Reactivity and burnout of wood fuels

This thesis deals with the combustion of wood in pulverised fuel power plants. In this type of boiler, the slowest step in the wood conversion process is char combustion, which is one of the factors that not only determine the degree of fuel burnout, but also affect the heat release profile in the boiler and thereby the overall operation and efficiency of the plant.

Chapter 1 consists of an introduction to thermal conversion of biomass fuels as well as a description of a Danish power plant where a measuring campaign was carried out as part of this project. Chapter 2 is a brief literature review of different aspects relevant to wood combustion, including wood structure and composition, wood pyrolysis, wood char properties and wood char oxidation.

The full scale campaign, which is the subject of Chapter 3, included sampling of wood fuel before and after milling and sampling of gas and particles at the top of the combustion chamber. The collected samples and data are used to obtain an evaluation of the mills in operation at the power plant, the particle size distribution of the wood fuel, as well as the char conversion attained in the furnace.

In Chapter 4 an experimental investigation on the relation between pyrolysis of wood in boiler-like conditions and wood char properties is presented. Chars from pine and beech wood were produced by fast pyrolysis in an entrained flow reactor and by slow pyrolysis in a thermogravimetric analyser. The influence of pyrolysis temperature, heating rate and particle size on char yield and morphology was investigated.

The applied pyrolysis temperature varied in the range 673 – 1673 K for slow pyrolysis and 1073 - 1573 K for fast pyrolysis. The chars were oxidised in a thermogravimetric analyser and the mass loss data were used to determine char oxidation reactivity. Char yield from fast pyrolysis (104 – 105 K/s) was as low as 1 to 6 % on a dry ash free basis, whereas it was about 15-17 % for slow pyrolysis (10 - 20 K/min); char yield decreased as pyrolysis temperature increased.

During fast pyrolysis wood particles underwent melting, yet to different extents for the two investigated fuels: pine wood produced chars of porous spherical particles, whereas beech sawdust chars showed a somewhat less drastic change of morphology with respect to the parent fuel. Char produced by low heating rate pyrolysis fully retained the original fibrous structure of wood.

Fast pyrolysis chars were significantly more reactive than slow pyrolysis chars (for the same activation energy, the pre-exponential factor was up to 2 orders of magnitude greater for chars increased. The amount and composition of the ash forming matter of the wood fuels is believed to play an important role in determining the differences in char yield, morphology and reactivity.

The modelling of wood char combustion is the subject of Chapter 5. The lowest and the highest reactivities obtained for the chars produced in the EFR are used in a simple single particle combustion model in combination with a description of Avedøreværket’s boiler. In the model the char particle is assumed to burn in a gas with constant temperature and constant oxygen fraction.

The particle temperature is on the other hand determined taking reaction heat, convection through boundary gas layer and radiation into account. The model accounts for external diffusion of oxygen to the particle outer surface, internal diffusion in the pores and heterogeneous chemical reaction (CO is considered the only product).

The model calculates an overall efficiency factor for combustion, yet assumes that all the reacting carbon is consumed at the outer surface of the char. The model predicts that at an average furnace temperature of 1200 K the conversion of char particles with radius 20-350 μm is very much affected by the reactivity of the char.

The influence of the particle’s reactivity is lower at higher temperatures: at furnace temperatures of 1500 K and 1700 K the combustion of the char is mainly controlled by transport processes. The effect of the oxygen concentration in the bulk gas is investigated and it is found that an increase of oxygen in bulk gas leads to higher particle temperatures and shifts the controlling step of the combustion towards diffusion.

As far as density is concerned, the model results indicate that the time of char conversion increases with increasing density. It is found that for larger char particles the residence time in the furnace can be considerably longer than for the gas (up to 6.4 s for 250 μm radius particle, compared with 4.6 s for the gas).

According to the model high reactive chars with radius up to 285 μm attain a conversion of at least 99.80% in the furnace when burning in gas at 1200 K and 2% oxygen; in order to obtain 99.80% conversion for gas temperatures of 1500 K and 1700 K low reactive particles should have radiuses no greater than 260 μm and 300 μm and highly reactive chars no greater than 315 μm and 335 μm, respectively.

Finally, the model results are compared to the full scale data from Avedøreværket power plant; the model provides a generally good prediction of the conversion attained by wood char in the furnace.