Resultaten en bevindingen van project

Ultra low NOX combustion of “green”fuels in an industrial boiler

Dit rapport is onderdeel van de projectencatalogus energie-innovatie. Tussen 2005 en 2011 kregen ruim 1000 innovatieve onderzoeks- en praktijkprojecten subsidie. Ze delen hun resultaten en bevindingen, ter inspiratie voor nieuwe onderzoeks- en productideeën. De subsidies werden verleend door de energie-innovatieprogramma's Energie Onderzoek Subsidie (EOS) en Innovatie Agenda Energie (IAE).

Datum Juli 2011 Status Definitief Stork Thermeq B.V., e.a. in opdracht van Agentschap NL

Colofon

Projectnaam Ultra low NOX combustion of “green” fuels in an industrial boiler

Programma Energie Onderzoek Subsidie Regeling Korte Termijn Onderzoek Projectnummer KTOT01006 Contactpersoon Stork Thermeq B.V. Hoewel dit rapport met de grootst mogelijke zorg is samengesteld kan Agentschap NL geen enkele aansprakelijkheid aanvaarden voor eventuele fouten.

Project No. KTOT01006

Title Ultra low NOx combustion of "green"

fuels in an industrial boiler

Publishable

Date 8 July 2011

Prepared for AgentschapNL

i

Colophon

Authors:

E. Gansekoele (BTG, BTG Bioliquids)

S. Dijkstra (Stork Thermeq (STQ))

I. Engelsma (Stork Thermeq (STQ))

J. Kalivodová (ECN)

S. Leiser (ECN)

F. Verhoeff (ECN)

BTG biomass technology group BV

P.O.Box 835

7500 AE Enschede

The Netherlands

Tel. +31-53-4861186

Fax +31-53-4861180

www.btgworld.com

office@btgworld.com

ii

TABLE OF CONTENTS

1 INTRODUCTION 1

2 PRODUCTION OF PYROLYSIS OIL 3

2.1 PROCESS DESCRIPTION 3

2.2 MASS BALANCE 4

2.3 FILTER TESTING 4

2.3.1 Self Cleaned Scraper Filter 4

2.3.2 Centrifuge 5

2.3.3 Brush centrifuge 7

2.4 CONCLUSIONS 8

3 COMBUSTION OF PYROLYSIS OIL 9

3.1 FUELS 9

3.2 INSTALLATION 10

3.3 BURNER DESIGN 11

3.4 RESULTS 13

3.5 CONCLUSIONS AND RECOMMENDATIONS 14

4 EFFECTS ON FOULING, CORROSION AND EMISSIONS 16

4.1 FIELD TESTS 16

4.1.1 Mobile deposition probe 16

4.1.2 Dust sampling probe and cascade impactor 18

4.1.3 Sampling locations 19

4.1.4 Results and discussions 20

4.2 LAB SCALE COMBUSTION SIMULATOR TESTS 23

4.2.1 Lab scale Combustion Simulator (LCS) description 23

4.2.2 Used fuels 24

4.2.3 Fouling propensity and deposition tests 26

4.2.4 Composition of the deposits 28

4.3 CONCLUSION LCS TEST 30

5 GENERAL CONCLUSIONS 31

1

1 INTRODUCTION

The amount of renewable energy (heat and/ or power) must be increased to be able

to meet the Dutch and international demands.

Current commercial technologies for renewable energy based on biomass are for

example direct (co-)combustion of biomass and gasification of biomass.

There are two main disadvantages of the above mentioned technologies:

1. The heat and electricity are directly used at the location where the biomass is

available. The transport of biomass is limited, because biomass has a low

energy density and handling can be very complex.

2. Biomass contains a lot of contaminants such as ash. Therefore not all

biomass can be used in boilers.

The above mentioned technologies are therefore only suitable for local biomass feed

stocks and the flexibility towards feed stocks is also limited.

Pyrolysis is a technology which can overcome the above mentioned issues. Almost

ash free oil is produced at the location where biomass is available. The energy

density of the oil is 5 to 10 times larger than the density of biomass. The oil can easily

be transported to a location where energy is required.

A possible application for pyrolysis oil is the use of the oil in existing boilers. The

potential is large in for example district heating plants and peak boilers.

Fast pyrolysis technology is developed at BTG. BTG has successfully demonstrated

the technology in Malaysia with a very difficult type of feed stock (Empty Fruit Bunch).

Because of the success BTG decided to start a spin-off company to commercialise

the pyrolysis technology BTG Bioliquids, (BTG-BTL). BTG-BTL will start the

construction of a 5 tons/h plant in the end of 2011. The unit has a capacity of 25,000

tons of pyrolysis oil per year. On the short term pyrolysis oil will become available

with 10,000 ton/year, on the large term (2 – 5 years) seems that enough oil becomes

available to penetrate a part of the energy market.

Therefore the co-firing of pyrolysis oil in existing boilers can substantially fasten the

introduction of renewable energy. The development, improvement and market

introduction of a suitable burner technology offers enormous possibilities to generate

renewable PJ’s and avoiding CO2 emissions in the Netherlands.

Research done in a 100 kW boiler has proved that diesel and natural gas can be

replaced with pyrolysis oil [BSE_DEN/040710/BNE, consortium of ECN, BTG, STQ)].

The research also shows that the amount of N in the oil is reduced with a factor of 2

to 3 in comparison to the original biomass. The presence of ash has a reduction

factor of more than 10-100. Pyrolysis of biomass results in a “cleaner” product in

comparison to the original biomass. Also pyrolysis oil is cleaner in comparison to

heavy fuel oil.

2

However in comparison to diesel and natural gas, pyrolysis oil contains more nitrogen

and pyrolysis oil contains a minimum amount of ash. Pyrolysis oil combustion can

give higher NOx and dust emissions and more fouling in comparison to diesel and

natural gas combustion.

The overall objective is to show successful combustion of pyrolysis oil with

acceptable NOx and dust emissions and limited effect on fouling of the boiler. The

main goal is to realise an industrial, simple application for pyrolysis oil in order to

fasten the implementation of pyrolysis and to increase sustainability of the energy

supply.

Specific objectives are:

1. The development of an innovative low NOx burner system;

2. Gain knowledge on the effect of fouling and corrosion during combustion of

pyrolysis oil;

3. Gain knowledge on the amount of dust emissions during combustion of

pyrolysis oil.

The combustion of three types of pyrolysis oils are compared with Heavy Fuel Oil

combustion. The three types of pyrolysis oils are:

1. Unfiltered EFB oil, which has a relatively high ash content with a low ash

melting temperature and high nitrogen content,

2. Filtered EFB oil, which contains less ash in comparison to #1 and has a high

nitrogen content,

3. Pine oil, which is clean pyrolysis oil. This oil will be produced in the

commercial demonstration plant.

