24-09-2014, 02:07 PM
When considering boiler energy savings, invariably the discussion involves the topic of boiler efficiency. The boiler suppliers and sales personnel will often cite various numbers, like the boiler has a thermal efficiency of 85%, combustion efficiency of 87%, a boiler efficiency of 80%, and a fuel-to-steam efficiency of 83%. Typically, 1.) Thermal efficiency reflects how well the boiler vessel transfers heat. The figure usually excludes radiation and convection losses. 2.) Combustion efficiency typically indicates the ability of the burner to use fuel completely without generating carbon monoxide or leaving hydrocarbons unburned. 3.) Boiler efficiency could mean almost anything. Any fuel-use figure must compare energy put into the boiler with energy coming out. 4.) \"Fuel to steam efficiency\" is accepted as a true input/output value.
8.PROJECT REPORT
(TO IMPROVE THE BOILER EFFICIENCY)
1. TO IMPROVE THE BOILER EFFICIENCY
Fig :- 01
When considering boiler energy savings, invariably the discussion involves the topic of boiler efficiency.
The boiler suppliers and sales personnel will often cite various numbers, like the boiler has a thermal efficiency of 85%, combustion efficiency of 87%, a boiler efficiency of 80%, and a fuel-to-steam efficiency of 83%.
Typically,
1.) Thermal efficiency reflects how well the boiler vessel transfers heat. The figure usually excludes radiation and convection losses.
2.) Combustion efficiency typically indicates the ability of the burner to use fuel completely without generating carbon monoxide or leaving hydrocarbons unburned.
3.) Boiler efficiency could mean almost anything. Any fuel-use figure must compare energy put into the boiler with energy coming out.
4.) "Fuel to steam efficiency" is accepted as a true input/output value.
Each term represents something different and there is no way to tell, which boiler will use less fuel in the same application! The trouble is that there are several norms to determine the efficiencies figures and it is practically very difficult to verify these without costly test procedures. The easiest and most cost effective method is to review the basic boiler design data and estimate the efficiency value on five (5) broad elements.
1) Boiler Stack Temperature: Boiler stack temperature is the temperature of the combustion gases leaving the boiler. This temperature represents the major portion of the energy not converted to usable output. The higher the temperature, the less energy transferred to output and the lower the boiler efficiency. When stack temperature is evaluated, it is important to determine if the value is proven. For example, if a boiler runs on natural gas with a stack temperature of 350°F, the maximum theoretical efficiency of the unit is 83.5%. For the boiler to operate at 84% efficiency, the stack temperature must be less than 350°F.
2) Heat Content of Fuel: The efficiency calculation requires knowledge of the calorific value of the fuel (heat content), its carbon to hydrogen ratio, and whether the water produced is lost as steam or is condensed, and whether the latent heat (heat required to turn water into steam) is recovered.
3) Fuel Specification: The fuel specified has a dramatic effect on efficiency. With gaseous fuels having higher the hydrogen content, the more water vapor is formed during combustion. The result is energy loss as the vapor absorbs energy in the boiler and lowers the efficiency of the equipment.
4) Excess Air Levels: Excess air is supplied to the boiler beyond what is required for complete combustion primarily to ensure complete combustion and to allow for normal variations in combustion. A certain amount of excess air is provided to the burner as a safety factor for sufficient combustion air.
5) Ambient Air temperature and Relative Humidity: Ambient conditions have a dramatic effect on boiler efficiency. Most efficiency calculations use an ambient temperature of 80°F and a relative humidity of 30%. Efficiency changes more than 0.5% for every 20°F change in ambient temperature. Changes in air humidity would have similar effects; the more the humidity, the lower will be the efficiency.
Comparing these five factors along with the stated efficiency will make you understand efficiency values more thoroughly. An important thing to note is to make the comparisons on equal footings. Consider the examples below:
a) If two boilers are stated as operating at the same stack temperature and one has less heating surface, stack temperature on the boiler with less heating surface should be challenged.
b) If two boilers are stated as operating at 15% excess air and one has a very complex burner linkage design or does not include a high-quality air damper arrangement, it is questionable that it will operate at the stated excess air level.
c) If two boilers of similar length and width are compared and one has more flue gas passes (number of times the flue gas travels through the boiler heat exchanger), the boiler with the greater number of passes should have a lower stack temperature.
1.1 Evaluating Boiler Efficiencies
The most basic efficiency norm which everybody agrees is the “input/output” ratio:
Where
• Eout is the energy needed to convert feed water entering the boiler at a specific pressure and temperature to steam leaving the boiler at a specific pressure and temperature. (This includes the energy picked up by the blow down and not converted into steam).
