Theoretical Study on the Effect of Glycerol Fraction in Slurries with

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Theoretical Study on the Effect of Glycerol Fraction in Slurries with Biomass Consumed by a Power-Generation Process Marcio L. de Souza-Santos* and Michael A. Camara* School of Mechanical Engineering Department of Energy, University of Campinas, Campinas, Sao Paulo 13083-970, Brazil ABSTRACT: Previous works have shown that glycerol, which is a residue from biodiesel production, can be applied to greatly increase the efficiency of power generation processes consuming biomass, such as sugar cane bagasse (SCB). The proposed process departs from a biomass and glycerol slurry, which is pumped into a pressurized gasifier. The produced gas is cleaned to reach particle content and sizes as well alkaline concentration allowed to be injected into commercially available axial gas turbines. The removal of alkaline is achieved through cooling the gas to temperatures below the dew-point of those chemical species. Energy recovered from that cooling and from the gas turbine exit drives two steam cycles. The present theoretical investigation assesses the effects of glycerol concentration in the slurry with wet biomass fed to the process on its overall efficiency. In addition, the gasifier operational pressure is increased to the limit allowed for air compression by axial compressors without intercooling. This work shows that efficiencies can reach levels nearing 46% and discusses the effect of glycerol fraction in the biomass slurry on the overall power-generation process. The present work is a technical feasibility study. Economic analysis or considerations are beyond its scope and have been left for future investigations.

1. INTRODUCTION On the one hand, thermoelectric power generation based on renewable sources, such as biomass, have gained substantial ground on the overall picture of power generation when compared with more traditional ones, such as coal and other fossil fuels. Such progress has been described in the literature,1−5 and that has led to positive environmental aspects.6,7 On the other hand, the production of biodiesel has increased reaching substantially. In Brazil alone, it reached around 4 000 000 m3 in the present year,8 and 11% of that is generated as glycerol.9 Applications to chemical industry are limited not just by the amount but also not justifiable refining in economic terms.10,11 However, that has also triggered inefficient consumption of crude glycerol in boilers or combustors12 and even when applied to enhance solid fuels fed to traditional power generation cycles.13−16 Gasification of solid fuels and glycerol mixtures is not new;17 however, no work had been published on applying that for power generation. Only recently, an inaugural proposition18 explores two alternatives depicted in Figures 1 and 2. At configuration A (Figure 1), 50% of wet sugar cane bagasse (SCB) leaving the mill is fed to a fluidized bed dryer (D) operating at atmospheric pressures. Thus, no special measure is required for that feeding that can be accomplished by Archimedes screws. Stream 28, which is a fraction of the gas turbine (equipment 3) exhaust is used for that drying. The dried fuel (DF) is mixed with the raw glycerol and injected into the pressurized gasifier (G) by commercially available slurry pumps. The stream exiting the gasifier (stream 16) goes through cleaning to drop the particle content and their maximum size to values feasible for injections into gas turbines. The alkaline concentration of that stream should also be decreased to reach acceptable levels by commercial turbines. Such is accomplished by dropping the temperature of that stream to values below the dew-points of those alkaline. In that process, steam is generated, which drives the Rankine cycle composed by equipment 11 to 15. © XXXX American Chemical Society

The clean gas stream 3 is injected into the combustor (equipment 2) and then into the gas turbine 3. Energy is recovered from that exhaustion by driving the steam cycle composed by equipment 4 to 8. At configuration B (Figure 2), SCB, with its typical 50% moisture when leaving the mill, is mixed with raw glycerol to form a slurry (stream 24), which is pumped by equipment 15 into the gasifier (G). As in the previously described process, cyclones and filters drop the particle content in the produced gas (stream 14) as well their maximum size of particles to values acceptable for injections into gas turbines. After that, stream 14 exchanges heat at equipment 9 until its temperature falls below the dew points of alkaline species. The recovered heat drives the Rankine cycle composed by equipment 9 to 13. Stream 3 enters the combustor and its exiting stream 4 is injected into the gas turbine (equipment 3). The energy recovered from the turbine exhaust (equipment 3) powers another Rankine Cycle composed by equipment 4 to 8. Additionally, it is important to stress that pumping slurries largely simplifies feeding solid particles into vessels. This method has been applied for a long time20 and explored in several previous studies.21−30 The preceding study18 has shown that efficiency levels around 42% might be achieved using the above proposed configurations. It has also demonstrated that, in spite of slightly higher efficiency obtained by configuration A, configuration B would be more attractive because it dispenses the drying step. Thus, savings of capital, operational, and maintenance costs would result. In view of that, the present work applies just to configuration B. Differently from the first study,18 in which 2 MPa was assumed for the gasification pressure, 4 MPa is used in the present one. That value allows for compression without intercooling because Received: October 5, 2017 Revised: November 23, 2017 Published: November 24, 2017 A

