Ignition and Reactivity of Coal and Char Particles

D. Samuel Perkins and Kyriacos Zygourakis

Introduction

An understanding of the ignition and reactivity behavior of coal and char particles is essential for the efficient design and operation of coal-fired power plants and industrial scale utility boilers.

Additionally, there is a great need for reductions in pollutant emissions from coal utilization processes. Emissions from coal fired power plants produce large amounts of NOx, SOx, and greenhouse gases.

Background/Experimental

Experiments designed to study the ignition and reactivity of coal and char particles are primarily of two types.

• Pyrolysis of coal followed by combustion of the pyrolyzed char particle.
  
• Direct combustion of coal particles.

These experiment types are performed in one of two reactors in our lab. The TGA/VMI reactor is a custom modified TGA which is equipped with a video microscopy system enabling visualization of transient ignition and reactivity phenomena.

The Hot Stage reactor is a custom built heated grid reactor capable of controlled heating rates of up to 1000 °C/sec. This reactor is also equipped with video microscopy.

Digital image processing is performed on the images obtained through microscopy. This allows for quantification of the changes in particle size, shape, and light intensity over time.

Summary of Results

Ignition Detection

The combination of video microscopy with thermogravimetry provides an effective means of detecting and characterizing coal and char particle ignitions.

Ignition Mechanism

Coal particles ignite homogeneously, heterogeneously, or through a combination of both mechanisms. Char particles, on the other hand, ignite primarily through a heterogeneous mechanism.

Coal and Char Reactivity

Pyrolysis and combustion conditions may significantly affect the reactivity and ignition behavior of coal and char particles.

Char Ignition Model

A simple heterogeneous char ignition model is effective in predicting the effects of various process conditions on char ignition observed experimentally.