1 Introduction
1.1 Methodology
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Operational parameters of cogeneration performance from both traditional and modern sugar mills are gathered from the literature and via direct correspondence with relevant persons at some of the sugar mills.
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Cogeneration performance parameters for both the traditional and modern sugar mills are analyzed.
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Modification of the traditional sugar mills is made and compared with the performance of the modern mills.
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A simplified economic analysis of the modification of the traditional sugar mills is made.
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A sensitivity analysis is made using two approaches.
1.1.1 Input data used for the cogeneration-based analysis
Parameter | Number of mills | Number of countries represented |
---|---|---|
Total number of raw data points (operational, projects, under construction) | 2330 | 99 |
Operational | 1953 | 94 |
Operational with cane crushing data | 221 | 42 |
As above and with electric power capacity data | 169 | 23 |
As above and with ethanol/sugar production data | 107 | 18 |
Parameters | Modern mills | Traditional mills | ||||||||
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B[21] | C[22] | E[25] | F [26] | G[27] | H[28] | I[29] | J[30] | |||
Name of sugar mill | Pioneer | Mackayb | Savannaha | Ugar | NRd | NRd | NRd | FSF | Pelwatte | Agroval |
Location | Australia | Australia | Mauritius | India | Brazil | Brazil | Brazil | Ethiopia | Sri Lanka | Brazil |
Cane crushed (tonne/h) | 565 | 500 | 425 | 417 | 875 | 500 | 500 | 178 | 150 | 125 |
Sugar production (103 t/year) | 265.2[20] | 264c | 286[17] | 184[17] | 230 | 238c | 220c | 100 | 50 | 264c |
Total bagasse (tonne/h) | 176 | 132 | 57 | 136 | 241 | 137 | 55 | 41 | 43 | |
Net bagasse (tonne/h) | 176e | 132 | 57 | 128 | 198 | 126 | 135 | 54 | 41 | 33 |
Excess bagasse (tonne/h) | 0 | 0 | 0 | 8 | 43 | 12 | 1 | 0 | 10 | |
Total steam flow (tonne/h) | 352 | 330 | 130 | 270 | 396 | 254 | 270 | 103 | 82 | 67 |
Mech power (kWh/TC) | 23 | 9 | 16 | 18 | 14 | 13.7 | ||||
Steam to process (tonne/h) | 223 | 225 | 99 | 240 | 396 | 246 | 270 | 103 | 82 | 67 |
Total el power (MW) | 61 | 43.3 | 28 | 44 | 9 | 7 | 6 | 5 | 2.2 | 2 |
El power for factory (MW) | 17 | 9 | 9 | 14 | 9 | 7 | 6 | 5 | 2.2 | 2 |
Surplus power (MW) | 44 | 34.3 | 19 | 30 | 0 | 0 | 0 | 0 | 0 | 0 |
Live steam T (°C) | 383/483 | 260/510 | 525 | 480 | 300 | 320 | 320f | 400 | 380 | 290 |
Live steam P (Bar) | 31/66 | 18/64 | 82 | 62 | 22 | 22 | 22f | 30 | 29 | 22 |
Steam to bagasse ratio | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 1.75 | 2 | |
El power consumed (kWh/TC) | 30 | 18 | 22 | 34 | 13 | 12 | 27 | 15 | 15 | |
El power generated (kWh/TC) | 108 | 86.6 | 66 | 106 | 10 | 13 | 12 | 27 | 15 | 15 |
Heat to process (MW) | 141 | 142.5 | 59.2 | 130.15 | 244.5 | 150.2 | 171 | 65.5 | 52.6 | 42.4 |
Power-to-heat ratio | 0.4 | 0.30 | 0.50 | 0.33 | 0.04 | 0.04 | 0.04 | 0.07 | 0.04 | 0.05 |
Boiler efficiency (%) | 69 | 68 | 88 | 74 | 69 | 72 | 71 | 70 | 62 | 73 |
Cogeneration efficiency (%) | 61 | 67 | 73.4 | 67.4 | 67.1 | 67.9 | 72.5 | 64.9 | 73.4 | 74 |
1.1.2 Heat balance equations
2 Analysis of operational parameters for efficiency improvement
2.1 Efficiency improvement due to the replacement of mechanical turbines with electric drives
2.2 Modification of traditional mills
2.2.1 Case 1
2.2.2 Case 2
2.2.3 Case 3
2.2.4 Model description
Parameters | Case 1 | Case 2 | Case 3 |
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Enthalpy (turbine inlet) | T and P from Table 2 | 500 °C, 40 bar | 500 °C, 40 bar |
Enthalpy (turbine exhaust) | Same as process steam conditions as the existing mill | Extraction steam has process steam condition as the existing mill Final exhaust @ 0.15 bar | Extraction steam has process steam condition as the existing mill Final exhaust @ 0.15 bar |
Bagasse mass flow | Net bagasse flow from Table 2 | Total bagasse flow from Table 2 |
