Evaluating coals for PFBC

Black coal and PFBC

PFBC is emerging as the coal-fired technology with the greatest potential for producing efficient, low cost power generation from medium to low sulphur coals.

Australian Coal

In late 1997 the CRC completed its first stage evaluation of the suitability of Australian coals for use in PFBC. Published by ACARP (Report C5056), the study provides a valuable insight into the impact of coal quality on PFBC performance and economics. This report is available from both ACARP and the CRC.

PFBC is emerging as the coal-fired technology with the greatest potential for producing efficient (43 - 45%HHV) low cost power generation from medium to low sulphur coals. PFBC technology currently supplies 500MW of electrical power, and this capacity is expected to double in the next three years (Figure 1).

Figure 1: World growth in PFBC capacity since 1990

Coal Quality and PFBC

A PFBC plant can be designed to successfully operate with any one of a number of feeds - from biomass through to anthracite. However, once built, the plant has limited fuel flexibility.

Studies at the Centre found that Australian coals would be ideal feeds to PFBC plants. Low sulphur content means that Australian thermal coals were predicted to have a lower cost of electricity than many international brands, as demonstrated in Figure 2.

Figure 2: Impact of sulphur content on PFCB coal value in use

Furthermore, the Australian coals' high ash fusion temperatures and low ash content were projected to reduce plant capital and operating costs.

However, the study also found that PFBC plant economics were sensitive to coal combustion efficiency - a potential area of disadvantage for Australian coals.

Char combustion efficiency was dependent on a number of factors.

• Char elutriation - the loss of uncombusted fine char particles by entrainment in the flue gas.
• Coal properties.

To investigate the mechanisms responsible for char elutriation, the CRC constructed a novel, bench scale PFBC facility at The University of New South Wales (UNSW). The combustor operates at industrial PFBC conditions of 16 atm, 8500^°C bed temperature, and 0.9ms-1 fluidising velocity.

PFBC facility at UNSW

The rig has already successfully characterised the elutriation behaviour of five Australian coals. In 2000, this database will be expanded to 20 coals, including a number of internationally traded coals currently being used in commercial PFBC plants.

The PFBC test facility is available to all CCSD participants interested in testing the elutriation behaviour of their coal brands.

The CCSD continues to further its knowledge of Australian coal combustion behaviour in commercial PFBC plants.

CCSD researchers at UNSW have almost completed an investigation of char in-bed combustion behaviour. UNSW researchers are also developing a model to predict the combustion performance of a coal in a commercial PFBC combustor.

The First Commercial Advanced Technology?

Utilities are constructing PFBC plants for a combination of reasons.

• Improved efficiency - PFBC plants are 3 - 4% more efficient than conventional PF power stations. This improvement equates to a fuel saving of about 10%.
• Retrofit adaptability - PFBC plants are compact, and can be sited within the boundaries of existing power plants.
• Fuel flexibility - PFBC plants can be designed to successfully operate with high ash or high moisture coals.
• Inherently low SO₂ and NO_x emissions.

Japanese Developments

The high cost of imported energy, and a commitment to high air quality standards has seen Japanese utilities take an interest in PFBC technology. IHI, MHI and Babcock-Hitachi are all developing PFBC plant manufacturing capabilities. Japanese utilities view PFBC as an efficient, clean technology which can be utilised whilst awaiting the commercialisation of IGCC.

Technology Description

In PFBC coal is injected into a pressurized (10 - 20 atm), 800 - 950^°C bed of material containing 90 - 95% coal ash and a desulphurisation sorbent. The coal rapidly combusts, and steam is generated within in-bed tubes. Particulates are removed from the hot flue gases, which are subsequently expanded through a gas turbine. The exhaust gases are cooled, generating more steam for power production (Figure 3). The steam turbine produces 80 - 90% of the generated power, and the gas turbine 10 - 20%.

Figure 3 - Technology description

For more information contact:

A/Prof John Frank Stubington
Associate Professor
University of NSW