OLGA Tar Removal Technology - b-dig.iie.org.mx

OLGA Tar Removal Technology

- biomass gasification to electricity -

Paper for POWER-GEN International 2007 in New Orleans

Author

Jan-Willem Könemann Technisch Bureau Dahlman B.V.

Co-author:

Robin Zwart ECN Biomass, Coal & Environmental Research

Contact:

Dahlman Industrial Projects B.V.

Tel: +31 10-5991114

Fax: +31 10-5991100

E-mail: [email protected]

www.olgatechnology.com

www.dahlman.nl

Dahlman OLGA Technology – Paper for POWER-GEN International 2007 New Orleans

2

INDEX

Page:

Summary 3

1 Introduction 4

1.1 Why biomass? Why gasification? 4

1.2 Gas purification overview 6

1.3 Utilisation of (bio)syngas & product gas 7

2 The tar problem 9

2.1 OLGA’s position in gas cleaning process 12

3 The OLGA process philosophy 14

4 Performance of OLGA; 0.5 MW(t) duration test 17

4.1 Process temperatures & dynamic gas composition 18

4.2 Gas composition & measured performance 19

4.3 Water condensate composition 20

4.4 Operating time of different components 21

4.5 Conclusion 0.5 MW(t) duration test 22

5 1 MW(e) biomass gasification plant in France 23

5.1 Plant description 24

5.2 Performance of OLGA 1 MW(e) 25

5.3 Test conclusions 27

5.4 Outlook 28

Appendix A: Tar Classification system 29

Appendix B: OLGA vs. other tar removal systems 30

References 33


3

Summary

The goal of this paper is to introduce you in gas treatment for (biomass) gasification and

especially the new OLGA technology. OLGA is a tar removal system developed to clean

product gas from biomass gasifiers but can also be applied for other purposes.

Although gasification is a very old technology, the utilisation of gasifiers to make a

combustible product gas for producing renewable energy in a gas engine or gas turbine is

relatively new. This report therefore contains an introduction in (biomass) gasification and

specifically the necessary gas purification. As biomass is the only renewable carbon source

and the gas market is expected to increase faster than any other energy source, a very

promising future is expected.

Tar related problems are the most important obstruction for the introduction of biomass

gasification as source of electricity and SNG on a large scale. Tars are responsible for major

operational problems due to tar condensation (fouling) and poisonous condensate leading to

unacceptable waste water costs. The tar problem is defined introducing critical parameters as

the tar dew point, removal of hetero-cyclic tars (like phenol) and the removal of naphthalene.

You are introduced in the OLGA tar removal process.

Performance of OLGA is demonstrated by showing the results of the duration test with a

0.5 MW(t) industrial pilot plant at ECN in the Netherlands. The OLGA process showed to be

stable and reliable and was operated for more than 650 hours. The gas quality was on

specification, the gas engine ran smoothly. Results on tar dew point (<10 °C), phenol removal

(completely) and naphthalene removal (>99%) were excellent.

Finally an outlook is presented to OLGA’s near future. An important tender in France

comprising 6 commercial gasification plants with a total electrical output of 81 MW(e). The

commercial installations are preceded by a 1 MW(e) industrial pilot plant. In this plant the

selected gasifier, gas cleaning and gas engine are tested together after which technical and

economical optimisation is studied for the benefit of the bigger commercial plants that

follow. First results are discussed in this paper.


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1 Introduction

1.1 Why biomass? Why gasification?

Nowadays, renewable energy is a hot topic. The stock of the fossil fuels is not inexhaustible

and therefore a lot of other energy sources (biomass, sun, water etcetera) are under

investigation at this moment. An energy source, with a large potential is biomass.

All organic material produced by plants or any other conversion process involving life is

called biomass. Biomass is renewable by definition. During growth of plants, crops and trees

CO2 is withdrawn from atmosphere. The release of this CO2 when biomass is used for energy

consumption just closes the short-CO2 cycle.

To achieve the Kyoto objectives, the share of renewable energy in the total consumption must

increase significantly. Biomass is generally considered as one of the most important

renewable energy sources. Biomass is the only renewable carbon source!

Gasification of biomass opens the full potential of biomass. Combustion is nowadays still

more used but gasification has more potential, both in efficiency for electricity production as

for the production of other products like SNG, Hydrogen or Fisher Tropsch Diesel.

A biomass gasification system can roughly be split up into four steps:

Biomass conditioning & transport

Dependant on the biomass feedstock and the type of gasifier, the biomass must be

conditioned (sized, dried etc.) and transported into the gasifier. Achieving a good continuous

and reliable biomass conditioning is often one of the most important points in operation of a

gasification plant.


