Oft Forgotten Relations…

“Often forgotten (or not realised), is the fact that there is a close relationship between boiler feedwater temperature and furnace gas exit temperature.” 

Over the years, the issue of minimising plant start-up times and costs has often arisen, especially in the context of large PF power plants starting from cold conditions. 

The initial boiler warming period is usually the most contentious, when firing is limited to the least number of oil burners (and the lowest-level burners), in order to manage boiler drum and steam pipe warming and differential temperatures, usually over many hours.

While the availability of steam bypass systems has provided major benefits, the necessary focus on managing boiler drum temperature rate and differential temperatures has often become the sole focus, at the expense of furnace flue gas exit temperature.  Consequently, further delays have occurred with progressing to the next stage of a start-up, where the first pulveriser and PF burners are introduced.

It is at this time that it is realised that primary air temperature is inadequate, as a consequence of low furnace gas exit temperature, and the only apparent solution is to introduce more (and higher-level), oil burners.  This is patently not the only or the best option, as temperature rates and metal differentials will certainly be exacerbated, at potential risk to plant condition and life.

Often forgotten (or not realised), is the fact that there is a close relationship between boiler feedwater temperature and furnace gas exit temperature.  Under low firing conditions, especially when the sole source of heat is provided by the oil burners alone, most of the available heat will be absorbed through the furnace riser tubes – and the lower the feedwater temperature, then the greater the amount of heat that will be absorbed at this early stage of the boiler.

Consequently, there will be only a minimal amount of heat available through the later stages of the boiler, including the Economiser section, and beyond the furnace gas exit to the primary and secondary air heaters.  Furnace conditions are further compromised by low secondary air temperatures, and a large portion of the available heat is consumed to heat the incoming cooler air, before the remainder of available heat reaches the furnace riser tubes.

Recognising that the steam bypass system is the best possible “boiler steam drain” available, the operator should aim to harness this energy to increase feedwater temperature as quickly as possible, not only to better manage temperature rate and metal temperature differentials, but especially to raise primary and secondary air temperatures as quickly as possible.

It should also be remembered that “heat soaking” is not only important for boilers, main steam pipes and turbines, but it is also important for pulverisers.  The age old principal, “Temperature-plus-Time”, is equally important to all large items of plant. 

If proper care is taken to raise the furnace gas exit temperature, and to ensure that at least the first pulveriser receives an adequate heat soak (and a second pulveriser as a contingency), then the pulveriser can be successfully brought into service without immediately losing PF outlet temperature when first coal flow is established.

Professional operating practices require informed thinking across all aspects of a process, that a well-planned sequence of actions are executed, and that contingencies are always part of the approach and planning.  This must also be strongly reflected in training and professional development activities, and these principles are a key focus of the Forum.

M. Cogitari

[Note that this conversation relates to a particular type of power plant, and that specific plant and control details may not be applicable at other sites.  The opinions expressed in this article are those of the author, and are not necessarily shared by the Forum].