With specific modifications, a bituminous coal-firing CHP system can be converted to operate with wood or other biomass pellets, write Thomas Krause and Yaqoub Al-Khasawneh

The Studstrupvaerket power station in Studstrup, Denmark features two units, 3 and 4, which are designed for combined heat and power (CHP) production. The maximum net power output is 350 MW and the maximum district heating output is 455 MJ/s per unit.

Dong Energy Thermal Power is planning a fuel conversion at unit 3 (SSV3) to 100 per cent wood pellet firing, while as far as possible keeping the maximum thermal heat output at 930 MWth with the option of using the present fuel qualities (coal, straw and oil) alternatively to the pellets.

The complete conversion of unit 3 comprises mainly new unloading and feeding systems, wood pellet storage, feeders and modification of existing mills and burners. The existing coal feeding system remains ready-to-operate to allow a fuel switch between hard coal firing and wood pellet firing.

In summer 2013 one of the upper mills (mill No 40) and the respective burners were upgraded at SSV3 to allow wood pellet operation as well as coal combustion. After the modifications, the transition from coal to biomass operation (and reverse) should be possible within a period of 30 minutes or less for each mill while the unit keeps running continuously.

Four different trial runs were conducted from October 2013 to February 2014 using bituminous coal and different kinds of wood pellets for testing under different load conditions, variable load ramps and fuel switching between coal and biomass at the modified mill. From December 2013 to March 2014, a total of about 70,000 tonnes of wood pellets were ground and burned with the upgraded combustion equipment.

Mill modifications

The boiler at SSV3 is equipped with four bowl and roller mills type MPS 190. Inside the mill (see Figure 1) the grinding process of the fed raw coal is initialised by rolling off stationary, rotating grinding rollers upon a turning grinding bowl.

Figure 1. MPS mill for coal and biomass operation
Figure 1. MPS mill for coal and biomass operation

The fuel is fed via a coal-chute centrally into the mill to the rotating grinding bowl, and afterwards is transported by centrifugal force to the grinding rollers. The grinding rollers are pressed on the grinding bowl due to their own weight and an additional external force, generated by a spring-loaded rope pull system.

The fuel gets crushed through pressure and shearing inside the milling gap between grinding rollers and grinding bowl. The primary air required for drying and pneumatic transport of the ground fuel is provided by a separate fan. The required primary air flow depends on the inserted mass flow rate of the fuel. The desired primary air temperature is achieved by mixing hot and cold primary air. The primary air is flowing into the grinding chamber through a nozzle ring, with a defined temperature, velocity and flow direction.

The mills at Studstrup unit 3 are equipped with dynamic classifiers of type HEP 33H6. This classifier uses adjustable, stationary blades and a speed-controlled rotor for fineness adjustment. Increasing the rotor speed leads to an improvement in the grinding fineness. The dynamic classifier is designed for coal operation with a grinding fineness of R_90 µm = 5%–25% (about 10%–30% residue on a sieve 75 µm).

Because of the differences in grinding, drying, pneumatic transport and the fineness requirements for combustion of pulverised coal and wood pellets, it is not possible to grind both fuels simultaneously. The parameters of the grinding system regarding the requirements of pulverised fuel drying, as well as fire and explosion protection, are set either for coal or for wood pellets. After the correct setting, the mill operates economically and reliably with the chosen material.

Necessary changes for adjusting the grinding system to the new fuel, besides the adapted control technology, are mechanical modifications to the mill and classifier regarding grinding, pneumatic transport of the pulverised fuel, and classification. The following modifications were made to adapt the grinding system:

  • Grinding bowl rotational speed increase;
  • Dam ring installation at the grinding table edge;
  • Coal-downpipe extension;
  • Modification of the classifier: installation of variable drive for louver blade angle adjustment, return hopper modification, change of the fan at the classifier motor;
  • Double wall installation (cylindrical airflow deflector) inside the grinding chamber and upstream classifier.

These mechanical modifications allow mill operation with both fuels. The classifier, for example, can be controlled by the angle adjustment of the louver blades and by adjusting the classifier’s rotational speed to fulfil the different fineness requirements for bituminous coal and biomass combustion.

The grinding system is equipped with additional temperature measuring points and an explosion suppression system.

Besides mechanical modifications, some adaptations to the control system had to be arranged. Mill operation with bituminous coal or wood pellets is realised individually by an independent operation programme which contains the required parameters and limit values for mill operation.

