SIST-TP CEN/TR 16788:2015
(Main)Characterization of sludges - Guideline of good practice for thermal processes
Characterization of sludges - Guideline of good practice for thermal processes
This Technical Report describes good practice for the incineration and other organic matter treatment by thermal processes of sludges.
Thermal drying, thermal conditioning and thermal hydrolysis are excluded.
This Technical Report is applicable for sludges described in the scope of CEN/TC 308 specifically derived from:
- storm water handling;
- night soil;
- urban wastewater collecting systems;
- urban wastewater treatment plants;
- treating industrial wastewater similar to urban wastewater (as defined in Directive 91/271/EEC);
but excluding hazardous sludges from industry.
Charakterisierung von Schlämmen - Anleitung für die gute fachliche Praxis thermischer Prozesse
Caractérisation des boues - Lignes directrices relatives aux bonnes pratiques pour les procédés thermiques
Le présent Rapport technique décrit les bonnes pratiques applicables pour l'incinération et les autres traitements de matières organiques reposant sur le traitement thermique des boues.
Le séchage thermique, le conditionnement thermique et l'hydrolyse thermique ne font pas l'objet du présent Rapport.
Le présent Rapport technique s'applique aux boues décrites dans le domaine d'application du CEN/TC 308, provenant spécifiquement :
— de la collecte des eaux pluviales ;
— des matières de vidange ;
— des systèmes de collecte des eaux usées urbaines ;
— des stations d'épuration des eaux usées urbaines ;
— des stations d'épuration des eaux industrielles assimilées aux eaux usées urbaines (telles que définies par la Directive 91/271/CEE) ;
à l'exclusion des boues industrielles dangereuses.
Karakterizacija blata - Smernica za dobro prakso toplotnih procesov
To tehnično poročilo opisuje dobro prakso sežiganja in drugih načinov obdelave organskega materiala s toplotnimi procesi obdelave blata.
Toplotno sušenje, toplotna obdelava in toplotna hidroliza so izključeni.
To tehnično poročilo se uporablja za vrste blata, opisane v standardu CEN/TC 308, ki izvirajo iz:
– sistemov za zbiranje deževnice;
– vsebine greznice;
– zbiralnih sistemov za mestno odpadno vodo;
– čistilnih naprav za mestno odpadno vodo;
– obravnavanja industrijske odpadne vode na podoben način kot komunalne odpadne vode (kot je opredeljeno v Direktivi 91/271/EGS);
vendar ne izvirajo iz nevarnega industrijskega blata.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-marec-2015
1DGRPHãþD
SIST-TP CEN/TR 13767:2005
Karakterizacija blata - Smernica za dobro prakso toplotnih procesov
Characterization of sludges - Guideline of good practice for thermal processes
Charakterisierung von Schlämmen - Anleitung für die gute fachliche Praxis thermischer
Prozesse
Caractérisation des boues - Lignes directrices relatives aux bonnes pratiques pour les
procédés thermiques
Ta slovenski standard je istoveten z: CEN/TR 16788:2014
ICS:
13.030.20 7HNRþLRGSDGNL%ODWR Liquid wastes. Sludge
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
TECHNICAL REPORT
CEN/TR 16788
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
November 2014
ICS 13.030.20 Supersedes CEN/TR 13767:2004, CEN/TR 13768:2004
English Version
Characterization of sludges - Guideline of good practice for
thermal processes
Caractérisation des boues - Lignes directrices relatives aux Charakterisierung von Schlämmen - Anleitung für die gute
bonnes pratiques pour les procédés thermiques fachliche Praxis thermischer Prozesse
This Technical Report was approved by CEN on 25 November 2014. It has been drawn up by the Technical Committee CEN/TC 308.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 16788:2014 E
worldwide for CEN national Members.
