Fire for the future

Written by: Tom Shelley | Published:

Tom Shelley reports on how sensing and control is saving fuel and improving efficiency in boilers and combustion



Specially developed sensors and control strategies can boost the combustion and overall efficiencies of industrial steam-raising boiler systems by as much as 10% – and there are even situations where fuel bills have been cut by more than 30%.
While the company that has developed these technologies is focused on a narrow range of specialist markets, many of its sensor technologies and strategies have potential applications across a wide range of industries and could even bring steam railway locomotives back into viability.
The problem of making steam-raising systems efficient has challenged engineers ever since those systems were first conceived. After two centuries of development effort, the basics are now fairly well understood. But there are still plenty of opportunities to improve efficiencies further by taking advantage of advances in control theory and systems.
Autoflame, based in south east London, has just won its third Queen’s Award for Innovation, most recently for its Mark 6 Evolution burner management system with water level control. The previous two awards were for Export Achievement in 1996 and Environment in 1997.
Managing director Brendan Kemp explains how the new controller works. “When there is a demand for more steam, the pressure in the boiler decreases, leading to the formation of superheated steam bubbles, which cause the water level to rise. In a simple system, this would turn off the water feed at a time when more water needs to be heated, leading to insufficient water being in the boiler and spurious shutdown. Our system instead raises the set level of the water at the same time as raising fuel feed. When normality is restored, the set level goes down. But this is only a tiny part of what is done.”
Water level is monitored to an accuracy of 2mm by two capacitance probes of Autoflame’s own design and manufacture, with PTFE dielectrics located towards each end of the boiler. The reason for two probes is to establish Wave signature. This depends on a number of variables. Breaking bubbles of steam precipitate little droplets as spray that tend to coat the surfaces of the level probes that, in turn, can be read as an increase in water level. Thermal convection currents in the water can result in turbulent, choppy waves. When the boiler is producing large quantities of steam, the steam travels over the surface and can produce swells and surges. And if Total Dissolved Solids (TDS) builds up, the surface of the water will foam, which will also be read as an increase in water level. As a result, the controller software, all written in-house in C++, has to be sufficiently sophisticated to be able to recognise exactly what is happening, take all these variables into account and use them to create a control algorithm that produces the proper level of control for each particular boiler.
If TDS gets too high, the result is ”like putting Fairy Liquid in boiling water,” says Kemp. “It plays havoc with the capacitance probes and everything else”. TDS is kept under control by monitoring the conductivity of the water, and blowing down and admitting fresh water when conductivity and TDS goes too high. But even in this instance monitoring TDS is not at all straightforward.
Autoflame’s system incorporates two enhancements to ensure it works on the basis of correct information. One is to compensate for temperature – since conductivity increases by about 2% for each ºC increase in temperature – and the other is to compensate for polarisation. It does this in two ways. The first is only to emit measuring current in the form of ten 300 microsecond pulses every second and the other is to measure any build-up of voltage potential above or below 0V in the water sample. A third potential problem that could affect the accuracy of TDS measurement is the build-up of scale on the probe electrode. This is avoided by designing the water sampling container in such a way that turbulence created during the blow-down sequence ensures the probe remains free of scale or deposited solids. The alternative strategy, adopted by most boiler systems round the world, is simply to blow down at regular intervals. If this is too frequent, hot water is wasted; too infrequent and there is a risk of damage to the boiler.
Another way in which Autoflame control systems ensure maximum efficiency is to monitor both ends of the combustion process, with ultra violet sensors to monitor flames, sensors to monitor combustion air and damper positions (if used to control airflow) and exhaust gas analysis to monitor what is coming out. This ensures optimum efficiency and minimal nitrogen oxides emissions, a major concern in the US. Here, with the ultraviolet sensors, there is also a unique, patented tweak or two. Each UV sensor incorporates a stepper motor driven shutter, so it can check itself for spurious output, and software reduces both voltage and the duration of pulses driving the sensor when the flame is at maximum to increase sensor life.
Preferably, air is supplied using variable speed driven fans. However, if dampers are used for control, Autoflame employs its own designed and built servo systems to move them, and also the fuel control valves.
“Quality control is absolutely everything to us, especially when it relates to dealing with large amounts of hot fuel,” states Kemp. “We make all our own valves and major components, because, if you want something to be phenomenally accurate, you have to ensure that you make all the bits in the loop yourselves, from the software to the hardware. Further, if you design all your own bits, you get the most cost-effective, sleek solutions.
And the payback can be significant. “If you previously had a system without electronic control, you could expect to achieve a 7-10% improvement. If you add a flue gas analyser, you could bump this up 1.5-2% more.
Co-director Nick Kemp says United Fishing Enterprises cut its fuel bills by more than 31% when it turned to Autoflame; and Kikkoman Foods has reduced its Soya sauce production costs by 3.06 euros per cubic metre of sauce. Meanwhile, Intel has standardised on Autoflame control systems worldwide; energy cost savings at just one site were more than $200,000 per annum. Autoflame is in the process of moving to larger premises.
Autoflame’s boiler water level control system appears much more sophisticated than those used in most plant operations and so could benefit a wide range of process industries. What impact might it have in raising the efficiencies of steam railway locomotives, for instance? Traditional locomotives achieved 13% overall thermal efficiency at best, while most of those built in the UK did not do even that well – but advanced electronic control was simply not available at the time. In a world where oil prices have risen sharply and are likely to stay there, interest in steam locomotives has been reawakened, since they can run on almost anything that will burn, including refuse. The latest project to reach our attention is the 5AT project being led by ex-British Rail consulting engineer David Wardale. More information at http://www.5at.co.uk.

Pointers

* A pair of capacitance probes and sophisticated software can accurately measure water levels inside a boiler, regardless of waves and surface effects, and make anticipatory adjustments in the event of sudden extra demands for steam

* Total dissolved solids are accurately measured and combustion systems monitored by a range of unique sensors

* Fuel savings of up to 10% can be expected, and more than 30% savings have been achieved on occasion

* The company makes all its own probes, sensors, servo systems and controls, and attributes much of its success to not outsourcing



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