A coal-fired power plant built in the UK in the 1970s had excellent potential for a biomass conversion, writes Graham Welford

Lynemouth power station

Credit: Lynemouth Power Ltd

Switching from coal to renewable woody biomass is a low cost way for old coal-fired power stations to meet new environmental legislation with a low carbon solution, whilst extending the life of these valuable assets by 10 or more years.

In the conversion of an existing coal fired power station to woody biomass pellets, the majority of the power station systems can be reused. However, this depends on the existing system capacities and conditions since, generally, the power stations proposed for conversion of this type are old and have reached the end of their life on coal, threatened by tightening emissions or the rising carbon floor price. However, the worn out components are easily replaced or upgraded.

Conversions are bespoke designs to suit the particular host coal-fired power station and the project drivers.

Biomass conversions of old coal units are not only a green technology in terms of sustainable generation with low CO2, but also recycle power stations which would otherwise be demolished, a wasteful and socially damaging process with loss of local jobs and infrastructure which have been supported for 40 years in many cases.

The Lynemouth project

Lynemouth is a 3×140 MW coal-fired power station dating from the 1970s. With no SOx or NOx controls and with the rising price of carbon, it faced closure at the end of 2015.

This site has excellent potential for conversion to biomass to solve these life-threatening problems and extend operations at the station. Given its good core attributes of upgraded turbines and cold North Sea cooling for an excellent underlying efficiency, it is probably the most efficient plant of its kind in the UK despite its relatively small unit size and modest steam conditions of 533/543˚C, 134 bar.

The high-cycle efficiency made this an excellent choice for a biomass conversion and continued operation of the power station on this site. The project benefited from a UK government subsidy in the form of a Contract for Difference (CfD), which has been ratified by the European Commission.

Full-scale burner test facility schematic

Credit: Doosan Babcock

The project targeted IED emissions compliance, maintaining the 140 MW generator output and 40 per cent gross cycle efficiency. It is now designed for virgin white wood pellets, although a design variant also considered a steam exploded black pellet.

Following a competitive front end engineering design (FEED) study and tender process starting in 2013, Doosan Babcock was engaged for the combustion and emissions work because of its extensive experience in biomass conversions over a 25-year period, including the UK plants at Tilbury, Ironbridge and Drax and the Atikokan power station in Canada.

Firstly, Doosan Babcock considered the safety requirements for the use of wood fuel and incorporated the necessary design changes. Then it conducted an overall evaluation of the existing boilers and modelled them with the pre-requisites of meeting full load on the biomass fuel, satisfying the pending emissions legislation and operating at maximum efficiency. The model was used to identify and resolve any bottlenecks limiting full load operation on biomass and to meet the other pre-requisites with a safe system.

Combustion and emissions system

The physical design of the existing boilers and the heating surface at Lynemouth was modelled from drawings and operating data using well-established Doosan Babcock design programs.

The model was validated using trial data for operation on coal and biomass, and it was thus calibrated. The model could then be used to predict with confidence the full conversion performance on the selected biomass fuels for the project, and importantly to test sensitivities to variations in fuel properties, operating regimes and loads.

The model was used to incorporate the revised low NOx biomass combustion system essential for meeting emissions and techniques to maximize the boiler and cycle efficiency. The resulting optimized process flow diagram (PFD) was used to assess the suitability of all existing plant components, identify bottlenecks, system by system, and find cost-effective solutions to upgrade or replace components and to make the system operate safely on biomass. This resulted in the replacement of fans, a new low NOx combustion system with over-fire air and dynamic classifiers, a primary air cooler for efficiency, and an ESP upgrade. The changes necessary for the key combustion and boiler systems are described below.

The existing boilers each had four coal mills, allowing full load on three mills with one spare mill, a generous 33 per cent spare capacity. Thus, by using all four mills for biomass with its much lower energy density, the mills had sufficient capacity to satisfy the 140 MW full load on biomass. The mills require some modifications to grind biomass, upgraded safety systems, and – to ensure consistency of the particle size – dynamic classifiers will be added.

