kg/K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

air air gasa gas gas gas waterb water water steam steam water water gas gas gasc steam steam water water water water water air air slurry slurry gas

298.00 763.43 800.00 1698.90 1044.83 384.50 298.00 298.00 354.58 1034.00 433.66 370.00 370.05 384.46 384.46 963.00 950.00 525.13 364.21 364.26 298.00 298.00 313.33 298.00 766.40 298.00 298.01 931.55

101 2000 2000 1990 120 110 110 130 110 10000 123 120 10100 108 108 2000 10000 500 480 10010 110 130 110 110 2200 110 2200 2200

213.00 213.00 320.60 245.06 245.06 245.06 500.00 500.00 500.00 50.00 50.00 50.00 50.00 190.06 55.00 32.06 2.50 2.50 2.50 2.50 100.00 100.00 100.00 15.00 15.00 40.63 40.63 55.00

−0.21044E0 0.48833E3 −0.27153E4 0.45241E2 −0.80832E3 −0.15643E4 −0.15906E5 −0.15906E5 −0.15668E5 −0.11960E5 −0.13179E5 −0.15603E5 −0.15592E5 −0.15644E4 −0.15644E4 −0.24405E4 −0.12162E5 −0.13007E5 −0.15627E5 −0.15617E5 −0.15906E5 −0.15906E5 −0.15842E5 −0.22843E0 0.49169E3 −0.15906E5 −0.15904E5 −0.94588E3

0.67402E1 0.68598E1 0.87851E1 0.79927E1 0.80856E1 0.69806E1 0.21844E1 0.21844E1 0.29147E1 0.10852E2 0.11100E2 0.30943E1 0.30949E1 0.69858E1 0.69858E1 0.90974E1 0.10648E2 0.10819E2 0.30278E1 0.30283E1 0.21844E1 0.21844E1 0.23947E1 0.67164E1 0.68366E1 0.21844E1 0.21846E1 0.71080E1

a

After cleaning to set alkaline concentration within acceptable levels. bWater = liquid water. cAfter cleaning to set particle size and content within acceptable levels. dEnthalpy values include the formation and sensible terms.

composition may change in the freeboard. Moreover, particles may exhibit large gradients of temperature and composition in the bed and freeboard. 17) Compositions and temperatures of all gas and solid phases vary in the freeboard and are computed using complete differential and energy balances.49 18) Particle size distributions modify due to chemical reactions, attritions between particles themselves, as well due to the entrainment and recirculation processes. Those are also taken into account to compute the size distributions of each solid phase in the bed and freeboard. 19) Heat and mass transfers in the axial or vertical direction within each phase are considered negligible when compared with the respective transfers in the radial or horizontal direction between a phase and neighboring ones. 20) At each axial position (z), mass transfers between phases result from differences of species average concentrations at each phase. As soon chemical species are consumed or formed by reactions, they are subtracted from or added to the respective phase. Therefore, these effects appear as sink or source terms in the mass continuity equations for each phase. 21) At each axial position (z), heat transfers between phases result from differences of temperature at each phase. These terms would appear as sinks or sources in the energy conservation equations.49 22) At the basis of the dense region (z = 0), the two-phase model49 is applied to determine the splitting of injected gas stream between emulsion and bubble phases.

6) Bubbles are assumed free of particles. 7) Emulsion gas is considered inviscid, therefore rises through the bed in a plug-flow regime. 8) The same as above is assumed for the bubble gas. However, dimensions, raising velocity, fraction of bed volume occupied by bubbles, as well other characteristics of bubbles are considered in all calculations regarding that phase. 9) Bubbles and emulsion exchange mass and heat. 10) Mass transfers also occur between particles and emulsion gas. 11) Heat transfers also occur between all phases, including particles. 12) Gases are assumed transparent regarding radiative heat transfers. 13) Emulsion gas exchanges heat with the vessel or reactor walls. Therefore, all heat transfers between the walls and other phases (bubbles and particles) take place indirectly through the emulsion gas. 14) All phases exchange heat with surrounding or eventually immersed surfaces (such as tube banks or jackets) in or around the bed and freeboard. 15) Heat transfers to tube banks or jackets are computed point-by-point between those and bed as well freeboard. Eventual phase changes inside the tubes or jackets are also computed. 16) The average composition for each solid particle is computed in the bed or dense region through convergence procedures involving the solutions of differential mass and energy balances described elsewhere. 49 However, their 7708


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Table 6. Description of Conditions at Each Stream of the Proposed Process (Configuration B) stream

fluid nature

temp (K)

pressure (kPa)

mass flow (kg/s)

enthalpy (kJ/kg)d

entropy (kJ/kg/K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

air air gasa gas gas gas waterb water water steam steam steam steam steam water water water gas gas water water gasc steam steam water water water water water air air gas slurry slurry

298.00 763.43 800.00 1698.89 1059.41 404.66 298.00 298.00 345.98 1049.00 805.40 805.13 805.13 445.05 370.00 290.86 290.91 398.86 398.86 298.00 298.00 867.04 857.00 463.78 396.74 396.79 298.00 298.00 369.54 298.00 766.40 949.53 298.00 298.01

