Huang Xiaoyan Wang Hua Wang Huitao
introduction
Aluminum is the second largest metal after steel. Aluminum metallurgy industry usually includes aluminum ore mining, alumina production, and electrolytic aluminum production [1]. At the same time, in this industrial process, there is also a large amount of energy consumption and a wide range of residual heat distribution [2]. Taking alumina produced by sintering as an example, China’s energy consumption index is 1447kg standard coal/t-Al2O3 [3], of which a large part is discharged in the form of low-temperature waste heat within 250°C [4], such as clinker Flue gas (temperature around 250°C [4]) takes away about 10% [5] of waste heat.
Waste heat power generation is an effective way for the recovery and utilization of low-temperature heat sources. It can not only increase the utilization of primary energy, but also reduce the heat pollution caused by waste heat. However, with the current level of technology, this part of the waste heat cannot be effectively recycled. Low-boiling point organic waste Rankine cycle technology is used to realize the thermal-electric conversion of low-temperature waste heat, which is currently the international focus of high-temperature waste heat recovery.
1. Selection of organic Rankine cycle working fluid driven by low temperature waste heat
Due to the large number of organic working fluids, their physical and chemical properties are very different. When recovering waste heat, it is necessary to select working fluids suitable for specific operating conditions based on the parameters of the temperature, specific heat capacity, and flow rate of the waste heat energy. The selected working fluid should generally reach the following levels: Claim:
1.1 Good thermodynamic properties: The normal boiling point and critical temperature of the working fluid should be within the appropriate range.
1.2 good heat transfer and flow properties: working medium should have a higher absorption and heat transfer coefficient to reduce the heat transfer area; have a lower viscosity to reduce the flow resistance in the pipeline.
1.3 Good physical and chemical properties: It has good chemical stability, ensures no decomposition within working temperature and pressure range, and does not have chemical reaction with exposed metal or non-metallic materials to ensure long-term reliable operation.
1.4 Environmental protection and safety are better: The working medium should have no ozone destructive power, but also have a small greenhouse effect; non-toxic, non-irritating, non-flammable, easily detected when leaked, such as: HC type, HFC type organic working medium.
1.5 The price is cheap and easy to buy.
Table 1 lists the normal boiling point, ozone depletion potential (ODP), global warming potential (GWP), and safety ratings for water and several organic media without ozone destructive capacity [6]: [7]
Table 1. Normal boiling points, ODP values, GWP values, and safety ratings for water and several organic media [7]
| Chinese name | Work quality code | Standard boiling point (°C) | ODP | GWP100 | Security Level |
| water | R718 | 100 | 0 | 0 | A1 |
| Methane | R50 | -161 | 0 | twenty three | A3 |
| Ethane | R170 | -89 | 0 | About 20 | A3 |
| Trifluoroethane | R143a | -47.2 | 0 | 4300 | A2 |
| Tetrafluoroethane | R134a | -26.1 | 0 | 1300 | A1 |
| Pentafluoroethane | R125 | -42.2 | 0 | 3400 | A1 |
| Propane | R290 | -42.2 | 0 | About 20 | A3 |
| Pentafluoropropane | R245fa | 15.1 | 0 | 950 | A1 [8] |
| Octafluoropropane | R218 | -37 | 0 | 8600 | A1 |
| N-butane | R600 | -0.5 | 0 | About 20 | A3 |
| Isobutane | R600a | -12 | 0 | About 20 | A3 |
For organic Rankine cycle power generation systems driven by low temperature waste heat, low boiling point working fluids are generally used due to the low temperature of the waste heat source (200-400°C). It can be seen from Table 1 that the low-temperature residual heat within 250 °C is the most suitable for the power cycle using the working medium R245fa, because its normal boiling point is not high, non-toxic and non-flammable; it belongs to the low-medium pressure range [7]. The strength of the pipeline equipment is not high. This article selects R245fa as the working fluid for achieving the power cycle.