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Gasification reactor

There are several different types, all with their own characteristics

o Fluidised beds (BFB of CFB)

o Fixed beds (updraft or down draft)

o Entrained flow

o Indirect gasifiers (separate combustion & gasification zones)

Gas purification

Biomass gasification results in a combustible gas. Tar removal is a critical gas purification

step and subject of this paper.

Gas utilisation

Sufficiently cleaned biomass gasification product gas can be applied for the generation of

different heat and electricity (by gas engines (CHP) or gas turbines (IGCC) but also for

advanced (future) products as ‘green’ synthetic natural gas (SNG), transportation fuels and

chemicals.


6

1.2 Gas purification overview

Purification of product gas is very important for all applications, except for direct combustion

after the gasifier. In case of direct combustion product gas purification must compete with

combustion exhaust gas purification. Which gas purification steps are necessary and how

efficient they should be is dependant upon:

1 Feedstock (e.g. biomass, waste) and it’s chemical components

2 Gasification technology & operational conditions of the gasifier

3 The application & downstream equipment; how clean should the gas be?

In general we can identify the following gas treatment steps, summarised in their most logical

order

� Particulate removal

Cyclones, filters, electrostatic filters, scrubbers

� Organic impurities

Tar removal is most important, OLGA is the topic of this paper

(see appendix B: OLGA vs. other systems for an overview of technologies)

� Removal of inorganic impurities

- Conversion of COS to H2S and HCN to NH3 by hydrolysis

- Removal of nitrogen and halogens (mainly NH3, & HCl), by scrubber technology

- Sulphur removal (H2S) by absorption and possibly conversion to elementary sulphur

� Removal of volatile (alkali) metals

Adsorption possibly on a pre-coat filter


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1.3 Utilisation of gasification product gas

This section shows some examples of the utilisation of product gas. As gas treatment is

highly dependant upon the application. A quick idea of the logical gas treatment is stated for

each application.

Power Generation

Product gas can be utilised for electricity production in several ways, below a quick summary

with an idea of the necessary gas treatment steps:

Firing on a boiler:

Injection of the product gas into the combustion chamber of a dedicated boiler or co-firing on

a coil boiler. Electricity is generated with a steam turbine after the boiler.

Necessary gas conditioning & purification:

Partial particulate removal by cyclones or complete particulate removal by Hot Gas Filters.

In-organics are mostly removed by an adsorption material on the filter or after the boiler. Tar

removal is not necessary as the gas is injected into the boiler above the tar dew point.

CHP, Combined Heat & Power

The product gas is fired on a gas engine


Particulate removal, tar removal (OLGA), gas cooling & water condensation (typically 30ºC)

and removal of in-organics in case applicable.


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IGCC (Integrated Gasification Combined Cycle);

Product gas is fired on a gas turbine; there are two options:

1 Atmospheric gasification

Gas is cooled down to approx. 40ºC (inlet gas compressor).


particulate removal, tar removal (OLGA), gas cooling & water condensation and

removal of in-organics in case applicable.

2 Pressurised gasification

Product gas can be injected in the gas turbine above the tar dew point temperature

(400-500ºC)


Complete particulate removal by Hot Gas Filters. In-organics can be removed on an

absorption material on the filter or after exhaust of the turbine.

Transportation fuels

In the future, biosyngas will be very important for the production of clean, sustainable fuels.

Most important products are Fisher Tropsch diesel and Methanol / DME. Both processes

need syngas (CO, H2) as feed. Biosyngas is different from product gas (it does not contain

methane or tars and has a lower calorific value, although it can be generated from product

gas. High temperature gasification (entrained flow) is the logical choice. In that case the gas

is tar-free.


Cooling, particulate removal and removal of in-organics, special attention for catalyst poisons

like sulphur (H2S), HCN, NH3 and COS.

Synthetic Natural Gas (bio-SNG) (product gas)

Product gas is preferred for the production of SNG as this already contains an important

amount of methane. SNG is formed by methanation, a catalytic reaction between H2, CO

forming CH4. SNG has similar properties as natural gas.


Particulate removal, tar removal (OLGA), gas cooling & water condensation and removal of

in-organics, special attention for catalyst poisons like sulphur, halogens and some metals.


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2. The tar problem

The presence of tars in the biomass product gas is seen as the biggest problem in its smooth

commercial application as source of sustainable energy. Tar is formed in the gasifier and

comprises a wide spectrum of organic compounds, generally consisting of several aromatic

rings. Simplified tars can be distinguished in “heavy tars” and “light tars”:

Heavy tars

Heavy tars condense out as the gas temperature drops and cause major fouling, efficiency

loss and unscheduled plant stops. The tar dew point is a critical factor.

