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  • Technical and economic analysis of flue gas denitrification process in household waste incineration

    Research background

    The denitrification technology of household waste incineration flue gas is mainly SNCR (selective non-catalytic reduction) and low temperature SCR (selective catalytic reduction). The principle of both is to generate harmless nitrogen and water through the reaction of reducing agents such as ammonia and urea with NOx (nitrogen oxides). The difference is that SNCR is a direct reaction of liquid reducing agent at high temperature (generally 850~1050℃), which occurs in the corresponding temperature range of the boiler, and the efficiency is limited by many factors. Low temperature SCR is the reaction of gaseous reducing agent under the action of catalyst at about 180℃, set up an independent reactor, the efficiency and stability is higher than SNCR.

    The waste incineration industry faces difficulties such as the decline of state subsidies and the increase of emission requirements, and the demand for technology cost reduction and efficiency is rising rapidly. Therefore, this paper establishes a technical and economic model of SNCR and SCR for waste incineration, studies the operating cost and its influencing factors in the denitration process of SNCR and SCR, and provides support for the efficient and low-cost application of denitration technology in the industry from an economic perspective.

    Research method

    The process models of waste incineration SNCR and low-temperature SCR were refined, and the matching operation cost model was established.

    Reducing agent reserve system: ammonia water is used as reducing agent, the process is simple, there is no need for heat preservation in most areas, so the energy consumption of ammonia water storage and transportation is mainly pump power consumption. The energy consumption of preparation of reducing agent is mainly the energy consumption of evaporation of ammonia water into ammonia gas, which is heated by steam. The use of urea solid as reducing agent needs to consider tank insulation, urea dissolution heat absorption, especially urea pyrolysis to produce ammonia gas consumes a lot of energy, need to use "steam heating + electric heating" two-stage heating process.

    Reaction system: SNCR reaction occurs in the furnace, without external energy, but ammonia evaporation may cause waste heat loss of flue gas. Low temperature SCR is located behind the dust collector, the smoke temperature is low, and steam is needed to heat the flue gas to more than 180℃, which has high energy consumption. And the reactor increases the flue gas resistance, increase the fan energy consumption.

    Finally, the material consumption and energy consumption in the denitration process are summarized and multiplied by their respective prices to obtain the cost of the denitration process of SNCR and SCR.

    Main research achievements

    Taking a 500 t/d waste incineration project as an example, different denitrification technologies were studied combined with technical and economic models. In terms of equipment investment cost, the investment cost of low-temperature SCR is much higher than that of SNCR, and urea process is slightly higher than that of ammonia. SNCR system is simple, low energy consumption, reducing agent is the most important factor affecting the operating cost. In addition, the power generation loss caused by droplet evaporation can not be ignored. The operating cost of low-temperature SCR is much higher than that of SNCR, in which the steam and electricity cost account for the highest proportion, and the flue gas heating cost is much higher than other costs. Urea process operation cost is significantly higher than ammonia water, because urea pyrolysis heating demand is greater than ammonia water, the overall electricity cost is more than 2 times of ammonia water. Emission standards were raised, reducing agent consumption increased, and SNCR and SNCR+SCR operating costs increased. The utilization rate of reducing agent of SNCR is lower than that of SCR, so the increase of operation cost is larger than that of SNCR+SCR, but the total cost is still much smaller than that of SNCR+SCR.

    Conclusion and prospect

    Reducing agent and evaporative heat loss are the main operating costs of SNCR, with reducing agent accounting for nearly 70%. The economy of ammonia process SNCR is not always better than that of urea, and the optimal scheme can be selected according to the local urea/ammonia price ratio. The steam heating cost of ammonia water and urea process accounted for 73% and 60% of SCR operating costs respectively. The annual operating cost of urea is about 730,000 more than that of ammonia water. The emission standard was increased from 200mg/m3 to 80mg/m3, and the operating cost of SNCR+SCR increased by 4.2%; SNCR increased by 83.3%, but the overall cost SNCR is still much lower than SNCR+SCR. SNCR is a more economical solution to NOx120 mg/m3 standard in Hebei, Tianjin, Hainan and other places, and it has been proved that it has the corresponding ability. However, there are still few researches and applications of high efficiency SNCR technology. Especially, how to ensure high efficiency denitrification and control ammonia escape within the standard 8 mg/m3 is a big problem that needs to be solved urgently. For projects with higher emission standards and higher stability requirements, low-temperature SCR technology is still the first choice. In the application, it is suggested to use ammonia water technology as far as possible. In technology, how to reduce flue gas heating energy consumption is the key.


    Source: https://mp.weixin.qq.com/s/j3UvcgQ0BVijaD274nKzeA

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