Burner modifications

The 24 burners were originally designed for the use of pulverised bituminous coal. Six burners are arranged in four elevations, located on the boiler front and rear walls. Each burner is connected to the combustion air supply system by a secondary air duct, including the necessary damper and flow measuring devices for individual combustion air control.

The target for the modification was to operate the swirl burners either in 100 per cent bituminous coal mode or 100 per cent biomass mode on a wood pellet basis.

For the realisation of coal/biomass combustion the six burners of the upper level 40 were upgraded with the essential DS Burner elements.

Figure 2. DS Burner components
Figure 2. DS Burner components

Concentric setup and swirling of all burner flows are essential features of this burner type. The DS Burner design is determined by the ignition, the flame start and the subsequent low-oxygen primary reaction phase. Further, the burner has been designed to handle the fuel particles in the pyrolysis process as well as their subsequent oxidation.

In general, the DS Burner consists of the following main components:

  • Ignition device;
  • Core air tube;
  • Primary air tube with integrated fuel nozzle, primary air swirl device and inlet elbow linked to the pulverized fuel line;
  • Secondary air pipe with swirl blades and tertiary air deflecting cone;
  • Tertiary air nozzle with swirl blades;
  • Burner wind-box.

The prerequisites for the process are created through the interaction between swirl devices and fuel nozzle, as well as the resulting heat transfer in the nearby burner zone. Early oxidation of the pyrolysis products with a defined oxygen volume is the precondition for stable low-NOx combustion in the core flame.

The core flame is surrounded by spatially staged flows of secondary and tertiary air, while the combustion process is supplied with the necessary oxygen by a continuous delayed supply from the peripheral burner flow.

For the burner modification, new primary air tubes with integrated fuel nozzles and inlet elbows, linked to the pulverised-fuel tubes, were installed. The fuel nozzles, fabricated as single component of spun-casting heat-resistant alloy, are welded to the pulverised-fuel tubes. With exception of the fuel nozzles, the pulverised-fuel tubes are fabricated of composite material, including an inside erosion-protective lining on a chromium-carbide base.

On the circumference of the core air tube, primary swirl blades with angle position of 35° to the flow direction are welded at a certain distance to the burner tip. In addition, the existing core air nozzle was replaced by a new one made of heat-resistant material, with length of about 650 mm.

Due to the new design of the pulverised-fuel tubes, new secondary air swirl inserts must be installed. With the exception of the SA nozzle with TA deflector, all burner parts on the combustion air side remained unchanged.

Process parameter and safety considerations

Coal and wood pellets have a clearly different grinding and combustion behaviour than coal. Thus the operating parameters of the firing system have to be adjusted to the individual fuel properties to comply with the following tasks: grinding and classifying, drying, pneumatic transport and distribution of the fuel dust into the individual pulverised fuel lines. Furthermore, stable ignition at the burners and the combustion process in the furnace has to be ensured. Mill operation with coal or wood pellets is realised individually by an independent operation programme.

Due to the highly volatile content, the reactivity of pulverised biomass is much higher compared to pulverised bituminous coal.

Special attention was paid to a sufficient distance between carrier gas temperature and self-ignition temperature of the pulverised biomass in a hot air atmosphere, and to preventing biomass ignition at hot surfaces in the grinding system.

During coal operation the required primary air temperature and, partly, the grinding system surface temperature during coal operation could lead to wood pellet ignition. Before switching from biomass to coal operation the grinding system must be emptied of biomass – in particular, the hot air annular channel below the mill’s nozzle ring, and the reject box. The primary air temperature may only be increased to the required values after this step.

Before switching from coal to biomass operation it must be ensured that the hot zones inside the mill are cooled down sufficiently to prevent self-ignition of the fuel during any operating mode or shutdown procedure. Therefore, additional temperature measuring points inside the mill housing were installed. While switching fuel types, dust samples were taken from the PF lines at certain times to determine the duration of the fuel discharge.

Figure 3 shows parameters and temperatures of the primary air upstream mill and of the carrier gas at mill outlet as well as fuel parameters to reach the boiler peak load of 930 MWth in four-mills operating mode.

