Development of Studies on Advanced Power Generation Based on

Article pubs.acs.org/EF

Development of Studies on Advanced Power Generation Based on Combined Cycle Using a Single High-Pressure Fluidized Bed Boiler and Consuming Sugar Cane Bagasse Marcio L. de Souza-Santos* and Juan Villanueva Chávez UNICAMP - University of Campinas, Faculty of Mechanical Engineering, Cidade Universitaria, Campinas, SP 13083-970, C. Postal: 6122, Brazil ABSTRACT: Following preliminary studies on power generation processes consuming sugar cane bagasse, the present one indicates the theoretical possibility of substantial gains of efficiency presently achieved at conventional mills. Although using the same basic proposition as before, i.e., a highly pressurized fluidized bed boiler to provide steam above the critical temperature to drive the steam-turbine cycle while the flue-gas is injected into gas turbines, the present configuration is an evolution from a preliminary study because it applies a more elaborate arrangement of equipment. As before, the present round also indicates that an important technical hurdle represented by problems of feeding solid particles to pressurized vessels might be overcome through the application of a slurry injection. Here, the alternative of a thicker slurry, with 40% (wt) solids, is considered. A comprehensive simulator for boilers and gasifiers and a process simulator to predict the main features of the steam and gas turbine branches are applied. Without too much effort, the present process might be applied to other biomass.

■

INTRODUCTION In many instances, biomass can be regarded as a renewable and sustainable energy source. This is particularly true for countries with tropical climate combined with large areas available for crops. The current power-generation units installed in most of the large sugar mills still employ steam-based cycles. Although some apply boilers to generate steam at very high pressure, their efficiencies remain in the neighborhood of 20% efficiency. BIG/GT (Biomass Integrated Gasification/Gas Turbine) technology is among the most proposed alternatives for a technical leap.1−8 On the other hand several hurdles are imposed when a gasification alternative is to be applied to fibrous fuels, such as sugar cane bagasse. One among those is the need of feeding the particulate biomass to a pressurized vessel. Usual systems apply cascade feedings, but those rely on the assembling and correct operation of several hoppers kept under an inert gas atmosphere. This method is too expensive not to mention prone to interruptions due to many combined parts which have to operate flawless. For instance, fibrous bagasse forms domes inside the hoppers and that prevents the continuous feeding to rotating valves or screws. Few solutions for that can be applied but are also cumbersome and not free of problems as well. The alternative of feeding by pumping fuel slurry has been used for a long time for combustors.9 Nonetheless, that is not feasible for most gasification processes because it would add more water to the already very wet fuel. In cases of coals, slurries with as much as 60% (wt) of solids have been applied.9 Others report no problem on pumping of slurries with up to 45% solids.10 Sugar cane bagasse leaves the mill with 50% moisture, and even if relatively low-water slurries could be achieved, we believe that the threshold of 45% wt of dry fuel should be near the limit. Therefore, one may end with fuel containing 55% water or more to be fed into the reactor. Hardly any gasification process would © 2012 American Chemical Society

overcome the loss of efficiency due to the energy requirements to evaporate substantial amounts of water. In addition, our simulation tests have confirmed that as well. Application of liquid fuels to constitute biomass slurries is not a very practical option, because such would take away the mill’s autonomy in terms of producing power based just on its own resources. On the other hand, boilers can operate well with slurry-feeding because the rate of steam generation and area of tube banks can be decreased. That can be combined with operations with low oxygen excesses in order to achieve sustainable combustion. The scheme of the process proposed here is shown in Figure 1. Actually, the present work is a third phase of developments since the first study11 and includes the following improvements: (1) significant changes in the power generation steam and gas branches and (2) greater concentration of dry solid in the feeding slurry. Such possibility has been justified above. Those improvements led to substantial gains on efficiencies when compared with the first study.

■

ASSUMPTIONS The basic assumptions for the present study were (1) Typical large sugar mill consuming 2 million tons of sugar cane per year, which provides 28% on bagasse with 50% moisture. That would lead to fuel input around 180 MW that can be consumed by the boiler. Having in mind timing for maintenance and other random factors, the input around 150 MW was assumed. (2) Bagasse leaves the mill with 50% moisture. (3) Since publications10 have shown the possibility of pumping slurries with 45% (wt) dry solid, it is believed that this should not be far from the truth for the case of bagasse. However, to be on the safe side, Received: December 21, 2011 Revised: February 18, 2012 Published: February 21, 2012 1952

dx.doi.org/10.1021/ef2019935 | Energy Fuels 2012, 26, 1952−1963

Energy & Fuels

Article

feeding bagasse. The choice was set in order to achieve an areavolume average particle diameter around 1 mm, which is assumed to be easily obtainable by simple gridding or cutting equipment. (11) The internal diameter of the boiler at the bed section was 9 m. This value was reached after a first series of simulations to keep the fluidization within usual values of superficial velocity. (12) The bed depth was 5 m. Such a value provides plenty of room for tube banks immersed in the bed. (13) Internal diameter of the boiler at the freeboard section set as 12 m and height as 10 m. Those values were reached after a first series of simulations to ensure enough decrease of superficial velocity in the freeboard to facilitate the inertial separation of particles. (14) The internal and external diameters of tubes immersed in the bed and freeboard set were 30 and 40 mm, respectively. These values seem to be reasonable within the range of possibilities for such tubes and allow usual half-life for the tubes immersed in fluidized beds. More details of the boiler geometry are summarized in Table 1. Of course, those assumptions and design choices could be modified or set as variables in future studies.

