Director of Sales
for Doosan Babcock
Converting older coal-fired power stations is one is one of the most powerful tools in a push towards greener energy. Instead of being decommissioned — a potentially wasteful and socially damaging process — stations can be switched from burning coal to biomass to meet new environmental legislation and extend their lifespan at the same time.
A great advantage of this approach is the relatively low cost: a large majority of the plant’s existing systems can be reused or repurposed, saving time and resources, while parts that have seen better days can easily be repaired and replaced.
Similarly, as well as saving on cost and environmental impact, allowing these stations to remain functional is of huge benefit to the surrounding community. Jobs that would otherwise be lost are retained, and infrastructure, often built specifically to service the station remains open.
Of course, converting a power station from coal to biomass, particularly older facilities, doesn’t come without its challenges. Each station is different, with specific requirements and systems which mean that every conversion requires a bespoke solution designed to maximise power and efficiency.
Fortunately, Doosan Babcock has 25 years of experience in performing such conversions, including projects at Tilbury, Ironbridge and Drax in the UK and Atikokan power station in Canada.
To achieve satisfactory emissions reductions, staged combustion systems, over-fire air streams and low NOx burners can all be added. To maximise combustion in the smaller space, the over-fire air systems can use booster fans to provide higher injection velocity and greater turbulence to maximise the combustion efficiency whilst giving low NOx emissions.
This is something of a Doosan Babcock speciality. We’ve been supplying low NOx burners since the 1980s for all sorts of coals, and in the late 2000s, we extended our work to younger fuels like wood. With a higher reactivity than coal, wood can be burned at larger particle sizes, but these also take longer to heat up in the furnace and the flames tend to stand-off the burner front. This means the burner can’t control the access of air to the fuel, essential for low NOx operation, and so lower burner exit velocities are required to get a stable and rooted flame.
Our variant of the standard Doosan Babcock low NOx burner is specifically optimised for wood pellet pulverised fuel and is an evolution of the standard burner already deployed in a number of stations throughout the UK as well as globally.
The most obvious design consideration for this sort of burner is the ability to slow down the higher velocity fuel and air stream, while still preventing drop-out of particles within the burner and reducing the burner exit velocity to ensure a fully rooted flame for sufficient NOx control.
The addition of over-fire air, a standard technology applied to pulverised fuel boilers, helps to reduce NOx further than is possible by low NOx burners alone. Because around 20 per cent of the normal combustion air is taken away from the low NOx burners and instead injected higher up the furnace, there exists a larger residence time which reduces conditions for NOx reduction reactions to occur. The over-fire air then burns off the resulting flue gas.
This delay in combustion takes more space, which is why modern furnaces are much larger than those in the 1970s. But, by accelerating the over-fire air with a booster fan, the burn-out can be maximized in the space and time available. Compared to unboosted over-fire air, the combustion efficiency is improved and, as a result, unburned carbon in ash and CO emissions are reduced.
Another major consideration for coal-fired to biomass conversions is the boiler heating surface. Biomass fuels have lower ash fusion temperatures than coals and also possess high calcium content within their ash that can coat the inside of the furnace with a white reflective deposit.
These effects can increase the furnace exit gas temperature and the chances of slagging in the first heating surface at the furnace exit, as well as upsetting the balance between furnace evaporation and that of the convective surface. The boiler becomes much more sensitive to the furnace exit gas temperature. But by carefully modelling the systems during design and installing appropriate systems, we are able to ensure that this will not impact boiler performance.
Doosan Babcock’s bespoke coal-fired to biomass conversions are a remarkable avenue not just for reducing emissions from older power stations, but for supporting the people who work in and near them. Through our comprehensive approach, backed by decades of experience and success, we’re not just ensuring maximum power and efficiency; we’re also helping to safeguard communities and the environment.
This blog was written by Graham Welford for Doosan Babcock voices.