\( {\dot{m}}_{f,\mathrm{new}}=\frac{\left({\dot{m}}_{f,\mathrm{net}}+{\dot{m}}_{f,\mathrm{ex}}\right)\bullet \left(1-{F}_{BC}\right)}{1-{F}_{\mathrm{new}}} \)
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Steam mass flow | Total steam flow from Table 2 | Determined using steam to bagasse ratio in Table 2 | Determined using the assumed steam to bagasse ratio of 3 |
Steam flow to process | Taken from Table 2 | Taken from Table 2 | Taken from Table 2 |
Electric power for factory use (existing turbine) | Electric power for factory Ṗel,net was taken from Table 2 | Electric power for factory, Ṗel,net taken from Table 2 | Electric power for factory, Ṗel,net taken from Table 2 |
Turbine power output (new turbines) |
\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}=\sum \dot{\mathrm{m}}\bullet \mathrm{h} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}=\sum \dot{\mathrm{m}}\bullet \mathrm{h} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}=\sum \dot{\mathrm{m}}\bullet \mathrm{h} \)
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Mechanical power | Taken from Table 2 | Taken from Table 2 | Taken from Table 2 |
Power for electric motors | \( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}=67\%\bullet \Big({\dot{\mathrm{P}}}_{\mathrm{me}}+{\upgamma}_{\mathrm{tr}}\bullet {\dot{\mathrm{P}}}_{\mathrm{me}} \)) | \( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}=67\%\bullet \Big({\dot{\mathrm{P}}}_{\mathrm{me}}+{\upgamma}_{\mathrm{tr}}\bullet {\dot{\mathrm{P}}}_{\mathrm{me}} \))a | \( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}=67\%\bullet \Big({\dot{\mathrm{P}}}_{\mathrm{me}}+{\upgamma}_{\mathrm{tr}}\bullet {\dot{\mathrm{P}}}_{\mathrm{me}} \))a |
Surplus power |
\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{ex}}={\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{t}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{net}} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{ex}}={\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{net}} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{ex}}={\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{drive}}-{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{net}} \)
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Heat to process | Taken from Table 2 | Taken from Table 2 | Taken from Table 2 |
Fuel power |
\( {\dot{\mathrm{P}}}_{\mathrm{f}}={\dot{\mathrm{m}}}_{\mathrm{f}}\bullet {\mathrm{LHV}}_{\mathrm{tot}} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{f}}={\dot{\mathrm{m}}}_{\mathrm{f}}\bullet {\mathrm{LHV}}_{\mathrm{tot}} \)
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\( {\dot{\mathrm{P}}}_{\mathrm{f}}={\dot{\mathrm{m}}}_{\mathrm{f}}\bullet {\mathrm{LHV}}_{\mathrm{tot}} \)
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Power to heat ratio |
\( \upalpha =\frac{{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{Q}}}_{\mathrm{ps}}} \)
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\( \upalpha =\frac{{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{Q}}}_{\mathrm{ps}}} \)
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\( \upalpha =\frac{{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{Q}}}_{\mathrm{ps}}} \)
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Cogeneration efficiency |
\( {\upeta}_{\mathrm{co}}=\frac{{\dot{\mathrm{Q}}}_{\mathrm{ps}}+{\dot{\mathrm{P}}}_{\mathrm{me}}+{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{P}}}_{\mathrm{f}}}\bullet 100\% \)
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\( {\upeta}_{\mathrm{co}}=\frac{{\dot{\mathrm{Q}}}_{\mathrm{ps}}+{\dot{\mathrm{P}}}_{\mathrm{me}}+{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{P}}}_{\mathrm{f}}}\bullet 100\% \)
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\( {\upeta}_{\mathrm{co}}=\frac{{\dot{\mathrm{Q}}}_{\mathrm{ps}}+{\dot{\mathrm{P}}}_{\mathrm{me}}+{\dot{\mathrm{P}}}_{\mathrm{el},\mathrm{tot}}}{{\dot{\mathrm{P}}}_{\mathrm{f}}}\bullet 100\% \)
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Electric motors have an overall efficiency of 90% including auxiliary losses [34].
