1. Introduction

Multi-phase flow is the co-flow of two or more phases and is crucial in many aspects of our lives.  Some examples of this are:

•           Our blood is a mixture of red blood cells that carry the oxygen and white blood cells that protect us against various attacks by viruses etc. in a liquid.

•           Our digestive system is based on efficient mixing of solid food, liquid drink and the fluids our body supplies to extract the nutrients we need.

On a larger scale, the landscape we live in is formed by glacial erosion that is a multi-phase flow phenomena involving ice, rock and water. The coastlines and sea beds are shaped by erosion when flowing water interacts with sand, mud or rock. The water courses are supplied by water falling as rain through the atmosphere.

Some industrial applications utilising multi-phase flow phenomena include:

•           The exploitation of aerosols in inhalers is making the lives of millions of asthma sufferers easier.

•           Filtration and sedimentation in sewage treatment plants have reduced the impact of water based pollution on the environment.

•           Scrubbers and precipitators have reduced the gas-borne pollution from power stations.

•           Vacuum cleaners have made it easier to keep our homes clean, and

•           The internal combustion engine where a spray of fuel is mixed with air and burned to produce a mixture of fine soot particles and gaseous combustion products.

These are only a few examples of our everyday lives that are dependent on multi-phase flow phenomena. This Combustion File discusses some of the multi-phase flow aspects that are important in industrial combustion applications, with special emphasis on coal fired power station boilers and the current issue of co-firing bio fuel and coal.

2. Multi-phase flow phenomena in power plants

There are many multi-phase flow environments in a coal-fired power station, such as:

•           Discharging of solid fuel from transport, storage, milling, classification, pneumatic transport to burners and combustion in a pf-flame.

•           Transport and deposition of ash/slag particles through the combustion chamber and convection section of the boiler.

•           Particle separation in precipitators, and

•           Emission of the remaining particles to the environment and their precipitation to the ground or transport in the atmosphere.

There are also other multi-phase flow situations in the water and steam side of a boiler with a water/steam mixture being fed to the steam drum where the two phases are separated more or less efficiently. Small water droplets can also condense out from the steam on the way to or in the steam turbines.

3. Multi-phase flow challenges in power plants

Many of the applications mentioned above have successfully managed to exploit the particular properties of multi-phase systems in power plants.  Though some multi-phase flow environments can be understood and controlled satisfactorily, many are still causing considerable concerns and problems.

The main difficulty in controlling multi-phase flow systems stems from the different properties of the phases. In the case of a solid particulate phase in air, the density, size and shape of particulates affect their flow properties considerably and any multi-phase flow system must be designed to handle the properties of the phases. There are numerous examples of where fuel handling systems, that were designed for and worked well with coal, have been completely inadequate when used for bio fuels like pulverised wood, which have a different density, specific surface, tendency for bridging, etc. Co-firing will increase the complexity of the system from two (coal and air) to three components (coal, co-fuel [eg wood] and air).

Solid particles suspended in air have a density that is considerably different from that of the air (for a mixture of coal and air, the ratio is in the order of 1400). Hence, these particles travel along a completely different trajectory when the flow direction changes. This difference is exploited in classifiers and cyclone separators (Cyclone or vortex vacuum cleaners are another example).  The same phenomenon causes considerable difficulties in metering flow and achieving satisfactory fuel distribution when several burners are fed from one common feed pipe.

Comminution and Combustion are at the centre of all pf type combustion systems. Various types of milling or grinding plants are used for power generation from low speed hammer mills to high speed vertical spindle mills. The fuel is ground to the size required for combustion. This material is presented to the mill’s classifier, where over sized material is returned for further size reduction.

Classification of particles after milling is one application where the different flow properties of particles with different sizes are exploited. Standard methods work well as long as the other properties remain constant. A problem occurs when particles of varying density or shape are classified. One extreme case is where a mixture of coal and wood particles is classified aerodynamically. The different density means that larger particles of the “lighter” wood can go through the classifier than the heavier coal particles. This difference in classification performance is further accentuated by the fact that wood has a more fibrous and porous structure that gives it a larger specific area than the coal.

Pulverised fuel is pneumatically transported to multiple burners, often with the fuel stream being sub-divided several times. The fuel is then burned in the furnace. The efficiency of the combustion process is dependent on matching the air to the fuel. For many power stations this can mean distributing the fuel from 6-8 mills to 30-50 burners. Tighter emission control requirements make the fuel/air balancing critical for a successful performance.

If a mixture of gas and particulates flow along a straight pipe during pneumatic transport, the particulate phase might eventually separate from the air due to gravity. If the mixture flows around a bend, the inertia of the particulates with their higher density will force them towards the outer wall of the bend. This volume with high particle density has a tendency to stay together, and the phenomenon has even been given a name, “rope” formation, due to the particle rich zones resemblance to a rope. The difficulty of sub-dividing this “rope” flow in a way that guarantees the same fuel/air ratio in all sub flows is evident.

Sedimentation is the process when a particulate phase separates from a gas or liquid phase by the influence of gravity. It can occur during pneumatic transport of solid fuels where it is not wanted since the particles are required to follow the air, or in a sedimentation tank in a water treatment plant where this phenomenon is essential for the function of the plant.

Flow measurement of a mixture of air with particles is difficult. The particle distribution in a cross section of a feed pipe cannot be assumed to be homogenous and sensors protruding into the multi-phase flow will be subject to substantial erosion, depending on the nature of the solids and the gas velocity. Many different approaches have been tested including:  

·          laser methods to measure velocity and size of the particles,

·          tomographical methods based on capacitance to measure the particle density distribution in a cross section of the pipe,

·          mechanical sensors measuring the impact of particles hitting the sensor.

Most of these methods can be used successfully after calibration for one type of solid with constant properties, but have had problems handling particles with strongly varying properties. One example of the latter is pulverised wood with fluctuating moisture content which affects both density and electrical properties such as capacitance. This can occur if the powder is stored in a bunker that has been filled with several batches having different moisture content.  The dampness of the fed powder can then vary considerably within minutes.

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