Testing of Australian Coals Under Gasification Conditions

Concern over the potential environmental impact of emissions to atmosphere from coal fired power generation plants has created an impetus for technological change. Advanced power generation systems based on gasification of coal, such as the Air Blown Gasification Cycle (ABGC), have the potential to be both cheaper and cleaner than conventional technology.

Project Aim

The overall aim of the project is to provide the Australian coal industry with information on the performance of seven Australian coals in the gasification stage of advanced coal-fired power plant systems, such as the Air Blown Gasification Cycle (ABGC), compared to world traded coals. The project comprised a programme of analytical, laboratory and pilot plant scale test work. The information gained from these studies will be compared to an existing extensive database of UK and internationally traded coals.

Project Method

All seven Australian coals have been fully characterised to assess their suitability for gasification in fluidised bed systems. Six of the Australian coals were classified as low sulphur bituminous coals and have been characterised in terms of fuel reactivity and ash characteristics; as was WA Collie coal which was classified as a sub-bituminous coal.

The coals were ranked, using an arbitrary reactivity scale, from low to medium. Their reactivities were comparable to certain UK, Polish, Colombian, USA and South African coals. The main minerals present in the coals were quartz and kaolinite, accounting for over 80% of the coal mineral matter.

Minor constituents identified in several of the coals included pyrite and calcium silicate. Of the three methods used to provide information of the melting characteristics of the coal ash, the in-house ash agglomeration correlation proved to be most reliable technique for predicting the temperature allowable for ash agglomerate free operation was predicted to be high for all Australian coals. This was attributed to their mineralogical compositions.

Subsequently, two of the Australian coals (Hunter Valley Mine Thermal and Low Volatile Queensland) were selected to those used in previous trials on UK and world traded coals. There were no operational problems encountered during operation of the gasifier with either Hunter Valley Mine Thermal or Low Volatile Queensland coal.

The specific energy of the fuel gas derived from Hunter Valley Mine Thermal and Low Volatile Queensland coals readily met the requirements of the ABGC process. The performance of Hunter Valley Mine Thermal coal was ranked higher than Low Volatile Queensland coal in terms of coal conversion.

The coal conversion efficiency attained for Hunter Valley Mine Thermal gasified at c.960°C (55%, dry ash free basis) was comparable to that obtained for certain UK and world traded coals, however the coal conversion efficiency for the Low Volatile Queensland coal was comparatively low (42%).

This is likely to be attributable to differences in volatile matter contents and char reactivities, but it could also be reflecting the variations in the operating conditions, caused by the different ash, volatile matter, moisture and sulphur contents of the coals.

The environmental performance of the Australian coals during gasification was good due to their low sulphur and chlorine contents. The trouble free operation experienced with the two planned tests enabled a third test to be carried out feeding Low Volatile Queensland coal at higher bed temperature of c.980°C. Improvements in both the fuel gas specific energy and the coal conversion efficiency were achieved.

A critical requirement of the ABGC is to ensure that the fuel is free of dust, in order to avoid damage to the gas turbine. Ceramic filter elements that are preferred technology for this clean-up, operating at temperatures around 600°C. During the gasification test programme the filter test facility took full flow of dust laden gas from the gasifier.

Stable, low pressure drop operation was achieved during gasification of both coals. The dust was easy to remove from the filters under normal operating conditions and there were no incidents of sticky, difficult to clean dusts.

Based on their low sulphur and ash contents and on the high ash melting characteristics, Australian bituminous coals are preferred coal for fluidised bed systems. In addition, the ability to operate the gasifier at relatively high temperatures without the risk of ash agglomerate formation should allow coal conversion to be optimised for low volatile coals, thereby making them ideal candidates for hybrid partial gasification systems such as the ABGC.

Project Outcomes

Coal characteristics

• All seven Australian coals have been fully characterised to assess their suitability for gasification in fluidised bed systems. Six of the Australian coals tested are classified as low sulphur bituminous coals and have been characterised as a sub-bituminous coal.
• The coals were ranked chronologically, using an arbitrary reactivity scale, from low to medium. Their reactivities were comparable to certain UK, Polish, Colombian, USA and South African coals.
• The main minerals present in the coals were quartz and kaolinite, accounting for over 80% of the coal mineral matter. Minor constituents identified in several of the coals included pyrite and calcium silicate.
• Of the three methods used to provide information of the melting characteristics of the coal ash, the in-house ash agglomeration correlation proved to be most reliable technique for predicting the temperature at which the gasifier bed agglomerates.
• The maximum gasifier operating temperature allowable for ash agglomerate free operation was predicted to be high for all Australian coals; ranging from 1030°C for Hunter Valley Mine Thermal coal to c.1040°C for Blair Athol and Low Volatile Queensland coals.
• The high ash melting properties of the Australian coals under gasification conditions was attributed to their ash compositions. All seven coals contained low concentrate of the minerals responsible for initiating ash agglomeration, namely pyrite and certain alkali containing aluminosilicate.

Gasification Performance

• There were no operational problems encountered during operation of the gasifier with either Hunter Valley Mine Thermal or Low Volatile Queensland coal.
• The specific energy of the fuel gas from the tests on the atmospheric pressure gasification test facility feeding Hunter Valley Mine Thermal and Low Volatile Queensland coals readily met the requirements of the ABGC process.
• The performance of Hunter Valley Mine Thermal coal was ranked higher than Low Volatile Queensland coal in terms of coal conversion. The coal conversion efficiency attained for Hunter Valley Mine Thermal gasified at c.960°C (55%, dry ash free basis) was comparable to that obtained for certain UK and world traded coals, however the coal conversion efficiency for the Low Volatile Queensland coal was comparatively low (42%). This is likely to be attributable to differences in volatile matter contents and char reactivities, but it could also be reflecting the variations in the operating conditions, caused by the different ash, volatile matter, moisture and sulphar contents of the coals.
• Gasification of Low Volatile Queensland coal at the higher gasifier temperature of c.980°C showed an improvement in both the fuel gas specific energy and the coal conversion efficiency. Operation of the gasifier at relatively high temperatures without the risk of ash agglomerate formation should allow conversion to be optimised for low volatile coals.
• The environment performance of the Australian coals during gasification was good. Emissions of hydrogen chloride were low due to the low coal chlorine content. Emissions of sulphur and nitrogen species were comparable to those found in fuel gas derived from low sulphur UK and world traded coals. The main advantages of low sulphur coals is the reduction in costs associated with limestone consumption and disposal, potential energy efficiency gains and the ability to operate the gasifier at higher bed temperatures without having an adverse effect on the agglomeration propensity of the bed.
• Stable, low pressure drop operation was achieved over the entire period of gasification with the Australian coals. The dust was easy to remove from the filters under normal operating conditions and there was no incidents of sticky, difficult to clean dusts.