The “contaminated” EFB oils are tested to be able to see the effect on fouling and

emissions with different types of oils.

Fouling and corrosion effects, and emissions are analysed during the tests.

The report is divided in three main chapters such as:

1. Production of pyrolysis oil,

2. Combustion of pyrolysis oil,

3. Emissions, fouling and corrosion effects during combustion of pyrolysis oil.

The individual reports from BTG, STQ and ECN are given in the annexes. More

details are given in the individual reports.

3

2 PRODUCTION OF PYROLYSIS OIL

The oil is produced in the 200 kg/h pilot plant of BTG. Filter and centrifuge tests are

performed in order to remove particles from the oil. EFB oil was already available in

the framework of this project. Part of the EFB oil is filtered.

2.1 Process description

Figure 2.1 shows schematically the pyrolysis production process.

18

1

3

4

6

7

9

11

12

15

8

13

5

REACTOR

CHAR COMBUSTOR

CONDENSER

14

PLATE HEAT EXCHANGER

COOLING TOWER

FLUE GASPYROLYSIS GAS

AIR

AIR

2

ASH

AIR FAN

AIR FAN

BIO-OIL PUMP

COOLING WATER

PUMP

WATER 10

19

NITROGEN

BIO-OIL

Biomass

AFTER BURNER

PROPANE 17

AIR

20 FLUEGAS

Figure 2.1: Pyrolysis process

Dried wood is transported to the reactor via a lock hopper system and a number of

screw conveyors

Biomass is mixed intensely with hot sand from 500 to 600 °C inside the reactor. Fast

heating of biomass takes place and pyrolysis takes place. Char, condensable

vapours and incondensable gases are formed during pyrolysis. The gases and

vapour will leave the reactor and are transported via two cyclones to the condenser.

The condensable vapours are condensed to pyrolysis oil by means of direct cooling

with sprayed pyrolysis oil while the incondensable gases are leaving the condenser

on the top.

The produced char and the hot sand are leaving the reactor on the bottom and will be

transported by means of pneumatic transport to the combustor. The char is

combusted in the combustor resulting in heating of the sand. However the amount of

heat produced in the combustor is so high, that it is necessary to cool down the sand.

Cooling down in the pilot plant is done by means of direct injection of cooling water.

Of course this way of operation will not be done in a commercial installation. In that

4

case a heat recovery boiler is installed after the combustor and steam is produced in

cooling coils to cool down the sand to desired temperature. The produced can be

applied for drying, steam turbine and/ or can be sold.

The flue gases from the combustor enter the after burner which is fired with

incondensable pyrolysis gases from the condenser.

2.2 Mass balance

A global mass balance is given in table 2.1.

IN

[kg/h] OUT [kg/h]

Yield [wt.%]

Biomass 121.4

Char 25.5 21.0

Ash 0.1 0.0

Pyrolysis gases 12.6 10.3

Bio-oil 83.1 68.5

Losses 0.2 0.2

Total 121.4 100.0

Table 2.1: Global mass balance for pyrolysis of pine wood

2.3 Filter testing

Three types of filters were tested, such as scraper filter, centrifuge and brush

centrifuge.

2.3.1 Self Cleaned Scraper Filter

Scraper filter type EF803 has been applied. The filter contains a scraper device in the

form of a screw (see figure 2.2).

Figure 2.2a: Filter element with scraper Figure 2.2b: Scraper

5

2.3.2 Centrifuge

Two types of centrifuges have been tested such as a batch-wise centrifuge (OTC2

Gea Westfalia Separator) and a self cleaned type (OSC4 Gea Westfalia Separator).

The centrifuges were connected to the pyrolysis pilot plant during the tests.

The principle of a separator (figure 2.3) is as follows:

A disc stack centrifuge separates solids and one or two liquid phases from each other

in a continuous process. The denser solids are forced outwards by centrifugal forces

while the less dense liquid phases form inner concentric layers. By inserting special

plates (disc stack) where the liquid phases meet, maximum separation efficiency is

ensured. The solids can be removed manually, intermittently or continuously

depending on the specific application. The clarified liquids overflow in the outlet area

on top of the bowl into separate chambers sealed off from each other to prevent cross

contamination.

As with disk separators with solid-wall bowl, the disk separators with self-cleaning

bowl are equipped with a stack of conical disks to create a large equivalent

clarification area within a relatively small bowl volume. These separators are likewise

available as versions for clarification and separation. Separators with self-cleaning

bowl are able to periodically discharge the separated solids at full speed. For this

purpose, several ports are spaced evenly around the bowl periphery. These ports are

opened and closed by means of a movable sliding piston located in the bowl bottom.

The opening mechanism is actuated hydraulically. Water is normally used as a

control medium; in special cases, low-viscous organic fluids can also be used.

6

Figure 2.3 a: Principle drawing of a batch separator (www.westfalia-separator.com)

Figure 2.3 b: Principle of self cleaning (www.westfalia-separator.com)

7

2.3.3 Brush centrifuge

Figure 2.5 shows a principle drawing of the centrifuge. A brush is assembled in a

drum. By rotating the brush and drum with the same speed an unique way of

separation is possible.

The liquid with solids is supplied at the bottom of the drum. The mixture flows

upwards through the hairs of the brush. The heavier particles are forced outwards by

centrifugal forces. A dry cake is formed. The clean liquid leaves on the top of the

drum. The cake grows from outside to inside and stays inside the hairs of the brush.

When the brush is filled, the system must be cleaned. The supply of “dirty” liquid is

stopped. The drum rotation is stopped. And the liquid inside the drum is drained.

When the drum rotation is stopped, the outside cylinder is moved upwards. When the

cylinder reached its upper position, the brush rotation is started again to sling off the

cake from the brush. The cake is collected on a splash screen. A scraper removes

the cake from the screen.

Figure 2.5: Principe

Figure 2.6: Dirty brush

8

2.4 Conclusions

When comparing the three technologies, the self-cleaned scraper filter system has

the lowest separation efficiency and the highest product loss.

Ash content [wt.%]

Oil in Oil out Oil loss

[wt.%]

Scraper filter 40

um

0.18 0.08 14

Centrifuge 0.15 0.03 3

Brush centrifuge 0.15 0.02 < 0.5 estimation

The separation efficiency of the two separator systems is more and less similar, but

the yield loss is lower for the brush centrifuge. However the supplier of the Westfalia

Separator claims that by the selection of the right equipment only a yield loss of 0.1

wt.% will be obtained.

A disadvantage of the brush centrifuge is that there are running only 2 commercial

machines in the world while the “standard” separator is common practice for a high

range of applications.

