Study on a novel district heating system combining clean coal-fired cogeneration with gas peak shaving

The existing coal-fired heating mode in north China has a higher energy consumption and causes heavy haze in winter. Combining with natural gas, deeply exploring the energy saving potential in coal-fired Cogeneration can provide an effective way for solving the problems. This paper proposes the methods to utilize exergy loss in the existing district heating system: 1) On the heat network, high temperature hot water or gas is used for driving the absorption heat exchangers (AHEs), by which the temperature of return water backing to CHP plant can be greatly reduced. 2) While in the CHP plant, a cascaded heating process with absorption heat pump (AHP) is constructed for efficient recovery of the condenser waste heat, by which the heating capacity of CHP plant can be significantly improved. Based on this, this paper proposes a new type of district heating system, in which clean coal-fired cogeneration is the dominant and novel gas peak-shaving is the supplements. Corresponding analysis model is established, taking a CHP plant with 2 × 330 MW turbine units as an example. Comparing with previous district heating system with gas peak-shaving boiler, the new system has apparent advantages: Firstly, the comprehensive exergy efficiency is increased from 47% to 66%, the coal consumption is reduced from 14.48 kg/GJ to 9.74 kg/GJ, while the natural gas consumption is reduced from 17.65 Nm³/GJ to 8.56 Nm³/GJ. Secondly, the emission of SO₂ is decreased by 50.6%, of smoke is decreased by 32.9%, and of NO_x is decreased by 35.6%. Finally, the energy cost is decreased from 55.1 CNY/GJ to 28.2 CNY/GJ, and the payback rate of incremental investment is 22%.