Paper No. FBC99-0005

Understanding the Behaviour of Australian Black Coalsin Pressurised Fluidized Bed Combustion

Proceedings of the 15th International Conference onFluidized Bed Combustion

May 16 - 19, 1999Savannah, Georgia

Copyright ©1999 by ASME

Understanding the Behaviour of Australian Black Coals inPressurised Fluidized Bed Combustion

John F. Stubington, Alan L. T. Wang and Yongbin Cui

Co-operative Research Centre (CRC) for Black Coal Utilisation and*School of Chemical Engineering and Industrial Chemistry,

University of New South WalesSydney 2052, Australia

15 International Fluidized Bed Combustion Conference, May 1999th

Email Phone (02) 9385 4338Fax (02) 9385 5966

*Mailing address

Abstract

Ultimately, this study aims to predict the coal combustion efficiency in an industrial pressurisedfluidized bed combustor (PFBC) for Australian black coals. This combustion efficiency dependspredominantly upon the rate of elutriation of fine carbon particles, which is proportional to bedcarbon loading in atmospheric experiments. The bed carbon loading is, in turn, dependent uponthe rate of combustion of char particles within the PFBC.

A novel batch-fed reactor has been designed, constructed and commissioned to enable separationand study of the mechanisms of coal devolatilisation, char combustion and fine carbon particleelutriation in a PFBC and extraction of coal-specific parameters to describe these processes. Theattrition and char combustion rates can only be determined experimentally and it is essential tomatch the environment around each coal particle, so that the results may be translated to theindustrial scale. Therefore, the rig was designed for identical conditions of pressure, temperature,particle size and fluidizing velocity within the bed to those used industrially. The exhaust gas isanalysed continuously for oxygen, carbon dioxide, carbon monoxide and hydrocarbons as afunction of time after coal injection, allowing separation and identification of the devolatilisationand char combustion stages as well as measurement of the combustion rates. The elutriatedcarbon particles undergo minimal freeboard combustion and are collected in a cyclone and anin-line filter over any period of time during the experiment, for subsequent analysis. The sandbed containing partially burnt char may be quenched at any time during the experiment andremoved from the rig for collection and characterisation of the partially burnt char particles. The rig is mostly computer-controlled and the design was subjected to a hazards analysisbefore construction. Results from the rig will be used in a mathematical model to predict theperformance of the coals in industrial-scale PFBC.

STATUS OF PRESSURISED FLUIDIZED BED COMBUSTION

Pressurised Fluidized Bed Combustion (PFBC) is a commercial Clean Coal Technology utilisingcombined gas turbine and steam turbine cycles to increase the efficiency of electricity productionfrom coal. Typically, this advantage is around 3-4 percentage points higher than a conventionalsteam plant with commonly used steam conditions, which corresponds to about a 10% fuel saving(Almhem and Lofe 1996).

ABB Carbon has designed and constructed five 70-80 MW PFBC power plants: at Tidd ineUSA, 2 at Värtan in Sweden, at Escatrón in Spain & at Wakamatsu in Japan, starting up between1990 and 1993. Currently, a sixth ABB unit of this size is being built at Cottbus in Germany forcombined heat and power and a larger 360 MW P800 power plant is under construction ateKarita in Japan. In addition, Mitsubishi Heavy Industries (MHI) have built an 85 MW PFBCepower plant in Hokkaido, Japan and Babcock-Hitachi are constructing a 2 x 250 MW PFBCepower plant for Chugoku Electric at Osaki, Japan. These larger units are scheduled for start-upbetween 1999 and 2002. All of these plants use bubbling-bed combustion technology operatingat 850-860 °C and 1-1.6 MPa.

Foster-Wheeler/Ahlström developed circulating-bed technology for PFBC and the U.S.Department of Energy committed partial funds for support of a demonstration of this variantof the technology. Given the competitive position of natural gas combined cycle plants in thede-regulated U.S. environment and the long term prospect for low gas prices in the U.S., thereis some doubt whether this PFBC demonstration plant will be built.

