Reducing steam system costs and emissions requires a systems view that encompasses generation, distribution, steam usage and condensate recovery. While generation is but one component of the steam system, efficient steam production (within the context of the technology and fuel employed) is a logical first goal when embarking on a steam system optimisation exercise. It is also a fairly straightforward goal to accomplish.
Simply put, boiler efficiency is a measure of the efficiency with which the energy added to the steam system (through fuels such as coal, gas, paraffin/kerosene, biomass etc.) is converted into energy that is usefully added to the boiler feed water to produce steam. In order to maximise boiler efficiency, one has to understand the various losses that occur in the steam generation process, and then take steps to minimise these losses as far as possible.
Boiler losses comprise losses from the boiler stack, losses as a result of blow down, losses from the surfaces of the boiler shell and, in the case of solid fuels such as coal, losses as a result of unburnt fuel that ends up in the ash stream.
Stack losses tend to be the biggest losses to consider. Essentially, with flue gases being at an elevated temperature, there is a continuous flow of heat (i.e. energy) leaving the boiler via the stack. While stack losses cannot be avoided altogether, there is much that can be done to minimise them.
When burning a fuel, the first consideration is to ensure that there is enough air being added to the boiler to ensure the complete combustion of the fuel. There should not however be so much air added that it contributes to a drain on efficiency. Remember that air is roughly 79% nitrogen, and that this gas does not participate in combustion. It simply enters as cold nitrogen and then leaves again as hot nitrogen, carrying energy with it. While a small amount of excess oxygen (i.e. above stoichiometric requirements) is necessary to account for imperfect contact between air and fuel, you should control the amount of excess air very strictly to minimise stack losses. “How much” excess air will depend on the fuel and the design characteristics of the boiler itself.
The second matter of interest in terms of stack losses is stack temperature. For a given flue gas composition, higher stack temperatures translate into greater amounts of heat being carried out of the boiler with the flue gases. Stack temperatures generally tend to increase with increasing boiler load, and you would also tend to see an increase as heat exchange surfaces (not just the generation bank, but also superheater tubes and economiser tubes if these are present) become fouled. For the generator bank, scaling on the water side will also cause increased flue gas temperature. Remember also that a boiler is a heat exchanger, and that the area for heat exchange is an important driver of stack temperature. If your boiler has too little area for the duty, no amount of water treatment, descaling or soot blowing will reduce stack temperatures to acceptable levels. In these instances, one option capable of delivering large savings is to recover heat from the flue gas using a heat exchanger, such as a feed water economiser, a combustion air pre-heater or both.
Shell losses refer to radiant and convective heat losses from the boiler shell itself. These are influenced by the design and condition of the refractory lining (for water-tube boilers) and the insulation of the boiler shell (for water-tube and fire-tube boilers). A further influence would be the ambient temperature and wind conditions the boiler is exposed to. Shell losses tend to be independent of boiler load, and hence become a bigger proportion of the total boiler losses at low loading levels.
Blowdown losses occur when the boiler is drained of water in order to reduce the concentration of salts inside the boiler (surface blow down) or to remove coagulated solids (bottom blow down). The water level inside the boiler is maintained or restored with additional feed water, which typically comprises make-up water and returned condensate. If not removed, salts tend to concentrate in a boiler over time, since while in the absence of carryover, steam is more or less free of salts, make-up water is not. Salts are hence continuously added to the system as steam and condensate losses or consumptive uses of steam are compensated for with make-up water. Since the water removed from the boiler during blow down is at the boiler operating pressure, it contains a significant amount of energy.
Insufficient levels of blow down can cause scaling and foaming, with the latter leading to carryover of water from the boiler into the steam. Besides downstream scaling problems in pipelines and on fittings and valves, this can also cause water hammer and overloading of the condensate removal system. In order to minimise blow down losses, you should seek to operate the boiler at the maximum salt concentration it can tolerate (this depends in part on the operating pressure of the boiler) and to use feed water with as low a salts concentration as possible (subject of course to economics). Blow down should be initiated at the target salt concentration and not below, since this will increase blow down losses.
Incomplete combustion can occur if an insufficient amount of air is added to the boiler, but can also occur if there are problems with air distribution, either due to boiler design problems or fuel characteristics. In the case of coal, the presence of an excessive amount of fines can contribute to the problem, impacting on the evenness of the distribution of air through the coal bed. The inherent characteristics of the coal used, in particular the combustibility of the organic fraction, also impact on the residence time required to ensure complete combustion.
A well-designed boiler with an appropriate fuel, and operated within a throughput range suited to its design, has the potential to deliver good boiler efficiency levels, but like any resource efficiency issue, this is not something that you can “set and forget”. Short-interval controls are absolutely vital, and the following are some minimum guidelines in terms of monitoring and management practices in order to effectively manage boiler efficiency:
- Routine measurement of boiler skin temperatures to identify hot spots and address issues such as damaged refractory, insulation problems, leaking inspection doors and the like. Handheld K-type and infra-red thermometers can be used.
- Routine measurement of flue gas temperature and oxygen concentration to ensure that these stay within defined limits – you can also use these to estimate stack losses. A simple electronic flue gas analyser can be used.
- Regular monitoring of total dissolved solids/conductivity, in feed water, make-up water and in the boiler itself. Where there are condensate contamination risks, condensate should also be routinely checked. Again, portable electronic conductivity meters are available at fairly low cost and on-line and in-line systems are also available.
- For solid fuels, physical inspection of ash is important as a means of identifying incomplete combustion (it is also possible to conduct loss on ignition tests). Flue gas analysis provides further information, and can be used for all fuels.
The point around these monitoring tools is of course to react as soon as you detect deviations.Without them you really are flying blind in terms of boiler efficiency. You should also equip shop floor staff with problem solving routines to deal with each individual deviation appropriately, shortening response times.
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