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COPYRIGHT RESERVED – PROPERTY OF JOHAN BRAND

JF Brand, African Carbon Energy, [email protected] JC van Dyk, African Carbon Energy, [email protected] FB Waanders, North-West University, [email protected]

THE USE OF VORTEX GAS SEPARATION IN UCG

mailto:[email protected]




Introduction

Description of UCG Syngas: content and contaminants

Proposed UCG syngas clean-up•Cold gas clean-up•Wet scrubbing•Ranque–Hilsch Vortex Tube

•Hot gas clean-up

•Candle Filters

•Vortex gas separation with Inertia-Vacuum filter

•Inertia-Vacuum Filter with Wet Scrubbing

Proposed UCG application

Conclusions

PRESENTATION ROAD MAP


INTRODUCTION

Impurities in the gasification feedstock (especially: sulphur, nitrogen, chlorine, and inorganic mineral matter), often ended up in the syngas.

High temperature syngas cleaning has been a focus of research for decades, • but commercializing has challenges

Reliability of these systems not yet proven in a commercially integrated gasification-based system.

Conventional high temperature gas cleaning technologies have fundamental limitations to improvement (intrinsic material properties of candle filters cause practical problems and lead to unacceptable availability). materials limitation which means that to increase the porosity of the material

(and thereby increase the filtration area and cost effectiveness of the filter), also causes the mechanical strength to decrease. use of exotic state of the art metals and ceramics has provided limited success.


INTRODUCTION

Woolcock and Brown eludes to gas clean-up technologies classified according to the process temperature range:

● Cold gas clean-up generally describes wet scrubbing processes that occur near ambient conditions, usually utilizing water sprays, and results in exit temperatures that allow water to condense where the contaminants will either absorb into the water droplets or serve as nucleation sites for water condensation.

Warm gas clean-up is often assumed to occur at temperatures higher than the boiling point of water, but still allow for ammonium chloride condensation. This typically implies a warm upper limit of about 300°C.

Hot gas clean-up occurs at temperatures >300°C, but is possible to still result in condensation of several alkali compounds.


4

INTRODUCTION TO UCG

COMBUSTION GASIFICATION PYROLYSIS DRYING

PYROLYSIS

PYROLYSIS

DRYING

DRYING

C + ½ O2 →COC + O2 →CO2H2 + O2 →H2O

C + CO2 →2 COC + H2O →CO + H2

Water-gas shift:CO + H2O→CO2 + H2Methane formation :C + 2 H2 →CH4CO + 3H2 →CH4 + H2O3C + 2H2O→CH4 + 2 CO

InjectionProduction

Slag / sintered ash

Raw coal

Roof (overburden)

Floor (under burden)

UCG is a coal mining method, using boreholes to chemically extract hydrocarbons through gasification.

UCG produce raw-syngas at the production well on surface, that is both hot and at pressure, ideal for the Fischer-Tropsch process which requires the clean syngas to be at >250°C and >25 bar.

Preservation of both temperature and pressure during gas clean-up will enhance the efficiency of the overall system.

The syngas production pressure designed for Africary’s Theunissen UCG (TUCG) project is: ±25 bar

Syngas will reach the surface with a target temperature of 250-350°C. The exit temperature is highly dependant on the mine design, gasification agent ratios, coal type, quantity of agent, residence time and groundwater in-flux.


DESCRIPTION OF UCG SYNGAS CONTENT AND CONTAMINANTS

Contaminants generally include particulate matter (mineral particulates, trace elements and char), water vapour, condensable hydrocarbons (i.e. oils and tars), sulphur compounds, nitrogen compounds, alkali metals and hydrogen chloride (HCl). The sulphur compounds and carbon dioxide (CO2) are usually removed by various adsorption-based acid gas removal systems, but novel alternatives like the Ranque–Hilsch Vortex Tube (RHVT) and Vortex Gas Separation (VGS) are proposed in this review.