Australian black coals have been fired in all the Japanese PFBC power plants and in the JapanesePFBC research programs. The larger commercial plants now under construction in Japan willcertainly be firing Australian coals, so it is essential that we understand the behaviour of our coalsin this new technology to support their marketing into Japan.

TECHNOLOGY ISSUES

Emissions

90-98 % of the sulphur emissions are captured by in-situ reaction with the limestone (or dolomite)used as bed material and nitrogen oxide emissions are inherently low due to the low combustiontemperature and NO reduction by the bed particles. Further reductions in NO emissions can bex xobtained by SNCR and SCR flue gas cleaning technology. Environmental performance data fromVärtan (Table 1) show the inherent cleanliness of this technology (Jansson and Anderson 1997).

Table 1. PFBC Performance Data Measured in the Värtan Plant(Jansson and Anderson 1997)

Emissions mg/MJ ppm (6 % O )2

SO2 15 15

NOx 10 14Dust < 1 < 5 mg/Nm3

Efficiency

PFBC and integrated gasification combined cycle (IGCC) technologies are often seen as compe-titors in the technology market for cleaner, more efficient electricity production from coal. However, PFBC is clearly commercial now, particularly in Japan, whereas IGCC is just reachingthe demonstration stage for commercial-size plant. Despite the low inlet gas temperature of850 °C to the gas turbine, ABB Carbon’s standard 425 MW P800 plant is reported to achieve aneefficiency of 44-45 % based on the lower heating value (LHV) of the coal. These efficiencies arealready in line with or above projections for IGCC and supercritical pulverised coal-fired steamcycles (Jansson and Anderson 1997). ABB Carbon quote a typical greenfield capital cost of $US1000/kW for the P800 unit (Jansson and Anderson 1997).

Advanced PFBC Technology

Current commercial technology uses cyclones for hot gas cleanup and special ruggedized gasturbines to handle the remaining fine ash particles in the gas. Commercialisation of ceramic filtersfor hot gas cleanup would allow the deployment of advanced gas turbines with higher efficiencies. Current demonstrations of such filters on large-scale PFBC plant are finally showing promise ofsatisfactory operation.

Advanced cycles (such as the Foster-Wheeler circulating-bed technology) raise the gas turbineinlet temperature to increase the efficiency further, usually by partially gasifying the coal toprovide gaseous fuel for a topping cycle. A similar approach is also being studied for improvingthe efficiency of the bubbling-bed technology by simply injecting additional fuel into thefreeboard. Together with more advanced steam cycles, higher PFBC plant efficiencies (up to50 %) can be expected as the technology matures.

PRELIMINARY ASSESSMENT OF AUSTRALIAN COALS FOR PFBC

In the prior CRC project, a preliminary feasibility study into the performance of Australian blackcoals in PFBC was completed by CRE in the UK (Laughlin and Sullivan 1997). From laboratorystudies of 7 Australian & 5 International coals followed by economic modelling of the PFBCcycle, it was concluded that the majority of Australian bituminous coals would make idealcandidates for PFBC. The high melting characteristics of the Australian coal ashes shouldminimise the potential for bed agglomeration problems and the lower sulphur content Australiancoals were predicted to have a lower cost of electricity or higher value for the same cost ofelectricity, as illustrated in Figure 1 (Stubington 1997a). This would provide an obviousadvantage for Australian coals in the PFBC market.

1.5

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0 1 2 3 4 5

Sulphur Content (% adb)

Bre

ak-e

ven

Coa

l Pric

e (A

us$/

GJ)

Australian

International

Figure 1. Break-even Coal Price for Constant Cost of Electricity

However, pilot- and demonstration-scale test firing of Australian coals in Sweden and Japanidentified potential problem areas, which were not addressed in the CRE tests. Followingassessment of the CRE work and extensive discussions with commercial PFBC operators andvendors by John Stubington(1997b), the areas of combustion efficiency, fly ash-related effects andoverall system modelling were identified as high priority for inclusion in the CRC's researchprogram on the performance of Australian coals in PFBC.

