Improving Direct Efficiency of Boilers- Role of Automation


Safety and efficiency are always given a prime importance by both boiler manufacturers and steam users. Over the time, there has been a significant improvement in boiler performance as far as these two parameters are concerned. As the technology advances, there is always a scope to perform still better on safety and efficiency.

This article explains how intelligent PLC (Programmable Logic Controller) can help steam users ensure a more efficient and safer boiler operation.

In case of conventional manual fired solid fuel boilers, the gap between the indirect and direct efficiency is quite high. Generally, the typical efficiency guaranteed is somewhere around 73-77%. The efficiency which is actually obtained lies in the range of 50-55%. This huge gap between direct and indirect efficiency is on the account of many losses taking place which can be certainly reduced with the help of boiler automation. With the help of automation it is possible to provide alerts/inputs to boiler operator which assist him to operate the boiler efficiently. The system can also be used by utility managers for monitoring performance. This system, thus, when used properly can lead to substantial savings for users.
 This article tries to understand the cause of gap between direct and indirect efficiency in case of manual fired solid fuel boilers and the role instrumentation can play.

Indirect and Direct efficiency- Why does the gap exist?

As we are aware, indirect efficiency is found out by calculating individual losses. Whereas direct efficiency is the ratio of heat energy generated by the boiler and energy supplied to the boiler in the form of fuel. This article explains the reasons behind gaps between direct and indirect efficiency.

How does manual operation lower the boiler efficiency?

In conventional manual fired boilers, boiler operations and adjustments like fuel feeding, setting ID fan damper position etc. are done by plant personnel. Just for an example, ideally, ID fan damper setting should not be same for the different boiler loads. It is impossible to manually optimise the damper settings along with the load and hence, boilers are run at same ID fan damper setting for all the loads. This significantly brings down the actual obtained efficiency- i.e. the direct efficiency.

Another typical example is that of manual feeding of fuel which significantly shoots up unburnt and stack losses taking place. Operators often overfeed or just front feed the boiler. Because of this, the air available for the combustion is insufficient to burn the fuel completely. This suddenly increases the unburnt losses. At the same time, when the operator keeps the door open for a time period longer than required, ambient air is sucked inside the furnace and leaves through the chimney at a higher temperature which results in increased stack losses.

From the two examples discussed above, it is quite clear that manual operation lowers down the actual obtained boiler efficiency because operators are not aware of real time parameters and hence are unable to take right steps accordingly. In other words, if operators are prompted by the automated alerts, these losses can be brought down. This is where intelligent PLC plays an important role.

Role of Automation


With the help of sensors and intelligent PLC, boiler operators can be given time to time alerts when they are following any inefficient operating practice. This kind of arrangement can even prompt the operators to change certain parameters when boiler operating conditions change. We will have a look at different kinds of alerts that can be provided and how they help to keep the losses under control.

1.    Close feed door


When feed door is kept open, due to the furnace pressure, air at room temperature is sucked in. This air also gets heated and then escapes through the chimney. A lot of heat is carried out by this air increasing the stack losses. With the help of a limit switch, operator can be alerted when the door is kept open and hence can considerably reduce the stack losses.
 
2.    Adjust ID fan damper

As the boiler operating conditions change, ID fan damper position needs to be changed. This ensures that correct amount of air is entering the furnace and hence, keeps the stack losses minimum. With the help of PLC, boiler operator can be prompted to change the ID fan damper settings.
 
3.    Clean the tubes

If the boiler tubes are not cleaned, due to the fouling nature of flue gases, a layer of soot develops on the inside wall of the tubes. This significantly lowers down the rate of heat transfer. At the same time, as the heat from the gases is not transferred to water, the stack temperature goes on increasing. By putting up a temperature monitor at stack exhaust, a pop-up to clean the tubes can be shown when the stack temperature goes higher than the normal value.
 
4.    Over feeding

Many times, boiler operators do not get an estimate of amount of fuel to be fed in to the boiler.  On the other hand, many times operators stuff the boiler with fuel sufficient for couple of hours to avoid the hassles of feeding frequently. This results in the over feeding. As a result of overfeeding, incomplete combustion of fuel takes place which results in increased unburnt losses. With the help of transmitters, intelligent PLC can alert the operator about overfeeding.

Apart from overfeeding, many times, boilers are fed from front side only which results in uneven distribution of fuel which again increases the unburnt losses. Intelligent PLC can detect such conditions by sensing the pressure changes and can prompt the operator to take corrective actions.
 
5.    Adjust fuel bed
If fuel is not distributed evenly on the bed, there will be some overfed pockets which will result in incomplete combustion of fuel. As a result, unburnt losses will go up. If such condition is detected, intelligent PLC can generate an alert and can prompt operator to adjust fuel bed.
 
6.    Possibility of  back firing

Monitoring and maintaining furnace pressure is an absolute necessity to avoid any occurrences of back fire. Intelligent PLC, with the help of furnace pressure transmitter can warn the operator when furnace pressure is likely to lead to back fire.
 
7.    Improve water quality

Blow down is essential to maintain boiler health. At the same time, blow down results in significant wastage of energy. With the help of intelligent PLC, an eye can be kept on blow down losses. Better the feed water quality, lower are the blow down losses. When blow down losses go up, PLC can prompt the boiler operator to take appropriate actions to ensure feed water quality. Acting upon this alert can significantly bring down the blow down losses.
 
Constant increase in fuel prices and increasing awareness about energy conservation and safety demand more efficient and safer boilers. Use of PLC in boilers is a next step to tackle these issues with the help of advancing technology.
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