SIST-TP CEN/TR 13767:2005

Characterisation of sludges - Good practice for sludges incineration with and without grease and screenings

This document  describes good practice for the incineration of sludges with and without grease and screenings.
This document is applicable for sludges described in the scope of CEN/TC 308 specifically derived from :
- night soil ;
- urban wastewater collecting systems ;
- urban wastewater treatment plants ;
- treatment of industrial wastewater similar to urban wastewater (as defined in Directive 91/271/EC) ;
but excluding hazardous sludges from industry.
This document is not applicable to co-incineration of sludge and other wastes, (either urban or hazardous) (see CEN/TR 13768) and to the use of sludge in cement kilns.
Annex A gives tables of data for different typical parameters for sludge, furnace and ash.

Charakterisierung von Schlämmen - Anleitung für die gute fachliche Praxis bei der Verbrennnung von Schlamm mit und ohne Fett und Rechengut

Caractérisation des boues - Bonne pratique d'incinération des boues avec ou sans graisse et refus de dégrillage

Le présent document décrit les bonnes pratiques d'incinération des boues avec ou sans graisse et refus de dégrillage.
Le présent document s'applique aux boues décrites dans le domaine d'application du CEN/TC 308, provenant spécifiquement :
-   des matieres de vidange ;
-   des systemes de collecte des eaux usées urbaines ;
-   des stations d'épuration des eaux usées urbaines ;
-   des stations d'épuration des eaux industrielles assimilées (telles que définies par la Directive 91/271/CE) ;
a l'exclusion des boues industrielles dangereuses.
Le présent document ne s'applique pas a la co-incinération des boues et d'autres déchets (urbains ou dangereux) (voir le CEN/TR 13768), ni a l'utilisation des boues en fours de cimenterie.
L'Annexe A regroupe des tableaux indiquant différents parametres caractéristiques des boues, fours et cendres.

Karakterizacija blata - Dobra praksa za sežiganje blat z maščobami in ostanki ter brez njih

General Information

• Status: Withdrawn
• Publication Date: 16-Sep-2014
• Withdrawal Date: 28-Jan-2015
• ICS: 13.030.20 - Liquid wastes. Sludge
• Technical Committee: KAT - Soil quality
• Current Stage: 9900 - Withdrawal (Adopted Project)
• Start Date: 22-Jan-2015
• Due Date: 14-Feb-2015
• Completion Date: 29-Jan-2015
• Ref Project: CEN/TR 13767:2004 - Characterisation of sludges - Good practice for sludges incineration with and without grease and screenings

Relations

• Replaced By: SIST-TP CEN/TR 16788:2015 - Characterization of sludges - Guideline of good practice for thermal processes
• Effective Date: 01-Mar-2015
• Replaces: SIST CR 13767:2001 - Characterization of sludges - Good practice for sludges incineration with and without grease and screenings
• Effective Date: 01-Feb-2005

Standards Content (Sample)

SLOVENSKI STANDARD
SIST-TP CEN/TR 13767:2005
01-februar-2005
1DGRPHãþD
SIST CR 13767:2001
SIST CR 13767:2001
.DUDNWHUL]DFLMDEODWD'REUDSUDNVD]DVHåLJDQMHEODW]PDãþREDPLLQRVWDQNLWHU
EUH]QMLK
Characterisation of sludges - Good practice for sludges incineration with and without
grease and screenings
Charakterisierung von Schlämmen - Anleitung für die gute fachliche Praxis bei der
Verbrennnung von Schlamm mit und ohne Fett und Rechengut
Caractérisation des boues - Bonne pratique d'incinération des boues avec ou sans
graisse et refus de dégrillage
Ta slovenski standard je istoveten z: CEN/TR 13767:2004
ICS:
13.030.20 7HNRþLRGSDGNL%ODWR Liquid wastes. Sludge
SIST-TP CEN/TR 13767:2005 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CEN/TR 13767:2005

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SIST-TP CEN/TR 13767:2005
TECHNICAL REPORT
CEN/TR 13767
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
August 2004
ICS 13.030.20 Supersedes CR 13767:2001
English version
Characterisation of sludges - Good practice for sludges
incineration with and without grease and screenings
Caractérisation des boues - Bonne pratique d'incinération Charakterisierung von Schlämmen - Anleitung für die gute
des boues avec ou sans graisse et refus de dégrillage fachliche Praxis bei der Verbrennnung von Schlamm mit
und ohne Fett und Rechengut
This Technical Report was approved by CEN on 26 February 2004. It has been drawn up by the Technical Committee CEN/TC 308.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2004 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 13767:2004: E
worldwide for CEN national Members.

