1

Gasification of Biomass and Sulfur-Containing Carbonaceous Fuels

Majid Charmchi

Department of Mechanical EngineeringUniversity of Massachusetts - Lowell

2

Introduction• Biomass: Black Liquor Gasification (BLG)

– What is Black Liquor?– Heat and Chemical Recovery:

• Current Recovery Process (Kraft Recovery Boiler)• Gasification Recovery Process (BLG)• Advantages of BLG Process

• Petroleum Coke Gasification (PCG)– Considering the current environmental issues, is

there a future for sulfur-containing low grade fuels?

– Could be a similarity between PCG and BLG?

3

What is Black Liquor?

4

Current Kraft Recovery Cycle

5

BL Gasification Recovery Cycle

6

BLG Effect on Emission• Efficiency of a typical Recovery Boiler

is 64% (power production < 15%)

• Efficiency of a typical BLG is 74%

• The industry’s vision is to become a net exporter of electricity

– Approximately 11% reduction in GHG emissions

– US paper production is about 100 million tones per year

– Approximate energy content in BL:• North America 1.35 EJ

• World 2.4 EJ

7

Petroleum Coke• Pet coke has:

– high heating value (~14,000 Btu/lb)– low ash levels (less than 0.5%)– low moisture (~ 8%)– There is plenty of pet coke (over 60

MT/yr most at coastal refineries)

• Pet coke contains:– high sulfur levels (~ 4-7%)– low grindability (Hardgrove index –HGI

~ 35-70)– some potentially harmful heavy

metals, such as Vanadium, Mercury, Nickel, etc.

– As a fuel source, It is limited due to: • It does not burn easily• Its direct burning has harmful emissions

8

Pet Coke & Black Liquor* Compositions*North American Wood Species

Typical Pet Coke Composition

% Wt

Kraft Black Liquor Composition

% dry Sodium 17 - 25

Potassium 0.1 - 2 Chloride 0.2 - 2

76 - 89 Carbon 34 - 39 0 - 0.1 Oxygen 33 - 38 2 - 7 Sulfur 3 - 7 3 - 4 Hydrogen 3 - 5

1.2 - 2.5 Nitrogen 0.04 - 0.2 5.5 - 15 Moisture

0 - 0.6 Ash Aqueous Phase %Wt Ash Elements

ppm Na2CO3 18 500 - 2000 Vanadium NaHS 2 250 - 450 Nickel H2O 80 50 - 250 Iron

There is a close similarity between concentrated black liquor and a slurry mixture of pet coke and green liquor

9

Objectives of BLG & PCG• Processing Aqueous Black Liquor to Obtain:

– Combustible gases (mainly CO, H2 and CH4) with:• High Heating Values (HHV) over 100 Btu/scf• H2/CO ratio ≥ 1

– Sulfide-rich melt -- sulfate reduction efficiency of Na2S/(Na2S + Na2SO4) ≥ 93%

• Gasification of Pet Coke to:– Upgrading low-value petroleum coke to clean fuel

gas or synthesis gas– Recover elemental sulfur and vanadium concentrate

10

Gasification Technology:R/D and Commercialization History

• Numerous techniques are developed for gasifying black liquor, coal and other carbonaceous fuels

– In 1983 EPRI listed 22 coal gasification processes– Recently, only 9 technologies are listed by NETL as being in use or

under R&D activity and all focused on coal gasification– Initially, over 20 BLG technologies were developed

• From that list, only two are currently under further development; none is fully commercialized

• Gasification on Molten Salt Pool: In 1980-85 period, a series of research efforts on both bench-scale and pilot-scale, funded by DOE, were conducted. These comprehensive tests have clearly illustrated that gasification of Kraft mill black liquor and pet coke/green liquor slurry is technically feasible, economically attractive, and environmentally cleaner

11

The Concept: Pilot-Scale Gasifier

Gasification on Molten Salt Pool

At start up:

70 % Na2CO3

30% Na2S

12

Laboratory-Scale Molten Salt Gasifier

13

The Gasification Process • The Stages of Gasification Process

– Atomization• High pressure liquid, splash plate, or steam assisted atomization

– Drying– Devolatilization (Pyrolysis) – Swelling

• Drying is aided by mixing and droplets swelling• The temperature range of devolatilization is fairly narrow (~250-

450 oC)• Droplet heating is rapid –droplets of about 1mm in size reach

above 450 oC before devolatilization is complete• Swelling of the particles during devolatilization enhances external

transport of oxygen during gasification• Devolatilization rate in oxidizing environment tend to be faster• Over half of the carbon in the solid would remain as residue after

devolatilization and arrive to the gasification zone

14

The Gasification Process • On molten salt pool (the gasification zone)

– Turbulent mixing with molten salt and air bubbles• Carbon oxidation (produces mainly CO, H2 and heat)

– Positive aspect of the inorganic salt pool• The inorganic matters increase the rate of char gasification• The presence of Na2SO4 and Na2S enhance carbon oxidation through the

sulfate/sulfide cycle that transports oxygen to the pool surface• The top area of the salt pool provides a surface for air/carbon contact, and it

protects the sulfide from re-oxidization

– Important reactions on the molten salt:• Reduction of the oxidized forms of sulfur (mainly sulfate to sulfide)

4C + Na2SO4 Na2S + 4CO

2C + Na2SO4 Na2S + 2CO2

• In absence of carbon and in case of excess oxygen (not likely), the sulfide may oxidize

• 2O2 + Na2S Na2SO4

15

BLG: Reduction of sulfate with CO reaction(Comparison of test data with equilibrium theory)

Source: Kelleher et al.