9

3 COMBUSTION OF PYROLYSIS OIL

To meet the growing industrial demands of biomass combustion at industrial scale,

the combustion performance of two pyrolysis oils, namely the pine oil and the empty

fruit bunch (EFB) oil is examined at the Stork test installation KMS ketel 9. A standard

Stork Double Register Burner (DRB) is used for this purpose (figures 3.3 – 3.4).

Combustion phenomena like flame-behavior, minimum air requirement, fouling

phenomena and emissions are examined.

The pine and EFB oils are supplied by Biomass Technology Group (BTG). The

combustion of the oil is monitored by emission analysis, visual inspection at the view

ports of the boiler installation, dust measurements and by deposit measurements.

Dust and deposits measurements are done by ECN. As a reference measurement

heavy fuel oil (HFO) delivered by Total Nederland was fired on the first day, after

which pine, filtered EFB and unfiltered EFB was fired.

3.1 Fuels

The following fuels are used during the tests

Natural gas (NG)

HFO

Pine oil

Filtered EFB

Unfiltered EFB

The oils have the following compositions

HFO Pine wood EFB

Density (kg/m³) 1050 1150 1150

LHV (MJ/kgar) 40.36 14.86 20.11

Air requirement (dry,

calculated) (kg/kg)

13.79 6.28 9.99

Composition Mass% - dry (wet) Mass% - dry

(wet)

Mass% - dry

(wet)

C (Carbon) 87.7 (87.7) 41.9 (41.9) 64.75 (64.8)

H (Hydrogen) 10.3 (10.22) 5.46 (7.9) 8.50 (9.4)

S (Sulphur) 0.94 (0.94) 0.01 (0.01) 0.08 (0.08)

N (Nitrogen) 0.42 (0.42) 0.1 (0.1) 0.4 (0.4)

O (Oxygen) 0.02 (0.64) 30.53 (50.09) 18.27 (25.4)

Variable

Moisture content 0.7% 22% 8%

Minerals (Si, Al, Fe,

Ca, Mg, K, Ti, etc)

(g/kg)

10.0 0.27 2.25

Vanadium (mg/kg) 26.3 0 0

Table 3.1 The oil properties and compositions. The LHV is calculated with the measured HHV done by ECN.

10

The most important difference between filtered and unfiltered EFB is a visual

difference, namely the phase separation. The filtered EFB has lesser visual phase

separation than the unfiltered EFB. Except the visual difference chemical analysis of

the filtered and unfiltered EFB did not show any significant differences.

3.2 Installation

The test installation KMS ketel 9 is located at Stork Thermeq in Hengelo,

Netherlands. It is a 9 MW boiler installation in which two different oils and natural gas

burner can be tested.

Figure 3.1 Boiler installation KMS Ketel 9.

11

Figure 3.2 The oil skid.

3.3 Burner design

The used burner is an off-the shelf Stork Low NOx Double Register gas- and

oilburner. It is an air staged burner with an oil gun in the center containing an

optimized Y-jet steam assisted atomizer, surrounded by eight low NOx gas spuds.

The flame stabilizes on an impeller, which is cooled by the combustion air.

The typical feature of the DRB burner is the staged supply of combustion air. Part of

the air is fed to the centre of the burner throat, while the rest of the air is skirting the

flame like a peel. Both air ducts are supplied with a register, which can induce a swirl

at the air flow. The swirl can be set with a handle at the burner plate. During the test

both registers are set at 27.5°.

12

Figure 3.3 (left) Photograph of a Double Register Burner. (right) Schematic drawing

of the main components of the burner.

Figure 3.4 The built-in DRB burner with the original impeller.

13

3.4 Results

The following results are achieved during the tests.

Oil

Date

Time interval

[hh:mm]

Power

[MW]

Oil flow

[kg/h]

O2

[vol% dry]

CO

[ppmvd]

NOx

[mg/m03@3%O2]

HFO 8th April 10:51-15:30 4.7 411 4.0 <5 550

Pine 13th April 10:25-11:50 2.6 606 3.0 16-383 133

Filtered

EFB

14th April 17:55-19:00 1.2 213 3.9 <50 395

Unfiltered

EFB

15th April 13:30-12:12 1.6 288 5.6 <5 844

Oil Oil temperature

[°C]

Air temperature LUVO

[°C]

Atomizing steam temperature

[°C]

HFO 97 95 215

Pine 60 95 210

Filtered EFB 70 100 225

Unfiltered EFB 67 100 227

Table 3.2 The results of the test. The pyrolysis oils were fired with a small natural gas

pilot flame of 0.6 MW

Figure 1 The most important process values for the used fuels.

Of the pyrolysis oils, pine was fired with the higher load, had the lowest NOx emission

and the lowest oxygen excess. The NOx emission of pine is significantly lower than

the NOx emissions of HFO, namely 133 mg/m03@3%O2 versus 550 mg/m0

3@3%O2,

respectively. This is probably due to the moisture content difference of pine and HFO,

namely 22% versus 0.7%, respectively. Another reason for the NOx reduction is the

nitrogen in the fuel of 0.1 vs 0.42 for HFO. Reduction in fuel nitrogen reduces the fuel

NOx formation.

14

The high moisture content reduces the adiabatic flame temperature. Another reason

for a low adiabatic temperature is the high oxygen content. This reduces the carbon

content, thus lowers the LHV and heat output. The adiabatic flame temperature of

HFO is 2180 °C at stoichiometric conditions and for pine 1649 °C at stoichiometric

conditions. Lower flame temperatures reduce the thermal NOx production, thus

reducing the NOx emission.

The air requirement for CO-free combustion is higher than for HFO. The oxygen

excess is namely 3.0 vol% dry. Air requirements for CO-free combustion increases

with an inhomogeneous flame and longer flame. The CO to CO2 reaction stops when

there is a oxygen shortage locally.

The NOx emission of filtered EFB is lower than the NOx emissions of HFO but higher

than the emissions of pine, namely 395 mg/m03@3%O2. This is probably due to the

moisture content difference of EFB and HFO, namely 8% versus 0.7%, respectively.

This high moisture content reduces the adiabatic flame temperature. Another reason

for a low adiabatic temperature is the higher oxygen wt%. This reduces the carbon

wt%, thus lowers the LHV and heat output. The adiabatic flame temperature of HFO

is 2180 °C at stoichiometric conditions and for EFB 1764 °C at stoichiometric

conditions. Lower flame temperatures reduce the thermal NOx production, thus

reducing the NOx emission.

The air requirement for CO-free combustion is higher than for HFO. The oxygen

excess is namely 3.9 vol% dry. Air requirements for CO-free combustion increases

with an inhomogeneous flame and longer flame. The CO to CO2 reaction stops when

there is an oxygen shortage locally.

The combustion of unfiltered EFB became more stable with increasing atomizing

steam pressure. This pressure was increased to 3.5 bar pressure difference with the

oil pressure, instead of the standard 1.5 bar with the other oils.

