Simplified Analysis of Energy Performance of Boilers
There are several factors that influence the energy performance of boilers including fuel characteristics, firing method, steam pressure, and heating capacity. The main challenge to ensure optimal operating conditions for boilers is to provide the proper excess air for the fuel combustion. It is generally agreed that 10% excess air provides the optimum air-to-fuel ratio for complete combustion. Too much excess air causes higher stack losses and requires more fuel to heat ambient air to stack temperatures. On the other hand, if insufficient air is supplied, incomplete combustion occurs and the flame temperature is reduced.
The general definition of overall boiler thermal efficiency is the ratio of the heat output, Eout, over the heat input, Ein (Krarti, 2011):
(8.1)��=�������
The overall boiler efficiency accounts for combustion efficiency, stack heat loss, and radiation and convection losses from the outer surfaces of the boiler. The combustion efficiency refers to the effectiveness of the burner in providing the optimum fuel/air ratio for complete fuel combustion. To determine the overall boiler thermal efficiency, some measurements are required. The most common test used for boilers is the flue gas analysis using an Orsat apparatus to determine the percentage by volume the amount of CO2, CO, O2, and N2 in the combustion gas leaving the stack. Based on the flue gas composition and temperature, some adjustments can be made to tune-up the boiler and to determine the best air-to-fuel ratio to improve the boiler efficiency. Monographs are available to determine the overall boiler efficiency based on measurement of flue gas composition and temperature. One of these monographs applies to gas-fired and oil-fired boilers and is reproduced in Fig. 8.11 (for IP units) and Fig. 8.12 (for SI units), and Example 8.2 illustrates how the monograph can be used to determine the boiler efficiency.
Example 8.2
A flue gas analysis of a gas-fired boiler indicates that the CO2 content is 8% with a gas flue temperature of 204°C (400°F). Determine the overall thermal efficiency of the boiler.
Solution
By reading the monograph of Fig. 8.12, the combustion occurs with an excess air of 48% and excess O2 of 7%. The overall boiler thermal efficiency is about 81%.
The long-term performance of boilers is expressed in terms of annual fuel utilization efficiency or AFUE and accounts for part-load operation. There are several measures by which the boiler efficiency of a heating plant can be improved such as the installation of condensing boilers, the periodic tune-up of the boilers, and control operations especially for modular boilers. The net effect of any energy efficiency measure is some savings in the fuel use by the heating plant. To calculate the savings in fuel use, ΔFU, related to the change in the boiler efficiency, the following equation can be used:
(8.2)���=�������−��������������⋅�����where �������, ������� are, respectively, the old and the new seasonal thermal efficiency of the boiler, and ����� is the fuel consumption before any retrofit of the boiler system.
Example 8.2 illustrates how the cost-effectiveness of a boiler tune-up kit can be evaluated.
Example 8.3
The boiler of Example 8.2 uses 1,500,000 L of fuel oil per year. An instrumentation kit is purchased at a cost of $20,000 and is used to adjust the boiler operation so its excess of O2 is only 3%. Determine the payback of the instrumentation if the cost of fuel oil is $0.20/L.
Solution
From Example 8.2, the existing boiler has an excess O2 of 6% and an overall boiler thermal efficiency of about 78% (i.e., ����=0.78).
After the boiler tune-up, the excess O2 is 3%. Using the monograph of Fig. 8.12, the new boiler efficiency can be determined from the flue gas temperature (343°C or 650°F) and the excess O2 (3%). It is found to be ����=84%=0.84. Using Eq. (8.4), the fuel savings can be calculated:
���=0.84−0.780.84⋅1,500,000=107,140 L/yr
Therefore, the simple payback period for the instrumentation is
���=$20,00010,714 L/yr*$0.20/L≍1.0 year
Other measures that can be considered to increase the overall efficiency of boilers include (Krarti, 2011) the following:•
Install turbulators in the fire-tubes to create more turbulence and thus increase the heat transfer between the hot combustion gas and the water. The improvement in boiler efficiency can be determined by measuring the stack flue gas temperature. The stack gas temperature should decrease when the turbulators are installed. As a rule of thumb, a 2.5% increase in the boiler efficiency is expected for each 50°C (90°F) decrease in the stack flue gas temperature.•
Insulate the jacket of the boiler to reduce heat losses. The improvement of the boiler efficiency depends on the surface temperature.•
Install soot-blowers to remove boiler tube deposits that reduce heat transfer between the hot combustion gas and the water. The improvement in the boiler efficiency depends on the flue gas temperature.•
Use economizers to transfer energy from stack flue gases to incoming feedwater. As a rule of thumb, a 1% increase in the boiler efficiency is expected for each 5°C (9°F) increase in the feedwater temperature.•
Use of air preheaters to transfer energy from stack flue gases to combustion air.
The stack flue gas heat recovery equipment (i.e., air preheaters and economizers) are typically the most cost-effective auxiliary equipment that can be added to improve the overall thermal efficiency of the boiler system.
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