101 2000 1990 1980 120 1100 110 130 110 10000 2400 2350 2350 123 120 115 10010 110 110 101 120 2000 10000 500 480 10010 110 130 110 110 2200 2200 110 2200

205.00 205.00 351.80 240.12 240.12 240.12 500.00 500.00 500.00 45.00 45.00 3.00 42.00 42.00 42.00 45.00 45.00 185.12 55.00 3.00 3.00 35.12 1.30 1.30 1.30 1.30 10.00 10.00 10.00 15.00 15.00 55.00 40.64 40.54

−0.21044E0 0.48833E3 −0.36677E4 −0.14397E3 −0.10001E4 −0.17653E4 −0.15906E5 −0.15906E5 −0.15704E5 −0.11923E5 −0.12440E5 −0.12440E5 −0.12440E5 −0.13157E5 −0.15603E5 −0.15936E5 −0.15926E5 −0.17718E4 −0.17718E4 −0.15906E5 −0.15906E5 −0.35509E4 −0.12385E5 −0.13129E5 −0.15495E5 −0.15479E5 −0.15906E5 −0.15906E5 −0.15605E5 −0.22843E0 0.49169E3 −0.11359E4 −0.15906E5 −0.15904E5

0.67402E1 0.68598E1 0.88402E1 0.80719E1 0.81646E1 0.64100E1 0.21844E1 0.21844E1 0.28113E1 0.10886E2 0.10970E2 0.10979E2 0.10979E2 0.11150E2 0.30943E1 0.20830E1 0.20836E1 0.70626E1 0.70626E1 0.21844E1 0.21844E1 0.89783E1 0.10402E2 0.10570E2 0.33898E1 0.33904E1 0.21844E1 0.21844E1 0.30891E1 0.67164E1 0.68366E1 0.71851E1 0.21844E1 0.21846E1

a

After cleaning to set alkaline concentration within acceptable levels. bWater = liquid water. cAfter cleaning to set particle size and content within acceptable levels. dEnthalpy values include the formation and sensible terms.

Table 7. Overall Efficiency Data for Configurations A and B mechanical power inputa (MW) mechanical power outputb (MW) net mechanical power output (MW) energy rate input by fuel (MW)c exergy rate input by fuel (MW) efficiency based on 1st Lawd (%) efficiency based on 2nd Lawe (%)

alternative A

alternative B

146.11 272.24 126.13 362.63 578.70 34.78 21.80

142.30 259.90 116.91 362.63 578.70 32.24 20.20

nuclei. Unreacted-core or exposed-core can be set as primary models. 24) Boundary conditions for the gas phases concerning temperature, pressure and composition at (z = 0) are given by the values of injected gas stream. 25) At each iteration, boundary conditions at z = 0 for the three possible solid phases (carbonaceous, sulfur absorbent, and inert) are obtained after differential energy balances involving conduction, convection, and radiative heat transfers between the distributor surface and the various phases. 26) The solution of differential equations describing the energy and mass transfers proceed from the distributor (z = 0) to the top of freeboard or lean region (z = zF). The values at the top of the bed or dense region (z = zD) are used as boundary conditions for the bottom of lean one. 27) For the first iteration, a carbon conversion is assumed. After solving the system of coupled nonlinear differential equations throughout the equipment, the new carbon conversion is computed. Conversions of all other solid-phases components are computed as well. 28) The cyclone system is simulated and all characteristics of the collected particles are obtained. If those are recycled to the

a

Due to compressors and pumps. bFrom steam and gas turbines. Based on the LHV. dDefined as follows: (useful mechanical power output)/(rate of energy input by fuel). eDefined as follows: (useful mechanical power output)/(rate of exergy input by fuel). c

23) For points above that (z > 0), the mass flow in each phase is determined by fundamental equations of transport phenomena. Those include mass transfers between the various phase as well homogeneous and heterogeneous reactions. The computation of the chemical species consumption or production rates by heterogeneous reaction include not just the chemical kinetics but also mass transfer resistances due to gas boundary layers around the particle, layers of converted porous solids, and porous reacting 7709


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Figure 18. Simplified scheme of CeSFaMB model.

coils, location in the equipment, lengths, pitch, arrange, angular position, internal and external diameter of tubes, linkage strategy between banks, material of tube walls, operational pressure). Additionally, other information may be fed such as material quality and thickness of vessel walls, water or gas-jacket geometries (if used), insulations (bricks of otherwise) thickness and quality, cyclone system design, etc. • Flow rates and characteristics of feeding of carbonaceous particles. These include type of the fuel, proximate and ultimate analysis, apparent and real densities, particle size distribution, and water fraction in slurry (if the case). CeSFaMB accepts up to five different fuels can be simultaneously fed into the reactor, including liquid fuels. • Flow rates and characteristics of feeding of absorbent and inert particles. If absorbent (limestone or dolomite or mixtures of those) and/or inert (such as sand or alumina) are used, similar information as for carbonaceous fuel should be provided. Up to five different absorbents and inert solids can be simultaneously fed into the reactor. • Flow rates and characteristics of injected gas and/or steam streams. The simulator accepts any composition and within a wide range of temperature and pressure of pure or mixture of gases or liquids injected into the equipment. • Flow rates and characteristics of eventually injected intermediate gas streams. Besides the gas flow through bottom distributor, the simulator also accepts intermediate injections of gas with any composition and conditions. The position of injections should be informed. CeSFaMB also accepts up to