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Contents

FOREWORD.3
INTRODUCTION .4
1 SCOPE.5
2 NORMATIVE REFERENCES.5
3 TERMS AND DEFINITIONS.5
4 SLUDGE PROPERTIES.6
4.1 CHEMICAL CHARACTERISTICS .6
4.2 PHYSICAL-CHEMICAL CHARACTERISTICS.7
5 COMBUSTION FUNDAMENTALS.9
6 EQUIPMENT CHARACTERISTICS.10
6.1 INCINERATION SYSTEMS.10
6.2 SUPPORT COMPONENTS.16
6.3 DESIGN ASPECTS.21
7 OPERATIONAL PROCEDURES.22
7.1 GENERAL.22
7.2 SPECIFIC.22
8 MANAGEMENT OF RESIDUES .25
8.1 FLUE GAS.25
8.2 ASHES .26
8.3 WASTEWATER.27
9 ENVIRONMENTAL IMPACT ASSESSMENT.27
ANNEX A.28
BIBLIOGRAPHY .32

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Foreword
This document (CEN/TR 13767:2004) has been prepared by Technical Committee CEN/TC 308 “Characterization
of sludges”, the secretariat of which is held by AFNOR.
This document supersedes CR 13767:2001.
Significant technical differences between this edition and CR 13767:2001 is taking account of the new Directive
2000/76/EC (incineration of waste).
The status of this document as CEN Technical Report has been chosen because the most of its content is not
completely in line with practice and regulation in each member state. This document gives recommendations for a
good practice but existing national regulations concerning the sludges incineration remain in force.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to announce this Technical Report: Austria, Belgium, Cyprus, Czech Republic, Denmark,
Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
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Introduction
The purpose of this document is to describe good practice of the sludge incineration in order to ensure a safe and
economical operation. The main goals are to :
 describe the principal design parameters relevant to different process schemes ;
 assess the operating procedures able to perform optimal energy consumption, emissions control and
equipment durability ;
 provide the responsible authorities with well established and easily applicable protocols for control purposes ;
 promote the diffusion of this practice and favouring the formation of a public opinion consensus ;
Potential advantages of high temperature processes include :
 reduction of volume and mass of sludge ;
 destruction of toxic organic compounds, if present ;
 energy recovery.
Anyway, priority should be given to reduction of pollutants at the origin and to recover if technically and
economically feasible valuable substances (phosphorous and potassium) in sludge and derived products.
The following abbreviated terms necessary for the understanding of this document apply :
COD Chemical oxygen demand
LOI Loss On Ignition
MHF Multiple Hearth Furnace
FBF Fluidised Bed Furnace
RKF Rotary Kiln Furnace
EF Electric Furnace
CF Cyclone Furnace
PCDF Polychlorodibenzofurans
PCDD Polychlorodibenzodioxins
PCB Polychlorinated biphenyls
PAH Polycyclic aromatic hydrocarbons
GCV Greater Calorific Value
LCV Lower Calorific Value
VOC Volatile organic carbon
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1 Scope
This document describes good practice for the incineration of sludges with and without grease and screenings.
This document is applicable for sludges described in the scope of CEN/TC 308 specifically derived from :
 night soil ;
 urban wastewater collecting systems ;
 urban wastewater treatment plants ;
 treatment of industrial wastewater similar to urban wastewater (as defined in Directive 91/271/EC) ;
but excluding hazardous sludges from industry.
This document is not applicable to co-incineration of sludge and other wastes, (either urban or hazardous) (see
CEN/TR 13768) and to the use of sludge in cement kilns.
Annex A gives tables of data for different typical parameters for sludge, furnace and ash.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references,
only the edition cited applies. For undated references, the latest edition of the referenced document (including any