Figure 2.1 & 2.2 Heavy tar fouls equipment,

Left a gas engine intercooler

Above a water scrubber grid


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Light tars

Light tars like phenol or naphthalene have limited influence on the tar dew point, but are not

less problematic. Light heterocyclic tars like phenol are very water soluble. They will be

easily absorbed by water and chemically pollute the bleed water of downstream condensers

and aqueous scrubbers. Purification of this water is very cost intensive and will jeopardise the

plants economic feasibility. Naphthalene is important as it is known to crystallise at the inlet

of gas engines causing a high service demand.

Tar defined

Tar defined

A well accepted definition states that tars are all organic compounds with a molecular weight

bigger than benzene. BTX (benzene, toluene and xylene) are components which are

considered as not important as they are not likely to influence the tar dew point and neither

are considered to form a big problem for waste water treatment. A better and more detailed

tar description is given by the classification of tars (see appendix A).

Figure 2.3 & 2.4 Light tar fouls equipment & seriously contaminates condense water,

Left a gas engine intercooler fouled with naphthalene crystals

Right contaminated condense water samples


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The tar dew point, a critical parameter

The lowest temperature in the process is determined by downstream equipment and the

application of the product gas. As typical tar dew points are between 150°C and 350 °C, and

the lowest process temperature is typically 30-40 °C massive tar condensation and tar

problems are inevitable. It is important to realize that the actual tar concentration is not the

most important parameter. It is the tar dew point which defines the point at which tars start to

be problematic. One of the most important goals for the OLGA technology is to lower the tar

dew point to a level at which problems can be excluded.


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2.1 OLGA’s position in gas cleaning process

To introduce you to the OLGA technology it is important to first show its position in a

generic line-up of an integrated air blown gasification system with gas engine for combined

heat & power (CHP) production:

ash

air

biomass

water

flue gas

1

2

4

6

7

9

8

Inorganicimpurities

tar

flue gas

5

solids3

ash

air

biomass

water

flue gas

1

2

44

6

7

9

8

Inorganicimpurities

tar

flue gas

5

solids33

1. CFB gasifier 4. Tar removal; OLGA 7. Booster

2. Gas cooler 5. Water quench / condenser 8. Flare

3. Dust removal; 6. Aqueous scrubber 9. Gas engine

Product gas cleaning can be split in the following logical steps:

1. Solids / dust removal

2. Removal of organic impurities; Tar

3. Removal of inorganic impurities (e.g. NH3 / HCl / H2S)

It is very important to consider the logical order of these cleaning steps. In principle it must

be avoided to mix dust, tar & water. It is best to separate the dust first, as dust can be

removed at a temperature in which water and tars are not present (> 400°C). After that tars

have to be removed above the water dew point to a level at which the tar problem cannot

occur in downstream equipment (minimal process temperature > tar dew point). After OLGA

inorganic impurities can easily be removed with aqueous scrubbers, tar is not a problem

anymore for operation of these scrubbers and does not contaminate the scrubber water.

Figure 2.5 Generic line up


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Dust removal with OLGA

Solid particles (dust) can be separated from the product gas upstream OLGA by a cyclone or

a hot gas filter (HGF). A hot gas filter will remove all particles but have the disadvantage that

it is a large and often an expensive system, especially for bigger gas capacities. When a

cyclone is used only the coarse particles are removed thus OLGA have to remove the

remaining fine particles. OLGA have proved to be capable of handling solids. An ESP is

integrated in the OLGA system to catch the aerosols which partly pass the first OLGA

column. In most projects the OLGA – ESP combination is more interesting that a HGF –

OLGA combination. Therefore this system is further chosen as basis of this paper. More

information of the alternative is available on request.

Aqueous scrubbers & condensers with OLGA

When gas is free of tar an aqueous scrubber column can be operated without problems. This

aqueous scrubber is normally used for:

1. Cooling the gas by quenching;

2. Further cooling the gas and remove the bulk of the water vapour by condensation;

3. Removing water soluble components like NH3, HCl, H2S, if applicable.


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Condensation

Tem

pera

ture

ºC

Dew points & process choices

TDP ± 350 ˚C

Water dew point ± 60 ˚C

Cooler

Particle separation

cyclone or

Hot Gas Filter

OLGA

Separation of:

tars & fine particles

Tar dew point < 10 ˚C

Absorption

Water Quench,

condenser & scrubber

(inorganics)

WDP ± 30 ºC

3 The OLGA process philosophy

Product gas produced by the gasifier contains solids (dust), tars and inorganic impurities

(depending on biomass feedstock). It is very important to consider the logical order for

cleaning the product gas. In principle mixing dust, tar and water must be avoided. In any case

it must be avoided to mix tar & water!