DOI: 10.1021/acs.energyfuels.7b02996 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Scheme of the fuel glycerol slurry integrated gasifier/gas turbine (FGSIG/GT) process, configuration A. C, compressor; CB, combustor; CD, condenser; CL, cleaning system; CY, cyclone; D, dryer; DF, dried fuel; FE, screw feeding; FS, fuel−glycerol slurry pumping; G, gasifier; GT, gas turbine; SG, steam generator; ST, steam turbine; P, water pump; and V, valve or splitter.

Figure 2. Scheme of the FGSIG/GT process, configuration B. C, compressor; CB, combustor; CD, condenser; CL, cleaning system; CY, cyclone; FS, fuel−glycerol slurry pumping; G, gasifier; GT, gas turbine; SG, steam generator; ST, steam turbine; P, water pump; and V, valve or splitter. gasification optimum condition, and the second sought the optimal overall power-production strategy. The gasification optimization was possible through the application of comprehensive simulator of fluidized and moving bed equipment (CeSFaMB, http://www.csfmb.com), which has been validated31−42 and applied18,22−42 to gasifiers and other equipment consuming a wide variety of fuels. Detailed description of its mathematical structure can be found elsewhere.41 That model has been expanded to include the possibility of consuming mixed solid and liquid fuels.43 The gasifier exergetic efficiency was chosen as the main objective function for that optimization that was carried in two rounds. Those are detailed later in this work. After those two phases, the optimal gasifier operation was found for each glycerol injection rate. The gas composition, temperature, and pressure, derived from each of those operations, was used as main fuel in the overall power generation process. Optimizations of every one of those situations were performed using the Industrial Process and

the exhaust temperature is kept below 950 K, which is the limit value imposed by materials composing axial compressors.19 This decision comes from the fact that increases in the gasification pressure impacts the efficiency of gas turbines and leads to higher overall process efficiencies. However, this picture may change at very high compression ratios, when intercooling should be applied to avoid surpassing temperature limits impose by axial compressor materials.30 Despite that, the main objective of the present work is to verify the sensibility of glycerol concentration on the fuel−glycerol slurry integrated gasifier/gas turbine (FGSIG/GT) process consuming SBC.

2. METHODOLOGY To find the effect of slurry glycerol concentration in the overall power efficiency, two main phases have been devised. The first sought the B

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Energy & Fuels Equipment Simulator (IPES) software. That program has been applied in previous works.22−30,44 It performs mass and energy balances around each equipment to satisfy the first and second laws of thermodynamics. The simulations provide the temperature, pressure, and composition as well other properties of every process stream. In addition, it provides a complete diagnosis of the process, including the first and second law efficiencies.

investigation would not be drastically modified by scaling up or down from that basic rate of fuel consumption. (5) According to the studied configuration B (Figure 2), wet SCB is mixed with glycerol to form the slurry pumped into the pressurized gasifier. The maximum dry-solid content in a slurry able to be handled by commercial pumps47,48 was observed to be around 50%. The present work departs from that condition and tests the effect on the gasification and on the overall power generation efficiencies when higher amounts of glycerol are added to the slurry. (6) The range of glycerol flow added to the biomass to form slurries have been between 3 and 8 kg/s. The lower figure would provide minimum glycerol concentration in a slurry that could be pumped by commercial equipment.47,48 The upper limit seems to be reasonable because the ratio between glycerol and dry biomass in the slurry would be well above one. It seems very difficult to find situations in which that ratio could be even higher. However, future works could explore such a possibility for very particular cases. (7) The exposed-core model is assumed for heterogeneous reactions between the solid biomass and gases between gases. The CIP (coated-inert particle) model is used to model the gas−liquid reactions. It assumes that the feeding liquid fuel coats solid particles in the bed. Details of those models can be found in the literature.41,43 (8) The gas leaving the gasifier should be cleaned to decrease particle content and their sizes to values acceptable for injections into commercial gas turbines. In addition, the alkaline concentrations in that gas must also be decreased for the same aim.49−51 Ceramic filters might be applied to tar removal,52,53 and dropping the temperatures to 800 K, which is below the dew-points of alkaline components, allows for the removal or, at least, significant decrease in the concentrations of alkaline species in the gas stream.54 That temperature is taken as 800 K. (9) The isentropic efficiencies of gas turbines and compressors are assumed as 87%.55 (10) The maximum temperature of streams leaving axial compressors is taken as 950 K.19 (11) Steam turbine isentropic efficiencies equal 87%.56 (12) Pump isentropic efficiencies are assumed to be 90%.57 (13) Minimum temperature difference between parallel streams entering or leaving heat exchangers is taken as 10 K. (14) Maximum injection temperature into gas turbines and steam turbines are set at 1700 and 900 K, respectively. (15) High-temperature heat exchangers can be used in some particular situations.58−60 Nonetheless, some precautions, such as minimizing the regions where those must be applied, is always advisable to avoid excessive capital and maintenance costs. Modifications on those assumptions might be made in future investigations. Despite that, the main results achieved here should not be severely altered. No economic considerations or computations are part of the present work.