Streams Raw gas 1 Raw gas 2

Major Components mol% mol%

H2 34.5% 43.1%

CO 46.0% 27.5%

CO2 7.3% 9.6%

CH4 7.0% 12.4%

C2H4 0.0% 0.0%

N2 0.6% 0.6%

O2 0.0% 0.0%

H2O 4.4% 6.6%

H2S + COS 0.2% 0.2%

Trace Components mg/kg

Bag2.4E-24 2.9E-24

Sbg2.6E-01 3.1E-01

Cdg4.6E-07 5.5E-07

Crg1.4E-24 1.6E-24

Beg5.8E-12 6.8E-12

Pbg4.1E-07 4.8E-07

Mog5.3E-19 6.2E-19

Cug3.0E-22 3.5E-22

Cog1.6E-31 1.9E-31

Mng2.2E-31 2.6E-31

Hgg1.7E-02 2.1E-02

Sng1.8E-02 2.1E-02

Asg1.4E-05 1.7E-05

Nig1.9E-18 2.3E-18

Zng1.4E-08 1.6E-08

Vg1.1E-18 1.3E-18

Clg2.6E+01 3.1E+01

Fg3.9E+00 4.6E+00

Liquids Mass%

Tar 0.6 0.7

Oil 0.6 0.7

Naphtha 0.3 0.4


PROPOSED UCG SYNGAS CLEAN-UP

Components/ Properties Limit value

Heavy Metals < 1 ppmw

Silica < 0.1 ppmw

O2 < 1 ppmw

Halogens as H-X (HCl etc.) < 5 ppbw

Alkali metals < 10 ppbw

Soot/dust/solids < 1 ppmw

Tars and aromatic components < 1 ppmw

Nitrogen Compounds including NH3, HCN and amines

< 10 ppbv

Sulphur (including H2S and organic sulphur)

< 5 ppbw

CO > 30%

H2 +CO > 60%

Wet gas dew point H2O at pressure

The level of cleaning based on the downstream consumer’s technology requirements, fuel specification and emission standards (i.e. FT Process of Power Supply).

Table 1: POST CLEAN-UP SYNGAS SPECIFICATIONS FOR FT

Table 2: GAS ENGINE FUEL GAS SPECIFICATIONSComponents/ Properties Limit value

Sulphur < 1,200 ppmv

Hydrogen sulphide equivalent

< 1,500 ppmv

Total Sulphur compounds < 57 mg/MJ or 2,000 mg/10kWh

O2< 2% vol.

C4 and Higher < 2% vol.

H2< 40% of total LHV

Gas humidity < 60%

Wet gas dew point > 15°C below gas temperature

Silicon and siloxanes < 0.56 mg/MJ or 0.2 mg/10kWh

Chlorine equivalent < 3.5 mg/MJ or 400 mg/10kWh

Ammonia < 1.5 mg/MJ or 55 mg/10kWh

Oils and tar <1.19 mg/MJ or 5 mg/10kWh

Particulate matter (soot, dust, ash)

< 1 - 3 m and

< 0.8 mg/MJ or 50 mg/10kWh

Calorific Value (CV) 4.7 to 7.6 MJ/m3n

Temperature < 40°C

Pressure < 200 mbarg


COLD GAS CLEAN-UP(Wet Scrubbing – the conventional way)

Cold Gas Clean-up (CGC) system is the predominant gas treatment technology chosen for TUCG in combination with gas engines due to its high efficiency and proven reliability.

For the Africary TUCG process design, a cyclone in combination with a wet scrubber

The system will recirculate and cool water to remove the moisture, tar, oil and particulates by wet scrubbing.

To minimize the wet scrubbing thermal penalty on the overall plant efficiency, Africary can utilize a duel stage cooling process, whereby the syngas will be cooled to ±150°C in the first stage and 80°C in a second stage with a trim water cooler making sure the fuel gas meets gas engine vendor requirements of <40°C

It deals with most of the condensable liquids and particulates, however only removes portions of the gaseous contaminants and does not do any gas separation and will have to be used in conjunction with a separate Acid Gas Removal (AGR) system.


Despite decades of research materials have achieved only limited commercial success due to a natural correlation between porosity, mechanical strength and thermal conductivity.