This inhomogeneous combustion is probably also the reason for the high NOx-

emission. The air requirement for CO-free combustion is also a lot higher than for

HFO, pine and filtered EFB. The oxygen excess is namely 5.6 vol% dry.

The air requirement for CO-free combustion is higher than for HFO. The oxygen

excess is namely 5.6 vol% dry. Air requirements for CO-free combustion increases

with an inhomogeneous flame and longer flame. The CO to CO2 reaction stops when

there is an oxygen shortage locally.

3.5 Conclusions and recommendations

The results of the tests conclude the following

Combustion of pyrolysis oil is successful with an off the shelf Stork Double

Register Burner in an industrial boiler.

The Pyrolysis oils were fired between 1.2 and 2.6 MW.

15

To stabilize the pyrolysis flame a small natural gas pilot flame between 19-

33% of the total load was needed. The reason for requiring a gas pilot is the

moisture content, which cools the flame at the stabilization point and makes

ignition difficult.

The flame of the pyrolysis oils stabilizes at a larger distance from the impeller

than the HFO flame, which is an indication of ignition issues.

Changing the holepattern of the atomizer tip and increasing the impeller does

not change the stability of flame significantly. This indicates that the ignition

and stabilization issue might not be an atomization or flow recirculation issue.

The most likely cause is the moisture content of the pyrolysis oils.

Unfiltered EFB clogs the atomizer gun. A sticky substance blocks the holes in

the atomizer gun. This is probably due to the sedimentation of this oil. Visual

inspection of the oil shows a phase separation.

The NOx emissions of filtered EFB and pine wood oil are lower than the

emissions of HFO. One reason for this reduction is the lower load at which

the pyrolysis oils were fired. The pyrolysis oils could not be fired at the load at

which HFO was fired, which was 4.7 MW, but this load difference is not

sufficient to explain the difference in NOx emissions in full. Another reason is

the lower adiabatic flame temperature, due to the moisture content and the

lower LHV value. Also pine wood oil has a lower nitrogen content in the fuel,

which explains the lowest NOx emission value obtained with this fuel

The unfiltered EFB has the highest NOx emission. The reason is phase

separation and sedimentation which causes an unstable combustion. The

required air for CO-free combustion also increased and with increasing air

the NOx emission also increases, due to the dilution correction of oxygen in

the flue gas.

The air requirement for CO-free combustion for the pyrolysis oils was higher

than that for HFO. One probable reason is the turbulent flow, due to

inhomogeneous combustion and flame length difference. The pyrolysis oil

flame was less homogeneous than the HFO flame, which means that locally

an oxygen shortage can appear. This increases the likelihood that the CO to

CO2 reaction stops. Increasing air excess decreases the possibility that an

oxygen shortage can occur locally.

The following points for future tests are recommended

Using preheated combustion air would probably increase the stability and

ignition point because the combustion air heat can be used to vaporize the

moisture content faster.

Handling unstable pyrolysis oil by storing the oil in a traceable or preheated

storage unit at temperature of 55°C. Preheating the oil decreases the

viscosity and makes the oil more homogeneous, due to better atomization.

Stable oil, which is not phase separated can be heated just before the burner.

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4 EFFECTS ON FOULING, CORROSION AND EMISSIONS

Replacing HFO with bio-oils is not a straight forward process and many technical

challenges during bio-oil combustion can occur (i.e. unstable combustion with soot

formation, burner dripping, flame stabilization, oil filter clogging and unstable feeding).

Generally, the most significant reason for these challenges lays in the properties of

the bio-oil. The crucial parameters for each combusted liquid bio fuel are viscosity,

water content, ash content and elemental composition of the fuel. Viscosity together

with water content plays an important role in the oil handling to the burner, the flame

stability and therefore the combustion performance. Ash is a solid contamination that

can deposit and cause localized overheating of metal surfaces. Excessive amounts of

ash may result in abrasive wear of the burner. Depending on the chemical

composition of the deposited ash not only slagging and fouling problems of heat

exchangers might occur, but also high temperature corrosion. The combination of

vanadium and sodium in HFO is known for being responsible for hot corrosion. But

also elements in bio-fuels originating from biomass (i.e. potassium, chlorine) have a

negative effect on fouling and corrosion propensities of the boiler materials. ECN

performed several measurements during the campaign. In this report the results with

respect to ash formation and particulate emissions are presented.

To further simulate and characterize the behaviour of the heavy fuel oil and pyrolysis

oil ash with respect to slagging, fouling and corrosion propensities during combustion,

the ECN Lab-scale Combustion Simulator (LCS) was used. Standard low NOx

combustion conditions, i.e. utilizing an air staged flame, were applied during the

experimental study. All of the collected ash deposit samples, as well as raw fuel were

subjected to standard chemical and physical analysis. The applied analytical

techniques include SEM/EDX and ICP analyses. The physical characterization

consists of particle size determination. In this report both the test series in the Stork

boiler and the LCS will be discussed.

4.1 Field tests

The following measuring probes were used during the field test campaign: the mobile

deposition probe to investigate the fly ash deposition on boiler tubes and the impactor

probe to determine the particulate emissions from the stack. Both probes are

described in this section.

4.1.1 Mobile deposition probe

A mobile deposition probe with replaceable deposition substrates was applied. In

Figure 4.1 a photograph of the probe is given, in which the individual measuring

instruments in the probe are indicated with an arrow.

17

Figure 4.1 Mobile deposition probe, the work-end of the probe

To withstand the high temperatures in a boiler, the probe consists of a water-cooled

lance, manufactured from high-grade stainless steel and therefore well-suited for

sampling in high temperature, high corrosive environments. A sketch of the mobile

ash deposition probe is given in figure 4.2.

Furnace/nozzles side (work-end) Operator/control side

Figure 4.2 Schematic drawing of the mobile deposition probe

The deposition coupon consists of a temperature controlled tube-shaped piece on

which fly ash can deposit. The coupon is detachable and after metallographic

preparation it can be further investigated by means of SEM/EDX.

Besides deposit sampling, the influence of the deposit on the heat transfer can be

investigated in real time by means of heat flux sensors placed close to the deposit

coupon. For the on-line measurement of the heat transfer in all four axial directions,

four heat flux/surface and temperature sensors are used. Those thermocouples are

also controlling very precisely the wall temperature of the deposition coupon.

The flue gas temperature can be independently measured by a retractable

thermocouple placed close to the deposition coupon.

ash sampling inlet

heatflux sensor

deposition coupon

thermocouple

18

The ash sampling system in the probe allows collecting of representative boiler fly ash

samples from the flue gas stream. Flue gas, together with the fly ash is isokinetically

sucked out of the boiler via a bended tube with the opening in the direction of the flue

gas stream. To prevent condensation in the sampling tube the temperature of the flue

gas sample is kept above the condensation temperature at all times. The dust is

separated from the flue gas sample stream by means of a small cyclone and a filter.