3. ASSUMPTIONS The basic main assumptions applied to the present investigations were: (1) The technique of a gasification reactor operating under a bubbling fluidized bed is used; however, others, such as a circulating bed or even entrainment flow, could be applied as well. It is believed that such choices would not drastically modify the main conclusions of the present work. However, it should be stressed that bubbling bed reactors are less stringent than those other techniques regarding the range of feeding particle size and density. One should have in mind that small rocks are easily mixed with the sugar cane during its harvesting. Those have quite different densities than SCB. Nonetheless, if the gas distributor is properly designed, such occurrences lead to no operational problems because heavier materials would fall at the bottom of the bed and could be withdrawn from the reactor without interrupting its operation.41,45 (2) The basic characteristics of SCB are presented in Table 1. Those have been taken from the literature46 and used in Table 1. Main Characteristics of the Fuel (Sugar-Cane Bagasse) Consumed by the Process60 property

value

high heating value (dry basis) 19.14 MJ/kg proximate analysis (wet basis) moisture 50.00% volatile 40.78% fixed carbon 7.57% ash 1.65% ultimate analysis (dry basis) C 49.66% H 5.71% N 0.21% O 41.08% S 0.03% ash 3.31% particle size distribution sieve opening (mm) retained mass (%) 1.680 82.00 0.841 3.91 0.354 9.86 0.250 3.23 0.177 1.00 densities apparent particle 720 kg/m3 real or skeletal 1400 kg/m3

previous works.22−24,28−30 The average size of feeding fuel particles is computed from the usual particle size distribution of SCB leaving sugar mills. (3) A cylindrical shape is assumed for the SCB feeding fibers into the gasifier. (4) SCB at a rate of 10 kg/s (wet basis) is continuously fed into the gasifier. This is compatible with the average production of large sugar-alcohol mills. Nonetheless, it is believed that the main conclusions of the present

4. RESULTS AND DISCUSSION 4.1. Gasifier. Several parameters can be applied as objective functions during gasifier optimizations. The exergetic efficiency is among the most commonly used and defined as: ηexe =

exergy of exiting gas total exergy of streams (gaseous or solid) entering the gasifier

(1) C

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Figure 3. Exergetic efficiencies achieved during the first round of simulations for gasification.

Figure 4. Cold efficiencies achieved during the first round of simulations for gasification.

Figure 5. Average temperatures in the middle of the bed computed for the first round of gasifier optimization.

The exergy of each stream is given by its mass flow (kg/s) multiplied by its specific exergy (kJ/kg). The cold efficiency is also applied and defined as: ηcold =

(given as kJ/kg). Enthalpies should include the formation of chemical species composing each stream or, when appropriate, low heating values of streams plus their sensible terms.41 In view of the present power generation process, the gas from the gasifier would enter a combustor after proper cleaning. During the cleaning, the gas would exchange heat to generate steam for a combined Rankine cycle. Therefore, not just the gas heating value but also its sensible enthalpy would contribute to

enthalpy of exiting gas at 298 K total enthalpy of streams (gaseous or solid) entering the gasifier

(2)

The enthalpy of each stream is represented by its mass flow (given as kg/s) multiplied by the respective specific enthalpy D

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Figure 6. Exergetic efficiencies achieved during the second round of simulations for gasification with 8 kg/s of glycerol injected (Case 6).