Figure 1. Scheme of the proposed process. Equipment symbols: C = compressor, GT = gas turbine, H = heat-exchanger, M = mixer, P = pump or pumps, S = splitter, ST = steam turbine, V = valve.

the present study assumes a feeding slurry with 40% (wt) of dry bagasse. It is also important to stress that the slurry cannot be used to start up the boiler. For that fuel-oil or other high heating value fuel should be applied. The slurry would be pumped into the combustion chamber only after it is well heated. The comprehensive simulator verifies the possibility of sustained combustion through rigorous mass and energy balances. However, even a simple energy balance allows one to verify that a steady-state is possible with waste slurry as fuel, as follows (a) The slurry contains 40% of dry solid. (b) A 150 MW boiler (feeding fuel basis) would consume around 18 kg/s of wet bagasse with 50% original moisture. Therefore, 9 kg/s of dry bagasse would be fed into the chamber. (c) Simulations show that a reasonable operation would require around 56 kg/s of air to be injected into the combustion chamber. (d) The boiling temperature of water at 2 MPa is 487 K, with a vaporization enthalpy of 1.89 MJ/kg. (e) The air stream would reach 765 K after compression to 2 MPa. Therefore, from that temperature to 487 K, the air provides 12.4 MW. (f) The mass of water added to compose the slurry would be 4.5 kg/s. (g) The original moisture contributes with 9 kg/s of water injected into the chamber with the fuel. Therefore, the total water would be 13.5 kg/s. (h) The power input due to fuel combustion plus heated air would be 162.4 MW. (i) As 25.5 MW would be required to evaporate all water (original moisture plus the added to form slurry), it represents around 16% of the power input, thus leaving plenty of energy to produce steam as well hot flue gas. (4) Maximum temperature of gas injected into the turbines was set as 700 K. This is to ensure proper cleaning of flue gas, including conditions for complete condensation of alkaline. There are some discussions on this point in the literature,12 and the chosen value seems to be conservative. (5) Turbine and compressor isentropic efficiencies equal 87%. (6) Pump isentropic efficiencies were assumed as 95%. (7) The minimum temperature difference between parallel streams entering or leaving heat-exchangers is taken as 10 K. (8) Pressure in the fluidized bed chamber is set as 2 MPa. That value was chosen to be well within the range of pressure for the flue-gas to be injected into commercial gas turbines. (9) Pressure inside the tubes immersed in the boiler bed and freeboard was set as 10 MPa. This is also within the range of commercially available boilers. (10) Particle size distribution of

Table 1. Summary of Boiler Main Configuration Parameters parameter internal diameter of the boiler at the bed section bed depth (to accommodate the tube banks) freeboard height (to ensure height taller than TDH) position of the lowest tube of 1st bed bank position of the highest tube of 1st bed bank position of the lowest tube of 2nd bed bank position of the highest tube of 2nd bed bank position of the lowest tube of 3rd bed bank position of the highest tube of 3rd bed bank position of the lowest tube of 1st freeboard banka position of the highest tube of 1st freeboard banka position of the lowest tube of 2nd freeboard banka position of the highest tube of 2nd freeboard banka number of serpentine tubes in each bed bank number of passes in each serpentine length of tubes in the 1st and 3rd bed bank length of tubes in the 2nd bed bank inclination of tubes in relation to horizontal (bed and freeboard) arrangement of tubes in the 1st and 3rd bed banks arrangement of tubes in the 2nd bed bank internal diameter of each tube (bed and freeboard) external diameter of each tube (bed and freeboard) tensile strength of tube material (bed) number of serpentine tubes in each freeboard bank number of passes in each serpentine length of tubes in both freeboard banks tensile strength of tube material (freeboard) arrangement of tubes in the 1st freeboard bank arrangement of tubes in the 2nd freeboard bank bed bank linked to 1st freeboard bank bed bank linked to 2nd freeboard bank internal insulation thickness average thermal conductivity of internal insulation (bed and freeboard sections) number of cyclones main diameter of each cyclone a

1953

value or condition 9m 5m 10 m 0.5 m 2.0 m 0.0 m 4.2 m 2.1 m 3.5 m 5.1 m 6.0 m 6.0 m 7.0 m 14 30 7m 5m 0 deg staggered in-line 30 mm 40 mm 500 MPa 10 50 9m 200 MPa staggered in-line 1 2 114 mm 0.22 W m−1 K−1 20 300 mm

Position above the distributor surface. dx.doi.org/10.1021/ef2019935 | Energy Fuels 2012, 26, 1952−1963