In atmospheric pressure fluidized bed combustion (AFBC), the elutriation of fine unburnt charpredominantly determines the combustion inefficiency but no research has been published on thiselutriation at high pressure. Research into the unburnt char elutriation from Australian black coalsin a PFBC environment is therefore the highest priority, for the following reasons:

1. One Australian black coal has exhibited this problem in a pilot-scale PFBC rig.

2. Australian black coals tend to have higher values of the Fuel Ratio (i.e., lower VolatileMatter), which correlate with lower combustion efficiencies in PFBC test rigs.

3. The overall system performance is sensitive to the combustion efficiency, as demonstratedin the CRE work.

4. Higher temperatures caused by combustion of unburnt char in the filter cake maycontribute to the problem of “sticky ash” in the ceramic filters.

AUSTRALIAN FACILITY FOR PFBC COAL COMBUSTION RESEARCH

A novel bench-scale combustor has been designed specifically to study the combustion behaviourof black coals in PFBC. Figure 2 shows a schematic diagram of the high-temperature, high-pressure, computer-controlled, batch-fed PFBC facility at the University of New South Wales,funded by the CRC for Black Coal Utilization and constructed with in-kind support from PacificPower. This facility has identical conditions in the bed to those used industrially, to ensure thatthe local environment around the coal particle matches that in an industrial PFBC. It providestime-resolved data on coal devolatilisation and char combustion to allow investigation of thedifferent mechanisms occurring during these processes, and it enables the collection forcharacterisation of both in-bed and elutriated coal char particles at different stages. The bed hasbeen commissioned and operates well at the design conditions given in Table 2.

Figure 2. Schematic Diagram of the PFBC Facility at UNSW

Table 2. Design Parameters of the PFBC Facility at UNSW

Bed Diameter 40 mm

Minimum bed height 60~80 mm

Operating pressure 1~1.6 MPa

Operating bed temperature 860 °C

Maximum Fluidization gas velocity 0.9 m/s

Bed material particle size 1 mm

Coal particle size <6 mm

Design metal temperature of combustor 850 °C at 1.6 MPa (abs)

Material of combustor 253MA stainless steel

Designed Service Life of Combustor 10,000 hours

Material of Pre-heater Inconel 600

In-bed Heater Rating 1 kW

Furnace Rating 13.5 kW

The major components of the PFBC facility include:

1. Two computer-controlled mass-flow controllers to control the flow rates of the gases N2

and air respectively. The dry N gas is supplied from a liquid N tank and the air from an2 2

air cylinder pack.

2. A fluidizing gas preheater made of a tube coil and an electrically heated furnace to preheatthe fluidizing gas up to about 750-800 °C.

3. A pressurised fluidized bed combustor with an internal electric heating coil to heat andmaintain the bed at a temperature of 850 °C.

4. A batch-feed coal hopper with a high-temperature solenoid valve, controlled by computer. This allows a batch of coal samples with a maximum particle size up to 6 mm to beinjected into the pressurised fluidized bed through a guiding tube from the top of thecombustor.

5. Two exhaust lines from the combustor, each containing a cyclone, a filter and a water-cooled condenser, to collect the elutriated fine char particles as soon as they are generatedin the bed. Solenoid valves located after the gas cooler allow collection of the elutriatedparticles over any selected time period during the burnout.

6. A N supply by-pass operated by two computer controlled three-way solenoid valves. 2

This allows the sudden change of the inlet fluidizing gas to either air, N or a mixture of2

the two, making it possible to quench or activate the combustion and characterise thecoal/char particles at any time during different combustion stages in the bed.

7. A high temperature metal-seated ball valve at the bottom of the combustor to drop the bedinto a nitrogen-purged catchpot and recover the in-bed solid combustion residues.

8. A set of on-line analysers for CO, CO , O and hydrocarbons to continuously analyse the2 2

exhaust gas.

The combustor design has been simplified to use a single wall for both temperature and pressurecontainment. It is fabricated of high temperature stainless steel 253MA with 10,000 hours servicelife due to the creep rupture limitation at the design metal temperature of 850 °C and systempressure of 1.6 MPa. An injection manifold for the fluidization gas surrounds the pressure vesselwall with horizontal holes into the combustor rather than a conventional gas distributor. Thecombustion bed is supported by inert sand above the bottom valve, which allows the bed to dropfor recovery of the in-bed solid combustion residues with minimum further damage. Thefreeboard has a minimum height, to collect the fine char particles as soon as they are generatedfrom the in-bed processes of coal. Post-bed combustion is minimised by exposing the wall of thefreeboard directly to the ambient environment to cool down the flue gas and the elutriatedparticles.