• Ein is the input energy into the boiler. The heat input is based on the high heat (gross calorific) value of fuel for efficiency calculations in India, USA and many other countries. Germany uses low heat (net calorific) value basis, implying that for an identical boiler, the stated efficiency will be higher.
There are two methods of assessing boiler efficiency;
a.) Input – output or direct method, and
b.) Heat loss or indirect method.
a.) Direct Method for Calculating Boiler Efficiency
Direct method compares the energy gain of the working fluid (water and steam) to the energy content of the fuel. This is also known as ‘input-output method’ due to the fact that it needs only
the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. The efficiency is than estimated using equation below:
Procedure:
1.) Measure quantity of steam flow kg over a set period, e.g. one hour period. Use steam integrator readings, if available, and correct for orifice calibration pressure
2.) Measure the quantity of fuel used over the same period. Use the gas or oil integrator, or determine the mass of solid fuel used.
3.) Determine the working pressure in Kg/cm2 (psi) and superheat temperature, °C (°F), if any.
4.) Determine the temperature of feedwater °C (°F)
5.) Convert steam flow, feedwater flow and fuel flow to identical energy units, e.g. kJ/kg or Btu/lb.
6.) Determine the type of fuel and gross calorific value of the fuel (GCV or HHV) in kJ/kg or Btu/lb.
7.) Calculate the efficiency using equation:
Direct method is simple in a way that it requires few parameters forcomputations and needs few instruments for monitoring. However this method may not be as accurate due to errors in metering fuel flow and steam flow. In practice, only very large industrial set ups and electric utility companies are instrumented well enough to obtain the required data.
b.) Indirect Method or Heat Loss Method for Calculating Boiler Efficiency
Here the efficiency is estimated by summing the losses and comparing with the heat input. The major heat losses from boiler are due to:
1. High temperature flue gas leaving the stack
2. Moisture in fuel and combustion air
3. Combustion of hydrogen (leaves boiler stack as water vapor)
4. Heat in un-burnt combustibles in refuse
5. Radiation from the boiler surfaces
6. Unaccounted for un-measured losses
Sum up the losses and calculate the efficiency using equation:
Efficiency (% E) = 100 – Σ Losses
Or
Fig :- 02
1.2 Evaluating Heat Losses from Boiler
The procedure for calculating boiler efficiency by indirect method is illustrated below.
1.) Dry Flue Gas Loss (LDG)
Heat is lost in the "dry" products of combustion, which carry only sensible heat since no change of state was involved. These products are carbon-dioxide (CO2), carbon monoxide (CO), oxygen (O2), nitrogen (N2) and sulfur dioxide (SO2). Concentrations of SO2 and CO are normally in the parts-per-million (ppm) range so, from the viewpoint of heat loss, they can be ignored.
Need to determine LDG
To determine dry flue gas loss, you need:
1.) Measurements of flue gas temperature and combustion air temperature. These can be measured using thermocouple type digital indicator. Sometimes these readings are directly available from the installed instrumentation.
2.) Flue gas analysis for CO2 and O2. These readings can be determined from ORSAT/ FYRITE combustion analysis kit or digital type portable flue gas analyzer. Some plants have continuous gas analyzers in place. By looking at O2 levels in the flue gas, conclusions can be drawn about the excess air levels.
3.) Heating Content in Fuel: This can be determined through lab testing of fuel sample. Typical indicative values are shown below for guidance.
Fig :- 03
It is important to note that the foregoing equations require the flue gas analysis to be reported on the dry basis; i.e. the volumes of CO2 and O2 are calculated as a percentage of the dry flue gas volume, excluding any water vapor.
Procedure to reduce Dry Flue Gas Loss (LDG)
The following can be concluded from the dry flue gas equations:
1) Reducing the quantity of dry gas (DG) or the excess air levels reduces the dry flue gas loss (LDG). Good burners and precise combustion controls are necessary for good results. Excessive emission of CO, unburned hydrocarbons, and unsafe boiler operation are factors that limit the extent to which excess air can be reduced. For best operating scenarios, ideally O2 levels in flue gas should be maintained close to 2% and not to exceed 4%.
2) The temperature differential of flue gas temperature (FGT) and combustion air temperature (CAT) should be lowered, or in other words reduce FGT and increase CAT. This is done by preheating the combustion air with a preheater which will raise the CAT, while installing an economizer to recover heat from the flue gases will lower the FGT. Any one or both of these parameters can be varied to reduce the LDG.
2.) Loss Due to Moisture from the Combustion of Hydrogen
The hydrogen component of fuel leaves the boiler as water vapor, taking with it the enthalpy (or heat content) corresponding to its conditions of temperature and pressure. The vapor is a steam at very low pressure, but with a high stack temperature. Most of its enthalpy is in the heat of vaporization. The significant loss is about 11 percent for natural gas and 7 percent for fuel oil.
