Knowledge Sharing : efficiency boiler

The efficiency of a boiler is quoted as the % of useful heat available, expressed as a percentage of the total energy potentially available by burning the fuel. This is expressed on the basis of gross calorific value (GCV). ASME Standard: PTC-4-1 Power Test Code for Steam Generating Units This consists of • Part One: Direct method (also called as Input -output method) • Part Two: Indirect method (also called as Heat loss method)
A. The Direct Method Testing

Measurements Required for Direct Method Testing Heat input Both heat input and heat output must be measured. The measurement of heat input requires knowledge of the calorific value of the fuel and its flow rate in terms of mass or volume, according to the nature of the fuel. For gaseous fuel:A gas meter of the approved type can be used and the measured volume should be corrected for temperature and pressure. A sample of gas can be collected for calorific value determination, but it is usually acceptable to use the calorific value declared by the gas suppliers. For liquid fuel: Heavy fuel oil is very viscous, and this property varies sharply with temperature. The meter, which is usually installed on the combustion appliance, should be regarded as a rough indicator only and, for test purposes, a meter calibrated for the particular oil is to be used and over a realistic range of temperature should be installed. Even better is the use of an accurately calibrated day tank. For solid fuel: The accurate measurement of the flow of coal or other solid fuel is very difficult. The measurement must be based on mass, which means that bulky apparatus must be set up on the boiler-house floor. Samples must be taken and bagged throughout the test, the bags sealed and sent to a laboratory for analysis and calorific value determination. In some more recent boiler houses, the problem has been alleviated by mounting the hoppers over the boilers on calibrated load cells, but these are yet uncommon. Heat output There are several methods, which can be used for measuring heat output. With steam boilers, an installed steam meter can be used to measure flow rate, but this must be corrected for temperature and pressure. In earlier years, this approach was not favoured due to the change in accuracy of orifice or venturi meters with flow rate. It is now more viable with modern flow meters of the variable-orifice or vortex-shedding types. The alternative with small boilers is to measure feed water, and this can be done by previously calibrating the feed tank and noting down the levels of water during the beginning and end of the trial. Care should be taken not to pump water during this period. Heat addition for conversion of feed water at inlet temperature to steam, is considered for heat output. In case of boilers with intermittent blowdown, blowdown should be avoided during the trial period. In case of boilers with continuous blowdown, the heat loss due to blowdown should be calculated and added to the heat in steam. B. The Indirect Method Testing The efficiency can be measured easily by measuring all the losses occurring in the boilers using the principles to be described. The disadvantages of the direct method can be overcome by this method, which calculates the various heat losses associated with boiler. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. An important advantage of this method is that the errors in measurement do not make significant change in efficiency. Thus if boiler efficiency is 90% , an error of 1% in direct method will result in significant change in efficiency. i.e. 90 ± 0.9 = 89.1 to 90.9. In indirect method, 1% error in measurement of losses will result in Efficiency = 100 – (10 ± 0.1) = 90 ± 0.1 = 89.9 to 90.1 The various heat losses occurring in the boiler are:

The following losses are applicable to liquid, gas and solid fired boiler L1– Loss due to dry flue gas (sensible heat) L2– Loss due to hydrogen in fuel (H2) L3– Loss due to moisture in fuel (H2O) L4– Loss due to moisture in air (H2O) L5– Loss due to carbon monoxide (CO) L6– Loss due to surface radiation, convection and other unaccounted*. *Losses which are insignificant and are difficult to measure. The following losses are applicable to solid fuel fired boiler in addition to above L7– Unburnt losses in fly ash (Carbon) L8– Unburnt losses in bottom ash (Carbon) Boiler Efficiency by indirect method = 100 – (L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8) Measurements Required for Performance Assessment Testing Flue gas analysis 1. Percentage of CO2 or O2 in flue gas 2. Percentage of CO in flue gas 3. Temperature of flue gas Flow meter measurements for 1. Fuel 2. Steam 3. Feed water 4. Condensate water 5. Combustion air Temperature measurements for 1. Flue gas 2. Steam 3. Makeup water 4. Condensate return 5. Combustion air 6. Fuel 7. Boiler feed water Pressure measurements for 1. Steam 2. Fuel 3. Combustion air, both primary and secondary 4. Draft Water condition 1. Total dissolved solids (TDS) 2. pH 3. Blow down rate and quantity The various parameters that were discussed above can be measured with the instruments that are given in Table 1.1.

Test Conditions and Precautions for Indirect Method Testing A) The efficiency test does not account for: • - Standby losses. Efficiency test is to be carried out, when the boiler is operating under a steady load. Therefore, the combustion efficiency test does not reveal standby losses, which occur between firing intervals •- Blow down loss. The amount of energy wasted by blow down varies over a wide range. • - Soot blower steam. The amount of steam used by soot blowers is variable that depends on the type of fuel. • - Auxiliary equipment energy consumption. The combustion efficiency test does not account for the energy usage by auxiliary equipments, such as burners, fans, and pumps. B) Preparations and pre conditions for testing • - Burn the specified fuel(s) at the required rate. • - Do the tests while the boiler is under steady load. Avoid testing during warming up of boilers from a cold condition • - Obtain the charts /tables for the additional data. • - Determination of general method of operation • - Sampling and analysis of fuel and ash. • - Ensure the accuracy of fuel and ash analysis in the laboratory. •- Check the type of blow down and method of measurement •- Ensure proper operation of all instruments. • - Check for any air infiltration in the combustion zone. C) Flue gas sampling location It is suggested that the exit duct of the boiler be probed and traversed to find the location of the zone of maximum temperature. This is likely to coincide with the zone of maximum gas flow and is therefore a good sampling point for both temperature and gas analysis. D) Options of flue gas analysis Check the Oxygen Test with the Carbon Dioxide Test If continuous-reading oxygen test equipment is installed in boiler plant, use oxygen reading. Occasionally use portable test equipment that checks for both oxygen and carbon dioxide. If the carbon dioxide test does not give the same results as the oxygen test, something is wrong. One (or both) of the tests could be erroneous, perhaps because of stale chemicals or drifting instrument calibration. Another possibility is that outside air is being picked up along with the flue gas. This occurs if the combustion gas area operates under negative pressure and there are leaks in the boiler casing. Carbon Monoxide Test The carbon monoxide content of flue gas is a good indicator of incomplete combustion with all types of fuels, as long as they contain carbon. Carbon monoxide in the flue gas is minimal with ordinary amounts of excess air, but it rises abruptly as soon as fuel combustion starts to be incomplete. E) Planning for the testing • - The testing is to be conducted for a duration of 4 to 8 hours in a normal production day. • - Advanced planning is essential for the resource arrangement of manpower, fuel, water and instrument check etc and the same to be communicated to the boiler Supervisor and Production Department. •- Sufficient quantity of fuel stock and water storage required for the test duration should be arranged so that a test is not disrupted due to non-availability of fuel and water. -Necessary sampling point and instruments are to be made available with working condition. • - Lab Analysis should be carried out for fuel, flue gas and water in coordination with lab personnel. • - The steam table, psychometric chart, calculator are to be arranged for computation of boiler efficiency. Boiler Efficiency by Indirect Method: Calculation Procedure and Formula In order to calculate the boiler efficiency by indirect method, all the losses that occur in the boiler must be established. These losses are conveniently related to the amount of fuel burnt. In this way it is easy to compare the performance of various boilers with different ratings. Theoretical (stoichiometric) air fuel ratio and excess air supplied are to be determined first for computing the boiler losses. The formula is given below for the same.