Figure 7. Cold efficiency achieved during the second round of simulations for gasification with 8 kg/s of glycerol injected (Case 6).

power generation. Therefore, the exergetic efficiency has been chosen as objective function for the gasification step. Because the objective of the present study is to access the sensitivity of glycerol concentration in the slurry consumed by the process, the following phases for the gasifier optimization, seeking the maximization of the exergetic efficiency, were performed: (1) In a first round, the rates of air injection and glycerol added to the feeding slurry pumped into the gasifier were taken as variables. At this stage, the bed diameter, along all other input parameters (such as operational pressure, feeding rate of solid fuel, bed and freeboard heights, as well all other variables) would be kept constant. (2) In the second round, the rates of air injection and bed internal diameter were taken as variables. Again, all other parameters, including the rate of glycerol added to the slurry, were kept constant. (3) Finally, for each rate of glycerol injection, the gasifier operation leading to the best exergetic efficiency was selected, and the produced gas composition, temperature, and pressure were used to optimize the whole power. That allowed us to draw a picture depicting on how rate of glycerol injection affected the overall power efficiency.

One should also be aware that, as every other technology, bubbling fluidized beds operate within a feasible range, which requires maintaining the superficial gas velocity above the minimum fluidization and below the second turbulent limit at all points in the bed, avoiding slugging flow due to too large bubbles taking the whole internal bed diameter, preventing conditions that might surpass ash-softening temperatures because those would probably lead to bed collapsing, ensuring steady-state operation (as an example, when the bed height cannot be maintained constant because high particle elutriation rate is established, and the loss of mass from the bed is not properly compensated by the feeding rate of solids into it), and avoiding segregation among particles, which usually happens when they present excessively large differences in size and density and the rate of their circulations in the bed cannot break that segregation. Those constraints were observed during preliminary gasifier simulations to select the range of viable operations for a given consumption of wet SCB and glycerol slurries. 4.1.1. First Optimization. As mentioned above, at this first stage, the rates of injected air into the gasifier and of glycerol added to the wet SCB slurry are assumed as variables. For SCB consumption rate at 10 kg/s (wet), the mass flow of injected air E

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Table 2. Most-Important Characteristics and Operational Parameters of Gasifier Leading to the Best Exergetic Efficiency for Each Rate of Glycerol Injection main input conditions or parameters mass flow of injected raw glycerol added to the slurry (kg/s) 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 fuel feeding position (above the distributor) (m) mass flow of feeding SCB (dry) (kg/s) mass flow of feeding SCB (wet) (kg/s) mass flow of injected air (kg/s) temperature of injected air (K) temperature of injected SCB−glycerol slurry (K) average pressure inside the equipment (MPa)

Case 1

Case 2

3 4 9 10 4 4 9 10 10 10 200 200 1 × 104 1 × 104 10 10 0.5 0.5 5 5 10 10 15 17 938.9 938.9 298 298 4.1 4.1 main output conditions or parameters

oxygen ratio mass flow of solids discharged from the bed (kg/s) mass flow of elutriated solids (kg/s) fluidization voidage (bed middle) fluidization superficial velocity (bed middle) (m/s) carbon conversion (%) average temperature at the middle of the bed (K) average temperature of produced gas (K) 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 fuel (MW) exergy flow brought with the injected gas (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 (%)

0.3279 0.5152 1.00 × 10−5 0.7709 4.93 × 10−3 95.69 1079.36 1079.48 0.08 22.42 4.476 132.36 142.65 4.73 4.37 130.42 98.15 91.16 68.6 165.1 10.09 175.19 131.13 132.20 75.46 74.85

0.3325 0.6003 1.47 × 10−3 0.7598 5.07 × 10−3 95.3 1091.43 1091.62 0.1 22.07 4.478 147.72 159.39 4.79 4.38 145.86 109.83 91.51 68.91 173.9 11.43 185.33 146.30 147.30 79.48 78.94

Case 3

Case 4

Case 5

Case 6

5 9 4 9 10 200 1 × 104 10 0.5 5 10 19 938.9 298 4.1

6 12 4 12 10 200 1 × 104 10 0.5 5 10 23 938.9 298 4.1

7 10 4 10 10 200 1 × 104 10 0.5 5 10 23 938.9 298 4.1

8 11 4 11 10 200 1 × 104 10 0.5 5 10 26 938.9 298 4.1

0.3720 0.4145 1.36 × 10−1 0.7869 4.03 × 10−3 96.78 1136.17 1136.56 0.18 19.3 4.385 178.46 194.24 4.63 4.05 178.36 130.96 91.83 67.42 191.8 15.46 207.26 177.14 178.30 86.03 85.47

