Effect of Pressure on Oxy-Fuel Gasification in Fluidised Beds

Nicolas Spiegl, Cesar Berrueco, Nigel Paterson, Marcos MillanImperial College London, Department of Chemical Engineering

IEA-FBC Meeting

Technical University of Vienna, 29th August 2011

Background

Power generation based on FBG + IGCC + CCS

A Fluidised Bed Gasifier operated in a high partial pressure of CO2

Aims

Establish the effect of operating conditions such as

Temperature

Inlet gas composition

Pressure

Feedstock

on the gasification performance:

Carbon Conversion

Fuel Gas Heating Value

Gas Composition

Gasifier Operability

Overview

Experimental Set up: Bench-Scale Pressurised

Fluidised Bed Reactor (PFBR)

Review of Results at Atmospheric Pressure

Effect of CO2 :C ratio at High Pressure

Effect of Steam addition at High Pressure

A closer look at the effect of Gasification Pressure

Conclusions

Experimental Set Up

Tar Trap

Fluidised Bed Reactor

Feed Hopper

Gas Analyser

NH3 Absorber H2 O Trap

H2 S Sampling Point

Gas Mixing

Steam Generator

Set up: the Pressurised FB Gasifier

34mm i.d., 504 mm height.

Up to 1000°C and 30 bar.

Solid feed rate of up to 6 g/min.

Gas flow rate up to 15L/min (~2NL/min used).

A German Lignite was used in these experiments.

Particle size range: 200-300um.

Sand Bed.

Constant fuel gas composition for about 30 min.

Spout

Thermocouple

Electrode

Atmospheric PressureEffect of CO2 /C Ratio on Carbon Conversion and LHV

CO2 to C ratio does have a significant effect at 850ºC, but not at 750 or 950ºC.

The LHV of the product gas drops due to dilution with CO2 .

950°C

□

850°C

Δ

750°C

Atmospheric PressureSolid feed rate: 1.7g/minNo O2 injected

CO, □CO2Δ

H2

, ×

CH4

( 950°C, □

850°C, Δ

750°C)

Atmospheric PressureEffect of Steam Injection on Carbon Conversion and Gas Composition

3

At 850ºC, replacing 25% CO2 with steam allows an increase in carbon conversion to be achieved.

Further increases in the amount of steam do not produce any further changes in conversion but they do in gas composition.

Atmospheric Pressure

Solid feed rate: 1.7 g/min

No O2 injected

Gas Inlet: 2.64NL/min

3

High PressureEffect of CO2 /C Ratio on Carbon Conversion and LHV

950°C

□

850°C

Δ

750°C

Experiments at 850ºC.As observed for atmospheric pressure there is an increase in carbon conversion with CO2 to C ratio.

The LHV of the product gas drops with the initial increase in CO2 /C ratio but remains constant afterwards.

High PressureEffect of Steam and Oxygen Injection on Carbon Conversion

At 850ºC, replacing 15% CO2 with steam allows an increase in carbon conversion to be achieved at all pressures.

Carbon conversion drops with increasing pressure. The reasons for this decrease were investigated in further detail.

Experiments carried out at constant CO2 :C ratios.

Both coal and CO2 flows were increased proportionally.

Wire Mesh Sample Holder + Sample

Tar Trap

Carrier Gas Entry Port

It measures behaviour of individual coal particles.

The Wire Mesh Reactor

Effect of Pressure Comparison: Pyrolysis – 850⁰C (WMR He, PFBR N2 )

Gas yield in the PFBR is similar to total volatile yield in the WMR

Tar is effectively destroyed in the PFBR

0

10

20

30

40

50

60

70

0 5 10 15 20

[wt%, daf]

Pressure [bar]

Gas Yield PFBR

Gas Yield WMR

Tar Yield WMR

Total Volatile Yield WMR

Effect of Pressure Comparison: Gasification 850⁰C (WMR CO2 , PFBR O2 /CO2 )

WMR: Carbon Conversion increases with Pressure

PFBR: Carbon Conversion decreases with Pressure

WMR – 10s holding time similar to PFBR 0

20

40

60

80

100

0 5 10 15 20

Carbon Conversion [%]

Pressure [bar]

WMR – 60s holding time

PFBR

WMR – 10s holding time

The similarities between the WMR carbon conversion at short times and the PFBR overall carbon conversion suggest that quick char

deactivation in the gasifier is the cause for the drop in conversions with pressure.

Conclusions

• A Bench-Scale FB Gasifier was set up to operate in continuous mode to study gasification in CO2 -rich atmospheres.

• Increases in CO2 :C ratio had a marked effect at intermediate temperatures (850ºC), enhancing the carbon conversion at all pressures.

• LHV dropped with CO2 :C ratio (more markedly at atmospheric pressure).

• Adding a 25% steam had the same effect as a 100ºC increase.• Pressure had a negative effect on carbon conversions, thought

due to enhanced contact between evolving tars and chars, which therefore undergo quicker deactivation.

Effect of Pressure on Oxy-Fuel Gasification in Fluidised BedsBackgroundAimsOverviewExperimental Set UpSet up: the Pressurised FB GasifierFoliennummer 7Foliennummer 9Foliennummer 11Effect of Pressure�Comparison: Pyrolysis – 850⁰C (WMR He, PFBR N2)Effect of Pressure�Comparison: Gasification 850⁰C (WMR CO2, PFBR O2/CO2)Conclusions