Figure 3. Gas and fuel parameter per mill for 930 MWth (boiler peak load) with guarantee fuels
Figure 3. Gas and fuel parameter per mill for 930 MWth (boiler peak load) with guarantee fuels

Operational results

In delivery state, the mill MPS 190 was equipped with a static classifier of type SLK, and designed for grinding a coal flow of 12.78 kg/s with a grinding fineness of R_0.09 = 20% (residue on a screen 90 µm) related to the coal parameters HGI = 55°H, moisture = 9% and ash content = 13.5%. Later the mill was equipped with a dynamic classifier of type HEP 33H6 for fineness improvement. Today, the average grinding fineness is about R_0.09 = 14% Related to the design fuel and the higher fineness degree, the coal mass flow for the same mill load is 11.65 kg/s.

The pulverised coal mass flow, the accessible grinding fineness and the dynamic behaviour of the mill should not be limited by the mill modifications. These are the requirements given by EnergiNet DK Grid Code: load changes at 50%–90% plant load not less than 4% per minute; below and above this range plant load changes not less than 2% per minute.Furthermore, the functionality of the modified burner must be demonstrated. The burners should have a stable ignition and fully satisfying coal combustion behaviour.

The performance of the modified mill 40 and the function of the dedicated burners were proven in a trial run in February 2014. The mill was operated safely with a coal mass flow of 12.5 kg/s and measurements were taken at the mill, as well as coal sampling at the pulverised fuel lines. Burner functionality was monitored by the flame scanners and checked by video recordings of the wing burners from the side wall inspection holes.

The dynamic behaviour of the modified mill had already been tested in January 2014. Load change behaviour of the modified mill has not deteriorated compared with unmodified mills. The required grid code parameters have been achieved.

A deterioration of part load behaviour appears while in coal operation mode. The modifications have led to an increased grinding capacity, while also increasing the achievable minimum load. The aim of a minimum load of 5.2 kg/s leads to a disturbed smoothness, so that the obtainable minimum load was increased to 6–6.5 kg/s. The desirable minimum load could be realised through adjustment of the grinding force during operation (state-of-the-art, but not implanted in SSV3) or by a moderate decrease of the modified mill’s grinding capacity.

With the exception of the temporary limitation of minimum load behaviour during coal operation, the performance of the modified grinding system as well as the burners turned out to be comparable to the capacity of the unmodified units. Thus, high expectations for biomass operation could be fulfilled and even exceeded.

Switching modes

The target was 30 minutes to shut down each mill and restart it with the other fuel – and the same process in reverse – or two hours for all four mills.Furthermore, it should be checked if a direct switch between operating modes without stopping the mill is possible.

A switch between the fuels in a period of 30 minutes with a shutdown and restart of the mill was possible from the beginning onward.

It is important to note that, during the switch from coal to biomass operation, the mill is cooled down sufficiently prior to the biomass feeder start in order to decrease the material temperature at the nozzle ring and the hot air inlet channel below the self-ignition temperature of biomass in particular. During the cooling-down process with the operational programme, the temperatures inside the mill were measured.

When switching from biomass to coal, it must be ensured that no biomass remains inside the mill, as self-ignition could occur due to the increased primary air temperature in coal operation mode. Complete clean-out of the mill is ensured with a special purge sequence. The mill reject box has to be cleaned completely as well.

Good dynamic behaviour of the grinding systems opened the possibility for a direct switch between fuel modes. Direct switching without any influence on flame stability at the burner was tested in a separate trial run, and comprehensive safety assessments were conducted. As a result of these assessments, the operation programmes were modified accordingly. The previously mentioned safety parameters must be fulfilled, and the experience gained from prior operating modes must be incorporated.

During coal operation, the material temperatures of the complete mill must be lower than 150°C prior to biomass feeder start. To ensure this, additional temperature measuring points have been installed. After switching to biomass feeding, the classifier’s rotational speed has to be continually lowered to the wood pellet classifier speed to protect the mill from overfilling. At the same time, the particles of the pulverised coal leaving the classifier must be fine enough to ignite safely.

Before starting with coal operation, it must be ensured that biomass can be discharged from the mill completely before setting the primary air temperature for coal operating mode. In addition, cleaning of the reject box is required prior to an increase in primary air temperature. After that the classifier rotational speed can be increased continually towards coal operating mode requirements. This must be done carefully as the coarse wood pellet particles have to leave the mill first.

Dr Thomas Krause is Head of Process Engineering Pulverizing Systems at Mitsubishi Hitachi Power Systems Europe. Dr Yaqoub Al-Khasawneh is Process Engineer for Pulverizing, Coal and Ash Handling Systems at MH Power Systems Europe Service.