1. Background

This file deals with the modelling of coal ash deposit formation and the impact of the deposit on the heat transfer in the furnace. The reader is also referred to the combustion file CF23 ‘What is biomass?’, where the composition of biomass ash is described. Coal ash and ash formation from coal are covered in the combustion files CF38 ‘What is coal ash?’ and CF39 ‘What factors affect coal ash formation?’

2. In-flight modelling

Deposits are mainly built-up by ash particles. The first step is to model how the particles are transported in the boiler. Particles can be modelled in two ways: as a separate phase where the statistics of this dispersed phase are modelled, referred to as Eulerian modelling, or as discrete particles, referred to as Lagrangian modelling. This latter approach is usually employed when modelling ash particles. Sub-models required for particle modelling are included in commercial [GLOSS]CFD[/GLOSS] codes. The option for the user to modify the drag force is typically offered. This may be required when modelling biomass combustion, where the particle shape may significantly deviate from that of a sphere.

Critical issues, apart from the drag coefficient, are the particle size distribution and the location at which the ash particle is introduced. In pulverized combustion applications, a straight forward approach is to assume that one fuel particle results in one ash particle. However, this is a simplified view. In coal, minerals are not homogeneously distributed, and included minerals may be released as particles (CF39). As the ash particle travels through the furnace it may also undergo changes in its chemical composition. Volatile components may be released, which makes the depositing ash composition different from that obtained from a lab ash. The ash particle may also undergo morphological changes, such as formation of an amorphous phase. Such changes may have a significant influence on the propensity of the ash to form deposits.

In other applications it is also necessary to take other entrained particles into account. In fluidised bed combustion, it may be necessary to include entrained bed material.

3. Deposition modelling

The deposition build-up rate depends on two factors:

·       the amount of particles transported to the surface, and

·       the probability that the particles deposit on the surface.

In certain gases erosion may also play a role, e.g. bed material particles may be erosive and have a cleaning effect. Large particles reach the surface due to their inertia, while smaller particles are transported by other mechanisms such as diffusion and [GLOSS]thermophoresis[/GLOSS]. These mechanisms are available in most commercial CFD solvers.

Models for the probability that a particle that reaches the surface will deposit are less well established and the modeller may have to implement their own criteria. Coal ashes form a glassy melt. Frequently, a model is used that assumes that the probability for a coal ash particle to deposit is proportional to the ratio between the coal ash viscosity and a reference viscosity. Such a model has been described by Walsh et al. [1]. In biomass firing, the ash chemistry differs.  Here, the ash chemistry is dominated by alkali and earth-alkali. These do not form highly viscous melts; instead they will form partial melts [2]. In this case, one possibility is to relate the probability of deposition to the viscosity. This concept was originally developed for [GLOSS]black liquor[/GLOSS][3]. A minimum melt fraction of 15% has been used. Below this level, particles do not stick. If the melt fraction exceeds 70% the particle sticks, and a linear relation between these two points has been assumed. In addition, the properties of the surface will influence the probability of sticking. Here, the thickness of the molten deposit is of importance. Straw, despite being a biomass, forms an ash which is viscous. Nevertheless, here too models based on the melt fraction have been utilized [4].

4. Heat transfer modelling

The deposits have a great impact on the heat transfer in a furnace. In boilers they reduce the heat transfer to the water side due to their added thermal resistance. If the deposit thickness can be estimated, it is possible to include this into the model. In certain cases it is possible to estimate the surface temperature of the deposit.

Inside the furnace, thermal radiation is the key heat transfer mechanism. The deposit will change the surface emissivity (CF140, CF146) of the furnace and consequently also the heat transfer.

5. Synopsis

The deposit build-up process is a transient process. In addition, it takes place over a long period of time. Theoretically it is possible to model this as a time dependent process [5]. However, it is important to remember that the boiler deposits have their own history. They have been formed over a long period of time, where the boiler may have been run with a number of coal qualities and at different loads. For a qualitative assessment of the locations prone to formation of deposits it may be enough to model the mass flux of particles to walls, especially if the temperatures of the impacting particles are also assessed.

Sources

[1] Walsh, P. M., Sayre, A. N., Loehden, D. O., Monroe, L. S., Beér, J. M., Sarofim, A. F., “Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth”, Progress in Energy and Combustion Science, Volume 16, Issue 4, 1990, Pages 327–345.

[1] Mueller, C., Selenius, M., Theis, M., Skrifvars, B-J., Backman, R., Hupa, M., Tran, H.N., “Deposition behaviour of molten alkali-rich fly ashes—development of a submodel for CFD applications”, Proceedings of the Combustion Institute, Volume 30, Issue 2, January 2005, Pages 2991-2998.

[3] Backman, R., Hupa, M., Uppstu, E., “Fouling and corrosion mechanisms in the recovery boiler superheater area”, Tappi J. 70 (6) (1987) 123–127.

[4] H. Zhou, P.A. Jensen, F.J. Frandsen, “Dynamic mechanistic model of superheater deposit growth and shedding in a biomass fired grate boiler”, Fuel 86 (10-11) (2007), 1519–1533.

[5] Wang, H., Harb, J. N., “Modeling of ash deposition in large-scale combustion facilities burning pulverized coal”, Progress in Energy and Combustion Science, Volume 23, 1997, Pages 267-282, 1997.

Acknowledgements

RELCOM; Reliable and Efficient Combustion of Oxygen/Coal/Recycled Flue Gas Mixtures.

Project undertaken with the financial support of the European Commission

FP7 Grant Agreement Number: 268191.