Energy & Fuels

■

Article

MATERIALS AND METHODS

Table 3. Summary of Boiler Simulation Outputs

The present work employed CeSFaMB or Comprehensive Simulator for Fluidized and Moving Beds (www.csfmb.com) as well as IPES or Industrial Plant and Equipment Simulator. The basic characteristics of the mathematical models behind those simulators can be found in the Appendix. Details and validations against real operations are available in the literature.13−23 The strategy used here was (1) Propose a configuration for the pressurized fluidized-bed boiler. That work departed from a relatively simple geometry with ensured bubbling fluidization within the usual range of combustor operations.13,14 (2) Using CeSFaMB, try various options to achieve relatively high exergy efficiency for the boiler. Of course, many variables related to the boiler geometry or configuration and operational conditions could be chosen. Nevertheless, to simplify the work of this first study, the following were taken as variables: (a) number of tube banks and number of tubes in each bank. The simulation program automatically verifies if the geometry of any proposed arrangement is physically possible within the available volumes set for the bed and freeboard. (b) Mass flow of air injected through the distributor at the bed base. The simulator checks if the combustion can be maintained and a steady-state regime is achieved within the boundaries of bubbling bed fluidization. (3) Using IPES and the best results for the boiler operation, simulate the steam and gas branches, as shown in Figure 1. Basic parameters regarding the operation of equipment and properties at streams are listed in the next section. (4) Improve the gas and steam branches having maximum exergy as the objective.

■

RESULTS AND DISCUSSION

As mentioned above, CeSFaMB simulation software was applied to maximize the exergy efficiency for that boiler. The resulting basic configuration is shown in Table 1 and other inputs to the simulation are listed in Table 2. The main results are shown in Table 3. Table 2. Other Inputs to Boiler Simulation parameter

value or condition

mass flow of dry bagasse mass flow of injected air average particle of feeding biomass proximate analysis of feeding biomass (w.b.) moisture volatile fixed carbon ash ultimate analysis of feeding biomass (d.b.) carbon hydrogen nitrogen oxygen sulfur ash

9.0 kg/s 56.0 kg/s 1.1 mm

parameter

value or condition

mass flow of flue-gas mass flow of solids reaching freeboard top mass flow of water injected in each bed bank fluidization voidage (bed middle) fluidization superficial velocity (bed middle) bed dynamic volume circulation flux of carbonaceous (bed middle) mixing index in the bed tar flow at the top of the freeboard total carbon conversion input energy rate due to fuel total energy rate input overall heat transfer coefficient (1st bed bank)a overall heat transfer coefficient (2nd bed bank)a overall heat transfer coefficient (3rd bed bank)a overall heat transfer coefficient (1st freeboard bank)a overall heat transfer coefficient (2nd freeboard bank)a temperature of steam leaving the 1st freeboard bank temperature of steam leaving the 1st freeboard bank temperature of steam leaving the 3rd bed bank total flow of produced steam total energy rate transferred to tubes mass held in the bed average residence time of particles based on feeding rate TDH average erosion rate of tube wall (1st bed bank) average erosion rate of tube wall (2nd bed bank) average erosion rate of tube wall (3rd bed bank) entering exergy flow exergy flow carried by flue-gas exergy flow carried by steam leaving exergy flow ratio between leaving and entering exergy flows

78.22 kg/s 19.15 kg/s 12 kg/s 0.9628 0.17 m/s 318 m3 0.59 × 106 kg m−2 s−1 1.000 0.000 kg/s 98.54% 153 MW 197.2 MW 375 W m−2 K−1 419 W m−2 K−1 376 W m−2 K−1 154 W m−2 K−1

a

50% 40.78% 7.57% 1.65

154 W m−2 K−1 916 K 914 K 760 K 36.0 kg/s 98.1 MW 9.6 × 104 kg 71.3 min 4.1 m 1.2 mm/1000 h 0.02 mm/1000 h 1.2 mm/1000 h 799.2 MW 67.9 MW 60.8 MW 128.8 MW 16.1%

At the middle of the bundle.

carbon is converted to gases at the same region. That last figure also illustrates how the remaining bed and freeboard just keeps the process stable without great changes in those concentrations. However, the reactions do not halt. Figure 4 presents the profiles at freeboard. The relatively low temperatures, when compared with those found inside conventional pulverized fuel combustion chambers, are typical of bubbling fluidized units.13,24,25 Actually, this is among the advantages of fluidized boilers, which require less expensive materials and insulations. In addition, temperatures in the range of 800−1000 K falls in the optimum range of SO2 absorption using limestone or dolomite. Of course, this is not the present case because the sulfur contents of biomasses are too low to justify the application of absorbents. Bubbling beds also show high heat transfer coefficients to tubes when compared with suspended fuel boilers. Table 3 shows that the average values for the tube banks in the bed were around 400 W m−2 K, well above the average 100 W m−2 K−1 achieved in conventional boilers. Obviously, this brings substantial capital savings.

49.66% 5.71% 0.21% 41.08% 0.03% 3.31%

Figure 2 illustrates the temperature profiles in the bed where the air is injected at the distributor (z = 0). Despite the uniform, average temperature in most of the bed, the narrow region near the distributor plays a very important part and many processes occur there, as for instance the fast consumption of oxygen. This is shown in Figure 3 along the profiles of average concentrations throughout the bed and freeboard of other important gases. It should be noticed that in the bed region the profiles are averages between concentrations in the emulsion and bubbles. Since the bulk of injected oxygen is consumed near the distributor, a sizable fraction of fuel 1954


Energy & Fuels

Article

Figure 2. Temperature profiles throughout the bed axial (z) positions (distributor surface at z = 0).

Figure 3. Concentrations of main gases in through the bed and freeboard.