The system is controlled and monitored by a PC with the data acquisition & control software,Visual Designer. The preheater has been protected by a thermocouple monitoring the coil metaltemperature and an overheat cut-off switch, since it was identified in the hazard analysis as apotential weak point of the facility. During operation, reaction gases (pure N and air) are filtered2

before entering the mass flow controllers, and then mixed before entering the pre-heating furnace. The gases are pre-heated up to 750~800 °C and injected into the combustor through the circulargas manifold. The exhaust gases are discharged through the exhaust by-pass line before and afterthe combustion test. When the desired test conditions are reached, the flow is switched from theexhaust by-pass to the testing line. As soon as steady-state is achieved, the coal particles areinjected into the fluidized bed combustor with some pressurising gas by opening the computer-controlled solenoid valve, which is then closed quickly.

Following devolatilisation of the coal particles and char combustion, the products of combustionexit with the flue gas. The elutriated char and fly ash particles are collected by the cyclone andthe in-line filter for subsequent analysis. Some of the flue gas passes into the gas analysis systemfor determining CO, CO , O and hydrocarbon concentrations. To quench the combustion and2 2

recover the char particles at an intermediate stage, the air line is switched to N and the fluidizing2

gas is switched from the preheater to the by-pass line to cool down the bed at the minimumfluidizing gas velocity. After cooling below the char ignition temperature, the bed materials aredropped into the bed collector by opening the bottom ball valve and the char particles are thencollected for further analysis and measurement.

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0.15

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Time (sec)

CO2 vol %

CO vol%

CO+CO2 vol%

HC(x100) ppm

Figure 3. Burnout of 4-4.75 mm Coal Particles in the PFBC Facility

Facility Operating Performance

The observed bed hydrodynamic behaviour and the pressure drop across the bed measured by adifferential pressure transmitter showed that the bed was evenly fluidized and the gas and solidswere mixed very well. Cold tests with a single coal particle proved that, in this novel design offluidized bed, the coal particle was well mixed with the particulate phase.

After commissioning, all design parameters have been achieved. The pressure was easily andsmoothly controlled at the prescribed operating pressure up to 1.6 MPa. For a range offluidization gas velocities 0~0.9 m/s, the gas temperature pre-heat reached 750~800 °C and thebed was heated up to 860 °C within one hour, solving the common problem of slow heat-upexperienced by most of the previous research rigs. The test temperature was controlled at 850 °Cduring combustion with variation of only ± 3~4 °C due to the release of reaction heat.

Initial Coal Burnout Tests in the Facility

Figure 3 shows the combustion profiles of 4 ~ 4.75 mm coal particles in the PFBC facility. Thetimespan of the hydrocarbon profile identifies the devolatilisation process. The char burnout timeis around 6 minutes with an oxygen concentration of 3.5 % in the fluidizing gas. Some variationsin the CO and CO profiles may indicate char particle fragmentation during char combustion, since2

initial combustion-quenching tests at the suspected time found fragmented char particles.

Initial Coal Swelling Tests

Swelling tests were performed under N by injecting coal particles into the hot pressurised bed,2

quenching the bed at the end of devolatilisation and recovering the char particles. The equivalentsurface diameter measured by Image Analysis was used to characterise the particle size. The ratioof mean particle sizes before and after devolatilisation defined a swelling index of about 1.1 forthe test coal particles with sieve size 4~4.75 mm.

Initial Fragmentation Tests

No primary fragmentation (i.e., fragmentation during devolatilisation) was observed in the aboveswelling tests. Secondary fragmentation of char (i.e., fragmentation during char combustion) wasfound at about 2 minutes after feeding coal particles into the bed, where the CO and CO profiles2

exhibit some rate changes. The ratio of the number of fragmented char particles (with sieve size> 1 mm) to the number of initial coal particles (sieve size 4~4.75 mm) was 33/20. Further work isessential to quantify such fragmentation, since it will certainly affect both in-bed char combustionand char elutriation.