amendments) applies.
EN 1085, Wastewater treatment – Vocabulary.
EN 12832, Characterization of sludges – Utilisation and disposal of sludges – Vocabulary.
EN 12255-8, Wastewater treatments plants – Part 8 : Sludge treatment and storage.
CEN/TR 13768, Characterization of sludges – Good practice for combined incineration of sludge and household
wastes.
EN 13965-1, Characterization of waste – Terminology – Part 1 : Material related terms and definitions.
EN 13965-2, Characterization of waste – Terminology – Part 2 : Management related terms and definitions.
3 Terms and definitions
For the purposes of this document, the following terms and definitions which apply are those given in :
 Directive 91/271/EC (concerning urban waste water treatment) ;
 Directive 75/442/EEC (the Waste Framework Directive) as amended by EU Directive 91/156/EEC ;
 Directive 89/369/EEC (concerning prevention of atmospheric pollution derived from urban solid waste
incineration plants) until 27/12/2005;
 Directive 2000/76/CE on Waste incineration
 EN 1085, EN 12832, and EN 13965-1 and -2.
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4 Sludge properties
Sludge characterisation for the assessment of combustion processes involves the evaluation of chemical and
physical parameters and specific properties.
4.1 Chemical characteristics
The main chemical characteristics to be taken into account are :
 organic and inorganic chlorine ;
 sulfur ;
 phosphorus and nitrogen ;
 other halogens ;
 organic micropollutants with main regard to chlorinated hydrocarbons, phenols and polyphenols,
polychlorinated biphenyls (PCB), pesticides and polycyclic aromatic hydrocarbons (PAH) ;
 elemental analysis of loss on ignition (LOI) ;
 trace elements.
The toxicity of emissions (gaseous, liquid, solid) from incineration generally depends on the presence of above
chemicals at origin, when improper operating conditions occur.
a) Sulfur
The sulfur content of sewage sludge ranges generally from 0,5 % to 2 % by dry mass. Because a fraction of the
sulfur is present in the oxidised sulfate form, not all of this sulfur is converted to sulfur dioxide during combustion.
Sulfur dioxide then combines with moisture, either in the waste gas treatment system or in the atmosphere, to form
sulfuric and sulfurous acids.
b) Phosphorus and nitrogen
Phosphorus can be present in sewage sludge in concentration ranging from 1 % to 5 % by dry mass. This
concentration mainly depends on the phosphorus load in the wastewater system and on the level of phosphorus
removal accomplished in the treatment plant. Nowadays in some countries the phosphorus concentration in urban
wastewater is decreasing due to substitution of phosphorus in detergents with other products. During combustion
phosphorus and phosphorus compounds are converted to calcium phosphate which can be present in the furnace
ash up to 15 % mass fraction of P O In certain conditions, leaching of phosphorus from ashes should be taken
2 5.
into account.
Nitrogen content of sewage sludge (2 % to 12 % dry mass) can be converted during combustion to molecular
nitrogen or to NO , depending on the temperature and atmosphere inside the furnace. NO formation from fuel
x x
bound nitrogen can be controlled by restricting the air flow to the minimum excess above the stoichiometric
requirement and by staging the air flow to the furnace (see 8.1).
1)
c) Chlorine and other halogens
Organic and inorganic chlorine compounds play an important role in the combustion processes for the tendency of
the chlorine radicals to bind to active radicals, like O*, H* and OH*. This determines a decrease in the combustion
rate with the possibility of toxic compounds formation. Chlorine and other halogens are also responsible for the
presence in the exhaust gases of acidic compounds which are undesirable for corrosion problems involved,
especially at high temperatures. The presence of organic chlorine in sewage sludge is generally negligible (less