Need to determine LH
Knowing the flue gas temperature (FGT), combustion air temperature (CAT) and fuel analytical data (HHV), LH can be calculated.
3.) Loss Due to Radiation and Convection (LR)
This loss occurs from the external surfaces of an operating boiler. For any boiler at operating temperature, the loss is constant. Expressed as a percentage of the boiler\'s heat output, the loss increases as boiler output is reduced; hence, operating the boiler at full load lowers the percentage of loss. Since the boiler\'s surface area relates to its bulk, the relative loss is lower for a larger boiler and higher for a smaller boiler. Instead of making complex calculations, determine the radiation and convection loss using a standard chart available from the Indian Boiler Manufacturers Association (IBMA). Refer to the figure below for illustration:
Fig :- 04
With modern boiler designs, this may represent only 1.5% on the gross calorific value at full rating, but will increase to around 6%, if the boiler operates at only 25 % output. Operating the boiler at full load will optimize this loss.
4.) Losses those are unaccounted for (LUA)
Reasonable assumptions concerning these losses are 0.1 percent for natural-gas-fired boiler systems and 0.2 percent for light oil-fired systems. For heavy oil, a value between 0.3 and 0.5% may be appropriate, to account for fuel heating and, perhaps, atomizing steam.
In general, the combustion efficiency of boiler falls in range of 75 to 85%.
1.3 TO ESTIMATE THE FUEL SAVINGS
All our efforts to reduce energy consumption are expressed in terms "percentage fuel saved". In case the corrective measures are taken to adjust the out-of-range conditions to increase efficiency from an "as is" situation to a new improved efficiency, the percent savings of fuel consumption can be estimated as:
It is not possible to capture each and every Btu from combustion in the boiler. The optimum efficiency of boiler lies in range of 75 to 85%.
The above analysis only highlights the part of thermal losses. Majority of energy efficiency improvements are typically found after the generation of steam. There are a lot of other controllable losses, for instance:
1.) Boiler blowdown rate
2.) Unreturned condensate
3.) Deaerator steam vent losses
4.) Steam use in end use equipment
5.) Identifiable losses in the distribution and use of process steam etc.
Here focuses on these aspects and suggests the possible improvements in three main parts:
1.) Combustion Efficiency
2.) Makeup, Feedwater, Condensate & Blowdown Management
3.) Steam Distribution & Use Management
2. (PART 1) COMBUSTION EFFICIENCY
The combustion efficiency test is your primary tool for monitoring boiler efficiency. A visual (opacity) technique to check change in flame shape, length, color, noise and smoke characteristics is the first early indicators of potential combustion related problems. But in practice, combustion efficiency is verifiable only with a flue gas analyzer. The stack temperature and flue gas oxygen (or excess air) concentrations are primary indicators of combustion efficiency.
Fig :- 05
The Logic of Combustion Efficiency Tests
The “combustion efficiency” test determines how completely the fuel is burned, and how effectively the heat of the combustion products is transferred to the steam or water.
Your boiler burns fuel efficiently if it satisfies these conditions:
1.) It burns the fuel completely.
2.) It uses as little excess air as possible to do it.
3.) It extracts as much heat as possible from the combustion gases.
The combustion efficiency test analyzes the flue gases to tell how well the boiler meets these conditions. The test is essentially a test for excess air, combined with a flue gas temperature measurement.
2.1 EXCESS AIR
The only purpose of bringing air into the boiler is to provide oxygen for combustion. Bringing in too much air reduces efficiency because the excess air absorbs some of the heat of combustion, and because it reduces the temperature of the combustion gases, which reduces heat transfer. The temperature of the flue gas indicates how much energy is being thrown away to the atmosphere.
There is theoretical or stoichiometric amount of air required for complete combustion of fuel. In practice, combustion conditions are never ideal, and additional or “excess” air must be supplied to completely burn the fuel. When the air falls below the stoichiometric value, there is some fuel that is not burned completely. This partially burned fuel creates smoke, leaves deposits on firesides, and creates environmental problems. Unburned fuel may also represent a significant waste of energy. The amount of waste depends on the energy content of the unburned fuel components. For example, the unburned components of heavy oil are mostly organic compounds that have high energy content. On the other hand, the unburned components of coal may consist largely of foreign matter that has much lower energy content than coal itself. One source estimates that each 0.1 percent of unburned combustibles in flue gas typically represents between 0.3 and 0.6 percent of the energy content of the fuel. This waste of energy is not measured by analyzing flue gas oxygen or carbon dioxide concentrations. The simplest way of determining the O2 and CO2 is to make an ORSAT or FYRITE gas absorbing test kits.
Excess Air V/s Boiler Efficiency
The table below relates the O2 levels to the excess air and combustion efficiency when seen together with stack temperatures.