Research On Low Temperature District Heating Of Combined Heat And Power Based On Green Heating

Combined heat and power (CHP) is considered as an important technology for efficient utilization of coal and emission reduction, which has superiority to separated production of heat and power with respect to enengy conservation, environmental protection, economic and social benefits. The thermal efficiency will be increased to about 80% if the technology of CHP is utilized on the coal-fired power units. Compared to the supercritical units with the coal consumption of 310 g/kW·h, the thermal efficiency of CHP nealy doubles. However, further reseach still needs to be conducted in the aspects of theoretical energy-saving potential, fuel consumption evaluation and technology application. There is still very large potential remaining for CHP.This paper presents a theoretical analysis of heating systems based on the 1st and the 2nd laws of thermodynamics, clarifies the theoretical potential limitation of heating systems and illustrates the theoretical minimum specific fuel consumption based on the analysis of generalized heating system. A novel concept (green heating, GH) is scientifically defined based on resersible heating process analysis:on the premise of scientific measurement, heating system with specific fuel consumption lower than bminI is termed as green heating system. The corresponding heating technology is GH technology. A new indicator (green-heating index, GH Index) is defined to characterise different options for heating and to quantitatively evaluate their green levels, which scientifically indicate the direction of energy conservation.To modify the specific fuel consumption method for GH, a common calculation model is proposed to evaluate the fuel consumption and GH levels of different kinds of heating systems, which lays the foundation for the rational develpoement heating systems. In this paper, the GH Index levels from low to high are electrical heating, boiler-heating, heat pump heating and CHP.Based on the concept of green heating, a practical technology was proposed and termed as low temperature direct heating (LTDH), which fully utilize the exergy at the cold end of turbine and supply heat with circulating water directly to heat users to improve the exergy efficiency and realize cascaded utilization of energy. Compared to traditional systems, LTDH has much higer energy-saving potential. For a 300 MW unit, comparing to the traditional system, the LTDH increases the power generation rate by 31 MW, which acounts for over 10% of the unit capacity.The adjusting methods of LTDH system were discussed. The heat users, the district heating (DH) network and the CHP plant are considered as one integrated unit from the overall system level. The adjusting method of the secondary DH network, the primary DH network and the CHP plant under part heat load condition was proposed. Taking LTDH with directly connected DH network as an example, the adjusting curves under part heat load conditions were presented. An energy-saving potential analysis of LTDH during the whole heating season was carried out. The research of LTDH may provide a guidence for the design and reform of the CHP heating systems in China.A platform of off-design simulation of LTDH was developed. A coupled mathematical calculation model of hydraulic and thermal characteristics of the DH network was established and program modules were developed. Taking a DH region as an example, a municipal DH network was built up and integrated with an existing CHP plant. Using the integrated model, the design and off-design characteristics of the heat users, the DH network and the CHP plant were analyzed. The model is reliable and meaningful for the integrated design and optimization of CHP system.Design and operational optimization of LTDH system was carried out. First, the control strategy of LTDH system was proposed and the design supply temperatures of primary and secondary DH network were optimized. Second, the operational supply temperatures were optimized based on genetic algorithm and the optimized operational curve was presented. In addition, the effect of decreasing the main stream flow rate to the fuel consumption of the LTDH system was discussed. The optimization results of LTDH system can serve as the quantitative guide for the practical utilization of LTDH.