A Pilot Investigation of a Dust Scrubber Coupling Heat Recovery from the Flue Gases of Sugar Mill Boilers

Transcription

1 2017 International Conference on Energy Development and Environmental Protection (EDEP 2017) ISBN: A Pilot Investigation of a Dust Scrubber Coupling Heat Recovery from the Flue Gases of Sugar Mill Boilers Shao-Ji ZHOU 1,a,*, Lu-Lu ZENG 1,b, Fa-Qiu LIU 1, Qiu-Ping TANG 1, Shang-Liao HUANG 2,c and Yu-Yu LIANG 2 1 School of Light Industry and Food Eng. Guangxi Univ. Nanning, Guangxi, PRC, COFCO Chongzuo Sugar Company Limited, Zongzuo, Guangxi, PRC a shaojiz@tom.com, b @qq.com, c shangliaohuang@163.com *Corresponding author Keywords: Flue Gases, Heat Recovery, Dust Removal. Abstract. A process for dust removal coupling heat recovery of the flue gases of sugar mill boilers is put forwards. A dust scrubber coupling with heat recovery from the flue gases was devised. The devise was basically a plate heat exchanger in which atomized hot water was spayed in the flue gas channels. A pilot test was carried out in a sugar mill to preliminarily investigate the performance of heat recovery and dust removal. The results shown that the heat exchange rate was significant, the dust removal rate for particles under 10µm in diameter was between 70-85%, and the emitted dust concentration was about 30~40 mg/nm 3. Introduction Sugar mills are major consumers of energy which is provided by the self-equipped boilers mostly burning bagasse. Energy saving will results in burning less bagasse or surplus of bagasse which in turn can be translated to revenues. One of the most effective measures of energy saving for sugar mills is to improve the thermal efficiency of boilers. The thermal energy loss from the emissions of flue gases at higher temperature is the largest heat loss from the boiler, the recovery of which is of great significant. The flue gases of sugar mill boilers belong to wet flue gases that contain significant amount of moisture in vapor form. The common bagasse feeding the boiler has a water content of about 50%, the flue gases would contain about 20% of moisture, the water dew point temperature is between 65~70 C. Therefore, a considerable amount of heat is available in the form of latent heat of water vapor in these gases and cannot be recovered if flue gases are not cooled down to temperatures lower than the flue-gas dew point. The recovery of this large amount of heat will improves the overall thermal efficiency of the boiler. A large number of studies have been conducted on the recovery of latent heat in the flue gases. Kuck [1] gives a thermodynamic analysis of the PAVE process in combination with indirect-contact condensing boilers. Rosa and Tosato [2] discussed the effects of various factors on the seasonal efficiency or annual efficiency of condensing boilers. Osakabe and his fellow worker [3-9] carried out experimental study on the condensation heat transfer of an actual flue gas from a natural gas boiler on horizontal stainless steel tubes. Jia et al. [10] theoretically and experimentally studied the effects of water vapor condensation on the convection heat transfer of wet flue gas in a vertical tube. Che et al. [11] and Liang et al. [12] analyzed the heat and mass transfer process of high moisture flue gases flowing downward in a bank of horizontal carbon steel tubes. Terhan and Comakli [13] designed and analyzed a flue gas condenser to recover latent heat from the exhaust flue gases of a 60 MW natural-gas fired district heating system. Wang et al. [14] carried out experimental and numerical studies on actual flue gas condensation heat transfer in a left right symmetric internally finned tube. 447