Contents Page
Foreword .4
Introduction .5
1 Scope .8
2 Normative references .8
3 Terms and definitions .8
4 Abbreviations .9
5 Regulatory aspects . 10
6 Sludge properties . 11
6.1 General . 11
6.2 Physico-chemical characteristics . 11
6.2.1 General . 11
6.2.2 Dry matter . 11
6.2.3 Loss on ignition . 12
6.2.4 Calorific value . 12
6.2.5 Grease, scum and screening . 13
6.2.6 Physical consistency and others . 13
6.3 Chemical characteristics . 13
6.3.1 General . 13
6.3.2 Sulphur . 14
6.3.3 Phosphorus . 14
6.3.4 Nitrogen . 14
6.3.5 Chlorine and other halogen . 14
6.3.6 Organic micro pollutants . 14
6.3.7 Trace elements . 15
7 Thermal processes fundamentals. 15
7.1 Incineration . 15
7.2 Gasification . 16
7.3 Pyrolysis . 17
7.4 Wet (air) oxidation. 17
7.5 Others . 18
8 Equipment . 18
8.1 Incineration devices . 18
8.1.1 General . 18
8.1.2 Fluidized Bed Furnace (FBF) . 20
8.1.3 Multiple Hearth Furnace (MHF) . 22
8.1.4 Combination of FBF and MHF . 22
8.1.5 Others . 22
8.2 Gasification devices . 23
8.3 Pyrolysis Devices . 25
8.4 Wet air oxidation devices . 26
8.5 Design aspects . 26
8.6 Auxiliary equipment . 27
8.6.1 General . 27
8.6.2 Transport, receiving area, storage and feeding systems . 27
8.6.3 Heat recovery . 27
8.6.4 Flue gas cleaning . 28
8.6.5 Ash and other residue handling . 28
8.6.6 Wastewater treatment . 28
8.6.7 Process monitoring . 29
9 Operational aspects . 29
9.1 General . 29
9.2 Incineration . 30
9.2.1 General . 30
9.2.2 Fluidized Bed Furnace (FBF) . 30
9.2.3 Multiple Hearth Furnace (MHF) . 32
9.3 Technologies without operational background in sewage sludge: Gasification/ Pyrolysis . 33
9.3.1 General . 33
9.3.2 Hazards . 33
10 Management of energy and material products. 33
10.1 General . 33
10.2 Incineration . 33
10.3 Gasification / Pyrolysis . 35
10.4 Resume 10.1 through 10.3 . 37
10.5 Wet Oxidation . 37
11 Management of residues . 37
11.1 General . 37
11.2 Flue Gas. 37
11.2.1 Composition/parameters . 37
11.2.2 Equipment . 39
11.3 Ashes . 42
11.3.1 Composition/Parameters . 42
11.3.2 Equipment . 42
11.4 Wastewater . 42
12 Economic aspects . 43
13 Co-management with other organic wastes . 43
13.1 General . 43
13.2 Specific considerations . 44
13.3 Additional storage and transports aspects . 49
13.3.1 General . 49
13.3.2 Storage . 49
13.3.3 Transport . 50
14 Assessment of impacts . 50
14.1 General . 50
14.2 Environmental aspects . 51
14.3 Economic aspects . 51
14.4 Social aspects . 51
Annex A (informative) Emission limit values . 53
Annex B (normative) Calorific Value calculations . 55
Annex C (informative) Tables. 57
Annex D (normative) Various systems to input sludge into a household waste incineration plant . 59
D.1 General . 59
D.2 Sludge whose dry matter content < 35 % . 59
D.3 Sludge whose dryness is > 65 % . 59
D.4 Sludge whose dry matter content between 35 % to 65 % . 60
D.5 Drying the sludge in the household waste incineration plant . 60
Bibliography . 61
Foreword
This document (CEN/TR 16788:2014) has been prepared by Technical Committee CEN/TC 308
“Characterization of sludge”, the secretariat of which is held by AFNOR.
This document supersedes CEN/TR 13767:2004 and CEN/TR 13768:2004.
Introduction
It is recognized that wastewater sludge is a potential source of valuable resources. Material recycling is higher
in the waste hierarchy (ref. 2008/98/EC Directive) than recovery (energy and material). Sludge incineration
and other organic matter treatments by thermal processes (gasification, pyrolysis and wet oxidation) should
deal with materials which do not meet beneficial use requirements. They represent a consistent year round
solution. To decide which type of solution is appropriate for a particular sludge, Figure 1 should be consulted.
