Eddy Break-Up: Coal Combustion

The combustion of pulverized coal is still one of the main primary energy conversion mechanisms worldwide.

In this tutorial, you simulate the flow and combustion of coal particles in an axisymmetric coal combustor. Coal furnaces are often used to provide heat for a specific purpose. Therefore, it is useful to simulate the temperature and monitor any products of incomplete combustion at the outlet—such as carbon particles or carbon monoxide gas. Since coal often contains sulfur impurities, it is also prudent to monitor sulfur dioxide emissions (a pollutant which contributes significantly to acid rain).

The coal particles enter the computational domain together with transport air at a velocity of $23 m / s$ and a temperature of $70 ° C$ . At a different boundary, combustion air inflow is specified as a velocity profile $V_{Z} (r)$ with swirl $V_{θ} (r)$ .

The following diagram shows the geometry of the coal combustor:

Initially, the combustion chamber is filled with hot air at a temperature of $527 ° C$ . Upon injection of the coal particles into the combustor, the coal particles devolatilize to form the combustible gaseous fuel. The high initial temperature around the coal particles ensures that devolatilization begins and also ignites the gas-phase combustion.

The Lagrangian Multiphase model is used to simulate the dispersed phase of solid coal particles, and the Radiation model is used to simulate radiative heat transfer within the system.

To set up a coal combustion simulation in Simcenter STAR-CCM+, you require the following information:

Proximate and ultimate analysis of the coal sample
Calorific value of the coal sample (LCV)
Specific heat of coal
Particle density of coal

In the particle phase, the multi-component coal particle consists of four components: raw coal, char, ash and moisture (water). The exact composition of these components depends on the proximate analysis of the coal. The coal particles undergo several processes once they are injected and flow through the domain. Simcenter STAR-CCM+ provides the following models to describe each of the processes:

Evaporation of the moisture content in the coal is modeled by the Coal Moisture Evaporation model
Coal volatile formation is modeled by the Coal Devolatilization model
Oxidation of the remaining char is modeled by the Char Oxidation model

The Coal Moisture Evaporation Model is based on a quasi-steady evaporation process where the driving force for evaporation is the departure from the vapor-liquid equilibrium using the Ranz-Marshall correlation.

The coal volatile that is released into the gas phase as a result of the coal Devolatilization model mixes with the surrounding gases to undergo an oxidation process. The gas-phase combustion reactions consist of two steps:

$C o a l V o l + O 2 \to C O + H 2 O + N_{2} + {S O}_{2}$

$C O + 0.5 O_{2} \to {C O}_{2}$

To properly define the gas-phase reactions, you require the coal volatile composition, which is computed based on the proximate and the ultimate analysis as well as the coal volatile specific heat.