Firing on all four?09 December 2014

Heat recovery should be a burning issue for engineers and managers – whether maintaining existing boiler plant or specifying new systems. Brian Tinham looks at available technologies, and what's coming

Walk around most middle-aged industrial boiler houses in the presence of a steam specialist, and he or she will point to issues of both plant and practice that are wasting 15–20% on efficiency. Even some relatively new boiler installations could be doing better. And industry pundits agree: in each case, plant managers would not necessarily need to spend huge sums on equipment. Additionally, ROI (return on investment) is typically less than two years, with many upgrades effectively self-funding.

Some improvement projects would involve boiler plant itself and mean modest capex in modern auxiliary equipment (such as add-ons to recover waste energy from flue gases or the condensate return lines) and/or advanced burner management systems. Others might look less at boiler efficiency and more at maximising existing energy potential – for example, harnessing oversized boilers' excess steam capacity to drive the new breed of micro-turbines for plant or factory electricity generation. But others again would doubtless focus on nothing more complicated than operational failings, almost certainly around managing water treatment and steam distribution system housekeeping (insulation, leaks, steam trap audits, etc) which are unforgivable sources of significant waste.

Not rocket science
Most of this is far from rocket science. Indeed, with few exceptions, technology and practice for boiler plant design, specification, installation, commissioning and maintenance have remained virtually unchanged for years. As Spirax Sarco marketing manager Chris Coleman says: "Up to 1,000kg/hr steam demand, most engineers still specify steam generators [steam tends to be wet, so the installation may also need separators]. Up to 30,000kg/hr you're into the range of three-pass boilers [most wet-back, but reverse flame where space is limited]. Beyond that, very large installations, such as those in thermal power stations, still use watertube boilers."

Unsurprisingly, the vast majority of serious plant is in the three-pass shell and tube boiler range, and among the few obvious changes to these is size: over the last 20 years, boilers have typically shrunk 25%. Why? Primarily because the steam space at the top has been reduced, reflecting the industry's drive for energy efficiency.
Another development will be the near universal appearance of economisers – flue gas energy recovery loops, used mostly to pre-heat feedwater (occasionally combustion air) from say 85–95ºC to 120–140ºC, hence delivering some 5% efficiency improvements on gas- or oil-fired plant.

If 5% off your fuel bill sounds attractive, here is the first and probably most obvious project. Importantly, economisers can be retrofitted to almost any boiler, including plant already so equipped, if an engineering survey reveals more low-grade flue gas energy still worth recovering and you could use the extra output. Even the old rules concerning below dew point operation can be bent, using bypass valves for recirculation and/or robust steaming economisers designed and built to withstand issues such as steam flashing and weak acid production.

Water treatment
The latter are now widely available – although, as Bosch Commercial and Industrial Heating contracts manager Matthew Walton says, going that route requires diligent attention to your water treatment, referring to the BS EN 12953 part 10 standard. "Economiser heat transfer surfaces are subject to corrosion product build-up and deposition of incoming metal oxides, which can sludge up during operational load and chemical changes. So good water treatment is crucial to the safe and efficient operation of boilers, but particularly where steaming economisers are involved." And he advises that dropping flue gas temperatures too low can result in chemical damage to the flue lining, due to condensation (particularly with biogas), as well as risking stack buoyancy and hence flue exit gas velocities.

Rotating equipment
Next up, however, will be rotating equipment – pumps and fans – which in many boiler houses are only crudely speed controlled. Fitting variable speed drives (VSDs) delivers on two fronts. On the combustion air side, for example, draft can be modulated much more precisely to match demand. That enables energy-efficient, near stoichiometric burner firing throughout the range, which in turn eliminates problems with excess air deviation – such as unnecessary tube cooling or soot production. It also saves electrical energy consumed by the motors themselves, which also run only at the demand speed.