Thermal processes involve, among others, reduction of volume and weight, highest destruction of toxic
organic compounds, possible recovery of phosphorus and other useful materials. Drawbacks include high
costs and complexity of plant operation.
In all cases, the energy balance (including energy for removing water etc.) and carbon footprint of the
processes should be calculated to verify the environmental benefit of the process.
A good performance of a thermal processing plant also depends upon the provision of proper auxiliary
equipment and devices, which include receiving and storage systems, pre-treatments equipment, feeding
system, flue gas cleaning, heat recovery, ash handling, wastewater disposal and process monitoring.
The purpose of this Technical Report is to describe good practice for sludge incineration and other organic
matter treatments by thermal processes 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 balance, emissions control and
equipment durability;
— provide the responsible authorities with well-established and easily applicable protocols for control
purposes;
— promote the diffusion of good practice;
— contribute to taking appropriate decisions.
Priority should be given to reduction of pollutants at the origin and to recover, if technically and economically
feasible, valuable substances (e.g. phosphorus) from sludge and derived products.
As part of a process and company quality approach, the relevant issues are therefore:
— exploiting the operating data and the statutory inspections carried out;
— rendering the process reliable, optimizing and of perpetuating it, as well as guaranteeing a permanent
development;
— maintaining a climate of confidence between the authorities, the sludge producers, the transporters, the
incineration plant and waste disposal site operators and allowing the services to be provided on a
contractual basis.
The local considerations to be taken into account are:
— the adoption of a more convenient solution with respect to other options;
— the geographical context, the client population and therefore the potential input material as well as the
expected developments;
— the proximity of the sewage treatment plant and the local transportation network;
— the capacity of treatment plants.
All of the recommendations of this document constitute a framework within which the thermal processes can
be proposed in addition to and/or as a substitution for land utilization, landfilling when allowed, or any other
process when relevant situations occur and appropriate conditions are met.
The management of sludges both upstream and downstream of the treatment process to ensure that it is
suitable for the outlets available is outlined in CEN/TS 13714:2013.
Figure 1 ― A basic scheme for deciding on sewage sludge use/disposal options and the relevant
CEN/TC 308 guidance documents
1 Scope
This Technical Report describes good practice for the incineration and other organic matter treatment by
thermal processes of sludges.
Thermal drying, thermal conditioning and thermal hydrolysis are excluded.
This Technical Report is applicable for sludges described in the scope of CEN/TC 308 specifically derived
from:
— storm water handling;
— night soil;
— urban wastewater collecting systems;
— urban wastewater treatment plants;
— treating industrial wastewater similar to urban wastewater (as defined in Directive 91/271/EEC);
but excluding hazardous sludges from industry.
2 Normative references
Not applicable.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
thermal treatment
reduction of organic matter by incineration, gasification, pyrolysis and wet air oxidation
3.2
thermal process
technique for the application of thermal treatment
3.3
combined thermal treatment
thermal treatment of sludge and other waste in the same device
3.4
pyrolysis
thermal treatment without supply of oxygen
3.5
gasification
thermal treatment with less than the stoichiometric supply of oxygen or air (partial combustion)
3.6
furnace
enclosed chamber where combustion of organic matter takes place
3.7
boiler
specific part of the thermal treatment plant where heat exchange takes place in view of recovering heat and
energy
3.8
flue gas treatment
any physical or chemical process aimed at cleaning the gas emission resulting from the thermal treatment with
the regard to their discharge into the atmosphere
3.9
bottom ash
combustion residue collected at the bottom of combustion furnaces
3.10
fly ash
solid material that is entrained in a flue gas stream
3.11
energy recovery
activity to use combustible waste as a means to generate energy through thermal treatment with recovery of
heat
3.12
recycling
activity in a production process to process waste materials for the original purpose or for other purposes,
excluding energy recovery
3.13
slag
partially glassy by-product obtained by cooling a mineral liquid phase
3.14
energy efficiency
amount of energy and/or heat recovery in relation to the energy content of input material
3.15
wet air oxidation
aqueous-phase oxidation of organics under pressure, using either air or oxygen as the oxidant
3.16
syngas
mixture of gases (including carbon monoxide, hydrogen, methane, etc.) produced from gasification or
pyrolysis process
3.17
char
combination of non-combustible materials and carbon produced from devolatization, gasification or pyrolysis
process
3.18
combustion
chemical and exothermical reaction with full oxidation of combustible materials
4 Abbreviations
For the purposes of this document, the following abbreviations apply.