The philosophy of OLGA is all about dew point control. In the figure below the tar and water

dew points are shown, together with the logical process steps.

Figure 3.1 Dew points are important for equipment selection

Logical equipment with typical temperatures:

1. Product gas cooler; gasifier exit 700-900 ºC– OLGA inlet 380 ºC

2. Separation of solids; 380 ºC

- coarse solids by a cyclone (OLGA for fine solid aerosols)

- all solids by a hot gas filter

3. OLGA tar separation; inlet 380 ºC outlet 70-90 ºC (safe above water dew point)

4. Water condenser; 70-90 ºC to 30 ºC

5. Water scrubber; 30 ºC

Key philosophy:

OLGA operates above the water dew point, but decreases the tar dew point to a level under

the lowest process temperature. Tar & water are not mixed!


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Principle of OLGA

The tar removal principle of OLGA is based on a multiple stage scrubber in which the gas

is cleaned by special scrubbing oil. In the first section of OLGA (the Collector) the gas is

gently cooled down by the scrubbing oil. Heavy tar particles condense and are collected, after

which they are separated from the scrubbing oil and can be recycled to the gasifier, together

with a small bleed.

In the second stage (the Absorber / Stripper) lighter gaseous tars are absorbed by the

scrubbing oil. Both stages are presented as separate columns in underneath process sketch. In

the absorber column the scrubbing oil is saturated by the lighter tars. This saturated oil is

regenerated in a stripper. In case of an air blown gasifier hot air is used to strip the tars of the

scrubbing oil. This air loaded with light tars can be used as gasifying medium in the gasifier.

Hence, the stripper column design is not only based upon the tar removal capacity but also

upon the amount of air that can be used by the gasifier. All heavy and light tars can be

recycled to the gasifier where they are destructed and contribute to the energy efficiency. Tar

waste streams are efficiently recycled this way.

Figure 3.2 Simplified Process Flow Diagram of OLGA


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Summarised advantages of OLGA

The advantage of OLGA is that it forms a reliable and solid solution for the tar problem. The

advantages can be summarised as follows:

� No more tar related problems

- Increased system stability and availability

- Minimisation of waste water treatment costs

- No tar waste streams

� Better gas quality compared to a thermal tar cracker

� More reliable and less vulnerable than a catalytic tar cracker

� No waste water poisoning as with tar removal in an aqueous scrubber


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4 Performance of OLGA; 0.5 MW(t) duration test

The OLGA process has proved itself in many lab & pilot scale tests. The performance of

OLGA is presented in this paper by the results of the 0.5 MW(t) duration tests. The main goal

of these tests are to show that the OLGA process is stable and reliable. The gas quality should

be on specification for usage in a gas engine / gas turbine. The duration test is also used to

study the duration effects.

Figure 4.1 Simplified process scheme of the 0,5 MW(t) gasification system in Petten

Figure 4.2 & 4.3 500 kW pilot OLGA in compact skid

Above ready for transport in Dahlman production

Left lifted for installation on site ECN Petten


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4.1 Process temperatures & dynamic gas composition

The figures underneath show the process temperature and dynamic gas composition data

from the duration test. Both the process temperatures and the dynamic gas composition show

that the process has been very stable. Operator action was limited to a guard function. Process

control was automatic and stable.

Figure 4.4 Dynamic Gas composition

Figure 4.5 Process temperatures


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4.2 Gas composition & measured performance

The table underneath summarises the data from gas analyses. The gas composition can be

seen as typical for a CFB gasifier. NH3 and H2S concentrations are low as clean woodchips

were gasified. The total tar and naphthalene data show good efficiencies (99%), phenol was

removed completely (under detection level of measurements). The tar dew points were

reduced to levels well below target (<10°C)

