Steam systems are often the most expensive energy systems to operate on industrial sites and can also easily be responsible for most of a site’s emissions. Fortunately, there are several well-defined approaches available to significantly improve efficiencies and reduce costs in steam systems. The starting point for such an exercise is to conduct a comprehensive steam system assessment and this involves an assessment of steam generation, steam distribution, steam users and condensate recovery. There are various tools available to enable an integrated assessment of these areas, some of which, like the US Department of Energy’s Steam System Modeler, are free. A holistic steam system assessment is however a potentially complex task, and you may want to consider taking action in more immediate ways as you build up to a system-level assessment. An assessment of steam generation is a great place to start.
The steam generation end of the system is where direct fuel costs are incurred, and also where significant improvement opportunities tend to exist. Maximising efficiency in this area makes sense, as every ton of steam passing through the steam system originates here. Actions taken to improve efficiency in the boiler plant still have a positive system impact when improvements are made in other areas e.g. reducing steam consumption or increasing condensate recovery. What then are the key issues to consider when assessing a boiler plant from an efficiency perspective?
Boiler Plant Infrastructure
The infrastructure in the boiler plant has a significant impact on the cost of operation and our ability to control these costs on an ongoing basis. It hence requires detailed evaluation. Here you should try to answer questions such as:
What meters and instruments are in place for water, steam and fuel, and do they allow for construction of mass and energy balances around the boiler?
How are the boilers controlled in terms of feedwater addition, level control, furnace pressure, blowdown, and the addition of air and fuel?
What equipment is in place which could influence energy efficiency e.g. secondary air fans, feedwater economisers, blowdown flash and heat recovery systems, air preheaters, superheaters?
How well insulated are the water and steam reticulation systems?
Where are steam traps installed and are they functional?
Are pressure relief valves seating properly?
Is there a de-aeration system and how does it operate?
How is level controlled in hotwells/de-aerators? Overflows can lead to significant losses.
What make-up water treatments are in place and are these sufficient for the conditions – more an asset integrity issue than an efficiency issue, but important enough to include in the assessment.
Infrastructural issues hence involve technology, but also the maintenance of the assets as well as their operation.
Since stack losses tend to dominate boiler efficiency losses, flue gas analysis is mandatory, and here we are most concerned with excess air levels and flue gas temperature. You will need a hand-held flue gas analyser for the assessment if your boiler is not equipped with a reliable in-line instrument. If boiler loads vary significantly, it would be important to measure flue gas characteristics at different loadings, as both temperature and excess air levels tend to vary as loads change.
Boiler shell losses are also important, and to assess these we need to determine surface area and skin temperatures to assess the condition of refractory and insulation, and also issues such as doors that are not properly sealed. In my experience shell losses can vary with boiler load, though they are generally accountable for a small part of boiler efficiency losses. An infra-red camera or a simple infra-red thermometer would be useful here, or even a simple k–type contact thermometer.
Boiler blowdown losses need to be evaluated, and for this conductivity checks will be needed, both for the feedwater and the blowdown itself. Many boilers these days are equipped with in-line conductivity measurement (remember that these will give a higher conductivity reading than a physical, neutralised sample). If yours is not, a sample cooler will be essential for sampling the blowdown safely prior to measurement with an offline instrument.
For solid fuels, it will be important to also evaluate the condition of the ash. Lab tests can be done to determine the calorific value of the ash, or for a start you can simply just evaluate it through visual inspection.
With the efficiency and infrastructure issues thoroughly investigated, the last consideration now becomes about boiler fuel. With your existing boiler fuel, you should always be seeking to optimise fuel costs by purchasing good quality fuel at the lowest cost. “Good quality” is a relative term and it depends on your requirements. You obviously want to ensure that efficiency is maintained or even improved, and that you receive the highest energy content per unit cost, but of course emissions are also very important. In general you need to attack this issue through trial and error. Often use of an alternative fuel can result in unexpected consequences e.g. increased levels of fouling.
For coal, which tends to be the most complex fuel to optimise, you need to consider not only energy content but also reactivity. This means ensuring that the coal you use is completely combusted by the time it is discharged as ash. For a given coal feed rate, this is about more than excess air, air distribution, secondary air rates, grate speed and bed height. Of course, your starting point must be to optimise these parameters. However, you may still find unburnt coal in the ash after doing so. Sometimes this is an operational issue, but if such problems can be ruled out, this is when you will need to carry out petrographic analysis on the coals you procure to determine which performs best. The point I want to stress here is that procuring fuel is about a lot more than selecting the fuel with the lowest cost per GJ, you need to consider fuel using “total cost of use” principles. So this is more of an ongoing optimisation issue than something I would tackle through an assessment.
Fuel switching is something that I do consider when conducting boiler plant assessments, and this can be one of the most powerful cost reduction strategies available. In plants that use more than one fuel, the solution can be as simple as loading some boilers more than others. In many cases it is however perfectly feasible to make significant changes to boiler plant in order to access the cost benefits of fuel switching. Note however that this is not only about cost, and attention must be paid to issues such as environmental impacts, regulatory requirements, supply chain risks and safety.
A boiler plant assessment is something that can be done fairly quickly and can yield significant cost benefits in a short space of time. If you haven’t evaluated your boiler plant, my advice would be to do so without delay.
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