BAT Best Available Techniques
COD Chemical oxygen demand
DM Dry Matter
ELV Emission Limit Values
GCV Greater (or gross) Calorific Value
HTFB High Temperature Fluidized Bed
LCV Lower (or net) Calorific Value
LOI Loss On Ignition
LPO Low pressure oxidation
MHF Multiple Hearth Furnace
MHV Medium heating value
MSW Municipal solid waste
NO Nitrogen oxides
x
PAH Polycyclic aromatic hydrocarbons
PCB Polychlorinated biphenyls
PCDD Polychlorodibenzodioxins
PCDF Polychlorodibenzofurans
RKF Rotary Kiln Furnace
SCR Selective catalytic reduction
SNCR Selective non-catalytic reduction
TOC Total organic carbon
UDG Up-draught or Counter-current gasifier
WWTP Wastewater treatment plant
5 Regulatory aspects
European regulations on thermal treatment of waste have been merged in Directive 2010/75/EU on industrial
emissions. This text merges seven previous European directives concerning the main industrial sectors and
especially the directives on the incineration of wastes (Directive 2000/76/CE) and on integrated pollution
prevention and control (Directive 2008/1/CE).
For the European regulation, an incineration plant is dedicated to the thermal treatment of wastes with or
without recovery of the combustion heat generated. This includes the incineration by oxidation of waste as
well as other thermal treatment processes such as pyrolysis, gasification or plasma processes in so far as the
substances resulting from the treatment are subsequently incinerated (this is not applicable if the gases
resulting from the thermal treatment of waste (pyrolysis or gasification) are purified to such extent that they are
no longer a waste and they cause emission no higher than those resulting from the burning of natural gas).
Incineration plant shall operate with a permit including provisions related to operating condition and emission
limit values. Best Available Techniques shall be considered for all stages of the life cycle of the thermal
treatment plant: conception, operation and closure.
Some of the operating conditions are, among others the following:
— a level of incineration, such that the slag and bottom ashes TOC content, shall be less than 3 % or their
loss on ignition less than 5 % of the dry weight of the material shall be achieved;
— the gas resulting from the process shall reach, after the last injection of combustion air, in a controlled
and homogeneous fashion and even under the most unfavourable conditions, the temperature of 850 °C.
If hazardous waste with a content of more than 1 % of halogenated organic substances is incinerated the
temperature shall be at least 1 100 °C. These temperatures shall be measured near the inner wall or at
another representative point of the combustion chamber as authorized by the competent authority and
kept for 2 s;
— an automatic system to prevent waste feed shall be operated when temperatures are below prescribed
values;
— any heat generated by the incineration process shall be recovered as far as practicable.
Limits are also given for (i) water discharges from the cleaning of exhaust gases, (ii) flue gas emission and (iii)
residues which shall be recycled, where appropriate, directly in the plant or outside in accordance with
relevant legislation.
The Emission Limit Values (ELV) are fixed in accordance with the emission values which are reachable with
the implementation of the Best Available Techniques (BAT). With the revision of the integrated pollution
prevention and control directive (2008/1/EC) and the publication of the Industrial Emission Directive
(2010/75/EU), the ELV cannot exceed the Best Available Technique associated emission level.
The emission limit values are reported in Annex A. It can be seen that the main difference from the previous
directives is the half-hourly average emission limit values.