Table 4.1 Typical gas composition at different locations

Raw product gas

After OLGA

Before booster

H2 vol% dry 7,2 7,3 7,4

CO vol% dry 17,4 17,1 17,4 CH4 vol% dry 4,6 4,5 4,6

CO2 vol% dry 15,5 15,5 15,5

C2H2 vol% dry 0,2 0,2 0,2

C2H4 vol% dry 2,0 1,9 1,9

C2H6 vol% dry 0,1 0,1 0,1

N2 vol% dry 51,3 51,6 52,0

H2O vol% dry 14,6 14,6 1,9

Benzene ppmv dry 3511 2415 2424 Toluene ppmv dry 448 156 158 NH3 ppmv

H2S ppmv 10 10 10

Total tar 1 mg/mn³ dry 16855 197 91

Naphtalene mg/mn³ dry 4023 38 35

Phenol mg/mn³ dry 386 < 2,5 < 2,5

Tar aerosols (incl. dust) mg/mn³ dry -- 10 < 5

Tar dewpoint (measured) ° C ≈ 350 -- -1 ± 1 Tar dewpoint (calculated) ° C ≈ 350 5 2 Tar dewpoint @ 2,5 bar (calculated) ° C -- 14 14 1accuracy ± 200 mg/mn³


20

4.3 Water condensate composition

Table 4.2 shows the water condensate composition. After OLGA a water condenser is

installed. The condensate analyses show some remaining BTX and naphthalene. BTX

removal is not a target of OLGA as these compounds do not form a problem for downstream

equipment. Further BTX is easily removed by the aqueous scrubber / stripper which is used

for NH3 removal. BTX and NH3 are recycled into the gasifier. In future tests OLGA

performance on naphthalene will be optimised to reach even better results.

Table 4.2 Tars in water after OLGA Benzene mg/l 49,9

Toluene mg/l 3,25

Ethyl benzene mg/l 0,012

m/p-Xylene mg/l 0,022

o-Xylene mg/l 0,024

Naphtalene mg/l 10


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4.4 Operating time of different components

The operation time of the different components is demonstrated in figure 4.6.

During start-up of the gasifier (combustion mode), the OLGA, NH3 scrubber and gas engine

are bypassed. The engine was deliberately started later in the test. Start-up of the engine is

with natural gas (NG) after which the engine was easily switched to product gas.

Figure 4.6 Operation time of different components

Availability of OLGA is more than 92%, most of the down time was caused by short stops

due to pipe fouling between the cyclone and the OLGA. As the test facilities are build up

over the last years and the set-up is frequently changed to accommodate different tests this

piping unfortunately is very complex and thus vulnerable for blockings. A good pipe

configuration and sufficiently traced piping should prevent this from happening in

commercial installations. Other stops were trivial; caused by a mechanical failure of a pump

and problems with fuel supply (temporarily crisis in beech wood pellets).

The OLGA proved to be reliable and never showed any kind of process defect. The system

worked smoothly. Frequent starts and stops should be prevented (cooling down and heating

up again). But the fact that the OLGA easily coped with starts and stops showed that the

system was very stable and very robust.

0 100 200 300 400 500 600 700 800 900

Stops

Gas engine

NH3 removal

OLGA

Cyclone

Cooler

Gasifier

Run time [hours]

Gasification Combustion

Product gas NG

Olga pumpFuel supplyPiping+valve


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4.5 Conclusion 0.5 MW(t) duration test

General performance

� The whole biomass gasification line, including OLGA is successfully operated for

more than 650 hours. The OLGA process showed to be stable and reliable;

availability of OLGA was 92%.

� The tar related problems in the piping before OLGA is an important lesson. Pipe

geometry, particularly between the cyclone and OLGA should be well considered

during plant layout design.

� The quench / condensation water after OLGA proved to be free of “problem tars.”

The small naphthalene and BTX concentrations that remain are not very difficult or

cost intensive to treat.

� Final inspection of the gas engine showed that it was completely clean; no carbon

deposition at cylinders and valves. The gas engine lube-oil quality was investigated

and shown to be similar to an engine operated at natural gas.

General gas specification goals:

� Efficiency on naphthalene is good (> 99%). But as naphthalene is a target component,

this efficiency will tried to be optimised in future tests.

� Efficiency on phenol is excellent, after OLGA phenol was not detectable.

� Aerosol concentration after OLGA (total of fine solids & entrained oil & condensed

tars) proved to be acceptable for gas engine applications

� Tar dew points were reduced to levels well below target (<10°C)


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5 1 MW(e) biomass gasification plant in France

The Commission of Energy Regulation (CRE) in France published a biomass energy tender

in 2005. As the electricity price in France is regulated it was not possible to develop biomass

energy projects until then. The main criteria for this tender were;

1. Primary energy source biomass or biogas

2. The project should be based on a combined heat and power concept (CHP)

3. Minimal output should be 12.5 MW(e) with a minimum of 4000 hrs/year

4. Electricity delivery should be guaranteed for 15 years.

The French company EBV (Energie Biomasse Viticole) was awarded with 6 biomass plants

with a total output of 81 MW(e). The biomass used in these plants is wood and wine distillery

residue. With this success the project partners started a phase were for each individual plant

the necessary permits are arranged, contracts are finalised and the plants feasibility is

considered in detail.

