Efficiency factors in natural gas fired steam boilers

A study to obtain maximum operating efficiency from a set of refinery steam boilers.

Tüpras¸ Izmir refinery

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Article Summary

In industry, steam boilers represent one of the areas where energy is consumed intensively. Increasing energy efficiency means decreasing fuel consumption and therefore the cost of steam production. The main parameters that affect a steam boiler’s efficiency are boiler load, combustion air, stack gas temperature, radiant heat loss, and acid dew point. In this article, natural gas fired conventional steam boilers are evaluated and potential savings are determined in the power plant of Tüpraş İzmir refinery.

The parameters that affect efficiency are traced and efficiency increase studies are focused on these areas (see Figure 1):
•    Number 1 represents the fuel system of the steam boiler. Natural gas is used as fuel and the composition of the gas is traced daily.
•    Number 2 represents combustion air feed which takes place close to the burners. Combustion air is taken from the atmosphere by forced draft fan and sent to the boiler through the preheater.
•    Number 3 represents the isolated surface of the steam boiler. Radiant heat loss is the heat escape from the surface of the boiler and is traced by thermal cameras.
•    Number 4 represents the stack gas where burned gases are given to the atmosphere after transferring their heat in the economiser. Stack gas temperature is traced and used in determining boiler efficiency.
•    Number 5 represents excess air which is removed with the stack gas. Excess air percentage is traced in order to ensure complete combustion and prevent efficiency loss.
•    Number 6 represents acid dew potential in the stack. In order to prevent acid dew, SOx and stack gas temperature is traced.
The efficiency of steam boilers can be calculated from:

Efficiency = 100 - Radiant heat loss % -
(0.044 + 0.325 * (           O2 %     ))
                                           18.16 - O2 %
*(Stack gas APH outlet temp-ambient temp) - 0.8)

Effect of load on steam boiler efficiency
Maximum efficiency is obtained generally when a boiler is used at 65-75% of load.1 When the load of the boiler falls below 50%, in order to burn all of the fuel, more excess air should be fed and this will increase heat losses. Therefore, a boiler load which is under 50% is not suitable in terms of efficiency.3

According to Figure 2, actual boiler loads are compared to 65% of the capacity of the boilers which is assumed to enable maximum efficiency in terms of load.

Boiler 1 and Boiler 2 are in a better situation when compared to other boilers due to load values of 65%. Boilers 5, 7, 8 and 9’s efficiencies are lowered due to low usage of capacity. The efficiencies of Boilers 1 and 2 are compared with Boilers 5 and 7 in Figure 3.

According to Figure 3, boiler efficiency increases with increased usage of boiler load.

When this trend is examined, an increase of 6% in boiler load increases efficiency by 3%. The boilers shown in Figure 2 have 350 t/h capacity. High pressure steam is produced at 38 bar and 440°C which supplies 4130 Gcal/h of thermal power. However, the average usage of this capacity is 205.4 t/h which corresponds to 2423 Gcal/h. When these boilers produce 2423 Gcal/h, a 72.7 Gcal/h loss is calculated. The total economic loss is $2394/h if the unit cost of high pressure steam is taken as $33/Gcal.

Effect of stack gas temperature on boiler efficiency
Stack gas temperature is a parameter that effects the efficiency of a steam boiler and needs to be traced closely. An increase in stack gas temperature affects boiler efficiency negatively. When the stack gas temperature is higher than the accepted value, heat is lost from the stack to the atmosphere. Every 20°C increase in temperature causes a 1% decrease in boiler efficiency.

The main reason for an increase in temperature is dirt and clogging of tubes in the radiant area of the boiler. The stack gas temperature should be higher than the acid dew point. In the case of lower temperature values, nitric acid and sulphuric acid forms which leads to corrosion in the stack.

In Figure 4, Boiler 2 has the lowest stack gas temperature and Boiler 7 has the highest value. When they are compared, a temperature difference of approximately 70°C is observed. By assuming a 1% decrease in efficiency for every 20°C increase in stack gas temperature, in Boiler 7 an efficiency decrease of nearly 3.5% is calculated.

Boiler 7 has 100 ton/h capacity. High pressure steam is produced at 38 bar and 440°C which represents 79 Gcal/h of thermal power. When this boiler works at 90% efficiency, the thermal power is 71 Gcal/h. Due to the high stack gas temperature of the boiler, a 3.5% efficiency loss occurs and the thermal power reduces by 2.4 Gcal/h to 68.6 Gcal/h. The economic loss is calculated as $79.2/h.

Effect of excess air on boiler efficiency
Combustion air is a critical parameter in order to enable complete combustion in a boiler. To prevent unburnt hydrocarbons, excess air is fed to the steam boiler.

The excess air ratio should be limited in order to save fuel consumption. A high level of excess air provides complete combustion but increases fuel consumption. A low level of excess air leads to movement of hydrocarbons to the stack and smoke forms.2

In order to understand whether the correct air ratio is applied, the percentage of oxygen is traced in the stack gas composition. At least 2% oxygen should be maintained, however the oxygen level should not be higher than 4%.6

Every 1% decrease in the oxygen level in the stack gas means a 0.5% increase in boiler efficiency.6

In the case of an increase in air, the stack gas is diluted and CO2 concentration decreases while the oxygen concentration increases. In safe operation, excess air should be 10-15%.

In Figure 5, the oxygen level changes between 3% and 6% in the stack gas. By lowering this ratio to 3%, the saving would be 1.4% in boiler efficiency.
Assuming the boiler has 75 t/h capacity, high pressure steam is produced at 38 bar and 440°C which represents 59 Gcal/h of thermal power. When this boiler works at 90% efficiency, thermal power becomes 53 Gcal/h. Due to a high oxygen level in the boiler, a 1.4% loss in efficiency occurs and thermal power reduces by 0.74 Gcal/h to 52.3 Gcal/h. The economic loss is $24.4/h.

Effect of radiant heat loss on boiler efficiency
A steam boiler has a higher temperature than its surroundings, therefore heat losses are observed through the surface of the boiler. This heat loss depends on temperature difference and isolation of the boiler’s surface. In general, heat losses are accepted as 2-3% of the heat produced in the system. Radiant heat loss can be explained by the Stefan-Boltzmann radiation rule:5

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