CeSFaMB applies published relations to estimate the erosion rates of immersed tubes in the bed.26,27 Among many factors, such rates depend mostly on the circulation flux of particles around the tube, tube tensile strength at local temperature, and bank characteristics.13 The computed results for the present case are shown in Table 3, and higher values were found for the staggered tube arrangement and the lower for in-line arrangement. Such is easy to understand, since in-line tubes tend to “hide” behind each other, thus avoiding high erosion rates caused by fast circulating particles. The circulation rates of particles along the bed height are shown by Figure 5. Here the inert phase is composed by ash freed from burning fuel. It is clear to observe the higher rate near the bed center. The reasons for that are extensively described in the literature.13,20,28−31

The simulation provides many other details as, for instance, the concentration profiles of gases along the bed and freeboard.13 These can be important during studies aiming boiler optimization. Figure 6 shows tar destruction by cracking and cooking in the bed just around the fuel feeding position at 1 m above the distributor (z = 0). This illustrates how effective deep bubbling fluidized beds can be in avoiding the presence of tar at the exiting flue gas. It is also interesting to notice that despite the overall oxidizing conditions, simultaneous gasification reactions lead to localized productions of reduced species such as H2S and NH3. The most important reactions are listed in the Appendix, and for instance, those two species are produced by the reactions in eqs R.2−R.6 as well during fuel pyrolysis. However, faster oxidations (eqs R.8−R.13) result in negligible concentration values 1955


Energy & Fuels

Article

Figure 4. Temperature profiles throughout the freeboard.

Figure 5. Particle circulation rates throughout the bed (since the simulations assume no sulfur absorbent feeding into the boiler, the respective line should be neglected).

and is shown in Figure 1. The conditions and properties of streams in such process are shown in Table 4. Having in mind that current power generating plants installed at sugar cane mills operate with overall efficiencies around 20%, the value shown at Table 5 indicates the possibility of substantial improvements on that respect. In addition, the proposed process can lead to even higher efficiencies. Among the various points to be investigated in that direction are (a) experimental confirmation if bagasse slurries might be pumped with higher contents of dry solid than assumed here. Any increase would substantially improve the process efficiency, (b) increases in the fluidized-bed pressure, (c) increases in the steam pressure, and (d) use of more elaborate cycles or processes for the steam and gas-turbine branches.

of these and other combustible gases at higher positions inside the equipment. The first bank in the freeboard is linked with the first bank in the bed, i.e., the fluid leaving the bank in the bed enters the tubes of the bank immersed in the freeboard. Figures 7 and 8 exemplify the typical wall and fluid temperature profiles of tubes belonging to bank no. 1 immersed in the bed and at the respective freeboard. The steam quality profile in the bed first bank is shown by Figure 9. The profiles for the remaining banks in the bed are similar. These show how all phase changes take place in the bed banks leaving the freeboard ones for additional superheating of steam. After a series of search and optimizations using the IPES simulator, a good strategy for the process has been achieved 1956


Energy & Fuels

Article

Figure 6. Tar and other concentration profiles throughout the bed and freeboard.

Figure 7. Wall and internal temperature profiles for tubes of bank no. 1 immersed in the bed.

■

■

CONCLUSIONS

The concept for an advanced thermoelectric power-generation process has been simulated and shows the theoretical possibility of achieving much higher efficiencies than the currently obtained in sugar cane mills. The simulations also show that feeding the bagasse as a slurry can lead to feasible boiler operation, thus avoiding the cumbersome and expensive cascade feeding systems, usually necessary to operate BIG/GT concepts. The work also presents important details of boiler operation, which can be useful for further design improvements. The next round of studies would confirm the limits of water content in bagasse slurries as well include pressures in the fluidized bed and inside tubes as variables. In addition, improvements on the strategies of gas and steam turbine branches will be tried.

APPENDIX

CeSFaMB Model

Previously called CSFB or CSFMB, the simulator is based on a mathematical model with a core composed by differential mass and energy balance equations. They describe axial or vertical point-by-point energy balances for each gas and solid phase in the bed and freeboard as well mass balance to every chemical species in each physical phase. Those balances are fundamental conservation equations, thus the simulator can be applied to a wide range of scales. The fluidizations dynamics are described by published semi-empirical correlations, and the complete model involves many sub-models to compute several auxiliary parameters. Of course, there is not enough space here to show all of the details, but the interested reader would find them elsewhere.13 The solution of such a system provides the 1957


Energy & Fuels

Article

Figure 8. Wall and internal temperature profiles for tubes of bank no. 1 immersed in the freeboard.

Figure 9. Steam quality profile in bank no. 1 immersed in the bed.