0.3425 0.4339 3.08 × 10−1 0.8040 5.50 × 10−3 95.2 1165.61 1166.01 0.18 17.92 4.551 193.82 209.61 5.09 4.44 197.46 146.70 94.21 69.99 200.7 15.46 216.16 195.15 197.10 91.18 90.28

0.3588 0.4394 2.09 × 10−1 0.8192 4.58 × 10−3 96.53 1189.14 1189.57 0.23 17.47 4.567 209.19 227.03 5.00 4.34 216.21 158.90 95.23 69.99 209.8 17.48 227.28 212.87 214.50 94.38 93.66

0.3363 0.6274 1.23 × 10−1 0.7921 5.72 × 10−3 94.08 1123.66 1123.86 0.12 18.47 4.574 163.09 176.13 4.83 4.34 160.69 119.78 91.23 68 182.8 12.78 195.58 160.22 162.20 82.93 81.92

“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 the substance is 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

was varied between 10 and 30 kg/s, while the mass flow of glycerol added to the slurry between 1 and 9 kg/s. Those ranges covered a wide spectrum of conditions in which the steady-state feasible operations were found. Situations above or below that range led to unfeasible operations or would not be realistic. Because the production rate of glycerol from biodiesel is usually much lower than of SCB, it is difficult to imagine that it would be physically or economically viable to operate with glycerol rates above the SCB consumption amounts. In this first round, the gasifier internal diameter was kept at 8 m; however, the bed diameter has been taken as variable during the second optimization phase.

Figure 3 shows the values of gasification exergetic efficiencies. The highest achieved value was 89.52% when the air injection was set at 29 and 8 kg/s of glycerol added to the slurry. It is possible noticing that the exergetic efficiency increases with the glycerol and air injection rates until the point when the bubbling fluidized bed starts presenting at least one among the abovementioned operational problems. Figure 4 shows the cold efficiencies computed within the same range of variables. It is interesting to observe that, contrary to the exergetic efficiency, high cold efficiencies were found at the lower ranges of glycerol and air injection rates. That can be understood because according to the coated inert particle or CIP model,43 F

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and, therefore, to faster combustion and gasification reactions, which might overcome the decrease of gas−solid reactions due to ticker glycerol layer around the particles. Thus, above a given rate of glycerol injection rate, higher carbon conversions as well cold efficiencies are observed. Summarizing, to a certain point, keeping the rate of solid fuel injection constant, the cold efficiency decreases when the rate of glycerol injection is increased. That trend is reversed for excessively high glycerol injections due to the significant increase in the temperature inside the bed. However, the exergetic efficiency always increases because, in addition to the heating value of gas, that efficiency also takes into account the temperature of the produced gas. 4.1.2. Second Optimization. For the operation leading to the highest exergetic efficiency with each rate of glycerol concentration in the feeding slurry, as found in the previous round of optimization, the rates of injected air and the bed internal diameter of gasifier were assumed as variables. As an example, Figure 6 shows the exergetic efficiencies when the rate of glycerol injected was 8 kg/s. The highest exergetic efficiency was found after simulations for bed internal diameter between 7 and 13 m. That range was selected because stable and steady-state operations were possible. The best value achieved for the exergetic efficiency was 93.66% and reached for airflow of 26 kg/s, 11 m bed internal diameter, and rate of glycerol injected at 8 kg/s. There are many factors influencing gasification to allow simple justifications for that result. However, it is worth noticing that high gasification efficiencies are commonly achieved for operations with superficial velocities near minimum fluidization conditions. Such leads to smaller bubbles, thus increasing the mass transfer between emulsion and bubbles, which, in turn, augments the contact between gases and reacting particles. Figure 7 shows the cold efficiencies of simulations. Comparing that with Figure 6, it is possible see that exergetic and cold efficiencies follow a similar trend when just the bed internal diameter and air injection rate are taken as variables. Therefore, there is no contrasting behavior between the tendencies of exergetic and cold differences as found during the first round of optimization. Such contrasts happen only when the rate of injected glycerol is involved as variable, which reaffirms the

