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直流锅炉的湿干态运行

Wet/Dry Operation of Once-through Boiler

一．

超临界直流锅炉启动特点

ⅠStart-up characteristics of super-critical pressure once-through boiler 超临界锅炉的启动系统是超临界机组的一个重要组成部分。

Start-up system of super-critical pressure boiler is an important part of super-critical pressure unit.

由于超临界锅炉没有固定的汽水分离点，在锅炉启动过程中和低负荷运行时，给水量会小于炉膛保护及维持流动稳定所需的最小流量，因此必须在炉膛内维持一定的工质流量以保护水冷壁不致过热超温。

There is no fixed W/S separation point in super-critical pressure boiler, feedwater flow will be less than the minimum flow of furnace protection and to stabilize circulation during boiler start-up and running with low load, so it’s necessary to maintain certain working media to ensure that water wall will not get over-temperature.

设置启动系统的主要目的就是在锅炉启动、低负荷运行及停炉过程中，通过启动系统建立并维持炉膛内的最小流量，以保持水冷壁水动力稳定和传热不发生恶化，特别是防止发生亚临界压力下的偏离核态沸腾和超临界压力下的类膜态沸腾现象，保护炉膛水冷壁，同时满足机组启动及低负荷运行的要求。

The main purpose of setting start-up system is that during boiler start-up/shutdown and running with low load, maintain minimum flow in furnace by start-up system to ensure stable water dynamic force of water wall and sound heat transfer, in particular, to avoid departure from nuclear boiling(DNB) under subcritical pressure, and film boiling under super-critical pressure, so as to protect water wall and satisfy the requirements for unit start-up and operation with low load.

超临界锅炉与亚临界自然循环锅炉的结构和工作原理不同，启动方法也有较大的差异，超临界锅炉与自然循环锅炉相比，有以下的启动特点：

The structure and working principle of super-critical pressure boiler are different from that of natural circulation boiler, so is the start-up method. In comparison with natural circulation boiler, super-critical pressure boiler is characterized by the following:

1．设置专门的启动旁路系统

Setting special start-up bypass system 直流锅炉的启动特点是在锅炉点火前就必须不间断的向锅炉进水，建立足够的启动流量，以保证给水连续不断的强制流经受热面，使其得到冷却。

The characteristic of start-up of once-through boiler is that before boiler light-up, it must continuously fill boiler with water to make start-up flow sufficient so as to ensure feedwater flows heating surface forcefully and continuously to cool it.

一般高参数大容量的直流锅炉都采用单元制系统，在单元制系统启动中，

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汽轮机要求暖机、冲转的蒸汽在相应的进汽压力下具有50℃以上的过热度，其目的是防止低温蒸汽送入汽轮机后凝结，造成汽轮机的水冲击，因此直流炉需要设置专门的启动旁路系统来排除这些不合格的工质。

Unit system is adopted by once-through boiler of high parameter and large volume. During start-up of unit system, superheat degree of steam to warm and roll for turbine should be over 50℃ under its corresponding admission pressure, so as to avoid water impact on turbine which may be caused by low temperature steam entering turbine and condensing. Therefore, it’s required to set special start-up bypass system to discharge these unqualified working media for once-through boiler.

2．配置汽水分离器和疏水回收系统

W/S separator and drain recovery system 超临界机组运行在正常范围内，锅炉给水靠给水泵压头直接流过省煤器、水冷壁和过热器，直流运行状态的负荷从锅炉满负荷到直流最小负荷，直流最小负荷一般为25%~45%。

When super-critical pressure unit runs within normal range, through pressure head, boiler feedwater flows directly by economizer, water wall and superheater, load under once-through operation is from full load of boiler to the minimum load of direct flow which is generally 25%~45%.

低于该直流最小负荷，给水流量要保持恒定。

When it’s below the minimum load of direct flow, keep feedwater flow stable. 例如在20%负荷时，最小流量为30%意味着在水冷壁出口有20%的饱和蒸汽和10%的饱和水，这种汽水混合物必须在水冷壁出口处分离，干饱和蒸汽被送入过热器，因而在低负荷时超临界锅炉需要汽水分离器和疏水回收系统，疏水回收系统是超临界锅炉在低负荷工作时必需的另一个系统，它的作用是使锅炉安全可靠的启动及其热损失最小。

For example, in 20% load, the minimum flow is 30%, which means there is saturated steam (20%) and saturated water (10%) in outlet of water wall, the W/S mixture must be separated in outlet of water wall, dry saturated steam is put into superheater, so W/S separator and drain recovery system are needed in super-critical pressure boiler; drain recovery system is another necessary system when super-critical pressure boiler is running with load low, whose main function is to make boiler start-up safe and stable and heat loss minimize.

3．启动前锅炉要建立启动压力和启动流量

Making start-up pressure and flow ready before starting boiler 启动压力是指直流锅炉在启动过程中水冷壁中工质具有的压力。

Start-up pressure is the pressure that working media in water wall possess during start-up of once-through boiler.

启动压力升高，汽水体积质量差减小，锅炉水动力特性稳定，工质膨胀小，并且易于控制膨胀过程，但启动压力越高对屏式过热器和再热过热器的保护越不利。

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Start-up pressure increases, volume and mass of W/S decrease, boiler water dynamic characteristic is stable, expansion of working media is minor, and it’s easy to control expansion process; however, the higher start-up pressure is, the worse for protection of platen superheater and reheat superheater.

