We recently had the privilege to supply and install a rather advanced boiler control system for a brewery in a neighbouring country. The system contained all the bells and whistles normally associated with a system of this nature and we set aside sufficient time for training of both the operating and management staff, which took place in three separate sessions.
At each session there was only one question during Q&A time – where can we see the daily coal consumption? I almost got the impression that we misinterpreted the client’s requirements for a control system – in reality they only needed an accurate coal counter, which we could have provided at a fraction of the cost of a full blown control system.
It wasn’t long before we discovered the somewhat overly interest in a coal counter – the performance of the steam plant was measured by one indicator only, namely coal usage per unit of liquor produced. Although it seems like a logical measure of performance, it would only make sense if the coal characteristics remained consistent for all time. Which brings me to the way one should look at coal and indicators based on its usage.
For many a person involved with steam production coal is a generic term for a black solid fossil fuel delivered in gravel size chunks, of which the quality is determined by the grade: A-grade being the best and D-grade being rather poor. To worsen matters coal procurement is often made the responsibility of the Buyer – a person who in all probability has never seen a boiler in operation. And his only perceived contribution is buying A-grade coal (which it often is not when delivered!) at the lowest cost.
Fact is that coal is merely the carrier of the energy required to produce steam. The energy content (referred to as calorific value or CV) is released during combustion of the coal. Now the peculiar thing about coal is that there are numerous characteristics determining its ability the convert it’s CV into usable energy. Even coal of various origins with similar CV may render different efficiencies when fired in the same boiler. (I will address significant coal characteristics in a future publication, so please accept as fact what is written here for now).
By implication then a kg of A-grade coal from various sources will not necessarily render the same quantity of steam. It thus becomes clear that the value added by coal is not so much in the CV of coal burned, as in the efficiency with which the CV can be converted into useful energy. In this respect the cost per unit of useful energy released by coal plays a much bigger role in the productivity equation than anything else. This then also implies that D-grade coal (at lower price per ton) can be more productive than A-grade coal (at higher cost per ton).
Time for an example.
- Coal X has a CV of 27800 kJ/kg and the user pays R1450 per ton delivered on site. Based on an analysis of the coal and its combustion properties it is calculated that one kg of this coal would yield 21,98 MJ of useful energy, and 8,74 kg of steam. The cost paid per GJ of useful energy thus amounts to R65,97.
- Coal Y has a CV of 25600 kJ/kg and the user pays R1320 per ton. Using the same methodology as with Coal X one kg of this coal would yield 20,28 MJ of useful energy. However, the cost paid per GJ of useful energy amounts to R65,09 in this instance, although the steam to coal ratio is only 8,07.
Since the quantity of energy required to produce a unit of steam (and product) remains the same in all instances, the cost per ton of steam is proportional to the cost per GJ of useful fuel energy. If we assume that one GJ of energy produces 0,4 tons of steam under specific operating conditions, then the fuel cost contribution to the cost per ton of steam produced is as follows:
- Coal X: R166,05
- Coal Y: R162,73
Those whose performance is measured against the steam-coal ratio indicator will probably jump at the opportunity to get hold of Coal X, ignoring the fact that it is not really such a great contributor to productivity as Coal Y would be. Furthermore, if a batch of Coal Y lands on site, the steam-coal ratio will drop significantly, despite the fact that productivity has improved.
My personal opinion of the matter is that the only sensible and accurate indicator for the efficiency of steam generation is the contribution of the cost of coal to the cost per unit of steam. And this number is really easy to calculate – the only data required is total steam (or even feed water) used, total coal consumed and the cost of coal delivered on site.
The great advantage of this indicator is that it can be put to good use to optimize combustion conditions. By changing coal suppliers (coal characteristics), excess air, steam pressure control set point, etc. one may be able to find a combination of coal and combustion settings which yield the lowest cost per unit of steam.
Please note that the above example does not consider the fouling tendencies of coal, which may contribute towards more regular cleaning of boiler tubes.
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.
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