Table 3. Produced Gas Composition Leaving the Gasifier, Case 6 chemical species

molar percentage

H2 H2O H2S NH3 NO NO2 N2 N2O O2 SO2 CO CO2 HCN CH4 C2H4 C2H6 C3H6 C3H8 C6H6 tar

14.7363 20.3583 0.0022 0.0322 0.0000 0.0000 39.0536 0.0000 0.0000 0.0013 11.8961 11.798 0.0003 2.0463 0.0341 0.0265 0.0013 0.0012 0.0123 0.0000

the liquid glycerol is assumed to coat the surface of solid fuel. This allows the gas around the particle to react with the glycerol layer but impairs the straight access of those reacting gases to the solid fuel. For the same rate of solid fuel feeding to the gasifier, increases in the rate of glycerol injection also increases the average thickness of glycerol layer around each particle solid particle fuel. Thus, the overall carbon conversion might decrease with increasing glycerol injection rate. This explains the initial decrease of cold efficiency for higher rates of glycerol injections. However, because the glycerol heating value is higher than the feeding wet solid fuel, the average temperature in the gasifier, as well of the produced gas, increases with glycerol injection rate. Thus, the exergetic efficiency always increase because it includes not just the gas heating value related to its composition but also the temperature or sensible enthalpy of that gas. In addition, higher glycerol injections led to higher temperatures (Figure 5)

Figure 8. Concentration profiles of CO2, CO, and O2 throughout the gasifier (Case 6). G

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Figure 9. Concentration profiles of H2O, CH4, and H2 throughout the gasifier (Case 6).

Figure 10. Concentration profiles of tar and other species throughout the gasifier (Case 6).

Figures 11 and 12 show the highest gasification cold and exergetic efficiencies achieved at each glycerol concentration in the feeding slurry with wet SCB. 4.2. Process. After the second state of gasifier optimization, at each case of glycerol injection rate, the properties of gas obtained from the gasifier operating at the highest exergetic efficiency were selected for optimizing the whole power generation process. Therefore, for each case (shown in Table 2), the optimization demanded many iterations applying the IPES simulator. The maximization of the first law overall efficiency was taken as the primary objective of those optimizations. To exemplify this, the gas produced under Case 6 (Tables 2 and 3) has been used as stream 14, and, after optimization, the main properties of process streams are listed in Table 4. A few points are worthy of commentary here:

reasoning described above concerning the effect of glycerol injection on heterogeneous reactions. Table 2 summarizes the operational conditions leading to the highest gasification exergetic efficiency for each rate of glycerol added to the slurry with wet SCB. As was seen, the highest exergetic and cold efficiencies are achieved at Case 6 or when the rate of glycerol added to the slurry was the highest within the studied range. Table 3 shows the composition of gas produced at Case 6. Considering that only air was employed as gasification agent, the operations led to good gas quality. Such results can be understood by the relatively high hydrogen content of the bagasse and glycerol, which after combustion provide enough water for important gasification reactions. Figures 8 and 9 show the concentration profiles of main gaseous species throughout the gasifier operating under Case 6 conditions. Figure 10 illustrates the fast release and destruction of tar by coking and cracking at positions near the feeding fuel position, thus allowing for the production of tar-free gas.

(1) For all six simulated cases using configuration B, the rate of energy exchanged during cooling of gas turbine exiting stream (stream 5) is much higher than that involved in the H

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Figure 11. Highest gasifier cold efficiencies achieved for each case of glycerol concentration in the slurry with SCB.

Figure 12. Highest gasifier exergetic efficiencies achieved for each case of glycerol concentration in the slurry with SCB.

rate of inputted energy by the fuel (SCB plus glycerol) fed to the process as denominator, while the second applies the rate of imputed exergy. It should be also remembered that Case 1 represents relatively low concentrations of glycerol in the slurry and that concentration progressively increases to reach the highest value in Case 6. In view of that, there are factors favoring the first Law efficiency when compared to the second law efficiency, as follows: (1) The energy imputed by the fuel is always smaller than the respective exergy (Table 5). (2) Table 5 also shows that the difference between in imputed energy and exergy decreases for higher concentrations of glycerol in the feeding slurry, or from Case 1 to Case 6. This is because the conversion of solids into gases involve thermodynamic irreversibilities derived from the masstransfer resistances for the gas to react with the solid core. That is not the case for liquid fuels. (3) Therefore, for Case 1, the first law efficiency is much higher than the second law one, but that difference decreases toward Case 6.