(6) Reacting emulsion gas passes through the bed following the axial or vertical plug-flow (inviscid) regime. The model involves a great number of homogeneous and heterogeneous chemical reactions; the main ones are listed below. (7) Despite a noncontinuous phase, bubbles are modeled as flowing through the bed under a reacting plug-flow regime as well. However, form, dimensions, and other characteristics of bubbles are considered in all calculations regarding that phase. During that travel, the gaseous part of the emulsion exchanges mass and heat with bubbles and particles. In addition, heat transfer occurs between all phases and vessel walls as well as tubes eventually immersed in the bed. The total area for heat and mass transfer between the bubble and emulsion phases is equivalent to the surface area of all bubbles in the bed. Diameter and velocity of the bubbles vary in the axial direction. (8) The reacting bubble

complete picture of the equipment operation, and a list of the most important is presented below. Besides boilers, the simulator can be applied to gasifiers, and other types of equipment. For cases of the bubbling technique, the model chart is shown in Figure 10. The fundamental hypothesis and solution procedure is summarized below:13,21 (1) The unit operates at steady-state regime. (2) The equipment is separated in two main regions: bed and freeboard. (3) The bed 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 a 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 emulsion holds all particles, and therefore bubbles are assumed free of particles. 1958


Energy & Fuels

Article

In addition, the differential energy balances for each phase also take into account a large number of heterogeneous chemical reactions, heat and mass exchanges with emulsion gas and gas in the freeboard region. (10) From the conditions at the feeding point, particles size distributions modify due to chemical reactions, attritions between particles themselves and with internal equipment walls. In addition, other processes or phenomena also influence the particle size distributions in the bed such as entrainment of fines to the freeboard, withdrawals from the bed, and eventual recycling to the bed of particles collected in the cyclone system. All those effects are considered by the model. (11) The freeboard is composed by particles and gases. The reacting gas phase passes through the freeboard region following an axial plug-flow (inviscid) regime. Particles also flow in the axial or vertical direction; however, heavier ones return to the bed due to the disengaging process. Proper relations provide equations to account for the gas−solid separation. Homogeneous and heterogeneous reactions are also considered in the freeboard. Thus, all compositions, temperatures, flows, and particle size distributions of every solid species are computed at each point in that region. (12) Heat and mass transfers also occur between all phases in the freeboard. Heat transfers take place between those phases and tubes or surfaces eventually immersed in that region are computed. (13) Gases are assumed transparent regarding radiative heat transfers. (14) Heat and mass transfers in the axial or vertical direction within each phase are considered negligible when compared with the respective transfers in the horizontal direction between a phase and neighboring ones. (15) At each axial position (z), mass transfers between phases result from differences of species average concentrations at each phase. As soon as 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. (16) 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. (17) At the basis of the bed (z = 0), the two-phase model35 is applied to determine the splitting of an injected gas stream between the emulsion and bubble phases. (18) For points above that (z > 0), the mass flow in each phase is determined by fundamental equations of transport phenomena. Those take into account: mass transfers between the main phases as well homogeneous and heterogeneous reactions. (19) Boundary conditions for the gas phases concerning temperature, pressure, and composition at (z = 0) are given by the values of the injected gas stream. (20) At each iteration, boundary conditions 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. (21) The temperature and composition profiles in the bed are achieved by iterative computation throughout the equipment. For the first iteration, a carbon conversion is assumed. After solving the system of coupled nonlinear differential equations describing the mass and energy balances at all phases and for all chemical species, the new carbon conversion is computed. Conversions of all other solid-phases components are computed as well. The solution provides temperature and composition in the emulsion and bubble phases, average composition, as well a temperature profile of solid phases in the bed. In addition, all variables related to heat transfers tothe tubes (if present) and internal walls are obtained. (22) The values at the top of the bed are used as boundary conditions for the bottom of the freeboard region. Then, the system of differential

Table 4. Summary of Stream Properties stream

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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

916.0 914.0 759.0 811.3 588.6 588.6 588.6 422.2 841.0 660.5 660.5 660.5 561.0 561.0 561.0 471.4 471.4 471.4 378.1 313.0 311.9 311.9 322.8 354.4 356.4 314.7 336.3 314.7 298.0 351.3 393.5 393.8 475.0 487.9 378.7 298.0 765.7 915.0 688.0 364.9 298.0

10000.0 10000.0 10000.0 10000.0 2100.0 2100.0 2100.0 437.0 437.0 123.0 123.0 123.0 54.0 54.0 54.0 23.0 23.0 23.0 8.0 8.0 8.0 123.0 123.0 123.0 54.0 8.0 23.0 8.0 103.0 102.0 123.0 10000.0 10000.0 2100.0 123.0 101.325 2020.0 2000.0 2000.0 105.0 103.0

12.00 12.00 12.00 36.00 36.00 6.00 30.00 30.00 30.00 30.00 1.00 29.00 29.00 1.00 28.00 28.00 1.00 27.00 27.00 27.00 29.00 29.00 29.00 29.00 1.00 1.00 1.00 1.00 300.0 300.0 36.00 36.00 36.00 6.00 6.00 56.00 56.00 78.22 78.22 78.22 300.0

Table 5. Overall Power Balance power inputa power outputa net power outputa rate of energy input due to fuel efficiency based on 1st Lawb

27.92 MW 85.15 MW 57.23 MW 153.02 MW 37.40%

a

Mechanical. bDefined as (net rate of useful power out)/(total energy rate in: fuel LHV × its feeding rate).

phase passes through the bed following an axial or vertical plugflow (inviscid) regime. (9) This model does not assume a complete stirred-tank approach. The well-mixed model is assumed only regarding the composition of solid particulate phases in the bed. The same is not imposed for their temperature. These are obtained for each phase at each point of the equipment interior after detailed energy balances, which consider all heat exchange processes between each solid and all others (gas and solid) phases as well as internals such as tubes and walls. 1959


