Evaluating coals for IGCC

The CCSD reactivity program is developing world class expertise and facilities for the assessment of coal reactivity.

The CRC for Coal in Sustainable Development aims to understand and predict the performance of Australian coals in entrained flow gasification plants. The output of this research will provide utility participants with the knowledge required to assess different IGCC plant options, and provide coal producers with the tools to sell Australian coals into future export IGCC markets.

The coal properties required for successful gasification vary significantly from those required for conventional pf combustion. Standard proximate and ultimate coal analyses tell plant designers and coal buyers little about a coal’s performance under gasification conditions. Specialised coal analyses are therefore being developed by CCSD and other research organisations to obtain more useful information about the properties of coal of most interest to IGCC plant operators, specifically measurement of a coal’s reactivity and slagging characteristics.

Why is coal reactivity important?
Accurate measurement of a coal’s reactivity under gasification conditions plays a large role in both gasifier design and the assessment of coal for use in a particular gasifier. Indeed, the rate of reaction determines the time required for conversion of the coal and therefore gasifier volume, char recycle system capacity requirements, final gas composition as well as oxygen and steam requirements. These factors in turn have an impact on the total capital and operating cost associated with an IGCC plant. Given the significance of coal reactivity, it follows that we need a comprehensive understanding of the coal conversion process and the performance of Australian coals.

What are the deliverables to industry? So far? In the future?
One of the first deliverables from the CCSD reactivity program are tools and procedures that enable the reliable measurement of coal and char reactivity. These procedures have been developed – and are available today. The program is also developing the fundamental mechanistic models that enable laboratory measurements to be used to predict industrial scale gasifier performance and results of this work will be available in early 2001, with more gasifier models available in 2002.

An ongoing experimental program – based around the use of the Pressurised Entrained Flow Reactor (PEFR) – is providing the technical backing that will support these procedures and models and make them credible to IGCC plant vendors and operators as a reliable coal performance assessment package.

Measuring coal reactivity
The coal conversion process can be simplified into two steps.

1. Pyrolysis or devolatilisation as the particle is rapidly heated.
During this stage the moisture and volatile matter in the coal are expelled and this is often accompanied by particle softening and swelling. This stage is important for two reasons:

• the amount and rate of evolution of volatile matter determines both the amount of char to be converted in the gasifier and the character of its surface area and porosity; and
• the nature of the volatile material can have an important role in the composition of the fuel or synthesis gas produced by the process.
  During pyrolysis most of the available oxygen is consumed in the combustion of the volatiles, providing much of the heat required for subsequent reactions.

2. Reaction of the residual char with CO2, H2O and any remaining O2.
Char reactions with CO2 and H2O are orders of magnitude slower than coal pyrolysis and char combustion. Consequently, they are rate determining – that is, char reactions have the strongest influence on the time required for coal conversion.

The CRC has established two facilities – wire mesh reactor (WMR) and thermogravimetric analyser (TGA) – for characterising the pyrolysis and char reactivity performance of coals. The facilities require only a few grams of sample and can provide fundamental coal characterisation information. The information is transportable – it can be used to provide a ‘first pass’ indication of relative coal gasification performance, or can be fed into gasifier models to precisely predict the utility scale performance of a brand.

Wire Mesh Reactor
The aim of this tool is to provide a direct measure of coal volatile yield, char yield and volatile composition under the high pressure (to 50atm) and high heating rate conditions commonly experienced in utility gasifiers. In general, use of volatiles yield information from atmospheric pressure tests can not be used as it overestimates the amount of volatiles produced under higher pressure conditions.

The wire mesh reactor (Figure 1) essentially acts as a high temperature toaster. A 20-mg coal sample is wrapped in a wire mesh and placed in a pressure vessel. The mesh is subsequently rapidly heated – up to 1,000°C per second and held at the pyrolysis temperature of ~1100°C for up to 10 seconds. Evolved gases, tars and the remnant char are collected, weighed and analysed.

Thermogravimetric Analysis
The aim of this tool is to provide a measure of the intrinsic chemical reaction rate of coal chars under gasification conditions. In this test, the product char from the WMR test (or from any other relevant char preparation procedure) is suspended in a furnace in the path of flowing reactant gases. By monitoring the change in sample mass, reaction rates can be calculated as a function of carbon conversion. The raw data from the TGA are analysed using proprietary software that accounts for the effects of drag and buoyancy at different pressures and temperatures.

The TGA can measure reaction rates of samples reacting with O2, CO2 and H2O at temperatures up to 1100°C and pressures up to 100 atm. By performing experiments at a variety of pressures and temperatures, kinetic parameters such as activation energies and reaction orders can be determined.

Mathematical modelling
The CRC is currently developing two types of models that will assist in predicting the performance of coals in utility-scale gasifiers. The first of these are fundamental models of the gasification processes. Essentially these models are designed to test, interpret and apply the theories of reaction processes being developed by researchers. These models allow WMR and TGA results to be interpreted beyond the limits of the laboratory conditions.

The second form are gasifier simulators. These predict the performance of coals in particular gasifier designs – extrapolating the performance of single particles into a complex system. Accurate outputs from the gasifier simulators rely on robust fundamental models of pyrolysis and char reaction processes.

Pressurised Entrained Flow Reactor (PEFR)
The PEFR facility is capable of reacting coals under realistic gasification conditions. It is not designed to simulate a specific gasifier, but rather to represent the most salient features of entrained flow gasification conditions. PEFR has two purposes:

• to provide a facility for testing coal under actual gasification conditions;
• to develop our understanding of gasification behaviour and extend the laboratory scale fundamental data thereby providing the technical rigour to support the test procedures and models developed by researchers;

and is designed to achieve two objectives:

• to provide a first pass assessment of the entire CRC reference coal suite of 14 coals; and
• to provide a detailed examination of up to four Australian coals.

Results from this work will be used to define the appropriate test procedures for determining the reaction conditions that result in the optimum gasification performance of Australian coals, and to validate the bench-scale measurements and mathematical models being developed in other projects.

Other applications of these facilities and models
The application of these facilities and models is not restricted to examining coal performance in IGCC plants. These tools are transferable to both PCI and PFBC, and can be used to assess the performance of coals and other fuels in these high intensity technologies.

For more information contact:

A/Prof John Frank Stubington
Associate Professor
University of NSW