With coal still being the predominant boiler fuel in South Africa users of coal fired steam boilers are forever under pressure to improve boiler efficiency with the worst possible fuel source that demands constant supervision of the boiler and manipulation of combustion parameters to extract the best performance from the age old energy conversion technology.
Pea coal firing is not particularly kind to boiler efficiency. It possesses all kinds of nasty characteristics which work against achieving any spectacular efficiency. Variations in energy content, volatile matter, particle size, reactivity, ash content, fouling tendencies, and many more requires constant human intervention to extract as much useable energy from the system as possible. No wonder coal must be considered an integral part of the total combustion system when looking at ways to optimize the combustion process.
It is not unusual to encounter only superficial knowledge among engineers and operators regarding the characteristics of coal and its influence on the efficiency and capacity equation. To many coal is merely a black solid fuel and the cause of many combustion problems. But as long as it is A grade it is the best, they say. The irony is that, having analyzed many A grade coals, I have found the majority of supposed A grade coal delivered on sites is actually B grade, even as low as D grade! So let us put coal through the combustion process and see what is of general importance.
- The main purpose of coal is to provide the energy required for steam production. And if one of our primary objectives is to produce steam at the lowest cost it follows that the price we pay for (usable) energy is a telling factor in achieving efficiency goals. So it is always advisable to calculate the cost per usable GJ of energy purchased. And most often one will find that A grade coal renders the highest cost per unit of energy (although it may have other properties which make up for the cost handicap).
- Lower grade (CV) coal will always be higher in ash content and lower in carbon content. High ash content affects boiler performance negatively in that unburned coal loss tends to increase, while combustion temperature decreases and hence boiler efficiency is lower.
- Always remember that your boiler has been designed to produce a certain maximum steaming capacity (MCR = maximum continuous rating) based on a certain calorific value of coal. Burning C and D grade coals may inhibit boiler capacity, especially if a boiler is required to operate at or near MCR.
- The next bottleneck in the production of sufficient steam is the rate at which the coal ignites, which is determined by the volatile percentage of the coal. Fires running away are mostly caused by low volatiles. With low volatile coal the operator is forced to run the stoker at slower speeds and higher furnace pressure to allow sufficient time and temperature under the ignition arch for the coal to ignite. Aim to purchase coal with volatiles exceeding 23% to be safe.
- Unfortunately volatiles are also the primary source of smoke production in the furnace. Volatiles percentage approaching 30% is cause for concern in terms of smoke from the stack. First thing to check when your boiler suddenly starts smoking is the volatiles content of the coal.
- A very important aspect of combustion efficiency is the reactivity of the coal, which can be established by means of a petrographic analysis. The higher the reactivity of the coal, the faster it burns out and the less excess air is required for its complete combustion. On the other hand, the less reactive the coal is (higher inertenite content) the more difficult it becomes to completely burn out the coal over the length of the stoker grate. To speed up combustion and to avoid live coal with the ash the excess air percentage needs to be increased, and so does dry heat stack loss. Bottom ash losses also tend to be higher with less reactive coal.
- Check the size grading of the coal when it is delivered. The stoker grate has been designed for pea grade coal, which means coal pieces between 5 and 20 mm in size will burn well and burn out completely over its length. Too much duff/dust in the coal causes segregation and inhibits air flow through the coal bed, causing carbon loss in the ash, combustibles loss in smoke and even carbon loss through the stack. Large pieces of coal may be difficult to ignite and to burn out completely over the length of the stoker.
- Receiving a load of heat affected (altered) coal does occur occasionally. Without going into any technical depth, this coal will be slow to ignite and slow to combust, and in most instances it will be unsuitable for firing on a grate stoker.
- Then lastly one has to be aware of the fouling tendencies of the coal. This can be established from the ash content analysis of the coal. We know that phosphorus compounds are notorious for a phenomenon called bird’s nesting, which completely blocks boiler tubes. But there are other nasty elements as well, containing low melting point metals such as calcium, sodium, iron and others and which accelerates fire side deposition inside fire tubes.
Unfortunately managing coal procurement is no easy task. By the time the coal has been delivered to site and is being fired for the first time it is rather late for effective remedial action. Most users just push through with the batch of poor coal and hope for a better product with the next delivery. A few suggestions:
- Know your coal better than your supplier, or involve someone who knows.
- Find a trustworthy coal supplier with whom a good working relationship can be established – this may however take time.
- Coal suppliers usually present a proximate analysis of the coal they market. Check the date of the analysis presented. It may be of their best coal six months ago.
- The proximate analysis does not say anything about fouling tendencies, reactivity or ash fusion temperatures. Carry out more advanced analyses from time to time to verify these important characteristics.
- Insist on a proximate analysis with every coal delivery. Regularly verify supplier analysis against own or independent analysis reports.
- Consider acquiring and installing devices and technology to alleviate the effects of poor coal characteristics, such as swinging chutes, over fire air systems, etc.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.