启动流量是指直流锅炉在启动过程锅炉的给水流量。

Start-up flow is the feedwater flow of boiler during start-up of once-through boiler.

二．

汽水分离器的控制方式

Ⅱ Control ways of W/S separator

启动分离器在湿态和干态的控制是不相同的，而且随着压力的升高，湿干态的转换是汽水分离器的一个显著特点。

It’s different control in wet/dry for start-up separator; with pressure increasing, conversion between dry and wet status, it’s another characteristic.

1．汽水分离器的湿态运行 Wet operation of W/S separator 如前所述，锅炉负荷小于25%时，超临界锅炉运行在最小水冷壁流量，所产生的蒸汽要小于最小水冷壁流量，汽水分离器湿态运行，汽水分离器中多余的饱和水通过汽水分离器液位控制系统控制排出。

As the said above, when boiler load is less than 25%, super-critical pressure boiler is running at the minimum water wall flow, and its generated steam is less than the minimum water wall flow, W/S separator runs in wet status, surplus saturated water in W/S separator drains by W/S separator level control system.

2．内置式汽水分离器的干态运行 Dry operation of in-built W/S separator

当锅炉负荷大于25%以上时，锅炉产生的蒸汽大于最小水冷壁流量，过热蒸汽通过汽水分离器，此时汽水分离器为干式运行方式，汽水分离器出口温度由煤水比控制，即由汽水分离器湿态时的液位控制转为温度控制。

When boiler load is over 25%, its generated steam is more than the minimum water wall flow, superheated steam passes W/S separator, at this moment, it’s dry operation, outlet temperature of W/S separator is controlled by coal/water ratio, namely, temperature control replaces level control in wet status.

3．汽水分离器湿干态运行转换

Dry/wet operation conversion of S/W separator

在湿态运行过程中锅炉的控制参数是分离器的水位和维持启动给水流量，在干态运行过程中锅炉的控制参数是温度控制和煤水比控制，在湿干态转换中可能会发生蒸汽温度的变化，故在此转换过程中必须要保证蒸汽温度的稳定。

In wet operation, the control parameters for boiler are water level of W/S separator and start-up feedwater flow; in dry operation, they are temperature control

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and coal/water ratio control; during dry/wet conversion, steam temperature may vary, thus maintaining steam temperature stable in conversion process.

对于启动分离器的超临界机组，具有干式和湿式两种运行方式。

For super-critical pressure unit of start-up separator, there are two operation modes: dry and wet.

在启动过程锅炉建立最小工作流量，蒸汽流量小于最小给水流量，锅炉运行在湿式方式，此时机组控制给水流量，利用疏水控制启动分离器水位，启动分离器出口温度处于饱和温度，此时直流锅炉的运行方式与汽包锅炉基本相同。

During start-up, boiler makes minimum working flow, steam flow is less than the minimum feedwater flow, boiler in wet operation, at this moment, unit controls feedwater flow, control water level of start-up separator by drain, outlet temperature of start-up separator is saturated, under the circumstances, operation mode of once-through boiler is generally the same as that of drum boiler.

控制策略基本是燃烧系统定燃料控制、给水系统定流量控制、启动分离器控制水位、温度采用喷水控制。

Basically，control way are : fixed fuel control in fuel system, fixed flow in feedwater system, start-up separator controlling water level, and temperature adopting spray control.

当锅炉蒸汽流量大于最小流量，启动分离器内饱和水全部转为饱和蒸汽，直流锅炉运行在干式方式，即直流控制方式。此时锅炉以煤水比控制温度、燃烧控制压力。

When steam flow of boiler is more than the minimum flow, saturated water in start-up separator shall convert into saturated steam, once-through boiler is in dry operation mode, namely, once-through control. At the moment, temperature is controlled by water/coal ratio, pressure by combustion in boiler.

我们讨论的超临界直流锅炉的控制策略主要讨论锅炉处于直流方式的控制方案。

Controlling plan of boiler under once-through is that of which we are herein discussing about super-critical once-through boiler.

假如直流锅炉处在定压力控制方式，则对于直流锅炉机组负荷、压力、温度三个过程变量中就具有两个稳定点，一个是压力，另一个是温度。因为压力一定，分离器出口的微过热温度也就确定了。

In case once-through boiler is under control of fixed pressure, then there are two stable points for three variables load, pressure and temperature of once-through boiler unit: one is pressure, the other is temperature. Because when pressure is fixed, minor superheat temperature of separator outlet is fixed, too.

在机组负荷变化过程中对压力和温度的控制应该是定值控制。

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It should take fixed set point control for pressure and temperature during unit load varying.

在锅炉变压力运行时，机组负荷、压力、温度是三个变化的控制量，在负荷发生变化时，压力的控制根据负荷按照预定的滑压曲线控制，分离器出口温度按照分离器出口压力的饱和温度加上微过热度控制。

When boiler runs with variable pressure, unit load, pressure and temperature are the three control variables; when load changes, control pressure according to load and preset sliding pressure curve, outlet temperature of separator by saturated temperature of outlet pressure of separator and minor superheat degree.

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