Elutriation of Char Particles

The cyclone with calculated cut-off size of 5 µm collected nearly all the elutriated particles. Theloss on ignition of this cyclone fly ash was up to 30 %, which was about 3~4 % of the carbon fedin the coal. The reactor was designed with minimum freeboard height to research only the in-bedprocesses, so this level of carbon elutriation is realistic. Freeboard combustion of such elutriatedchar should be included in the combustor modelling and is not studied in this rig. Visualexamination of the fly ash identified some cenosphere formation, which may explain the poorperformance of the test coal in Japanese pilot plant tests for ash stickiness in ceramic filters.

PFBC COMBUSTION EFFICIENCY RESEARCH PROGRAM

The overall objective of the CRC’s PFBC Combustion Efficiency project is to characterise thecombustion efficiency of Australian black coals in commercial Pressurised Fluidized BedCombustors. Combustion inefficiency is determined predominantly by the elutriation of fineunburnt char, which is proportional to the carbon loading in the bed (at least for AFBC). Twosub-projects are currently underway, with the following specific technical objectives:

1. Investigate which mechanisms for fine char particle generation and combustion aresignificant in PFBC, develop model(s) for the most important mechanism(s) and measurethe coal-specific parameters of the model(s) for 5 coals.

2. Investigate which mechanisms for in-bed char combustion are significant in PFBC,develop model(s) for the most important mechanism(s) and measure the coal-specificparameters of the model(s) for 5 coals.

The first sub-project aims to describe the rate of carbon elutriation from the bed for each coalparticle fed to the bed. The second sub-project aims to allow estimation of the in-bed carbonloading in a commercial PFBC; so that, together with the first sub-project, it will describe the rateof carbon elutriation from the top of the bed for all the coal/char particles in the bed. These data,together with modelling of freeboard combustion reactions, will enable prediction of the carbonelutriation loss from the top of the combustor; including all the relevant processes in an overallcombustor model.

CONCLUSIONS

The results of the commissioning tests indicated that the facility performed very well. All thedesign parameters and capabilities have been achieved. The bench-scale characterisation methodsdeveloped in this program have been demonstrated to provide relevant information for assessingthe behaviour and performance of Australian coals in PFBC. Preliminary tests are now underway to fully characterise the operational performance of the facility and establish the range oftest conditions for the detailed characterisation of Australian coals. In future tests, the keymechanisms governing unburnt carbon elutriation and the combustion efficiency will be identifiedand quantitatively studied.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support provided by the Cooperative Research Centrefor Black Coal Utilisation, which is funded in part by the Cooperative Research Centres Programof the Commonwealth Government of Australia. The authors thank Mr. Michael Roesch,Mr. David Murphy and Mr. Naranjin Sharma of Pacific Power for their invaluable efforts inconstruction, assembly and commissioning of the facility; Dr. David Harris of CSIRO for the coalsample; and Dr. Allen Lowe of Pacific Power for his guidance.

REFERENCES

Almhem, P., and Lofe, J. (1996). “PFBC Power Plants: A Competitive Alternative.” POWERGEN '96, Orlando, Florida.

Jansson, S., and Anderson, L. (1997). “Pressurised Fluidised Bed Power Plants.” Indo-EuropeanSeminar on Clean Coal Technology and Thermal Power Plant Upgrading, New Delhi.

Laughlin, K., and Sullivan, K. (1997). “Evaluation of Australian Coals for Pressurized FluidizedBed Combustion.” Final report to CRC for Black Coal Utilisation.

Stubington, J. F. (1997a). “Preliminary Evaluation of Australian Coals for Pressurised FluidizedBed Combustion.” First Annual Conference of Participants, Cooperative Research Centre forBlack Coal Utilisation, Brisbane, 12 November, pp. 6.

Stubington, J. F. (1997b). “Research necessary to predict the performance of Australian blackcoals in Pressurised Fluidized Bed Combustion technology.” Report to the CRC for Black CoalUtilisation on project 5.5.