To increase the porosity of the material and thereby increase the filtration area, also means that the mechanical strength is decreased, which is a fundamental limitation for candle filters and implies that an increase in the porosity leads to an increase in both the filtering rate and surface area (m2/g).

WARM GAS CLEAN-UP(Candle Filters)

High temperature filter technology can deal with most of the particulates, however it only removes a portion of the contaminants and does not remove any condensable liquids;

nor does it perform gas separation and will therefore have to be used in conjunction with a separate AGR system including an internal CGC wet scrubber.


Sample number O₂ (mol%) N₂ (mol%) Total (mol%)

Control (air) 22.3 75.9 98

C1 21.8 76.0 98

H1 23.9 77.0 101

C2 24.1 75.9 100

H2 22.8 77.6 100

Various authors reported on gas separation of components in a gas mixture when it is passed through a Ranque–Hilsch Vortex Tube (RHVT).

In 2018 Brand and Esterhuyse conducted similar experiments on a commercial RHVT.

20% cold-cut samples taken (C1 and H1) provide the expected O2 reduction at the

cold side (±0.5%) and the increase of O2 at the hot side of about 1.6%.

Best results (C2 and H2) of about 2% increase in O2 at the cold side and the same

increase of ±2% N2 at the hot side obtained using a 10% cold-cut flow orifice.

HOT GAS CLEAN-UP(Ranque–Hilsch Vortex Tube)


Chentsov, et al. published the successful testing of HGC using a vortex gas separator. A test was completed to remove fly ash with a particle size of 5-100 μm, utilizing Inertia-m, utilizing Inertia-Vacuum cleaning as an aerodynamic filtering method, at the Ekibastuz coal power plant. The installation had a capacity of 15,000 m3/h and was equipped with measuring equipment, and electronic scales with a measurement accuracy of ±0.1 gram. Forty-five tests were carried out, with up to -35 mbar vacuum, which achieved filtering of up to 99.25% of all particles with the latest upgrades operating at 99.99%.

HOT GAS CLEAN-UP(Vortex gas separation with Inertia-Vacuum filter)


The opposite of a Cyclone separator!

Three important effects are observed with regards to gas flow in an APG (Adverse Pressure Gradient) which underpin the work principle of a Vortex gas separation in an inertial-vacuum:1. An axial gas flow accelerating in a converging nozzle maintains its laminar structure, while transforming its potential energy into a laminar vortex.

2. If two streams containing dispersed dust particles in a APG or in a "over-expanded state" intersect, then after crossing their flows, the particles transfer to the jet with the highest velocity.

3. If a high-velocity gas stream travels over a large stationary gas volume, separated by small slots, this large stationary gas volumes act as a "sump" for suspended particles.

HOT GAS CLEAN-UP(Vortex gas separation)

All three effects mentioned here can be observed in nature where a dust devil forms a funnel-like chimney that sucks dust particles into the centre of a vortex.

Gu, et al - vortex could entrain solid particles and carry them into the vertical swirling wind field.

Another trend identified was for the fine dust grains to rise along the inner helical tracks, while the larger dust grains were lifting along the outer helical tracks.


HOT GAS CLEAN-UP(Vortex gas separation with Inertia-Vacuum filter)

1 - Gradient Separation channel2 - Inlet3 - Outlet4 - Parabolic nozzle5 - Cyclone nozzle6 - Parabolic diffuser7 - Dust exhaust pipe8 - Peripheral degas scroll9 - Pipe connection for vacuum pump control of the boundary layer


Moiseev, et al. focus in separating less than 0.5%

vol methane from a mine ventilation gas flow

by recovering the methane in concentrations of up to 80%.

the total energy redistribution in a VGS is caused by a combination of: The kinetic energy flux; directed from the periphery to the

centre. The thermal energy flux; which occurs due to turbulent

molecule movement within a high gradient field of radial static pressure.

The huge variability in gas specific heat induced by the adverse pressure gradient.

HOT GAS CLEAN-UP(Vortex gas separation)


HOT GAS CLEAN-UP(Inertia-Vacuum Filter)

The same principles used to separate dust particles and light gasses can also be used in a filter mechanism.

The collected light fraction containing dust particles, obtained from the gradient separator, is vacuumed through an accelerating nozzle (stream A).