Energy & Fuels

Article

Figure 10. Scheme of CeSFaMB strategy.13

addition, the simulation uses a data bank and correlations for computations of physical and chemical properties of streams that may be composed by many chemical species and different physical phases and taken from the available literature.36,37 The composition of gases and solids along the bed and freeboard include 18 solid and 20 gaseous chemical species. Those are provided by the differential equations that include kinetics of 95 chemical reactions. The most important are

equations related to the freeboard is solved. It provides all temperature and composition profiles of every phase in that region. (23) If the equipment operates with recycling of elutriated particles to the bed, the cyclone system is simulated and all characteristics of the collected particles are obtained. Those are used to compose the mass and energy balances during the next iteration in the bed. (24) Steps 20−23 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. As seen above, despite taking into account heat and mass transfer between phases in the radial or horizontal direction, all variations on temperatures and concentrations are assumed to occur only in the axial direction (z). Therefore, this is a onedimensional model. Regardless of its apparent simplicity, the mathematical model involves a large nonlinear and tightly coupled system of nonlinear differential equations. Many subroutines are used to compute various constitutional parameters. In

a a a ⎛1 ⎜ + 531 − 551 + 546 ⎝2 4 2 2 a531 ⎞ + a563⎟O2 → CO + H2O + a546 NO ⎠ 2

CHa531Oa551Na546Sa563 +

+ a563SO2

(R.1)

CHa531Oa551Na546Sa563 + (1 − a551)H2O a a ⎛ ⎞ ↔ ⎜1 + 531 − a551 − a563⎟H2 + CO + 546 N2 ⎝ ⎠ 2 2 + a563H2S 1960

(R.2) dx.doi.org/10.1021/ef2019935 | Energy Fuels 2012, 26, 1952−1963

Energy & Fuels

Article

CHa531Oa551Na546Sa563 + CO2

Detailed modeling of fuel pyrolysis is also included and based on several important published works.32−34 A complete description of the model equations and assumptions cannot be included here due to paper size limitations. They can be found elsewhere.13,21 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 (1) 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 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). In addition, other information may be fed such as material quality and thickness of vessel walls, water or gas-jacket geometries (if used), insulation (bricks of otherwise) thickness and quality, cyclone system design, etc. (2) 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). Liquid fuels are also allowed. (3) 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. (4) 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 injected into the equipment. Liquid streams are also allowed. (5) Flow rates and characteristics of eventually injected intermediate gas streams. Besides the gas flow through the bottom distributor, the simulator also accepts intermediate injections of gas with any composition and conditions. The position of injections should be informed. (6) Flow rates and characteristics of the eventual intermediate gas withdrawals. Besides the usual withdraw of gas stream at the top of the freeboard, the simulator allows verifying the effects of intermediate withdrawals of gases. For that, just the position and flow of withdrawal should be informed. (7) Other operational conditions, such as internal average pressure in the bed and external wind velocity. CeSFaMB provides the following information: (1) 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. (2) 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. (3) 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. (4) Composition, particle size distribution, and flow rates of solids or liquids at each point inside the equipment as well of those streams leaving the

⎛a ⎞ 3a ↔ 2CO + a551H2O + ⎜ 531 − a551 − 546 − a563⎟H2 2 ⎝ 2 ⎠ + a546 NH3 + a563H2S

(R.3)

a ⎛ 3 CHa531Oa551Na546Sa563 + ⎜2 − 531 − a551 + a546 ⎝ 2 2 ⎞ + a563⎟H2 ↔ CH 4 + a551H2O + a546 NH3 ⎠ + a563H2S

(R.4)

CHa531Oa551Na546Sa563 + (2 − a551)NO ↔

a a ⎞ ⎛ a531 ⎞ ⎛ ⎜ − a563⎟H2 + CO2 + ⎜1 + 546 − 551 ⎟ ⎝ 2 ⎠ ⎝ 2 2 ⎠ N2 + a563H2S

(R.5)

CHa531Oa551Na546Sa563 + (1 − a551)N2O a ⎛a ⎞ ⎛ ⎞ ↔ ⎜ 531 − a563⎟H2 + CO + ⎜1 + 546 − a551⎟N2 ⎝ 2 ⎠ ⎝ ⎠ 2 + a563H2S

(R.6)

CO + H2O ⇔ CO2 + H2

(R.7)

2CO + O2 ⇔ 2CO2

(R.8)

2H2 + O2 ⇔ 2H2O

(R.9)

CH 4 + 2O2 ⇔ CO2 + 2H2O

(R.10)

2C2H6 + 7O2 ⇔ 4CO2 + 6H2O

(R.11)

4NH3 + 5O2 ⇔ 4NO + 6H2O

(R.12)

2H2S + 3O2 ⇔ 2SO2 + 2H2O

(R.13)

N2 + O2 ⇔ 2NO

(R.14)

tar + O2 → combustion gases

(R.15)

tar → gases

(R.16)

NO + CO ⇔ 1/2N2 + CO2

(R.17)

CO + N2O ⇔ N2 + CO2

(R.18)

N2O ⇔ N2 + 1/2O2

(R.19)