2 The current situation in the sugar mills in China is that the flue gases leaving the boiler is first dedusted with scrubbers, and then emitted to the atmosphere, the latent heat in the flue gases is discharged with the emission of gases and waste water. A proposal for dust removal coupling heat recovery of the flue gases of sugar mill boilers is put forwards in this paper, a pilot experimental rig was set up in a sugar mill. The objective of the investigation is to evaluate the efficiencies of the heat recovery as well as the dust removal. Proposed Process and Experimental Rig Proposed Process A process for dust removal coupling heat recovery of the flue gases of sugar mill boilers is put forwards and sketched in Fig. 1. Boiler system cyclone Atomized hot water spray heat exchanger Hot water from sugar process Legend: Air Flue gases Water Induced fan Water/gases separation To chimney Figure 1. A Process for Dust Removal Coupling Heat Recovery of the Flue Gases of Sugar Mill Boilers. As is shown in Fig 1, the dust scrubber coupling heat recovery is composed of three parts, including: (1) The atomized hot water spray at the top, which is the abundant condensate in sugar process with a temperature ranging between 70~100 C. The spraying of water is to improve dust removal efficiency, avoid deposit of fly ash on the surface, and reduce corrosion due to condensation of acids; (2) The heat exchanger in the middle, which is a surface type heat exchanger with flue gases in one side and the air feeding the furnace at the other side. As the sugar mills in China usually operate in winter time and the air is mostly below 15 C, the flue gases can be cooled down below 60 C as far as the heat exchange surface is adequate, as such, the vapor in the flue gases will condense and the latent heat is recovered; (3) The waste water and flue gases separator at the bottom, which is to separate dust washing water from flue gases. The flue gases leaving the boiler is primarily dedusted by the cyclone, thus to reduce the dust load in the scrubber, reducing water consumption. Experimental Rig To waste water treatment Table 1. Main Equipment in the Experiment. Item Size or Specification Functions Induce fan 2000 Pa, 1000 m 3 /h Air and flue gases transport Cyclone Ф Flue gases dust removal Vortex flowmeter 100~1000 m 3 /h Air and flue gases flow rate measurement 448

3 Rotameter 100~1000 L/h Hot water flow rate measurement LD-5Laser dust monitor 0.01~100 mg/m 3 Emitted flue gases dust content monitor For validation of the proposed process, an experimental rig was set up in the COFCO Chongzuo Sugar Company Limited, where part of the flue gases were drawn from the main flue gases duct of the sugar mill. The flow sheet of experimental set up is similar to the proposed process but with necessary measuring and controlling facilities. Details of the major equipment for the experiment are listed in Tab. 1. The dust scrubber coupling heat recovery used in this experiment was a plate type heat exchanger, configuration of which is shown in Fig. 2. The heating surface was two 0.4 3m plates. To intensify the mixing of water and the flue gases, sections of packed corrugated plates (Fig. 3) was placed at the inlet of the heat exchanger in the flue gas ducts. The flue gases were mixed with the hot water through atomizing and spraying the hot water in the inlet stream, and were demisted at the outlet. Figure 2. Configuration of Heat Exchanger. Figure 3. Section of Packed Corrugated Plates. 449

4 Results and Discussions Heat Recovery Efficiency Tab. 2 is the experimental results of heat recovery experiment. During the experiment, the flue gas temperature was about 130 C, the hot water temperature 95 C. The ratio of water to gas was 3.5 L/m 3. From Tab. 2 it could be seen that the flue gases entering the heating surface was around 70 C, as the hot water was atomized in the gas stream, it could be well assumed that the flue gases was close to saturation at this point. The outlet temperature of the flue gases was below 60 C, indicating that the vapor in flue gases was condensed, accordingly the latent heat recovered. The air temperature rose from 13 C to 50 C. If all boiler feeding air is treated, the equivalent boiler thermal efficiency can increase by about 2%, the heat recovery was significant. Table 2. Heat Recovery Experimental Results. Series Flue gas flow rate, m 3 h Humidified flue gas temperature, C Flue gas outlet temperature, C Air flow rate, m 3 h Air inlet temperature, C Air outlet temperature, C Heat transfer, W Overall heat transfer coefficient,w m -2 C Dust removal Efficiency The dust contents of flue gases at the inlet (before mixing with water) and the outlet were detected, results of the detection are shown in Tab. 3. Table 3. Dust Removal Test Results. Experimental series Flue gas flow rate, m 3 h Inlet dust content, mg Nm Outlet dust content, mg Nm Removal efficiency, % From Tab. 3, it could be seen that the dust removal efficiency were between 80% by the dust scrubber, the dust content of the outlet flue gases was between 30.4 ~ 42.7 mg/nm 3, indicating that the experimental device had a satisfactory dust removal efficiency. As is discussed above, the vapor in the flue gases condensed in the process of heat exchange, In the process of condensation, with the submicron particles around, the vapor tended to condense on the particles which served as the nucleus for phase change [15], the removal effect was thus enhanced. Contents of particles of different sizes in the flue gases both at the inlet and outlet were detected, the removal efficiencies varying with particle sizes are plotted in fig. 4. It could be seen from Fig. 4 that the dust removal efficiency increased with the increase of particle sizes. For particle size below 2µm, the removal efficiency was below 70%, for particle size over 4µm, the removal efficiency was over 90%. 450