In front of the bigger commercial plants, EBV decided to build a 1 MW(e) demonstration

plant as a pilot for the 12.5 MW(e) plants. For the demonstration plant a special updraft

gasifier was selected. OLGA technology was selected as the preferred tar removal

technology.


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5.1 Plant description

The gasifier is operated at an under pressure. Product gas leaves the gasifier at a temperature

between 690 °C and 850 °C. Downstream the gasifier the product gas is cooled down in heat

exchanger to 350 °C. A cyclone downstream the cooler removes the coarse particles. After

the cyclone a booster is used to give the product gas a positive pressure. After the booster the

product gas can be transported to OLGA or directly to the flare. Downstream OLGA the

product enters a water condenser, which partly removes water cooling the gas to 25-50 °C.

Finally, the product gas can be transported to the flare or to the 1 MW(e) gas engine.

Figure 5.1 1 MW(e) OLGA shown in 3D parametric design Figure 5.2 Equipment skid in Dahlman production


25

5.2 Performance of OLGA 1 MW(e)

The Moissannes demonstration plant was used for two main test runs (fully measured) in

2006. In the first period the plant was running on a lower capacity caused by limited biomass

drying capacity. Dry biomass was made available for the second period. During both main

testing periods gas analysis were executed to verify OLGA performance.

1. First period measurements were executed with wood as gasification feedstock.

2. Second period measurements were executed with grape residue as gasification

feedstock.

1st period measurements; tar aerosols and dust concentration

Dust measurements were carried out at 350 °C upstream OLGA. The dust concentration

upstream OLGA fluctuates very much. Concentrations between 168 and 1459 mg/m3 were

measured. This large deviation in concentration might be explained by the fact that the

presence of a large amount of fines in the gas influences the accuracy of the used

measurement method. Another method is selected to perform the upstream dust

measurements in the future.

Downstream OLGA a series of tar aerosol & dust measurements was carried out with a filter

at 70 °C. The amount of dust or tar aerosols measured was below the detection limit of 25

mg/Nm3. As the filters remained white; the concentration of dust or tar aerosols must be far

below the 25 mg/Nm3.

1st period measurements; tar concentration and tar dew point

Depending on changes in gasifier conditions like gas residence time, temperature and fuel/air

ratio the total tar concentration of the raw product gas fluctuated between 1.2 and 5.5 g/Nm3.

The tar dew point at the inlet of OLGA exceeded 190 °C due to the presence of heavy tars.

After the first test it could be concluded that the OLGA process worked correctly. The

collector & ESP (heavy tar separation) was operating according to expectations. The absorber

/ stripper (light tar separation) also showed good performance, the key compounds phenol

(undetectable after OLGA) and naphthalene (99%) were sufficiently removed. The measured

tar concentration was decreased to values between 76 and 546 mg/Nm3. The tar dew point


26

was still too high caused by a start-up effect (evaporation of a fraction from the start-up

scrubbing oil). It was shown during the first day of start up that the concentration of this tar

condensate decreases rapidly. As soon as the oil in the collector is stable, the remaining

concentration of tar oil in the product gas will disappear and the tar dew point will be get

below 20 °C.

2nd period measurements; tar concentration and tar dew point

Measurements were executed with the plant running at full capacity. The total tar

concentration of the raw product gas was 11 g/Nm3, considerably higher as measured in the

first period (with wood and not at full capacity) The tar dew point at the inlet of OLGA was

measured as 180 °C due to the presence of heavy tars.

Again the Collector / ESP combination (heavy tars) was running stable and removed the

heavy tars satisfactory. The absorber / stripper combination (lighter tars) was also running

stable but surprisingly showed poor efficiency on e.g. naphthalene. Investigating this

unexpected result concluded in a much too low stripper air flow. Further investigation

showed that the stripper air fan could not reach it’s design flow due to a high back pressure at

the inlet of gasifier. Logically this problem occurred during operation at full capacity. Using

the OLGA simulation model we looked at the expected efficiency at the lower stripper air

flow capacity and reproduces the same efficiency figures. We can therefore say that also the

absorber / stripper loop is operating as expected, although the limited capacity of the stripper

air fan should and will be corrected.

During this test run the total energy balance of the plant was measured by an independent 3rd

party. Running at full capacity, the plant reached its target efficiency and output of 1 MW(e).