1) Bromine can exert similar effects than chlorine but the organic compounds are easier formed and they can also be easier
destroyed at high temperatures.
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than 50 mg/kg dry mass) but the concentration of inorganic chlorine can be some units per cent dry mass
depending on chlorine presence in the sludge water fraction and on the use of inorganic conditioners. The
industrial sludges similar to sewage sludge mentioned in Directive 91/271/EC, which derive from food and/or
beverage transformation and production, do not contain organic chlorine. As for sewage sludge, inorganic chlorine
can be present in such sludges if FeCl is used as conditioner.
3
d) Organic micropollutants
Although the presence of organic micropollutants in sewage sludge can be in some cases noticeable, they
generally do not pose problems in incineration. Chemical analysis can include, for particular cases of contaminated
sludges, the compounds which are recognised to be recalcitrant to a thermal degradation.
e) Elemental analysis
Elemental analysis of loss on ignition (C, H, N, S, O) is important to predict flow rate and composition of flue gas
and therefore to design the purification gas line. Typical elemental analysis of primary, secondary, mixed and
digested sludge is given in Table A.1.
f) Trace elements
Trace element presence in sewage sludge has to be considered for their potential tendency to be transferred in the
gaseous phase (especially for mercury). They (except mercury) can be concentrated in fly ashes collected in bag
and electrofilters (arsenic, lead, cadmium and zinc). Mercury generally escapes with flue gases but can be filters
condensed in scrubbers or captured by activated carbon filters.
Trace elements are generally present in sewage sludge in very variable concentrations depending on the presence
of industrial effluents in the wastewater. Table A.2 gives an indication of the most common range of variation and
the typical values of trace element concentrations, but it has to be pointed out that, currently, the trace element
presence in sewage sludge is decreasing due to a more effective control of undesirable pollutants input to the
sewerage system.
4.2 Physical-chemical characteristics
The main physical-chemical characteristics to be taken into account are :
 dry matter ;
 physical consistency ;
 loss on ignition (LOI) ;
 calorific value ;
 presence of grease, scum and screenings.
Rheological properties also play an important role, especially as far as the design of feeding system is concerned.
a) Dry matter
In incineration of sewage sludge dry matter is a variable affecting both fuel requirement and exhaust gas
production. Generally any increase in dry matter is believed to be beneficial in the combustion for the reduction in
fuel requirement. Until the condition for autogenous combustion is reached the increase in dry matter corresponds
also to a decrease in combustion gases production. It should be pointed out that any further increase of dry matter
beyond the limit of autogenous combustion could be not very convenient because this entails a more abundant gas
production, especially if dilution air is used instead of water for the control of the combustion chamber temperature.
The use of water, on the contrary, reduces the quantity of recoverable heat in the boiler.
Moreover, if after burning of combustion gases should be accomplished, the feeding of too dry a sludge to the
furnace implies also very abundant fuel requirements in the after burning chamber due to high gas production.
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Therefore, thermal drying of sludge before incineration has to be properly designed and operated in order to attain
an optimal drying level. Dried and dewatered sludges can be mixed, if necessary, to avoid too high not needed dry
matter concentrations.
b) Physical consistency
Physical consistency of sludge should be adapted to furnace and its feeding system.
Depending on the dewatering device a crumbly product could be needed before feeding the incineration furnace.
