Development of heating retrofit using waste heat from coal-fired CHP system cold end

doi:10.3969/j.issn.1674-1951.2021.03.008

Abstract

Abstract:

The insertion of high-proportion intermittent renewable energy has increased the requirement for flexibility of coal-fired units.With the mounting demand for heating in residential areas,insufficient peak-regulating capacity and lack of heat supply can be alleviated by heating retrofits using waste heat from coal-fired CHP system cold end.Three transformation methods,high back-pressure,heat pump waste heat recovery and low-pressure cylinder "zero output" heating can be taken in this work.The high back-pressure heating transformation was evaluated based on practical cases.Then,focusing on the absorption heat pump,the heat pump waste-heat recovery heating transformation was introduced,including its operational principles and performance parameters.Taking cutting low-pressure cylinder and shaft heating reform as the example,the characteristics of low-pressure cylinder "zero output" heating transformation were listed. Finally,the applicability of the three heating transformation methods was summarized,which is expected to provide a reference for the following heating retrofits at coal-fired CHP system cold end.

Key words: coal-fired CHP system, waste heat of cold end, high back-pressure heating, waste heat recovery, zero output from low-pressure cylinder, peak shaving of coal-fired units, renewable energy, absorption heat pump

CLC Number:

TK01 ⁺9：X783

FANG Xu, PENG Xuefeng, ZHANG Kai, MA Jingbang, ZHAO Ruixiang, WANG Jinxing. Development of heating retrofit using waste heat from coal-fired CHP system cold end[J]. Huadian Technology, 2021, 43(3): 48-56.

Figures/Tables 11

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Comparison on the waste heat heating renovation schemes of cold end

改造方案	原理	优点	缺点	使用情况
高背压供热改造	供暖期更换为高背压低压缸转子,利用汽轮机排汽对热网循环水进行一级加热	最大限度地回收中压缸排汽余热,供热能力强,供热经济性好	改造工作量较大,改造需要更换转子,运行检修工作量大,维护成本较高,对热负荷要求高	热网回水温度较低的采暖地区适宜采用高背压供热改造方案;改造时,应使实际供热负荷接近最大供热负荷^[38]
吸收式热泵余热回收	在发生器内采用高温蒸汽作为驱动热源将工质溶液进行分离	运行成本较低,制冷工质溴化锂溶液无毒,且没有损耗,设备变化负荷不受影响,机组容量大制热出水温度较高	设备比较复杂,发生器内压力对热泵性能影响较大,制热吸收较小,占地面积较大,在热负荷变化时,余热水源无法保障,会出现抢水问题,空冷岛容易结冻	用于热电厂乏汽余热回收供暖;适用于冷却循环水温度高、抽汽压力大的情况^[39]
电驱动压缩式热泵余热回收	采用电能作为驱动热源将工质溶液进行分离	结构简单,操作灵活,占地面积小,相对于吸收式热泵投资少,能效高	运行成本高,制热温度较低,制冷工质多为有毒有机物,对环境产生破坏	适用范围广。适用于单个房间或数个房间,既能夏季制冷也能冬季供热,较适用于电能廉价的地区^[40]
切除低压缸进汽	供热期切除低压缸进汽运行,低压缸不做功,中压缸排汽直接供热	可以大量回收中压缸排汽余热量,汽轮机本体基本不需要改造,运行维护费用底,投资少,供热经济性好,运行方式灵活	没有长期运行经验,存在低压缸小容积流量运行工况。长期偏离设计工况,运行存在一定危险	低压缸转子需要冷却的蒸汽量较大,余热利用率低于光轴改造,但机组改造成本远低于光轴改造成本^[41]
光轴改造	用光轴代替原低压转子,切除低压缸,中压缸排汽直接供热	可以大量回收中压缸排汽余热,供热能力强,改造工作量少,运维成本低,供热经济性好	机组电负荷随热负荷的变化而变化,调节方式单一;需更换转子,维护成本高,热负荷要求高,机组发电功率低	同切除低压缸进汽改造一致,在参与电网深度调峰时,有效缓解供热问题。应根据具体经济效益进行选择^[42]

Tab.4

References 42

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机组及容量	改造方式	效益	参考文献
300 MW高背压供热机组	利用旧转子改造用于高背压供热运行工况的低压转子主轴,末级及次末级拆除原动叶,并安装假叶根,拆除末级、次末级隔板,更换为导流环	供电煤耗由改造前289.48 g/(kW·h) 降低至151.04 g/(kW·h),热电比由改造前41.31%提高至183.74%	[18]
330 MW空冷高背压供热机组	利用汽轮机排汽余热和机组的低压缸抽汽对热网循环水进行加热	热网循环水供热温度提高至68 ℃,年供热量增加	[19]
330 MW直接空冷机组的高背压供热改造机组	采用循环水高背压改造方案,改变循环水流量、回水温度和机组排汽流量	采暖期间的平均电负荷为250 MW,平均供热量1.30 TJ/h。机组平均供电煤耗下降32.10 g/（kW·h）	[20]
200 MW三缸三排汽凝汽式汽轮机组	通过连通管蝶阀控制中压缸排汽压力在合理范围内,保持双分流低压缸不动,将中压缸后单排汽低压缸末级隔板和动叶去掉,末级叶轮安装假叶根,用以保护轮槽。对凝汽器A与凝汽器B之间进行隔断	机组供热期的煤耗由366.50 g/（kW·h）降至321.64 g/（kW·h）,机组可以低负荷运行,更加灵活地参与电网的深度调峰	[21]
300 MW直接空冷机组	使EBSILON软件进行建模,验证了模型的精确性,根据此热力模型对高背压供暖改造后的机组进行了性能分析	热电比可达200%,有效缓解了用热多、用电少的矛盾,同时提高了机组的调峰能力	[22]

热泵种类及数量	热泵容量/MW	机组容量/MW	供热功率/MW	效益	参考文献
溴化锂吸收式热泵×8	24.887	774	199.096	煤耗下降16.49 g/（kW·h）,供热期盈余1 107.41万元	[26]
溴化锂吸收式热泵×2	46.600	300	159.856	供热量增加 10.85%,电负荷调节裕度增加27 MW	[27]
溴化锂吸收式热泵×4	32.500	300×2	121.952	回收乏汽余热130 MW,供热收入1 819万元。减少了大量SO₂,CO₂,NO_x及灰渣烟尘的排放	[28]
溴化锂吸收式热泵×1	108.600	300×2	78.692	回收乏汽余热217 MW,静态投资为23 852万元,投资回收期5.61 a	[28]

机组及容量	改造方式	收益	参考文献
200 MW供热机组	切除低压缸进汽	相同锅炉蒸发量的情况下,增加采暖抽汽量140 t/h,机组发电负荷下降约25 MW;相同供热能力的情况下,降低机组发电负荷约58 MW	[30]
300 MW供热机组	光轴改造	供热能力提高429 GJ/h。增加供热面积约238.5万m²,提高了电厂热效率,回收乏汽量155 t/h	[31]
350 MW超临界热电联产机组	切除低压缸进汽	供热蒸汽量大幅度提升,在可调节范围内,采暖抽汽量每增加100 t/h,供热负荷增加约70 MW,电负荷调峰能力增加约50 MW	[32]
200 MW纯凝机组	低压转子光轴改造	节约煤炭资源,发电煤耗低至268.50 g/（kW·h）,NO₂,CO₂,SO_x排放量大大减少	[33]