It's a similar story with modulating feedwater pump speed, instead of the more usual on/off control – in this case, again, saving on electrical energy but also pump maintenance, because of the absence of shock loads. And as Leigh Bryan, director of Robey-Wellman Boilers & Furnaces (recently the subject of a management buyout), says: "You also get much greater control over steam production. We'd envisage savings of 1–2% by switching from on/off to VSD-based pump control on boiler feedwater."

The list goes on. For ageing boiler plant with plenty of life left, but serving plant where steam demand has reduced, installing a new single-stage micro-turbine may make a lot more sense than replacing the boiler. Passing steam through such equipment enables plants to generate their own electrical power, while simultaneously dropping pressure at the outlet for downstream use, as required. Spirax Sarco says a micro-turbine producing 300kW could generate cost savings of £150,000 per year. And equipment is available to run from 50kW up to 3MW.

Other options for some include advanced steaming condensate recovery, as again offered by Spirax Sarco with its FREME (flash recovery energy management equipment), with flash energy being used to pre-heat boiler feedwater downstream on the high pressure side of the pump. That company also recommends its EasiHeat, which involves condensate sub-cooling designed to extract remaining useful energy and usually provide hot water via heat transfer units.

Worth revisiting your steam plant?

Low NOx burner and boilers
New boilers and burners may well be with us in the next year or two, driven largely by legislation bearing down on flue gas NOx emissions. Robey Wellman, for example, is currently reviewing its conventional three-pass boiler designs not only to improve steam production efficiency, but also to re-profile the furnaces to accommodate new and future low-NOx burner equipment.

Director Leigh Bryan says it's mostly about bringing large boilers in line with the proposed Medium Combustion Plant Directive (part of the European Commission's Clean Air Policy Package), set to impact some 17,000 plants in the 1– 50MWh range in the UK by around 2020. "We're developing boilers that will work in tandem with low-NOx micro-modulating burners, which allow very fine control of air and gas rates," he explains. "The benefit will be very precise oxygen trim control, which will not only cut NOx emissions but probably save another 1– 2% on gas or oil costs – as long as site engineers specify good oxygen sensors, such as Zirconia cells."

His company is far from alone. Babcock Wanson is leading the French DEMOXYA (development and demonstration of burners with very low nitrogen oxide emissions) research project, which for the last year has been working on designs to reduce NOx emissions while improving energy efficiency from fossil fuel-fired boiler plant.

Charles Tetard, business development manager, explains that Babcock Wanson was awarded funding by ADEME, the French Environment and Energy Management Agency. Others involved include Bertin Technologies (carrying out digital combustion simulations around flame lengths and furnace geometry) and the Pprime Institut at Poitiers (fundamental research into the mechanisms of NOx formation).

"The first prototype has been installed on an 8MW natural gas boiler and is currently being tested with modulating loads and steam demands to assess its efficiency and emissions characteristics," reveals Tetard. "The second phase will be a prototype for light fuel oil. That is currently being designed and we hope to be ready for testing in the first half of next year."

Making this work, he says, is about first redesigning the combustion head itself and, second, moving up to multi-jet fuel injections through the cycle – both aimed at improving air and fuel mixing to minimise NOx production. "But we're also working on micro-modulation of the mix ratio, using a PLC that works with the load on the boiler, measured using a pressure sensor. The PLC then modulates the fuel valves and air supply via a variable speed drive. We aim to improve turndown from 1:8 up to 1:40, while also reducing NOx and improving efficiency."

Treat water treatment with respect
Beware of oxygen pitting, which is caused by the presence of oxygen in water as temperatures rise. It can be a major problem, particularly with economisers, and hence the importance of limiting and maintaining oxygen within the feed water. The inlet especially is subject to severe pitting, because it is often the first area after the deaerator and hotwell to be exposed to increased heat.

As Bosch Thermotechnology contracts manager Matthew Walton says: "Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems. Because pits can penetrate deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines, economisers, boiler tubes and condensate lines. Additionally, iron oxide generated by the corrosion will eventually concentrate within the boiler."

Brian Tinham

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