c) Loss on ignition and calorific value
Calorific value of sludge is probably the most important parameter for the evaluation of combustion processes. It
represents the heat quantity developed in the combustion by the unit mass of material in standard conditions.
As a first approximation the Greater Calorific Value (GCV) can be evaluated by the Du Long equation, if the
elemental analysis of combustible material is known :
GCV = 32 810 C + 142 246 (H – O/8) + 9 273 S (1)
where
GCV is in kJ/kg LOI ; and
C, H, O and S are the mass fraction of the elements in the loss of ignition.
The above formula gives an overestimation of the heat value of sludges with high organic nitrogen content
because : a) the nitrogen will be associated with the hydrogen as an amine, b) the production of nitrogen oxide in
the amine combustion reduces the hydrogen heat release.
The following equation can be used to take into account the above effects :
GCV = 32 810 C + 142 246 (H – O/8) + 9 273 S – [2 189 N (1 – µ) + 6 489N µ] (2)
where
µ represents conversion (mass fraction) of nitrogen to nitrogen oxide, generally in the range 2 % to 7 %.
Lower calorific value (LCV) can be also evaluated (Nielsen & Simonsen, 1994) by measuring the chemical oxygen
demand COD and the total Kjeldahl nitrogen (TKN) (ammoniacal + organic nitrogen) and using the formula :
LCV = 13 700 COD + 19 000 TKN (3)
where
LCV is in kJ/kg LOI ; and
COD and TKN are expressed in kg/kg LOI.
COD of sludge generally varies in the range of 1,5 kg to 1,8 kg O /kg LOI and TKN in the range 0,02 kg/kg LOI
2
to 0,09 g/kg LOI.
Typical calorific values of municipal wastewater sludges range from 22 100 kJ/kg LOI to 24 400 kJ/kg LOI
(anaerobically digested primary) to 23 300 to 27 900 (raw primary). Secondary sludges display values between
20 700 kJ/kg LOI and 24 400 kJ/kg LOI.
The variability of the calorific value mainly depends on the elemental analysis of sludges : when the hydrogen
content is higher also the calorific value displays higher values as for primary sludge in comparison with secondary
and with digested sludge.
LCV can be estimated considering the water present in the sludge (1 – X), being X the fraction of dry solids, and
the combustion water (9 H LOI) :
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LCV (kJ/kg sludge) = GCV X LOI – 2 440 (9 H LOI + 1 – X) (4)
where
LOI is the loss on ignition with respect to dry solids (kg/kg).
If the lower calorific value of loss on ignition is known (LCV ) the lower calorific value of wet sludge can be easily
LOI
evaluated by :
LCV = LCV X LOI – 2 440 (1 – X) (5)
LOI
As a first approximation for LCV a value of 23 000 kJ/kg LOI can be assumed.
LOi
d) Presence of grease, scum and screenings
Grease, scum and screenings can be incinerated together with sludges but generally they pose several problems.
Screenings clog feed mechanisms for certain types of furnace and therefore a grinding or shredding process is
advisable before feeding.
Screenings also contain bulky and non-combustible materials, which create problems in the ash disposal system.
Skimming generally contains more than 95 % moisture and therefore they should be thickened to at least 25 %
solids before incineration. Skimming is difficult to handle in the thickened state due to their viscosity and a heating
process to 70 °C to 80 °C is generally requested to get skimming pumpable. This scum solids should be ground to
a size not exceeding 6 mm. GCV of skimming and screenings are in the range 37 000 kJ/kg to 44 000 kJ/kg dry
solids and 23 000 kJ/kg to 25 600 kJ/kg dry solids, respectively.
-6 3 3
Quantities of screenings are strictly dependent on the screen opening : they can vary in the range of 3·10 m /m
-6 3 3
to 40·10 m /m of sewage for opening of 12 mm to 25 mm (the upper limits apply to the reduced openings). As
3
dewatered sludge production can be approximately evaluated in 1 l/m of sewage the screenings production can
be accounted in approximately 0,2 % to 4 % in mass of sludge production, considering that the density of wet