2.R245fa cycle performance calculation
2.1. Calculation of R245fa Thermal Parameters
In this paper, the thermodynamic performance of R245fa is calculated using the well-known (Peng-Robinson) PR equation: (2.1a)
This equation is a two-parameter state equation and is based on the correction of the RK equation. The free energy a, ratio ç„“h, and specific entropy s are all calculated using the residual function equation [9]. The various residual function equations are as follows:
(2.1b)
(2.1c)
(2.1d)
2.2.R245fa cycle performance calculation After the working medium is pumped out of the working medium, it turns into a high-pressure liquid and enters the waste heat boiler. In the waste heat boiler, the waste heat-aluminum clinker flue gas passes heat to the high-pressure working medium, and the high pressure After absorbing heat, the working fluid becomes high-pressure steam and enters the organic turbine to do work, thus driving the generator to generate electricity.
Taking R245fa condensation temperature 30 °C, the evaporation pressure of the working medium in the waste heat boiler is 1926 kPa, the superheat degree is 30°C, the waste heat inlet temperature is 250°C, and the waste heat flux is 300kg/s.
Table 2. Calculation of Organic Rankine Cycle Driven by Low Temperature Residual Heat
| Location, numbering | Temperature/°C | Pressure/MPa | Density/(kg·m-3) | Ratio/(kJ·kg-1) | Specific entropy / (kJ/(kg·K)-1) |
| Turbine exit 4 | 89.083 | 0.179 | 8.356 | 478.3 | 1.9097 |
| Pressure pump inlet 1 | 29 | 0.179 | 1353.96 | 237.4 | 1.129 |
| Waste Heat Boiler Inlet 2 | 30 | 1.926 | 1360.998 | 238.5 | 1.129 |
| Turbine inlet 3 | 150 | 1.926 | 96.202 | 529.183 | 1.9097 |
| Specific work/(kJ·kg-1) | 50.883 | ||||
| Thermal efficiency/% | 17.13 | ||||
| Exhaust heat temperature/°C | 99.65216 | ||||
3. Power cycle fire loss calculation
Variable temperature heat source in the waste heat boiler from the inlet temperature to the discharge temperature, assuming no phase change in the exothermic process of the flue gas, evaporation temperature 120°C, condensing temperature 30°C, superheat 30°C, residual heat flux 300kg/s, The power cycle exergy loss is:
Table 3. Exergy loss calculations
(ambient temperature is 25°C)
| name | Preheater | Evaporator | Superheater | Condenser | Smoke exhaust |
| Exergy loss/KW | 2708.058 | 2232.456 | 910.9113 | 820 | 2370 |
Table 4. Calculation of heat transfer area
| name | Preheater | Evaporator | Superheater |
| Area/m2 | 3782.767 | 2359.912 | 608.9441 |
From Table 3, it can be seen that there is a clear difference between organic Rankine cycle and steam Rankine cycle. In the organic Rankine cycle, the exergy loss in the preheater is the largest, accounting for about 13% of the total heat transfer, which is mainly because The heat absorbed by the organic heat medium in the hot section accounts for most of the heat, which is different from that of water. From Table 4, it can also be seen that the area of ​​the preheater accounts for about 50% of the total area of ​​the waste heat boiler, so the optimal design of the preheater is necessary.
4 Conclusion:
a. This article uses R245fa as the working fluid;
b. When the residual heat flow temperature is 250°C and the evaporation pressure in the waste heat boiler is 1926 kPa, the cycle thermal efficiency can reach 17.13%, which is much higher than water (when the working medium is water, the cycle thermal efficiency is about 5%), so When the Rankine cycle method is used for power recovery of low temperature waste heat, water should not be used as the circulating working medium;
c. In the organic Rankine cycle, the heat loss of the preheater is the greatest, which is different from steam steaming;
d. The optimization design of the preheater is necessary.
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