4.1.2 Dust sampling probe and cascade impactor

A mobile dust sampling probe equipped with a flat filter and/or cascade impactor has

been used during the particulate sampling, see figure 4.3.

Figure 4.3 Mobile dust sampling probe

Cascade Impactor

The particle size distribution and the particulate emissions with the different fuel oils

have been investigated with the cascade impactor. A general layout with a number of

stages defining the PSD-resolution is given in figure 4.4.

Figure 4.4 Photo of a PILAT MARK V cascade impactor

Configuration

19

To ensure the best possible separation of the particle size fractions, it is advisable to

use as much jet stages as possible, in combination with a pump rate as high as

possible under the circumstances. Generally, the MARK III/V impactors can be

applied within the pump rate range of 0.1 to 1.0 acfpm (atmospheric cubic foot per

minute), 2.83 – 28.3 Nl/min. Naturally, isokinetic sampling must be performed. It

means an appropriate combination of nozzle size and pump rate should be verified

prior to the sampling. Preferred are large nozzles, in order to maximise the inlet area

of the impactor. The sampling time for the impactor measurements has been

calculated in line with the maximum load of the impactor, which should be kept at less

than 2.0 grams.

Precutter/right angle adapter

In order to allow sampling at high dust conditions and perpendicularly to the gas

stream, the so-called pre-cutter/right angle adaptor must be applied, which is

depicted in figure 4.4, together with the appropriate nozzles. The pre-cutter was

placed on top of the impactor by means of the ½”/3/4” adapter.

4.1.3 Sampling locations

Two sampling locations for the ECN probes were selected. The first location (referred

to as location 1), for the ash deposition measurements, was located in the top of the

boiler after the superheater bank, as shown in figure 4.5, the second location (referred

to as location 2), for the particulate emissions measurements, was located in the

ductwork between the induced draft fan and the stack.

Figure 4.5 Schematic view of STORK boiler 9 with measuring points

The furnace is schematically shown in figure 4.5. On the left, the burner is located and

heat exchanger surfaces are indicated.

20

The deposition probe accessed the boiler horizontally as can be seen in figure 4.6 a).

The impactor probe accessed the flue gas duct horizontally as well, as shown in

figure 4.6 b).

Figure 4.6 ECN sampling location Stork boiler Hengelo; a) location 1- in the boiler; b)

location 2-before the stack

4.1.4 Results and discussions

4.1.4.1 Chemical characterisation of the fuels

The following fuels were used during the tests: natural gas, HFO (heavy Fuel Oil),

Pine oil, filtered EFB oil (F) and unfiltered EFB oil (UF).

The liquid fuels were analysed according to standard European norms at ECN. The

chemical compositions of all liquid fuels combusted during the campaign are given in

table 4.1.

a) b)

21

Table 4.1 Chemical composition of the liquid fuels

Fuel HFO Pine Oil EFB Oil (F) EFB Oil (UF) Test method

HHV [MJ/kg] 42.625 16.6 22.18 24.133 NEN-ISO 1928

Ash content wt.% d.l. d.l. 0.27 0.31 ISO 1171

Moisture content wt.% 0.17 27.00 23.00 21.00 EN ISO 12937

volatile matter wt.% 90.10 94.60 88.50 88.00 ISO 562

C wt.%. 85.67 40.00 53.65 56.04 NEN-ISO 17247

H wt.% 9.70 7.40 6.70 6.10 NEN-ISO 17247

N wt.% 1.23 0.20 2.03 2.11 NEN-ISO 17247

O wt.% 0.34 50.00 36.85 33.40 NEN-ISO 17247

Elements

Si mg/kg db 164 34 326 513 NPR 6425

Al mg/kg db 118 55 26 28 NPR 6425

Fe mg/kg db 32 11 55 50 NPR 6425

Ca mg/kg db 49 32 86 91 NPR 6425

Mg mg/kg db 6 8 47 40 NPR 6425

K mg/kg db 109 d.l. 609 420 NPR 6425

Ti mg/kg db 5 9 d.l. d.l. NPR 6425

P mg/kg db 21 d.l. 43 42 NPR 6425

S mg/kg db 9440 83 814 795 NPR 6425

Cl mg/kg db 69 38 235 269 NEN-EN-ISO 10304-1

Na mg/kg db 13.97 d.l. d.l. d.l. NPR 6425

Ba mg/kg db 3.95 d.l. d.l. d.l. NPR 6425

Ni mg/kg db 25.14 d.l. d.l. d.l. NPR 6425

Li mg/kg db 0.03 d.l. 12.39 d.l. NPR 6425

Sr mg/kg db 0.84 d.l. d.l. d.l. NPR 6425

V mg/kg db 26.30 d.l. d.l. d.l. NPR 6425

Zn mg/kg db 4.73 d.l. d.l. d.l. NPR 6425

Ba mg/kg db 3.95 d.l. d.l. d.l. NPR 6425

Mn mg/kg db d.l. d.l. d.l. d.l. NPR 6425

4.1.4.2 Fouling effects

Fouling propensities of the four liquid fuels has been studied. From the trend of the

heat flux measurement, it can be concluded that the combustion of Pine Oil will not

bring significant problems with fouling of the heat exchanging area. Further it can be

concluded that the highest fouling propensity was recorded during the combustion of

unfiltered EFB oil at a steam tube temperature of 216°C, followed by filtered EFB oil

and HFO.

22

Chemical compositions of the deposited ashes in the boiler area strengthen the

conclusions with respect to fouling results. While the main ash forming elements in

the deposit when Pine oil was fired are alumina-silicates accompanied with calcium,

the main elements detected in ash after EFB oil combustion are potassium silicates

with elevated concentration of sulphur and chlorine. An alkali-rich bio-oil, i.e. EFB,

exhibits a clearly higher fouling potential, while woody bio-oil is less troublesome.

Nonetheless the formed deposits have not shown in any case extensive sintering nor

melt formation, which predicts that deposits may easily be removed by conventional

soot-blowing techniques.

4.1.4.3 Dust emissions

The results of the dust emission are summarized in table 4.2.