3 3
screenings is 640 kg/m to 1 000 kg/m .
Quantities of scum are very much dependent on the quality of the sewage and on the collecting system in the
3
wastewater treatment plant : the highest values can be as high as 17 g of dry solids/ m of sewage which means
up to 1,7 % of sludge production. At a concentration of 25 % this value increases to 6,8 %.
Addition of scum and grease can result in operating and safety problems : due to their high energy content an
increased volume of exhaust gases are suddenly produced in the heating space. It could happen that suction
blowers, responsible for vacuum production in the furnace are not able to draw off the developed explosive gas
immediately, which, therefore, can escape in the ambient air.
An operator has to take measures against such a situation and has to control appropriately any addition of
combustible material different from sewage sludge.
5 Combustion fundamentals
Combustion is an oxidation reaction carried out at high temperature: the union of oxygen with carbon, hydrogen
and sulfur yields energy and products of combustion, namely, carbon dioxide (CO ), water (H O) and sulfur
2 2
dioxide (SO ). Organic nitrogen is preferentially converted to nitrogen gas but a certain amount (2 % to 7 %) can
2
also be further oxidised to nitrogen oxide (NO).
The nitrogen in the air is also candidate to be converted to oxides of nitrogen (NO ). This phenomenon begins to
x
be noticeable at temperatures higher than 1 100 °C and increases with any further increase of temperature.
The maximum temperature achieved by the combustion of a fuel will result from the balance between the energy
produced and/or the energy input and that of combustion products. The heat released by the combustion of a
substance is then used to increase the temperature of the combustion products to the equilibrium temperature. The
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amount of heat required to raise a unit weight of gas, liquid, or solid of one degree is the specific heat of the
material. In Tables A.3 and A.4, the main properties of gaseous products of combustion are shown.
All oxidising combustion reactions require some excess air to ensure that the reaction proceeds rapidly to
completion. The amount of excess air required is a function of time of stay, temperature and turbulence, commonly
referred to as the "3Ts of combustion". Generally as turbulence is maximised, excess air can be decreased.
Turbulence provides more opportunities of contact between fuel and oxygen and changes substantially for various
types of combustion units. High efficiency burners may employ as low as 20 % to 30 % excess air while less
efficient furnaces, like multiple hearth and rotary kiln furnaces, need 100 % to 125 % excess air at least. As excess
air quenches the combustion temperature it is desirable to minimise the quantity to be employed especially when
auxiliary fuel is needed to sustain combustion. This effect can be reduced by air pre-heating. If insufficient excess
air is added to the furnace or if one or more of the "3Ts" concepts are lacking, the combustion operation will
generate smoke and products of incomplete combustion), thus making incineration operation not acceptable.
6 Equipment characteristics
6.1 Incineration systems
Incineration plants, independently on the system and type of furnace used, is designed, equipped, built and
operated to respect limits and prescriptions of the Directive 2000/76/EC.
Temperature and residence time is accordingly strictly observed. Each line of the incineration plant should be
equipped with at least one auxiliary burner and with an automatic system preventing sludge feed when improper
conditions occur. Exhaust gases should be discharged in a controlled fashion and in conformity with relevant
European, national and regional air quality standards by means of a stack the height of which is calculated in such
a way as to safeguard human health and the environment.
The type of incinerator most commonly in use for sludge incineration is the Fluidised Bed Furnace (FBF). Other
types are : Multiple Hearth Furnace (MHF), Rotary Kiln Furnace (RKF), combination of MHF and FBF, Electric
Furnace (EF) and Cyclone Furnace (CF).
They can be combined with a dryer.
a) Fluidised Bed Furnace (FBF)
It is a cylindrical refractory lined shell containing a sand bed fluidised during operation by air through a distributor
plate below the bed. The temperature of the bed is controlled at about 750 °C. FBFs fall into two categories :
bubbling and circulating. They are based on the same principle, but in the circulating bed unit a higher fluidisation
velocity creates very intensive mixing of air and fuel. Particles are carried out of the vertical combustion chamber by
the flue gas and are removed in a cyclone to be returned to the FBF through a loop seal. A cross section of a
bubbling FBF is shown in Figure 2. Typical design parameters of a bubbling FBFs, which are much more common
than circulating types, are reported in Table A.5.
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Key
1 Exhaust and ash
2 Pressure tap
3 Sight glass
4 Burner
5 Tuyeres
6 Fuel gun
7 Pressure tap
8 Start-up preheat burner for hot windbox
9 Windbox
10 Fluidizing air inlet
11 Refractory arch
12 Fluidised sand bed
13 Freeboard
14 Sludge inlet
15 Thermocouple
16 Sand inlet
Figure 1 — Bubbling fluidised bed furnace (typical cross section)
Advantages of FBFs are low excess air requirement, due to the high turbulence, low NO production, due to
x
effective control of combustion temperature, reliability (no moving parts), flexibility for shock load, adaptability to
sludges at different moisture content (dewatered, partially dried, full dried), heat storage capacity by sand bed, and
possible abatement of acidic compounds within the bed using additives, like limestone and dolomite.
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Disadvantages include ash and sand carry-over, and possible formation of a block of vitrified sand when salts with
low melting points are present. This problem can be attenuated by an addition of chemicals to bind the alkaline
salts.
b) Multiple Hearth Furnace (MHF)
It consists in a vertical cylindrical-refractory lined reactor containing a number of horizontal hearths. Rabble arms,
supported by a single central shaft, rake the sludge radially across the hearths from the top to the bottom, in
counter-current with air and hot gases. A cross section of a multiple hearth furnace is shown in Figure 2. Three
zones can be distinguished in the furnace: drying, with gas temperature up to 400 °C, burning (temperatures of gas
and solid phases of 850 °C to 900 °C), ash cooling (temperatures of ashes and air generally lower than 200 °C).
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Key
1 Sludge cake, screenings and grit
2 Burners
3 Supplemental fuel
4 Combustion air
5 Shaft cooling air return
6 Solids flow
7 Drop holes
8 Rabble arm drive
9 Shaft cooling air
10 Ash discharge
11 Clinker breaker
12 Gas flow
13 Rabble arm (2 or 4 per hearth)
14 Auxiliary air ports
15 Scum
...