The stoichiometry parameters of many reactions are a function of the composition of solid fuel at each particular position in the bed or freeboard. Therefore, those are also a function of mass and energy balances at each position. Since the reactivity of the processed fuel is an important parameter in combustion and gasification reactions, the kinetics parameters are taken from various publications (listed elsewhere13,21) and applied accordingly. The heterogeneous reactions take into account the resistances to mass transfers of gases through the layers inside the reacting particles and around them. As seen, the model computes the kinetics of each reaction at each point of the equipment and does not assume the simplification of chemical equilibrium; however, equilibrium conditions are computed in order to limit the progress of each reaction. 1961


Energy & Fuels equipment. (5) Overall elemental mass balance verification. (6) Temperature profiles of each gas (emulsion and bubbles) and solid (carbonaceous, absorbent, and inert) throughout the entire equipment are provided. (7) If tube banks are present, point-by-point profiles of temperature inside the tubes and their walls. (8) If the unit is equipped with water or gas jacket, profiles of temperature inside the jacket and walls throughout the entire height as well steam quality. (9) Process parameters, which includes specific aspects of boilers, gasifiers, dryers, shale retorts, pyrolyzer, or any other type of simulated equipment. (10) Rates and parameters related to heat transfer to ambiance and internals with a detailed account of heat transfer rates, coefficients to tube banks (if any), internal walls, and external ambient. (11) Rates of erosion at tube bank walls and the respective mean-life times. (12) General warnings to the user related to possible operational problems as well as a list of various aspects that might interest the user are presented. Among the most critical there are the possibility of slugging flow, surpassing solid particle softening temperatures, excessive elutriation rates, low cyclone efficiencies, etc. (13) Pointby-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 throughout the freeboard. (14) Composition profiles of each chemical species (18 possible components) throughout the entire equipment and at each phase (emulsion, bubbles, gas in the freeboard). (15) Rate profiles for each reaction at each phase throughout the entire equipment. (16) Main pressure losses at various points or sections of the equipment. (17) Overall exergy analysis of the unit operation. (18) If sulfur absorbent is fed into the unit, several efficiency-related parameters to sulfur capture.

ACKNOWLEDGMENTS

■

REFERENCES

This investigation has been supported by the Faculty of Mechanical Engineering, University of Campinas (UNICAMP).

(1) Williams, R. H. Biomass Gasifier/Gas Turbine Power and Greenhouse Warming. IEA/OECD Expert Seminar on Energy Technologies for Reducing Emissions of Greenhouse Gases, OECD Headquarters, Paris, France, April 12−14, 1989. (2) Ogden, J. M.; Williams, R. H.; Fulmer, M. E. Cogeneration Applications of Biomass Gasifier/Gas Turbine Technologies in the Cane Sugar and Alcohol Industries. Proceedings of the Conference on Energy and Environment in the 21st Century, Massachusetts Institute of Technology, Cambridge, MA, March 26−28, 1990. (3) Larson, E. D. Biomass-Gasifier/Gas-Turbine Cogeneration in the Pulp and Paper Industry. Proceedings of the 36th ASME International Gas Turbine and Aeroengine Congress and Exhibition, Orlando, FL, June 3−6, 1991. (4) Larson, E. D. ; Marrison, C. I. Economic Scales for FirstGeneration Biomass-Gasifier/Gas Turbine Combined Cycles Fueled from Energy Plantations. Proceedings of the 41st ASME Gas Turbine and Aeroderivative Congress, Birmingham, England, June 10−13, 1996. (5) Larson, E. D.; Hughes, W. E. M. Performance Modeling of Aeroderivative Steam-Injected Gas Turbines and Combined Cycles Fueled from Fixed or Fluid-bed Biomass Gasifiers. Proceeding of the 41st ASME Gas Turbine and Aeroengine Congress, Birmingham, England, June 10−13 , 1996. (6) Consonni, S.; Larson, E. D. Biomass-Gasifier/Aeroderivative Gas Turbine Combined Cycles; Part A: Technologies and Performance Modeling. Proceedings of the American Society of Mechanical Engineers’ 8th Congress & Exposition on Gas Turbines in Cogeneration and Utility, Industrial and Independent Power Generation, Portland, Oregon, October 25−27, 1994. (7) Consonni, S.; Larson, E. D. Biomass-Gasifier/Aeroderivative Gas Turbine Combined Cycles; Part B: Performance Calculations and Economic Assessment. Proceedings of the American Society of Mechanical Engineers’ 8th Congress & Exposition on Gas Turbines in Cogeneration and Utility, Industrial and Independent Power Generation, Portland, Oregon, October 25−27, 1994. (8) Larson, E. D.; Williams, R. H.; Leal, M. R. L. V., A review of biomass integrated-gasifier/gas turbine combined cycle technology and its application in sugarcane industries, with an analysis for Cuba, Energy Sustainable Dev., 2001 5 (1). (9) Anthony, E. J. Fluidized bed combustion of alternative solid fuels; status, successes and problems of the technology. Prog. Energy Combust. Sci. 1995, 21, 239−268. (10) He, W.; Park, C. S.; Norbeck, J. N. A Rheological Study on the Pumpability of Co-Mingled Biomass and Coal Slurries. Proceedings of the International Pittsburgh Coal Conference 2008, Pittsburgh, PA, September 2008. (11) de Souza-Santos, M. L.; Chavez, J. V. Preliminary studies on advanced power generation based on combined cycle using a single high-pressure fluidized bed boiler and consuming sugar-cane bagasse. Fuel 2012, 95, 221−225. (12) Oakey, J.; Simms, N.; Kilgallon, P. Gas turbines: gas cleaning requirements for biomass-fired systems. Mater. Res. 2004, 7 (1), 17−25. (13) de Souza-Santos, M. L. Solid Fuels Combustion and Gasification: Modeling, Simulation, and Equipment Operation, 2nd ed.; CRC Press: New York, NY, 2010. (14) de Souza-Santos, M. L. Modelling and Simulation of Fluidized-Bed Boilers and Gasifiers for Carbonaceous Solids. Ph.D. Thesis, University of Sheffield, Department of Chemical Engineer and Fuel Technology, England, United Kingdom, 1987. (15) de Souza-Santos, M. L. Comprehensive Modelling and Simulation of Fluidized-Bed Boilers and Gasifiers. Fuel 1989, 68, 1507−1521.