Table 4.2 Results of the dust emissions measurements in the stack of STQ boiler 9

Day Time FUEL Total dust Total dust

[h]

[mg/Nm3] @ 6% O2 [mg/Nm

3] @ 3% O2

08/04/2010 12:15 HFO 25 30

13/04/2010 10:02 Pine oil 36 42

10:39

13 20

11:23

10 13

14/04/2010 14:51 EFB oil (F) 66 80

15/04/2010 11:27 EFB oil (UF) 66 79

13:42

59 71

Standard emission limit for dust; NeR 20

Apart from two results for pine oil combustion, all other results are surpassing the

allowed Dutch emission limit. The standard emission limit for dust in The Netherlands

is 20 mg/m3. As expected the highest dust emissions were monitored during EFB oil

combustion. The concentration was three time higher than the emission limit. The

lowest value was obtained during the HFO combustion and Pine Oil combustion

when stable combustion is maintained. This fact is mainly due to the ash content in

the original liquid fuel and stability of the flame during combustion.

According to the chemical composition of the fly ash there is a difference between

filtered and unfiltered EFB oil in silica content compared to the original ash in the fuel.

The particle size distribution characteristic of filtered EFB oil combustion shows

bimodal particle size distributions; the fine particle classes consist of potassium,

sulphur and chlorine while studied HFO ash consists mainly of a fine fraction,

containing (V, Ni, Zn, Cr). Most of the trace elements are partially or fully vaporised

during combustion. It is generally known that the fine particles (dp < 2.5 µm) are more

harmful to health than larger ones. Due to dust emission limits, a boiler installation

producing electricity from firing bio-oil might need a dust removing facility, which

removes also the finest particles.

23

4.2 Lab scale combustion simulator tests

To further simulate and characterize the behaviour of the heavy fuel oil and pyrolysis

oil ash with respect to slagging, fouling and corrosion propensities during combustion,

the ECN Lab-scale Combustion Simulator (LCS) was used. Since the LCS facility

does not allow for combustion of liquid fuels (without major modifications) a similar

approach as in the first pyrolysis oil project (called BOBO1) was chosen: ash mixtures

were applied in order to simulate the ash composition of the studied pyrolysis oil and

heavy fuel oil. These ash blends were prepared from raw biomass (pine wood), rather

than from the oil, by gradually ashing it, while keeping the temperature under 400 °C,

to avoid volatilization of inorganics. The latter approach was chosen because ECNs

lab facilities do not allow ashing of oils in a controlled manner. Furthermore, the

ashing of oil would have taken substantially more time due to the low ash content of

the oils. The composition of the ash obtained in abovementioned way was compared

with the composition of the original oil ash and, where necessary, corrected for

missing components by adding an adequate amount of relevant minerals.

Standard low NOx combustion conditions, i.e. utilizing an air staged flame, were

applied during the experimental study. All of the collected ash deposit samples, as

well as raw fuel were subjected to standard analysis. The applied analytical

techniques include SEM/EDX and ICP analyses.

4.2.1 Lab scale Combustion Simulator (LCS) description

The ECN Lab-scale Combustion Simulator (LCS), depicted schematically in figure

4.7, is a flexible facility for the characterization of solid fuel behaviour under typical

combustion and entrained flow gasification conditions.

Legend

I Devolatilisation zone

II Combustion zone

1 Solid fuel feed

2 Multi-stage flat flame gas burner

3 Inner burner

4 Outer burner

5 Shield gas ring

6 Reactor tube

7 Optical access

Figure 4.7 Staged flat flame gas burner and reactor (drop) tube in ECN

combustion simulator

oxygen

gas

+

particles

gas

temperaturehigh

high

I

II

1

2344 5

6

7

24

The facility comprises a drop tube reactor together with a primary/secondary gas

burner to simulate a flame/flue/syngas environment in which the conversion

behaviour of fuel particles can be studied as a function of time.

For fouling tests, a Horizontal Deposition Probe (HDP) was utilised as shown in figure

4.8.

Figure 4.8 Photo and schematics of the LCS heat flux/deposition probe

This probe, located at the exit of the furnace of the LCS tests rig, is designed to mimic

the gas/particle flow around a single boiler tube. It consists of a ring-shaped

quadruple heat-flux sensor and a detachable deposition substrate. While the sensor

yields on-line data on the influence of the deposit on the effective heat flux through

the tube wall, the tubular substrate is used to collect samples for off-line deposit

morphology studies (by means of visual/electron microscopy, also in combination with

EDX analyses). The surface temperature of the probe can be set within a broad

temperature range (250-750 °C), allowing for a proper representation of the

temperature regimes throughout a modern steam boiler.

4.2.2 Used fuels

The applied combustion conditions were “standard low NOx conditions”, i.e.

simulating gradual (staged) air introduction into the flame as applied in state-of-the-art

industrial burners. In order to simulate proper concentrations of relevant combustion

by-products, such as sulphur dioxide, the SO2 precursor H2S was added to the

25

combustion gases via the burner in order to account for sulphur bond in the fuel (see

also below).

The duration of each LCS test was set to one hour at a fixed solid fuel feeding rate of

two grams of ash per hour. Two fuel ashes were used in the tests, corresponding to

the fuels fired during the measurement campaign at Stork Thermeq:

1. Pine oil ash (PO), produced at low temperature from pine wood

2. Heavy fuel oil (HFO) ash, prepared from laboratory compounds, representing

the mineral composition of the parental fuel

In the Stork measurement campaign pyrolysis oil from empty fruit bunches (EFB) was

tested as well, but since EFB pyrolysis oil ash was already tested in the LCS during

the BOBO1 project, no additional EFB tests were done in the framework of this

project.

In order to compensate the loss of chlorine during the ashing of pine wood, the ashes

were blended with ammonium chloride (100 % NH4Cl) in order to match the chlorine

content of a deposit sample taken during combustion tests of pine wood pyrolysis oil

during the measurement campaign at Stork Thermeq. Synthetic heavy fuel oil ash

was produced from a mixture of SiO2, Al2O3, CaO, FeSO4, KOH and NH4Cl in order to

reproduce the elements most important for slagging and fouling phenomena. The

achieved elemental ratios for the different fuels are shown in figure 4.9 and figure

4.10. It can be seen that overall a good agreement between the elemental ratios of

the elements most crucial for slagging and fouling could be obtained.

Comparison of elemental ratios of pine wood ash, synthetic pine oil ash and pine oil

deposit (sampled at Stork Thermeq campaign)

0

1

2

3

4

5

6

7

8

Si/Al Si/Ca Si/K K/Cl S/Cl

ele

men

tal ra

tio

s b

y m

ass [

-]

pine oil deposit, campaign

synth. Pine oil ash

pine wood ash, ECN

Figure 4.9 Elemental ratios of crucial components for pine wood ash, synthetic pine oil ash (pine wood ash + NH4Cl) and pine oil deposit sampled at measurement campaign at Stork Thermeq. (Sulphur originates from addition of H2S to the burner during the LCS tests).