5 η/% d p /um Figure 4. Variation of Removal Efficiency with Particle Sizes. Summary A process for dust removal coupling heat recovery of the flue gases of sugar mill boilers is put forwards. The primary pilot experiment showed that: 1) The heat recovery was significant, the air temperature rose averagely from 13 C to 50 C for equivalent flow rate to the flue gases. The temperature of flue gases decreased to below 60 C, indicating that vapor in the flue gases had undergone condensation. 2) The dust removal efficiency of the scrubber was between 80%, the dust content of the flue gases decreased to 30.4 ~ 42.7 mg/nm 3. For the size separation efficiency, the dust removal efficiency increased with the increase of particle sizes. For particle size below 2µm, the removal efficiency was below 70%, for particle size over 4µm, the removal efficiency was over 90%. Acknowledgement This research was financially supported by the Guangxi Science and Technology Development Scheme (contract code: Guike AB ). References [1]J. Kuck, Efficiency of vapor-pump-equipped condensing boilers, Appl. Therm. Eng. 16 (3) (1996) [2]L. Rosa, R. Tosato, Experimental evaluation of seasonal efficiency of condensing boilers, Energy Build. 14 (3) (1990) : [3]M. Osakabe, Thermal-hydraulic behavior and prediction of heat exchanger for latent heat recovery of exhaust flue gas, Proc. ASME Heat Transfer Div. 2 (1999) [4]M. Osakabe, T. Itoh, K. Yagi, Condensation heat transfer of actual flue gas on horizontal tubes, in: Proceeding of 5th ASME/JSME Joint Thermal, Engineering Conference, AJTE , [5]M. Osakabe, Latent heat recovery from oxygen-combustion flue gas, AIAA 2 (2000) [6]M. Osakabe, K. Itoh, T. Ohmasa, Condensation heat transfer on sprially finned tube in actual flue gas, J. MESJ 35 (4) (2000) [7]M. Osakabe, T. Itoh, Thermal-hydraulic prediction for wet region of economizer using spirally finned tubes, Proc. ISME J. (2000)

6 [8]M. Osakabe, K. Ishida, K. Yagi, T. Itoh, K. Ohmasa, Condensation heat transfer on tubes in actual flue gas, Heat Transfer Asian Res. 30 (2) (2001) [9]M. Osakabe, K. Yagi, T. Itoh, K. Ohmasa, Condensation heat transfer on tubes in actual flue gas (parametric study for condensation behavior), Heat Transfer Asian Res. 32 (2) (2003) [10]L. Jia, X.F. Peng, Y. Yan, J.D. Sun, X.P. Li, Effects of water vapor condensation on the convection heat transfer of wet flue gas in a vertical tube, Int. J. Heat Mass Transfer 44 (22) (2001) [11] D.F. Che, Y.D. Da, Z.N. Zhuang, Heat and mass transfer characteristics of simulated high moisture flue gases, Heat Mass Transfer 41 (2005) [12]Y.B. Liang, D.F. Che, Y.B. Kang, Effect of vapor condensation of moistened gas, Heat Mass Transfer 43 (2007) [13]M. Terhan, K. Comakli, Design and economic analysis of a flue gas condenser to recover latent heat from exhaust flue gas, Appl. Therm. Eng. 100 (2016) [14]Y. Wang, Q. Zhao, Q. Zhou, et al. Experimental and numerical studies on actual flue gas condensation heat transfer in a left right symmetric internally finned tube, In. J. Heat Mass Transfer 64 (2013) [15] S. Heidenreieh, F. Ebert, Condensational droplet growth as a preconditioning technique for the separation of submicron particles from gases, Chemical Engineering and Processing 34(1995)