16

Bench-Scale Gasification Test Results(BLG Case Study)

Case APredried BL solids

injected into the salt pool

Case BDirect injection of

wet BL into the salt pool

Case CWet BL is sprayed into

the hot gas (drying zone) above the salt

pool

Feed (wt. % solids) 98 64.7 64.7

Gasification temperature (oC)

925 997 992

Product gas composition, dry basis (vol. %)

CO2 8.5 16.6 13.7

CO 20.6 7.4 14.3

H2 11 10.4 15.1

CH4 1.2 1.4 2.5

C2’s 0.1 0.05 0.5

N2 56.5 62.8 53

H2S - 0.3 0.2

HHV, dry basis, kJ/m3 (Btu/scf)

4322 (116) 2607 (70) 4795 (129)

Smelt sulfur reduction (%) 100 64.7 96.5Source: A.L. Kohl

17

Pet Coke Gasification Test Results(PCG in Air and in O2/steam )

Gasification in air Gasification in O2/steam

Source: A.L. Kohl & J. A. Ashworth

18

Conclusions • Molten salt gasification process provides a means

of converting sulfur containing biomass fuel and low-value petroleum coke to clean fuel gas

• The process recovers:– inorganic pulping chemicals in BLG– Elemental sulfur and vanadium in PCG

• The presence of molten salt pool serve an important role as catalysts in gasification of fixed carbon– Almost any carbonaceous material, e.g., pet coke,

residual oil, coal, and wood can be converted to clean fuel gas

• Gasification in pure O2/steam provides hydrogen rich product gas with medium-Btu HHV

19

Challenges and Barriers

• Capital cost• Market penetration (conservative industries)• Technical issues

– Corrosion problem (presence of alkali metals)• Advances in refractory material

– Slurry atomization• spray system design (critical drying zone issue)

– Process scaling• Moving from small scale R/D tests to full scale

commercial size gasifier

20

References1. E. Larson, S. Consonni, and R. Katofsky, “A Cost-Benefit Assessment of Biomass Gasification Power Generation in

the Pulp and Paper Industry,” Final Report, Prepared by Navigant Inc., Princeton University, and Politecnico di Milano, 2003

2. A. L. Kohl, “Black Liquor Gasification,” The Canadian Journal of Chemical Engineering Vol.64, 19863. E. G. Kelleher, “Gasification of Kraft Black Liquor and Use of the Products in Combined Cycle Cogeneration,” Final

Report – Phase II, DOE/CS/40341-T5, 19854. E. G. Kelleher and A. L. Kohl, “Black Liquor Gasification Technology,” Chemical Engineering Technology in Forest

Product Processing, AIChE, Vol.2, P40, 19885. A. L. Kohl and J. C. Newcomb, “Effect of Pressure on Smelt-Water Explosions,” TAPPI Journal, P82, September 19866. P. J. Hurley, “Energy Balances for Alternative Kraft Recovery Systems,” Chemical Eng. Progress, P43, 19807. A. L. Kohl, R. B. Harty and J. G. Johanson, “The Molten Salt Coal Gasification Process,” Chemical Eng. Progress,

P73, 19788. R. E. Henry and H. K. Fauske, “Nucleation Processes in Large Scale Vapor Explosions,” ASME Journal of Heat

Transfer, P 280, 19799. D. Stelman, A. J. Darnell, J. R. Christie, and S. J. Yosim, “Air Oxidation of Graphite in Molten Salts,” The

Electrochemical Society Inc., p. 299, 197610. D. R. Stull and H. Profet, “JANAF Thermodynamic Tables,” 2nd Ed., NSRDS-NB537, National Bureau of Standards,

197111. Electric Power Research Institute (EPRI) Report AP-3109, “Coal Gasification Systems: A Guide to Status,

Applications, and Economics," Prepared by Synthetic Fuels Associates, Inc. June 1983.12. DOE report, "Molten Salt Coal Gasification Process Development Unit," Phase 2 Final Report, Rockwell Inter­national,

DOE/ET/10296-l96, April 198213. A. L. Kohl, R, B. Harty, J. G. Johanson, and L. M. Naph­thali, "The Molten Salt Coal Gasification Process," Chem.

Eng. Prog. Vol. 74, p73, 197814. A. L. Kohl and J. A. Ashworth, "Process Upgrades Coke to Gas," Hydrocarbon Processing, pp. 97-100, July 198315. A. L. Kohl and F. C. Riesenfeld, "Gas Purification," 4th ed. Gulf Publishing Co. Houston Texas, 1985