26

Comparison of elemental ratios of original HFO ash and synthetic HFO ash

0

1

2

3

4

5

6

7

8

9

10

Si/Al Si/Ca Si/K K/Cl S/Cl

ele

men

tal ra

tio

s b

y m

ass [

-]

HFO measured

synthetic HFO ash

Figure 4.10 Elemental ratios of crucial components for raw heavy fuel oil and synthetic heavy fuel oil ash. (Sulphur originates from addition of H2S to the LCS burner during the LCS tests).

4.2.3 Fouling propensity and deposition tests

Four LCS deposition experiments were performed employing the horizontal

deposition probe, simulating fouling of heat exchanging surfaces in convective areas

of a boiler. The two reference tests with synthetic heavy fuel oil ash and the synthetic

pine oil ash were carried out at a simulated steam-tube surface temperature of 500

°C. The subsequent additional test with the synthetic oil ashes were performed at a

temperature 250 °C, simulating the temperature of the ECN deposition probe used

during the measurement campaign at Stork Thermeq.

Each experiment includes ash deposition on the sensor ring and ash deposition on

the deposit ring (see figure 4.11). From the data obtained during collecting the ash on

the sensor ring part of the probe a fouling factor was calculated – which is a measure

of the deposits heat transfer resistance. Results of the deposits formed from the

same ash are very similar if one concentrates at the slope of the calculated fouling

factor at the later stages of the experiments, i.e. starting from a cumulative ash feed

of 0.8 grams. It can be seen that the lines of the same ashes for varying probe

surface temperatures run almost parallel, indicating a higher influence of the ash

composition than of the probe surface temperature on the heat resistance of the

deposit (while the offset of the curves is a function of the initial heat transfer at the

onset of the experiment, which is necessarily a function of the probe surface

temperature). Furthermore, it can be seen that the thermal resistance of the heavy

fuel oil deposit is significantly higher than the one of pine oil ash.

27

Fouling factors of different oil ashes at varying probe surface temperature

-0.001

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.00000 0.20000 0.40000 0.60000 0.80000 1.00000 1.20000 1.40000 1.60000 1.80000 2.00000

Cumulative ash feed [g]

Fo

ulin

g f

acto

r [K

*m^

2/W

]pine oil ash, T_probe=250 degC

pine oil ash, T_probe=500 degC

heavy fuel oil ash, T_probe=250 degC

heavy fuel oil ash, T_probe=500 degC

Figure 4.11 Fouling factors of oil ash deposits at different probe surface

temperatures

Plotting the above-mentioned slopes of the fouling factors (referred to as specific

fouling factors) reveals the absolute differences of the different fuels in terms of fouling

propensity, see figure 4.12. It can be seen that the specific fouling factors of pine oil ash

and HFO ash differ by approximately a factor of two, while the change in probe surface

temperature causes an alteration of specific fouling factor by approx. 35 %.

Furthermore, it can be seen that the fouling propensity of EFB ash (data taken from

(Assink, 2009)) is twice as high as HFO ash and four times higher than of pine oil ash.

Hence, the fuel ashes can be grouped in terms of fouling propensity as: EFB > HFO >

Pine oil. The latter is in line with the observations made during the full-scale

measurement campaign carried out at Stork Thermeq, reported earlier in this

document.

28

Comparison of specific fouling factors of different oil ashes at varying probe surface

temperature

0

0,001

0,002

0,003

0,004

0,005

0,006

pine oil ash,

T_probe=250 degC

pine oil ash,

T_probe=500 degC

heavy fuel oil ash,

T_probe=250 degC

heavy fuel oil ash,

T_probe=500 degC

EFB oil ash,

T_probe=550 degC

sp

ec

ific

fo

uli

ng

fa

cto

r [K

*m^

2/W

*g_

fue

l_a

sh

]

Figure 4.12 Specific fouling factors of oil ash deposits at different probe surface

temperatures. Data for EFB oil ash taken from BOBO1 final report (Assink, 2009)

4.2.4 Composition of the deposits

Each sample of the deposited ash on the deposit ring (SS310) was fixed with epoxy

resin and subsequently cross sectioned. Care was taken to prepare samples in a

water-free environment, to prevent any dissolution, re-crystallisation and removal of

alkaline salts. SEM/EDX analyses were used to study the composition and structure

of the deposited ash on the substrate. Results are summarised in table 4.3. The said

data are normalized and expressed on carbon- and oxygen-free bases.

29

Table 4.3 SEM/EDX-derived average composition of deposited ash in mass fractions

(carbon- and oxygen-free)

Test

code

Ash

T [°C]

LCS

496

Pine

250

LCS

497

Pine

500

LCS

498

HFO

500

LCS

499

HFO

250

Al 0,47 0,78 29,64 27,72

Si 12,61 12,50 40,12 38,84

Cl 5,56 10,52 3,51 3,74

K 0,92 1,41 6,45 6,76

Ca 47,63 40,79 9,59 8,45

Fe 3,19 1,68 8,91 9,18

S 5,82 7,22 1,77 3,49

Cr 0,33 0,48 d.l. 1,07

Mn 19,34 14,37 d.l. 0,77

Ni 0,82 2,32 d.l. d.l.

P 3,34 1,64 d.l. d.l.

Mg d.l. 5,05 d.l. d.l.

Ti d.l. 1,23 d.l. d.l.

LCS 496 /deposit ring/ 250°C/ synthetic pine oil ash

An overview of the cross-sectioned deposited layer onto base material is depicted in

table 4.3. Two different types of particles in the deposit can be observed. Fluffy

agglomerates of small particles rich in calcium and big ball-shaped particles

consisting mainly of silicium, calcium, manganese and smaller portions of phosphorus

and iron have been seen. The ash layer was thoroughly analysed at several different

positions. Overall the deposit was relatively homogeneous in composition. There

were no interactions of the deposit at the metal interface visible.

LCS 497 /deposit ring/ 500°C/ synthetic pine oil ash

The deposit formed from synthetic pine oil ash at a tube surface temperature of 500

°C was very similar to the previous one formed at a lower temperature. The main

differences were higher concentrations of magnesium and chlorine and lower

concentrations of calcium and manganese compared to the LCS 496 test. A very thin

layer at the metal interface enriched in chlorine could be detected that may indicate

initial signs of corrosion. However, to verify the latter, longer exposition times of the

deposition probes would be required.

LCS 498 /deposit ring/ 500°C/ synthetic heavy fuel oil ash

The inspection of the deposit formed by synthetic heavy fuel oil ash revealed that

mainly two types of particles were deposited. On the one hand molten ball shaped

structures composed mainly of silicium, calcium, iron and potassium, while aluminium

was found mainly as pure alumina, which formed the second class of particles. Also

in this case a very thin layer at the metal interface slightly enriched in sulphur could

be detected that may indicate initial corrosive attack of the tube material.