27

5.3 Test conclusions

Both for dust & heavy tars (collector & ESP) as well as the light tars (absorber & stripper)

OLGA performs according to its design. A new air fan with more power is installed to make

sure the absorber and stripper can also reach their design efficiencies at full capacity.

In general we can say that of-course some set-back’s were experienced during the

commissioning of this demonstration plant, but these were limited to trivial mechanical

problems which were time consuming. The demonstrated process – gasification, gas cleaning

and gas-engine – showed to work. Duration tests are planned to further optimise the plant

both technically and economically.


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5.4 Outlook

At this moment optimisations are realised and some maintenance is done. More optimisation

& duration tests are scheduled for the second half of 2007. During these tests more detailed

results will become available. In the same period a decisions will also be made to invest in

the first 12.5 MW(e) commercial plants.

Flare foto

Figure 5.3, 5.4 & 5.5

4MW(t) Gasification plant in France.

Please note the night shot; the bright blue flame from the

flare is a good indication for clean, tar free gas.


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Appendix A: Tar Classification system

According to the ECN definition, tar comprises all organic components having a molecular

weight higher than benzene. Benzene is not considered to be a tar. ECN uses a tar

classification system comprising six classes (see Table B.1). This classification system is in

particular developed to provide ‘easy’ insight in the general composition of tar. Trends are

easier recognised on the basis of these classes. However, for more specific problems or issues

the detailed data will remain necessary.

Class Type Examples 1 GC undetectable tars. Biomass fragments, heaviest tars

(pitch) 2 Heterocyclic compounds. These are components that

generally exhibit high water solubility. Phenol, cresol, quinoline, pyridine

3 Aromatic components. Light hydrocarbons, which are important from the point view of tar reaction pathways, but not in particular towards condensation and solubility.

Toluene, xylenes, ethylbenzene (excluding benzene)

4 Light poly aromatic hydrocarbons (2-3 rings PAHs). These components condense at relatively high concentrations and intermediate temperatures.

Naphthalene, indene, biphenyl, antracene

5 Heavy poly aromatic hydrocarbons (≥4-rings PAHs). These components condense at relatively high temperature at low concentrations.

Fluoranthene, pyrene, crysene

6 GC detectable, not identified compounds. Unknowns Table B.1: Tar classification system

From the practical viewpoint, the classification comprises only tar components that can be

measured. Classes 2 to 6 are sampled using the solid phase adsorption (SPA) method and

measured by gas chromatography (GC). Although class 6 tars are sampled and measured (a

peak is found in the chromatogram), it is unknown what the individual components are. In

principle components in this class belong to the other classes, but are here lumped to a single

concentration representing the ‘unknowns’. Class 1 represents the heavy tar fraction (roughly

≥7-ring PAHs). These components cannot be determined by the combination of SPA and GC.

The components are measured by weight and thus represent the gravimetric tars.


30

Appendix B: OLGA vs. other tar removal systems

Tars from biomass product gases can be removed with a thermal tar cracker, a catalytic tar

cracker or a physical process. The thermal and catalytic tar cracker are installed directly

downstream the gasifier and operate at high temperature. The physical processes like an

aqueous scrubber or OLGA are installed downstream a product gas cooler. The inlet

temperature of a tar cracker is typically 850°C and of a physical process 400°C.

Thermal tar cracking

A thermal tar cracker heats up the product gas to a temperature of 1200°C. At this

temperature the tars are removed almost completely leading to a very low tar concentration

(<100 mg/mn3) and tar dew point (<10°C). The disadvantage of this application of a thermal

cracker is the reduction in efficiency. To increase the temperature of the product gas a part of

the product gas is combusted with oxygen. Consequently, the system efficiency (biomass to

electricity) is reduced as well as the calorific value of the product gas. The reduction in

calorific value makes the application of the product gas from a direct air blown gasifier in a

gas engine difficult.

Catalytic tar cracking

A catalytic tar cracker does not heat up the product gas and thus eliminates the disadvantages

of a thermal cracker. In theory the tar removal efficiency can be complete. However, soot

formation and deactivation of the catalyst is a serious problem to be dealt with, resulting in

limitations in the process. At the moment, the tar concentration at the inlet of the cracker

should remain below 2 g/mn3 and the presence of alkali metals and sulphur should be

controlled. Several projects have shown that a catalytic tar cracker can be a vulnerable part of

the system. Bad tar removal by e.g. catalyst deactivation directly leads to heavy tar problems

downstream. In principle the tar removal efficiency is less compared with a thermal cracker

but good enough for the application of the product gas in a gas engine.