cooling of gas leaving the gasifier (stream 14). Despite that, the power generated by the Rankine cycles associate with the cooling of gas from gasification are not only required to decrease the alkaline content of that gas stream but also useful to compose an efficient power generation architecture. (2) At the present instance, simple Rankine cycles were applied. Future works should be preoccupied in improving those cycles to provide higher overall power generation efficiencies. Table 5 summarizes the overall power generation parameters for all simulated cases. The first and second law efficiencies are illustrated by Figures 13 and 14. As was seen, relatively high levels of efficiency have been achieved, reaching values close to 46%, which surpassed those achieved in the inaugural work.18 The apparent contradiction between the behavior of first and second law efficiencies achieved for each case can be understood. First, it is important to notice that the both definitions of first and second law efficiencies use ratios involving the net process power output as numerators. Nonetheless, the first has the total I

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(5) That last effect favors the gasification exergy efficiency in relation to the cold one. It also favors the values of overall process second Law efficiency in relation to the computed by first Law. (6) Higher air ratios also demand more power for compression at equipment 14 (Figure 2), which brings penalties to the net power obtained from the process. Nonetheless, the first Law efficiency is more affected by that loss than the second Law efficiency because the denominator of the ratio used to compute the first is smaller than the second. Finally, the combination of those opposing tendencies led to relatively high first law efficiencies at Case 1. That advantage initially decreases when going toward Case 6 but tends to match the two efficiencies when approaching the situation represented by Case 6. Such behavior can be seen in Figure 13. However, the second law efficiency always increase with higher glycerol flow added to the slurry, as portrayed in Figure 14. Another aspect worth of commenting is how the increase in operational pressure from 2 MPa, used in a previous work,18 to 4 MPa as applied in the present study, combined with variations on the rate of injected glycerol, allowed a jump of near 4% on the overall power efficiency. One should also have in mind that no limitation on the temperature of steam turbine injections were taken in the previous study,18 while here, that has been set at 900 K. That value was found after an extensive survey of feasible conditions informed by turbine manufacturers.61−67 Obviously, glycerol enhances the heating value of biomass, and the efficiencies for power generation reflect that in comparison to the process using just biomass−water slurries.22−24,28,29 Additionally, they also greatly surpass the level of 33% estimated for the BIG/GT process consuming SCB68 and, by far, the nowadays 20% achieved in traditional high-pressure Rankine cycles69 operating with SCB. As we commented above, the concentration of glycerol in the slurries are limited at the lower end due to the feasibility of slurry pumps operations and on the higher end to the availability of glycerol from biodiesel units compared to the rate of SCB discharged by a large sugar-alcohol mills. Nonetheless, those limits might be stretched in future investigations. The present work also provides valuable information for decision-making on the proportion of glycerol to be applied in mixtures with SCB aiming power generation process under configuration B. Of course, there are limitations on the available amount of raw glycerol from biodiesel production. Convenience and economic factors should also be taken into account. However, from the fundamental thermodynamic point-of-view, the second law efficiency (Figure 14) instructs for the maximization

Table 4. Main Characteristic of Streams at Configuration B Using Case 6 Gas Conditions stream

fluid nature

temperature (K)

pressure (kPa)

mass flow (kg/s)

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

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

298.00 932.39 800.00 1698.11 963.83 410.00 298.00 298.00 312.95 899.97 388.18 386.80 387.07 1189.57 899.19 387.76 386.80 387.07 298.00 298.00 312.84 298.00 917.38 298.00 298.10

0.101 33 × 103 0.400 00 × 104 0.399 70 × 104 0.398 00 × 104 0.120 00 × 103 0.117 00 × 103 0.101 33 × 103 0.130 00 × 103 0.127 00 × 103 0.999 70 × 104 0.165 00 × 103 0.162 00 × 103 0.100 00 × 105 0.400 00 × 104 0.999 70 × 104 0.165 00 × 103 0.162 00 × 103 0.100 00 × 105 0.110 00 × 103 0.130 00 × 103 0.127 00 × 103 0.110 00 × 103 0.410 00 × 104 0.110 00 × 103 0.410 00 × 104

92.60 92.60 43.43 136.03 136.03 136.03 960.00 960.00 960.00 27.10 27.10 27.10 27.10 43.43 8.13 8.13 8.13 8.13 290.00 290.00 290.00 26.00 26.00 18.00 18.00

a

After cleaning to set alkaline concentration within acceptable levels. Water: liquid water. cAfter cleaning to set particle size and content within acceptable levels. b