LCS 499 /deposit ring/ 250°C/ synthetic heavy fuel oil ash

The observations are largely identical with the ones of LCS 498. The biggest

difference is the higher sulphur content found both in the deposit and close to the

30

metal interface. The latter is probably due to enhanced condensation of gaseous

sulphates.

4.3 Conclusion LCS test

In general it can be concluded that for both fuels and at all the experimental

conditions condensation of gaseous alkalis have been observed. This is confirmed by

the existence of the thin fine particulate film visible on the whole circumference of the

steam model tube in all tests. This can be due to both direct condensation of the

alkalis from the gas phase onto the surface of the tube and/or by thermophoretic

attraction of alkali-rich aerosol formed in the flue gas. The detailed mechanism cannot

be pinpointed in this limited series of tests. Nonetheless the existence of alkali

condensates is a crucial indication of three possible operational problems, anticipated

for pine oil combustion. The first of the said hurdles could be aerosol-related

emissions, while the second is increased fouling propensity and corrosion of the

steam tubes, as well as the flue gas conditioning equipment downstream of the boiler.

Furthermore, even though the LCS tests were carried out at extreme conditions in

terms of ash load which can be seen as a time-lapse of a real installation firing oil, the

results clearly indicate that pine oil –even though the fuel with the lowest fouling

propensity- shows a slagging and fouling tendency implying that a soot blowing

system is needed when firing this fuel within the temperature range investigated.

However, the conditions studied in this work are typical for industrial scale thermal

power plants and the extrapolation to different process parameters, i.e. different

temperature regimes, is difficult. The data generated suggests that the fouling

propensity decreases with decreasing heat exchanger surface temperature which

implies that for low temperature applications, such as building and water heating, less

or even no fouling at all might be observed. However, the latter should be

investigated separately, since it was not within the scope of the project.

Comparing the data generated in the LCS with the information gathered from the

industrial scale campaign at Stork Thermeq, it can be concluded that the approach

undertaken for the LCS tests, using prepared fuel ashes instead of raw fuels, is a

simple and successful way of reproducing properties of ash found in large scale

combustion systems. Furthermore, the results generated during the LCS tests can be

seen as worst case when firing the investigated fuels since the arrival rate of ash

particles on the deposition probe in the LCS is much higher than in real combustion

systems. It was also shown that the differences in fouling propensities of the different

fuels recorded in the LCS are in line with the observations made on the large scale.

However, a further in-depth comparison of the deposition mechanisms was not

possible due the limited amount of deposit collected during the full-scale campaign.

Furthermore, the differences in temperatures at which measurements were carried

out make a direct comparison rather difficult since the ash chemistry is a strong

function of temperature.

31

5 GENERAL CONCLUSIONS

Filtration tests with self-cleaning filter and two types of centrifuges have shown that

the centrifuge techniques have the highest efficiency towards solids separation and

have the lowest oil losses.

Based on the results of the combustion tests, it was concluded that:

Combustion of pyrolysis oil is successful with an off the shelf Stork Double Register

Burner in an industrial boiler. The Pyrolysis oils were fired between 1.2 and 2.6 MW.

To stabilize the pyrolysis flame a small natural gas pilot flame between 19-33% of the

total load was needed. The reason for requiring a gas pilot is the moisture content,

which cools the flame at the stabilization point and makes ignition difficult. The flame

of the pyrolysis oils stabilizes at a larger distance from the impeller than the HFO

flame, which is an indication of ignition issues.

Changing the holepattern of the atomizer tip and increasing the impeller does not

change the stability of flame significantly. This indicates that the ignition and

stabilization issue might not be an atomization or flow recirculation issue. The most

likely cause is the moisture content of the pyrolysis oils. Unfiltered EFB clogs the

atomizer gun. A sticky substance blocks the holes in the atomizer gun. This is

probably due to the sedimentation of this oil. Visual inspection of the oil shows a

phase separation.

The NOx emissions of filtered EFB and pine wood oil are lower than the emissions of

HFO. One reason for this reduction is the lower load at which the pyrolysis oils were

fired. The pyrolysis oils could not be fired at the load at which HFO was fired, which

was 4.7 MW, but this load difference is not sufficient to explain the difference in NOx

emissions in full. Another reason is the lower adiabatic flame temperature, due to the

moisture content and the lower LHV value. Also pine wood oil has a lower nitrogen

content in the fuel, which explains the lowest NOx emission value obtained with this

fuel The unfiltered EFB has the highest NOx emission. The reason is phase

separation and sedimentation which causes an unstable combustion. The required air

for CO-free combustion also increased and with increasing air the NOx emission also

increases, due to the dilution correction of oxygen in the flue gas.

The air requirement for CO-free combustion for the pyrolysis oils was higher than that

for HFO. One probable reason is the turbulent flow, due to inhomogeneous

combustion and flame length difference. The pyrolysis oil flame was less

homogeneous than the HFO flame, which means that locally an oxygen shortage can

appear. This increases the likelihood that the CO to CO2 reaction stops. Increasing

air excess decreases the possibility that an oxygen shortage can occur locally.

Summarising both the full-scale measurement campaign at Stork Thermeq and the

lab-scale investigations carried out at ECN, the following can be concluded

concerning fouling and corrosion behaviour:

32

Even though the investigated bio-oils have very low ash contents, they do show

slagging and fouling behaviour. The propensity is highest for EFB oil, lower for heavy

fuel oil and the lowest for pine oil. From the findings concerning slagging and fouling

of bio-oils, it can be concluded that a boiler (co-)fired with EFB and heavy fuel oils

must be equipped with a soot blowing system to prevent uncontrolled growth of

deposit on boiler and heat exchanger surfaces. Pine oil has a smaller fouling

propensity. The data generated suggests that the fouling propensity decreases with

decreasing heat exchanger surface temperature which implies that for low

temperature applications, such as building and water heating, less or even no fouling

at all might be observed. However, the latter should be investigated separately, since

it was not within the scope of the project.

Even though effective exposure times of metal coupons were generally too short to

make a final judgement concerning corrosion risks when (co-)firing bio-oils, there are

indications that the concentrations of alkali-salts (especially KCl and K2SO4) in the

flue gas which deposit on heat exchanger tubes cause increased corrosion of these

surfaces. The latter could be verified in the LCS test firing pine oil ash with a tube

surface temperature of 500 °C (LCS497), after which a thin layer at the metal

interface enriched in chlorine could be detected which in turn is most likely a sign of

initial corrosion. However corrosion effects at lower tube temperatures are expected

to be less.

The measurement of emission (NOx, dust, CO) during the full-scale measurement

campaign at Stork Thermeq indicate that for the combustion of the pine oil the local

emission limits can be met (<5 MW th, BEMS). For EFB and HFO combustion

additional flue gas cleaning is required. This is industrially common practice for HFO

combustion