The furnace at Lynemouth predates NOx requirements and so is small by modern standards. To achieve the NOx emissions now demanded requires a staged combustion system and a new over-fire air stream to be added in addition to low NOx burners. To maximize the combustion efficiency in the small furnace, the over-fire air system uses a booster fan to provide a higher injection velocity and greater turbulence.

Doosan Babcock has been supplying low NOx burners since the 1980s for all types of coals and test work was extended to ‘younger’ fuels akin to wood during the late 2000s. With a higher reactivity than coal, wood can be burnt at larger particle sizes. However, the larger particles take longer to heat up in the furnace and biomass flames tend to stand-off the burner front. This would not allow the burner to 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. The test work gave the basis of modifications required to the burners which Doosan Babcock has applied at previous biomass conversions such as Atikokan, Tilbury and Ironbridge. The projects have given Doosan Babcock real-life experience which has been integrated into a variant of the standard Doosan Babcock low NOx burner, shown on page 27, specifically optimized for wood pellet pulverized fuel, and this is being applied to the Lynemouth project.

Doosan Babcock low NOx biomass burner

Credit: Doosan Babcock

The most obvious design consideration for the burner is to be able to slow down the higher velocity fuel and air stream whilst preventing drop-out of particles within the burner and at the same time reducing the burner exit velocity to ensure a fully rooted flame for good NOx control.

The burner has been proven at full commercial scale on both steam exploded black and virgin white wood pellets on the Doosan Babcock full scale test facility, and also on one mill group within a utility boiler. The performance in these circumstances has allowed Doosan Babcock to be confident about meeting the required emissions limits within the Lynemouth boiler when the burners are used in a staged configuration using over-fire air.

Over-fire air is a standard technology applied to pulverized fuel boilers to reduce NOx further than is possible by low NOx burners alone. About 20 per cent of the normal combustion air is taken away from the low NOx burners and instead injected higher up the furnace, giving a larger residence time under reducing conditions for NOx reduction reactions to occur. The over-fire air then burns off the resulting flue gas high in reducing species such as CO.

Full-scale burner test facility

Credit: Doosan Babcock

This delay in combustion takes more space, which is why modern furnaces are much larger than those in the 1970s. By accelerating the over-fire air, the burn-out can be maximized in the space and time available within the existing Lynemouth furnace dimensions. Hot secondary air is taken off the supply ducts to the burner windbox and routed to a booster fan before ducting back to the injection nozzles above the top row of burners. Compared to unboosted over-fire air, the combustion efficiency is improved and, as a result, unburned carbon in ash is reduced.

Boiler heating surface

The boiler modelling described above was used to study the impact of the fuel switch from coal to biomass on all aspects of the heating surface and its ability to pick up heat from the flue gas and convert it into steam without overheating the boiler pressure part tubing and maintaining critical steam temperatures to the turbine at the required maximum duty.

In moving to biomass firing from coal, there are differences that are of importance to the boiler heating surface:

1) Biomass fuels have lower ash fusion temperatures than coals; and

2) Biomass fuels have high calcium contents in the ash, and this tends to coat the inside of the furnace with a white reflective material.

These effects tend to increase the furnace exit gas temperature and increase the chances of slagging in the first heating surface at the furnace exit. They may also upset the balance between furnace evaporation and that of the convective surface. The boiler becomes much more sensitive to the furnace exit gas temperature. This has been studied in great detail to ensure it will not impact the boiler’s performance.

The project builds on the experience and previous success in biomass conversions and co-firing by Doosan Babcock. Biomass conversions are bespoke, drawing on a toolkit of products and technologies. The combustion and emissions system for Lynemouth is a comprehensive approach designed for maximum power and efficiency and to maximize availability throughout the 10 years of the CFD subsidy contract, whilst complying with emissions standards.

Contracts were let by Lynemouth Power Limited for all parts of the project at the end of January 2016, with final notice to proceed at the end of April 2016. The final unit is planned to be operational in early 2018. At July 2016, all major subcontracts have been placed for the boiler works and demolition and refurbishment activities within the boiler house are well underway.

After the conversion the resulting power station will have been recycled and fit for the next 10 years of profitable operation using a green, low CO2, renewable and sustainable fuel meeting all emissions legislation.

Graham Welford is Sales Director at Doosan Babcock. www.doosanbabcock.com