However, there are factors that disfavor the first law efficiency in relation to the second law. Those are (1) The thickness of glycerol around SCB particles fed to the gasifier increases from Case 1 to Case 6. (2) Greater liquid thickness tends to impose a barrier for the reactions between gases and the solid carbonaceous particles. That leads to decreases in carbon conversion. (3) In general, to compensate the above effect, optimizations forced the use of increasing oxygen ratios (Table 2). From Case 1 toward Case 6, it led to higher produced-gas temperatures. (4) Higher air ratios imply on more fuel diverted to combustion leaving less fuel for gasification reactions. Thus, the gas quality decreases while the gas temperature increases. Table 5. Main Overall Parameters for Configuration B

value main parameter a

mechanical power input (MW) mechanical power outputb (MW) net mechanical power output (MW) rate of energy input by fuel slurryc (MW) efficiency based on first lawd (%) rate of exergy input by fuel slurry (MW) efficiency based on 2nd lawe (%)

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

44.71 104.6 59.89 132.36 45.25 165.10 36.27

52.298 118.48 66.185 147.72 44.80 173.90 38.06

58.462 130.2 71.741 163.09 43.99 182.80 39.25

62.41 142.26 79.86 178.46 44.75 191.80 41.64

73.89 161.77 87.87 193.82 45.34 200.70 43.78

80.61 176.19 95.58 209.19 45.69 209.80 45.56

a

Due to compressors and pumps. bFrom steam and gas turbines. cBased on LHV. dDefined as (useful mechanical power output)/(rate of energy input by fuel slurry). eDefined as (useful mechanical power output)/(rate of exergy input by fuel slurry). J

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Figure 13. Overall first law efficiencies achieved by the power generation process using the gas from the gasifier that led to the highest exergetic efficiency at each case of glycerol concentration in the slurry with SCB.

Figure 14. Overall 2nd law efficiencies achieved by the power generation process using the gas from the gasifier that led to the highest exergetic efficiency at each case of glycerol concentration in the slurry with SCB.

level of 33% estimated for the BIG/GT process consuming SCB68 and, by far, the 20% achieved in traditional high-pressure Rankine cycles69 operating with SCB. The present work also provides valuable information for decision-making on the proportion of glycerol to be applied in mixtures with SCB aiming power generation process under configuration B. Of course, there are limitations on the availability of raw glycerol from biodiesel production. Convenience and economic factors should also be taken into account. However, the second law efficiency advises maximizing the proportion of glycerol, while the first law efficiency suggests that lower proportions might be considered as an alternative when glycerol availability is limited. Economic analyses are not in the scope of the present study neither the feasibility of glycerol to couple with demands of large power units consuming sugar cane bagasse. Future studies may focus on these aspects as well on improvements in the power generation architecture.

of the proportion of glycerol in the slurry with SCB, while the first law efficiency (Figure 13) suggests that lower proportions of glycerol would provide not too distant efficiencies than those achieved at very high proportions. The reasons for that have been explained above.

5. CONCLUSIONS The investigation of power generation consuming slurries of sugar cane bagasse and glycerol has been expanded to evaluate the effect of glycerol concentration in the slurry. In addition, the gasification pressure has been increased from 2 to 4 MPa, which is near the limit pressure allowed for air axial compressors without intercooling. The configuration of power generation cycle is known as FGSIG/GT, which is a variation of the fuel slurry integrated gasifier/gas turbine (FSIG/GT) proposed to overcome the barrier of feeding particulate fibrous biomass to pressurized vessels through pumping it as a slurry. The inaugural work18 had shown that overall power efficiency could be largely increased by using glycerol to form the slurry. The present one demonstrates how the surge in operational pressure from 2 to 4 MPa, combined with variations of glycerol injection rates, allow the first law efficiency close to 46%. That figure is around 4% higher the found in the previous study with glycerol as slurry media, and despite the obvious enhancement of biomass heating value, the present figures surpasses the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +55-19-97107134. Fax: +55-19-3513278. *E-mail: [email protected]. ORCID

Marcio L. de Souza-Santos: 0000-0002-8493-0146 K

DOI: 10.1021/acs.energyfuels.7b02996 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Notes

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The authors declare no competing financial interest.

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