1 Introduction
With the traditional coal-fired and oil-fired heating in the northern part of our country, with the development of economy and urban expansion, the shortcomings of these traditional heating methods have become more and more prominent and can not meet the requirements of sustainable development. According to statistics, coal-fired heating has been Be the culprit in winter air pollution in northern cities. For example, in Beijing, terminal energy consumption in Beijing was 38.28 million tons of standard coal in 1999, of which coal consumption was 2011 million tons, accounting for about 53% of the total energy consumption of terminals, which was the main source of air pollution. In order to avoid the low temperature corrosion of the tail heated surface, the traditional coal-fired heating boiler exhaust temperature is usually higher than 150 ℃, steam boiler even higher than 200 ℃, most gas-fired hot water boiler exhaust temperature 140 ℃ -200 ℃ between. Excessive exhaust gas temperature not only consume a large amount of energy, but also increase the boiler operating costs [2,3]. Therefore, exploring some clean, energy-efficient and efficient heating technologies in these areas is of great realistic and important significance and has gradually become a hot issue that is widely concerned by engineering and academic circles.
To this end, since the 1970s, Western Europe and the United States have successively developed condensing boilers, that is, based on the traditional boilers, a condensing heat exchange heating surface is added to reduce the temperature of the exhaust gas to 40-50 ° C, In the steam condensate down and the release of latent heat, can increase the thermal efficiency to 100% (calculated as low calorific value); the same time found that in the flue gas condensation, flue gas SOx, NOx, CO2, CO and fly ash And soot can partially or completely dissolve in the water, so that the harmful substances discharged into the atmosphere greatly reduced, is conducive to environmental protection [2,3]. Therefore, the use of condensing boilers for energy saving and environmental protection are of great significance. However, our country adopts the traditional 95/70 ℃ closed heat supply system, a considerable number of boilers run at 80/70 ℃ [4]. When using clean fuel natural gas, the flue gas dew point temperature is generally about 55 ℃, through the return of the original heating system to complete the flue gas condensation is not feasible. And the higher temperature of the smoke, compared with air source heat pump heating system, should be a good low-temperature heat source [5]. In view of the problems existing in the application of condensing boilers to the traditional heat supply in our country, in order to achieve the purpose of energy saving and environmental protection, the waste heat recovery system of flue gas widely used abroad is introduced - the combined system of condensing boilers and heat pumps.
2 condensing boiler and heat pump system
The combined system of condensing boilers and heat pumps not only makes use of the principle of energy saving and environmental protection of condensing boilers but also utilizes the principle that heat pumps increase the heat energy of low grade to high grade. In this system, in order to make the heat pump system recover heat energy of lower grade as much as possible and reduce heat pump system installed capacity and power consumption so as to save energy as much as possible, the heat network backwater and the smoke from the condensing boiler body heat Heat the flue gas to a certain temperature, and then pass the low-temperature flue gas through the condensing heat exchanger (as anti-corrosion material, such as stainless steel) to recover part of the latent heat and sensible heat in the flue gas. In order to maintain the stability of the temperature of the low-temperature heat source at the evaporation end of the heat pump and reduce the average heat transfer temperature difference at the evaporation end, the flue gas is firstly passed through a wall-type condensing heat exchanger (such as a plate and a tube) or a contact condensing heat exchanger Liquid column, packing, etc.), first with the low-temperature water (cooling water) and flue gas heat transfer, and then the cooling water is heated by the pump pumping to the evaporation side of the heat pump and heat pump working medium (such as R11, etc.) heat exchange. The following describes the condensing heat exchanger and heat pump combined heat recovery system, the system has two types: as shown in Figure 1 wall heat exchanger and heat pump heat recovery system and Figure 2 contact heat exchange heat pump and heat recovery system .
Figure 1, a wall-type condensing heat exchanger and heat pump combined heat recovery system
1 - boiler body 2 - flue gas duct for condensation 3 - flue gas bypass duct 4 - smoke exhaust duct to chimney 5 - wall condensing heat exchanger 6 - condensate discharge duct to be treated 7, 13 - Backwater 8-flue gas cooler 9-cooling water circulation pump 10-heat pump evaporator 11-throttle valve 12-heat pump condenser 14-compressor
Figure 2 contact condensing heat exchanger and heat pump combined heat recovery system
1 - boiler body 2 - flue gas duct for condensation 3 - flue gas bypass duct 4 - smoke exhaust duct to chimney 5 - contact condenser heat exchanger 6 - flue gas cooler 7, 15 - return water from the zone heat network 8 - Cooling water circulation pump 9 - Treatment liquid circulation pump 10 - Sewage discharge pipe 11 - Water processor 12 - Heat pump evaporator 13 - Compressor 14 - Heat pump condenser 16 - Throttle valve
In FIG. 1, there are four heat exchange processes in addition to the heat exchange of the boiler body in the combined heat recovery system of the wall-type condensing heat exchanger and the heat pump. (1) In the flue gas cooler 8, the high-temperature flue gas and the heat from the area (2) a heat exchange process between the low temperature warm flue gas from the flue gas cooler and the cooling water (low temperature heat source of the heat pump) in the inter wall condenser heat exchanger 5; (3) a heat exchange process between the cooling water from the inter-wall condensing heat exchanger and the heat pump low-temperature working fluid in the evaporator 10 of the heat pump; (4) in the heat pump condenser 13, The heat exchange process between the backwater of the district heating network. Condensate generated in the condensing heat exchanger 5 is discharged through the condensate discharge pipe 6 to be treated, and the flue gas condensed in the condensing heat exchanger 5 and the flue gas from the bypass flue 3 are mixed into the chimney.
