Combustion of char

The combustion rate of char depends on local, fraction specific concentration of char (εchar,i ρchar), molar concentration of oxygen (CO2), particle size (dp,i) and temperature (T):

, , , (4.13)

, ,

exp (4.14)

Due to a relatively coarse calculation mesh and the transient, fluctuating flow, complete mixing of oxygen and char within a calculation cell cannot be assumed and the actual combustion rate is smaller than the kinetic reaction rate. In the current model, this limitation of incomplete mixing is considered by setting the model parameters experimentally so that the model results match the measurements, e.g. the determined total carbon conversion. Another possibility would be to limit the reaction rate based on kinetic reaction rate and the mixing rate, similar to Equation 2.12. Yet another possibility would be to define a separate sub-model for determining "effective oxygen", which would indicate the actual oxygen concentration available for char combustion.

New modelling methods are currently being developed with the support of transient simulations, which can then be used for determining alternative correlation models.

In usual calculation studies, the exponent of the oxygen content has been in the range cchar = 0.75...1.0 and the exponent for the particle size effect has been bchar = -1, which makes the reaction rate proportional to the surface area of the group of particles. The temperature effect has been often eliminated (Echar = 0) to improve the convergence and the coefficient achar has been tuned based on measured carbon conversion (Myöhänen et al., 2005).

The combustion rate of char is slower than the devolatilization rate. Consequently, with a typical fuel, char has time to penetrate to the furnace and flow to the bottom of the furnace. This results in a high local concentration of char at the bottom of the furnace.

Thus, the maximum char combustion rate is located at the bottom of the furnace instead of the location of the fuel inlets (Figure 4.15). Naturally, local maximums are found at the location of the fuel inlets.

The elemental composition of char can be defined by user input or by using the correlations presented in Chapter 4.1. The elements of char burn in the presence of oxygen producing different gas species:

Char + O₂ → NO, N2O, SO2, H2O, CO, CO2 (4.15)

Figure 4.15. Modelled char combustion rate.

Nitrogen in char produces nitrogen oxides: NO (nitric oxide, nitrogen monoxide) and N2O (nitrous oxide). The distribution is determined by the model parameters of the NOx-model.

The sulphur combusts to sulphur dioxide (SO2). In real conditions, other forms of sulphur oxides may exist as well, e.g. SO3, SO and S2O. The amount of these is considerably smaller and these have been neglected in the model.

Hydrogen burns to water vapour.

Most of the char consists of carbon. The carbon in char (Cchar) combusts to carbon monoxide and carbon dioxide as follows:

Cchar + (1 – 0.5 γ_char) O₂ → γ_char CO + (1 – γ_char) CO₂ (4.16)

Parameter γchar is a user given input value, which determines the distribution of CO and CO2 during combustion of char. In literature, many correlations have been presented for determination of CO/CO2 –ratio of char combustion (Ma, 2006, p. 84). In general, the share of CO is increasing with increasing temperature, and the ratio could be estimated by an Arrhenius-type expression. At usual CFB temperatures, the molar share CO/CO2

has been 1...10, which corresponds to γchar = 0.5...0.9.

The following table presents the composition of gas species produced from the combustion of char of different fuel types presented in Chapter 4.1.

Table 4.6: Predicted production of gas species from char combustion.

Fuel Production from char combustion (kg/kg,char) (γCO = 0.5) Oxygen consumption Char-N SO₂ H₂O CO CO₂ O₂ (kg/kg,char)

Petroleum coke 0.016 0.115 0.047 1.074 1.687 1.939

Anthracite 0.015 0.012 0.051 1.135 1.783 1.996

Medium volat. bituminous coal 0.017 0.010 0.050 1.133 1.781 1.992 High volat. bituminous coal 0.022 0.008 0.044 1.130 1.776 1.980 Subbituminous A coal (high S) 0.019 0.117 0.040 1.070 1.681 1.927 Subbituminous A coal 0.016 0.037 0.044 1.120 1.760 1.977 Subbituminous B coal 0.018 0.017 0.046 1.130 1.775 1.985 Subbituminous C coal 0.017 0.036 0.048 1.119 1.759 1.979

Lignite A coal 0.016 0.012 0.040 1.135 1.783 1.987

Lignite B coal (1) 0.013 0.017 0.037 1.136 1.785 1.989 Lignite B coal (2) 0.015 0.009 0.049 1.137 1.786 1.995 Lignite B coal (3) 0.018 0.076 0.040 1.096 1.722 1.951

Peat, foreign 0.022 0.014 0.037 1.128 1.772 1.973

Peat, domestic 0.026 0.007 0.029 1.129 1.773 1.963

Wood, Salix 0.012 0.002 0.038 1.146 1.800 1.998

Wood, chips 0.012 0.001 0.031 1.147 1.803 1.994

Wood, bark 0.010 0.001 0.033 1.149 1.806 1.998

Demolition wood (1) 0.024 0.005 0.033 1.131 1.778 1.970 Demolition wood (2) 0.018 0.006 0.025 1.139 1.790 1.977 Waste, recovered fuel (REF) 0.019 0.014 0.030 1.131 1.778 1.973 Waste, refuse derived fuel (RDF) 0.024 0.009 0.029 1.129 1.774 1.965

As most of the char is carbon, the production of carbon monoxide and carbon dioxide from burning of carbon is dominating. When considering the heat effect of char combustion, the other elements could be neglected and the char could be assumed to be 100% carbon without making any large errors. However, the nitrogen and sulphur in char have a large effect on formation of nitrogen oxides and sulphur dioxide, thus they must be known in order to determine the formation of emissions correctly. The amount of hydrogen in char is small so it has very little effect when char is combusted. For modelling gasification of char, the presence of hydrogen in the char model is beneficial as it allows simulating the release of sulphur as H2S during gasification of char.

In real conditions, char contains some oxygen as well (Ma, 2006). In the above study, the oxygen in char was found to be small and it would not have a large effect on heat release or emissions. In the model, the oxygen in char can be specified, if the elemental composition of char is defined by user and not by the built-in correlations. During combustion, the oxygen in char is used for oxidizing the different combustible elements.

During gasification, the oxygen in char is released as molecular oxygen, which then reacts in combustion reactions.