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1、Condensing boiler applications in the process industryQun Chen, Karen Finney, Hanning Li, Xiaohui Zhang, Jue Zhou, Vida Sharifi, Jim SwithenbankSUWIC, Department of Chemical fax: +44 (0) 114 222 7501.E-mail address: q.chensheffield.ac.uk (Q. Chen).Applied Energy 89 (2012) 3036Contents lists availabl
2、e at ScienceDirectApplied Energyjournal homepage: of the fuel. The fact that one device can recover both com- ponents suggests that this is a preferable strategy to drying the feed and using two separate devices.Sheffield University Waste Incineration Centre (SUWIC) has conducted an extensive liter
3、ature review of industrial condensing boilers, looking into various technologies and the associated costs. In addition, extensive calculations have been carried out as part of a case study to investigate the thermal design of a condensing boiler in a large scale district heating plant (40 MW). This
4、paper presents the results obtained from the above studies.2. Industrial condensing boilers and their applicationsCombustion in air yields water vapour and CO2. Conventional boilers transfer most of the sensible heat of this reaction to hot water or steam, whereas condensing designs capture the late
5、nt heat the energy released by condensing the vapour; extractingthis heat achieves higher efficiencies.2.1. Flue gas condensersThe application of condensers to recover latent heat from fluegas is wider than stand-alone gas-fired condensing boilers, for power plants and commercial/industrial faciliti
6、es. In general, there are two types of condensers: indirect and direct contact. Indirect contact condensers recover the latent heat by passing gas through condensing heat exchangers 3. These can be sub-categorised into pipe condensers, lamella condensers and combi condensers 4. Thesecondoption,direc
7、tcontactcondensers,consistofa vapour-conditioning chamber and a counter-current spray cham- ber, in which small droplets of cool liquid come into contact withthe hot flue gas, providing a non-fouling heat transfer surface. The droplets cool, condense and remove water vapour. To improve contact betwe
8、en the spray and gas, the chamber may be equipped with packing. This offers high heat transfer and water recoverycapabilities. Such condensers require site-specific engineering de- sign and an understanding of their effects on the system.2.2. Advantages of condensing boilers2.2.1. Latent heat recove
9、ryLatent heat recovery is the most significant advantage of con-densing boilers, greatly improving thermal efficiency. Fig. 1 shows this for a wood chip boiler with a condensing heat exchanger at dif- ferent excess air ratios (k). The fuel (wood chips) used in the boiler has 50% moisture content. As
10、 shown, the heat exchanger recovers the latent heat of the moisture when it is condensed, resulting inefficiencies exceeding 100% with reference to the lower heat valueof the fuel. On the other hand, increasing k decreases the vapour partial pressure in the gas and lowers its dew point; more sensibl
11、e heat is carried by non-condensable gas, less vapour is condensed under higher k and at the same temperature, higher k lead to lowerefficiencies.2.2.2. Emission abatementFlue gas condensers can also reduce flue gas emissions, provid-ing several environmental benefits. The condensate forms a con-sta
12、nt film that can remove large particles and dissolve most of the highly dissociated inorganic matter, such as sulphuric acid,and chlorides 5. Combustion generates fine particles, contribut-ing significantly to energy sector emissions 6. Sippula et al. 7studied emissions of four wood chip-fired distr
13、ict heating units. All were equipped with cyclones to remove coarse particles. Onehad a condensing flue gas scrubber, which removed 44% of PM1 and 84% of total solid particles, resulting from thermophoresis (induced by the temperature gradient between the gas and the surface) and diffusiophoresis (c
14、aused by steam condensing on cool surfaces). Other separation mechanisms include inertial impaction and gravitational settling for large particles and Brownian diffusion for small particles. Particle sizes grew inside the scrubber, causing a shift in the particle size distribution 7.2.3. Technical b
15、arriers of condensing boilers and potential solutions2.3.1. Corrosion Corrosion by corrosive combustion products causes cracks in low-temperature steel heat exchangers, where mechanical stresses are high 8. Microscopic analyses reveal inter-granular corrosion, showing complete material grains becomi
16、ng detached. The amount and composition of condensate needs to be known to avoid this 8,9. To reduce corrosion and capture latent heat, condensing boil- ers need corrosion-resistant fabrication materials, careful installa- tion and sophisticated controls. The specialised materials and terminal units are expensive and the installation costs are higher than conventional boilers 10. Characterised by austenitic/ferritic material and high