Tar removal by aqueous scrubbers

Aqueous tar removal systems cool down the product gas and remove the tars by

condensation. In most aqueous systems dust and tars are collected simultaneously. The

product gas is cooled down and aerosols of dust and tars are collected with a wet ESP


31

downstream. Some systems use a dry hot gas filter (HGF) upstream for dust removal instead

of a wet ESP. The HGF reduces the risk of fouling of the aqueous system with dust. The tar

dew point downstream an aqueous system is similar to or higher than the operating

temperature of the system. Therefore, the total tar content downstream an aqueous system can

exceed 1 g/mn3. To avoid tar condensation and fouling of piping the gas should not cool

down. In the aqueous scrubber system a tar/water problem is created. Mixing (heavy) tars

with water will lead to operational difficulties in the scrubber and huge maintenance costs.

The most important disadvantage is formed by waste water handling. Waste water handling is

often so expensive that the plants economical feasibility is at stake.

Tar removal by OLGA

Based on the problems with aqueous scrubbers ECN and Dahlman developed the oil based tar

removal system OLGA. In OLGA the tars are removed by condensation and by absorption.

The temperature remains above the water dew point to avoid mixing of dust and tar with

water. Due to the absorption step in OLGA the tar dew point is decreased far below the

operating temperature of OLGA, typically below 10°C. The total tar concentration is reduced

to 200 mg/mn3. Tars downstream OLGA are composed of light compounds like Xylene and

Indene. These compounds do not cause fouling problems in the downstream system. Phenols

are almost completely removed in OLGA to avoid the production of hazardous condense

water and expensive wastewater cleaning.

Test experience with OLGA & aqueous scrubbers

ECN operated and tested two aqueous systems and one oil based system, OLGA, downstream

the 100 kg/h (500 kWth) air blown circulating fluidized bed gasifier. The gasifier produces a

product gas with a tar load of 10 to 20 g/mn3 on dry basis, divided in 12% heavy tars, 84%

light tars, and 4% hetero-cyclic tars. The heavy tars are compounds with a molecular mass

between phenanthrene and coronene. The light tars are compounds with a molecular mass

between xylene and phenanthrene. The hetero-cyclic tars are compounds with an oxygen or

nitrogen atom in the molecular structure, like phenol and pyridine. These compounds have an

increased polarity and exhibit higher water solubility.


32

Figure B.1 compares the tar removal efficiency of the three tested gas cleaning systems. In

the wet scrubbing system (aqueous scrubber) the hetero-cyclic tars were removed for 80%,

the light tars for 60% and the heavy tars for only 50%. The gas was not on specification for a

gas engine. With the addition of a wet-ESP the heavy tars were almost completely removed

(99%) and the tar dew point decreased to 60°C. The product gas could be applied in a gas

engine, but the system suffered from wastewater problems. The OLGA removed the heavy

tars totally and the light as well as hetero-cyclic tars for 99%. The tar dew point was reduced

well below a temperature of 10°C. The water condensate did not contain phenols and the gas

could be applied in a gas engine.

Figure B.1 Tar removal efficiency OLGA technology compared to wet scrubbing

Conclusive you can say that the performance of the OLGA system is better than aqueous

systems. The catalytic tar cracker competes with OLGA. However, deactivation of the

catalyst is still a drawback of these systems and reduces the flexibility of the application.

40%

60%

80%

100%

tar

rem

ova

l

heavy tars 49% 99% 100%

light tars 62% 74% 99%

hetero-cyclic 79% 79% 99%

Dew point [°C] 180 60 10

Wet scrubbing Idem+ESP OLGA


33

References

[1] Boerrigter, H.; Rauch, R., Review of applications of gasses from biomass gasification,

ECN Biomass, Coal & Environmental Research, report ECN-RX-06-066, June 2006.

[2] Boerrigter, H.; van Paasen, S.V.B. ; Bergman, P.C.A. ; Könemann, H.W.J. ;

Emmen, R. ; Wijnands, A., “OLGA” Tar Removal Technology Proof of concept

(PoC) for application in integrated biomass gasification combined heat and power

(CHP) systems, ECN Biomass, Coal & Environmental Research, Dahlman Industrial

Group, report ECN-C—05-009, January 2005.

[3] Camozzi, D. ; Lucarno, R. ; Wolff, J., Sannazzaro Gasification Plant, project update

& start-up experience, Gasification Technologies 2006, Washington DC,

October 2006

[4] IEA: World energy outlook, ISBN 92-64-17140-1 – 1999, 225 p. (1999).

[5] Könemann H.W.J. ; Egas, F. ; van Paasen, S.V.B., Tar Removal in Biomass

Gasification Processes; Paper for ERTC Paris 2006, November 2006

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