In Figure 2, the contact condensing heat exchanger and heat pump combined heat recovery system and the partition system significantly different from the condensing heat exchanger with flue gas, the use of contact heat exchangers, flue gas and cooling water in direct contact with the heat exchange , And the cooling water is contaminated, requiring regular treatment and sewage disposal.
Intercrystalline condensing heat exchangers and contact condensing heat exchangers have been used abroad, each with its own advantages and disadvantages. Such as the wall with flue gas and water separation, water pollution, but there is a large temperature difference between the heat exchanger, limited by the size of the structure, heat transfer area is limited, there is thermal resistance and fouling resistance, low heat transfer efficiency. Contact with gas-liquid two-phase discrete contacts, heat transfer temperature difference is small, heat transfer area increased greatly, flue gas pollutants have been cleaned, which is conducive to environmental protection, but the water quality affected, need to be dealt with.
3 condensing boiler and heat pump combined system application
Abroad, the advanced method has been developed to recover the heat energy in the flue gas. The combined system of condensing boilers and heat pumps is widely used in areas such as district heating boiler rooms, especially the wet areas such as domestic waste, wood chips and the like Of the fuel and natural gas-fired boiler room [6,7,8]. For example, Sweden installed a combined system of condensing boilers and heat pumps with a heat recovery of 20 MW at its largest municipal waste incineration plant (Uppsala, southeast) in 1985. The annual benefits of installing major equipment (including installation) and waste heat recovery are shown in Table 1 [2].
Table 1 Major Equipment Investment (Including Installation) and Waste Heat Recovery Annual Benefit
Through economic calculations, although the initial investment is larger, the investment used is lower than the capital used to build up a heat by building a boiler, and it removes the flue gas while increasing heat production efficiency Acid gas and mercury, which is very beneficial from the standpoint of energy saving and environmental protection. Therefore, in some urgent need to expand, transform and the new central heating boiler room, the use of condensing boiler and heat pump system should be an energy saving and environmental protection.
According to the "Ninth Five-Year Plan" and the 2010 Development Plan for the Construction of Beijing Central Heating Network, the existing seven heat source plants, including one heat source and two heat sources, will be expanded and expanded, and the designed heating area will increase by 5.92 million m2. Another proposed to build two thermal power plants, the new heating scale of 10 million square meters. In 2005, the heating area is expected to reach 83.59 million square meters, accounting for 31.13% of the total floor area of ​​the city [1]. And Beijing began to focus on environmental assessment as the central goal, focusing on the impact of various heating modes on environmental protection, and as a benchmark and starting point for analysis and comparison, taking into account the energy efficiency, resource utilization, investment and economic benefits Therefore, the research on the combined system of condensing boiler and heat pump in this area is of great significance. And in the combustion of clean fuel natural gas, theoretically about 1.6kg of condensate per standard cubic meter, such as WNS4.2-1.0 / 95/70-Q-type hot water boiler fuel consumption of about 440Nm3 / h, if Sixty percent of the water vapor produced is cooled to produce 440 x 1.6 x 0.6 = 422.4 kg / h of water, and it can be seen that the amount of water obtained is appreciable. If some measures are taken to utilize this part of water, it will undoubtedly play a positive role in some water-scarce regions [2, 9].
4 Conclusion
The combined system of condensing boilers and heat pumps is a kind of energy recovery system with high efficiency and energy conservation and environmental protection. In view of the current heating situation in our country, such as serious pollution, insufficient heat, urgently needed expansion and some new boiler room fund shortage, The joint system should be a way to save energy and protect the environment that is worth exploring and applying. The system can also generate considerable condensate, which can be treated and utilized as part of industrial water and animal husbandry for less demanding water quality. And the heat pump part of the system can also provide users with cooling capacity in the summer, for small central air conditioning use, with greater energy efficiency.
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