The area of the accelerating nozzle "F1" is smaller than the cross-section of the exit "F2" and the velocity of the gas increases and expands.

The dusty input stream, passing through the cross section (F1), has a greater velocity than the flow in the cross section (F2) and therefore the particles in this flow also have a higher velocity (greater kinetic energy), which allows them to overcome the drag force of slower exiting stream-B flowing over cross section (F2). Thus, particles go into the channel, but get trapped inside the circulating stream C.

The rotating gas circling the rotary chamber becomes a highly efficient filter, consisting of layers of gas (acting as layers of filtering material). Any remaining (less than 2 - 3%) ultra-fine particles that may accumulate in the rotating stream eventually coagulate and fall into the collecting chamber (D), due to gravitational force.


PROPOSED UCG APPLICATION

Combining all the information presented, it is possible to design a novel UCG gas clean-up and separation unit that will allow the hot and efficient separation of syngas into its useful components, together with partial cleaning for further gas utilization.

Vortex gas separation in combination with an inertia-vacuum filter can be utilized

directly from the Hot UCG production well to achieve the following: Removal of any ash, dust and vapour from the High Temperature syngas. Remove most of the H2S and a portion of the CO2.

Separate H2 for H2:CO ratio control

As the UCG production well is pressurised, it is expected that the use of an Axial Flow Pressure Regulator will be required. This device must control the upstream flow, in such a manner, that the downstream compressor can create a pressure drop and allow the implementation of APG of about 50 mm H2O over the vortex separator and

the filters.

Although the proposed system can only clean the syngas partially it may still be reasonable to implement as the advantages of obtaining hot ratio correction with acid gas removal will allow smaller downstream equipment.

COPYRIGHT RESERVED – PROPERTY OF JOHAN BRANDPROPOSED UCG APPLICATION

1

2 3

4


STEP 1 - the incoming gas is separated and an initial vortex created by a stationary swirler guide vane, as well as the addition of a portion of recycled pure gas will be used in a cyclone injector to add significant spin to the outside of the incoming raw gas.

STEP 2 - the spinning gas will be allowed to inter-mix with the central dusty cord which will allow the transferral of dust to the central axial flow of light gasses (mainly H2 and

CH4), to be removed by the dust exhaust pipe. This gas may be cooled to generate

process steam and to protect down steam vacuum compressors. Before the cooled gas is fed to the compressors it is passed through an inertia-vacuum filter system or a CGC system that will remove any condensable liquids and dust.

STEP 3 - the vortex strength is increased by internal swirler guide vanes, which allows the heavier molecular gasses to move to the periphery where they are removed by an inverted cyclone nozzle. The gas may be cooled to generate process steam and to protect down steam vacuum compressors. From literature mentioned it was shown that this heavy gas can pass through a smaller vortex separator to separate the very heavy sulphur components from the CO2. The sulphur components will be routed to a sulphur

recovery plant and the CO2 may be further processed on site.

STEP 4 - the remaining cleaned gas is passed through the high temperature compressor; where after a portion of the gas is used in the first step.

PROPOSED UCG APPLICATION


CONCLUSION

The incorporation of hot gas clean-up based on a Ranque–Hilsch Vortex Tube or Vortex gas separation with Inertia-Vacuum filter may provide both:

1) particle separation, and;

2) gas separation directly form a UCG production well.

The advantages of such a hot gas clean-up system in combination with RTI’s Warm Gas Desulfurization Process may provide for efficient syngas clean-up for clean coal power generation.

This process can further be combined with deeper acid gas removal, as the carbon to hydrogen ratio control may allow the efficient blending of syngas produced by different UCG operating regimes to supply a cost-effective FT syngas for poly-generation operations.


ACKNOWLEDGEMENTS

AFRICAN CARBON ENERGY

NORTH-WEST UNIVERSITY

Part of the research presented in this paper was hosted by the South African Research Chairs Initiative (SARChI) of the Department of Science and

Technology and National Research Foundation of South Africa (Coal Research Chair Grant No. 86880).

Any opinion, finding or conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard.

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