bed, CeSFaMB includes such a stream into the mass and energy balances during iterations. 29) Steps 25 to 28 are repeated until convergence regarding a weighted overall deviation is achieved. That weighing considers deviations between assumed and computed conversions of chemical species as well between assumed and computed heat transfers among phases and immersed surfaces in the bed and freeboard. This and the tight coupling of all chemical and physical phenomena involved in the equipment, ensures consistency regarding all mass and energy balances. Once the simulation is concluded all internal and overall details of the equipment operation such as temperature, concentration, and all other variable profiles throughout the entire equipment, are printed. A graphical interface facilitates the input of data for simulations as well consultation and study of outputs.

■

APPENDIX B

CeSFaMB Inputs

The simulator input list includes various equipment geometry and operational characteristics. Those allow proper description of the unit and allow studies on mechanical as well process design. The main data are related to the following: • Equipment Geometry. Among the most important characteristics there are: hydraulic diameter according to the height of the equipment and positions of bed top, freeboard top, positions of solid particle injections, positions of gas injections, positions of tube banks (if used) in the bed or in the freeboard, as well all aspects of those banks (if straight tubes or 7710


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• Rate profiles for each reaction at each phase throughout the entire equipment. • Main pressure losses at various points or sections of the equipment. • Overall exergy analysis of the unit operation. • If sulfur absorbent is fed into the unit, several efficiencyrelated parameters to sulfur capture. • Most results are offered in the form of graphs or tables. • Details on the mathematical model behind CeSFaMB can be found elsewhere.49

four different gas streams simultaneously injected into the reactor at diverse positions. • Flow rates and characteristics of eventual intermediate gas withdrawals. Besides the usual withdraw of gas stream at the top of freeboard, the simulator allows verifying the effects of intermediate withdrawals of gases. For that, just the position and flow of withdrawal should be informed. Up to four different withdrawals from the reactor at diverse positions can be set. • Other operational conditions, such as internal average pressure in the bed and external wind velocity.

■

CeSFaMB Outputs

CeSFaMB provides the following information: • Equipment performance parameters, which include all important overall aspects of the unit operation such as: flow rates of gases and solids leaving the equipment, carbon conversion, mixing rate (allow verification of eventual segregation among solids), residence time of each solid species, TDH, flow rates of tar or oil leaving with gases, etc. • Devolatilization parameters with all aspects of the volatile release during the operation including rates, composition of released gas (includes amount of tar) and average time for complete pyrolysis. • Composition, flow rates and thermodynamic, transport phenomena properties, and adiabatic flame temperatures (in the case of gasifiers) of gas streams. These are supplied at each point inside the equipment (including bed and freeboard) as well of those of produced streams under molar and mass bases. • Composition, particle size distribution, and flow rates of solids or liquids at each point inside the equipment as well of those streams leaving the equipment. • Overall elemental mass balance verification. • Temperature profiles of each gas (emulsion and bubbles) and solid (carbonaceous, absorbent, and inert) throughout the entire equipment are provided. • If tube banks are present, point-by-point profiles of temperature inside the tubes and their walls. • If the unit is equipped with water or gas jacket, profiles of temperature inside the jacket and walls throughout the entire height. • If the unit contains tube bank or water jacket, profiles of steam quality throughout each bank and water jacket. • Process parameters, which includes specific aspects of boilers, gasifiers, dryers, shale retorts, pyrolyser, or any other type of simulated equipment. • Rates and parameters related to heat transfer to ambiance and internals with detailed account of heat transfer rates. • Rates of erosion at tube bank walls and the respective mean-life times. • General warnings to the user related to possible operational problems as well a list of various aspects that might interest the user is presented. Among the most critical, there is the possibility of slugging flow, surpassing solid particle softening temperatures, excessive elutriation rates, low cyclone efficiencies, etc. • Point-by-point information related to the dynamics of fluidization, such as: diameter and rising velocity of bubbles in the bed, void fractions and particle size distributions of all solid species throughout the bed and freeboard, superficial velocities, circulation rates of particles in the bed, and fluxes of solids in throughout the freeboard. • Composition profiles of each chemical species (18 possible components) throughout the entire equipment and at each phase (emulsion, bubbles, gas in the freeboard).

AUTHOR INFORMATION

Corresponding Author

*Phone: +55-19-997107134. Fax: +55-19-3513278. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to express their gratitude to our friend, Senior Engineer Francisco Domingues Alves de Sousa, for his assistance on the matter of feeding systems of particulate fuels to pressurized vessels as well by indicating limits for thermal resistances of materials.

■

REFERENCES

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