IPES Model

The Industrial Process and Equipment Simulator is a typical zero-dimensional model composed by equations describing the mass and energy balance at each equipment (or control volume, CV) belonging to a processing or power-generation plant. The model uses the 1st and 2nd Laws of Thermodynamics applied to each CV in the process to compose a system of equations. It also uses a sophisticated data bank for computations of physical and chemical properties of streams that may be composed by many chemical species and different physical phases (gas, liquid, and solid). The user should provide information on how the CVs are linked by streams as well other information related to every CV (efficiency, operational limits, etc.). IMSL (International Mathematics and Statistics Library) routines are used to solve the system of equations and provide the temperature, pressure, and composition of each stream leaving a CV. The simulation output also provides a large series of information related to each stream and equipment. Further details can be found in Chapter 5 of a published book.13

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +55-19-35213278 (office); +55-19-97107134 (mobile). Fax: +55-19-35798275. Notes

The authors declare no competing financial interest. 1962


Energy & Fuels

Article

(16) de Souza-Santos, M. L. Application of Comprehensive Simulation to Pressurized Fluidized Bed Hydroretorting of Shale. Fuel 1994, 73, 1459−1465. (17) de Souza-Santos, M. L. Application of Comprehensive Simulation of Fluidized-Bed Reactors to the Pressurized Gasification of Biomass. J. Braz. Soc. Mech. Sci. 1994, 16, 376−383. (18) de Souza-Santos, M. L. A Study on Thermo-chemically Recuperated Power Generation Systems Using Natural Gas. Fuel 1997, 76, 593−601. (19) de Souza-Santos, M. L. A Feasibility Study on an Alternative Power Generation System Based on Biomass Gasification/Gas Turbine Concept. Fuel 1999, 78, 529−538. (20) Costa, A. M. S.; de Souza-Santos, M. L. Studies on the Mathematical Modeling of Circulation Rate of Particles in Bubbling Fluidized Beds. Powder Technol. 1999, 103, 110−116. (21) de Souza-Santos, M. L. A New Version of CSFB, Comprehensive Simulator for Fluidized Bed Equipment. Fuel 2007, 86, 1684−1709. (22) de Souza-Santos, M. L. Comprehensive Simulator (CSFMB) Applied to Circulating Fluidized Bed Boilers and Gasifiers. Open Chem. Eng. J. 2008, 2, 106−118. (23) de Souza-Santos, M. L. CSFB Applied to Fluidized-bed Gasification of Special Fuels. Fuel 2009, 88, 826−833. (24) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; John Wiley: New York, 1991. (25) Geldart, D. Gas Fluidization Technology; John Wiley: Chichester, U.K., 1986. (26) Bouillard, J. X.; Lyczkowski, R. W.; Folga, S.; Gidaspow, D.; Berry, G. F. Hydrodynamics of erosion of heat exchanger tubes in fluidized bed combustors. Can. J. Chem. Eng. 1989, 67, 218−229. (27) Bouillard, J. X.; Lyczkowski, R. M. On the erosion of heat exchanger tube banks in fluidized-bed combustors. Powder Technol. 1991, 68, 37−51. (28) Soo, S. L. Note on Motions of Phases in a Fluidized Bed. Power Technol. 1986, 45, 169−172. (29) Soo, S. L. Average Circulatory Motion of Particles in Fluidized Beds. Power Technol. 1989, 57, 107−117. (30) Soo, S. L. Particulates and Continuum, Multiphase Fluid Dynamics; Hemisphere: New York, 1989. (31) Lin, J. S.; Chen, M. M.; Chao, B. T. A novel radioactive particle tracking facility for measurement of solid motion in gas fluidized beds. AIChE J. 1985, 31 (3), 465−473. (32) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Very rapid coal pyrolysis. Fuel 1986, 65, 182−193. (33) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Kinetics of volatile evolution in coal pyrolysis: experiment and theory. Energy Fuels 1987, 1, 138−152. (34) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. General model of coal devolatilization. Energy Fuels 1988, 2, 405−422. (35) Toomey, R. D.; Johnstone, H. F. Gaseous Fluidization of Solid Particles. Chem. Eng. Progress 1952, 48 (5), 220−226. (36) Reid, C. R.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th. ed.; McGraw-Hill: New York, 1987. (37) Karapetyants, M. K. Chemical Thermodynamics; Mir: Moscow, Russia, 1978.

1963


Development of Studies on Advanced Power Generation Based on

Recommend Documents