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Electrical and mechanical efficiencies of the power turbines are taken as 96% each.
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Based on the finding from Birru et al. [12], the power absorbed by the mechanical equipment such as rollers and crushers is 67% of the mechanical power produced by the steam turbines.
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The isentropic efficiency of the CEST is taken as 75%.
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The discount rate is taken as 6% [38].
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Equipment lifetime is taken as 20.
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The baseline electricity sales price is assumed to be 0.08 USD/kWh.
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Steam-to-bagasse ratio corresponding to 40% bagasse moisture content for case 3 is considered to be 3.0 [35].
2.2.5 Economic analysis
Equipment | Modifications | Cost considerations | |||||
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Case 1 | Case 2 | Case 3 | Case 1a | Case 2a | Case 3a | ||
Installed capital cost (USD/kW) | Installed capital cost (USD/kW) | Installed capital cost (USD/kW) | Installed capital cost (USD) | ||||
Electric drive | X | X | X | 150 [34] | 150 [34] | 150 [34] | |
BPT | X | 350 [40] | |||||
HP Boiler | X | X | 2000 [39] | 2000 [39] | |||
CEST | X | X | 600 [42] | 600 [42] | |||
Dryer | X | Variesb |
2.2.6 Sensitivity analysis
Varied parameter | Equipment | |||||||||
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Electric drive | BPT | CEST | HP Boiler | Dryer | ||||||
Min. | Max. | Min. | Max. | Min. | Max. | Min. | Max. | Min. | Max. | |
Installed cost (USD/kW) | 100 | 400 | 300 | 400 | 500 | 700 | 1880 | 2100 | ||
Installed cost (103 USD)b | 150 | 500 |
2.2.7 Results
Regions | Regional-installed grid power in MW [45] | Potential total surplus power by mills (MW) | Percentage contribution* |
---|---|---|---|
Alagoas | 277.14 | 88.2 | 31.8 |
Amazonas | 437.5 | 29.4 | 6.7 |
Espirito Santo | 143 | 29.4 | 0.2 |
Mato Grosso | 223 | 29.4 | 13.2 |
Paraiba | 224.6 | 88.2 | 39.3 |
Pernambuco | 158.77 | 118 | 74.1 |
Rio de Janeiro | 342 | 117.6 | 34.4 |
Rio Grande do Sul | 318 | 29.4 | 9.2 |
Rondônia | 534.6 | 29.4 | 55 |
Case 1 | Case 2 | Case 3 | |||||||
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LCOE ($/MWh) | 8 | 11 | 17 | 50 | 58 | 72 | 50 | 58 | 72 |
Values of varied parameters | |||||||||
Min | Baseline | Max | Min | Baseline | Max | Min | Baseline | Max | |
Capital cost ($/kW) | 324 | 386 | 497 | 2395 | 2623 | 2861 | 2399 | 2630 | 2868 |
Annual O&M cost ($/MWh) | 0 | 3 | 7 | 4 | 8 | 18 | 4 | 8 | 18 |
3 Conclusion
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Traditional sugar mills have higher steam consumption due to the poor efficiency of the mechanical steam turbines which can be improved by replacement with electric drives.
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The size of a sugar mill and the mechanical power consumption are not necessarily proportional.
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Mills without steam-driving mechanical turbines produce more turbine power than those that have such turbines.
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Electricity tariffs among other factors have a significant influence on the decisions related to retrofit activities on the cogeneration units of sugarcane mills.
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High cost-incurring investments like installation of high-pressure boilers may not always be the necessary modification that needs to be made if a surplus power export is required. Other cheaper options such as bagasse drying and replacement of mechanical steam drives with electric ones can be introduced.
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One indication of underutilization of energy potential in modernized mills is that the total useful energy extracted from each tonne of cane does not in practice become higher with the change to electrical drive.
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The retrofit of a sugar mill’s cogeneration unit for the purpose of surplus power production may not always be feasible due to, among others, the seasonality of the sugarcane production and the higher costs associated with modern equipment.
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Considering the lower production cost of electricity from bagasse than from other energy sources, there should be a clear motivation to produce electricity from sugarcane for export to the national grid.