6 Sludge properties
6.1 General
Sludge characterization for the assessment of thermochemical processes involves the evaluation of both
technical and economic parameters. The main technical characteristics to evaluate the suitability of
thermochemical processing are dry matter or moisture content, calorific value, ash content. The main
economic parameters are cost of processing, collection and transport, and the characteristics of the recovered
materials and by-products.
6.2 Physico-chemical characteristics
6.2.1 General
The main physico-chemical characteristics to be taken into account are:
— dry matter (or moisture content);
— loss on ignition;
— calorific value;
— amount of grease, scum and screenings.
Physical consistency, together with rheological properties, also play an important role, especially as far as the
design of feeding system is concerned.
6.2.2 Dry matter
The dry matter (DM), or moisture content, is of primary importance for thermal processes because it strongly
affects the Lower Calorific Value (LCV) of organic material which decreases when the moisture content
increases.
In thermal processing of sewage sludge dry matter is a parameter affecting both fuel requirement and exhaust
gas production. Generally speaking, any increase in dry matter is believed to be beneficial in the combustion
for the reduction in fuel requirement. When the condition for autogenous combustion, at a given temperature,
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
involves a more abundant gas production, due to dilution air or water needed for the control of the combustion
chamber temperature depending on design of incineration plant. However, the use of water, 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.
6.2.3 Loss on ignition
The loss on ignition represents the portion mass escaping as gas as a result of the ignition of the dry mass of
sludge.
The loss of ignition is generally used as a measure of the volatile matter content but it should be noted that
inorganic substances or decomposition products (e.g. H O, CO , SO , O ) are released or absorbed and some
2 2 2 2
inorganic substances are volatile under the reaction conditions.
It is measured by heating sludge in a furnace at (550 ± 25) °C and expressed as percent of the dry mass. The
loss on ignition can be used as an assessment of the organic part of the sludge, and is therefore related to its
heat value.
The presence in the sludge of iron with oxidation during ignition from iron (II) to iron (III), and of calcium
hydroxide or calcium oxide, when sludge is conditioned with lime, can involve decreasing of the loss on
ignition value (EN 12879).
6.2.4 Calorific value
Calorific value of sludge is a very important parameter for the evaluation of thermal processes, as it
represents the heat quantity developed in the combustion process by the unit mass of material in standard
conditions.
The Calorific Value can be expressed as (see EN 15170):
— Greater (or Gross) calorific value (GCV) at constant volume with both the water of the combustion
products and the moisture of the sludge as liquid water;
— Lower (or Net) calorific value (LCV) obtained by calculation from the Gross calorific value provided that
either the hydrogen content of the sludge or the amount of water found in the combustion test has to be
determined.
Sludge usually contains much water, combustible and incombustible solids. Therefore their calorific value,
especially on the “as received” basis – is quite low.
The calculation of calorific value of sludge is based on LOI (loss on ignition or organic matter content).
Typical calorific values of municipal wastewater sludge 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 sludge display values between
20 700 kJ/kg LOI and 24 400 kJ/kg LOI.
GCV and LCV values can be calculated according to the standard method EN 15170, while the procedures for
the theoretical calculation of GCV and LCV are reported in Annex B.
6.2.5 Grease, scum and screening
Grease, scum and screenings can be thermally treated together with sludge but generally they pose several
problems.
Screenings clog feed mechanisms for certain types of furnace and therefore grinding or shredding is advisable
before feeding. Screenings also contain bulky and incombustible materials, which create problems in the ash
disposal system.
Skimmed material generally contains more than 95 % moisture and therefore it should be dewatered to at
least 25 % solids before treatment. Skimming is difficult to handle in the dewatered state due to its viscosity
and a heating process to 70 °C - 80 °C is generally requested to get skimming pumpable. After dewatering,
scum solids should be ground to a size not exceeding 6 mm. GCV of skimming and screenings are in the
range 37 000 to 44 000 kJ/kg DM and 23 000 to 25 600 kJ/kg DM, respectively.
Quantities of screenings are strictly dependent on the screen openings: they can vary in the range of
−6 3 3 −6 3 3
3x10 m /m to 40x10 m /m of sewage for openings of 12 mm to 25 mm (the upper limits apply to the
reduced openings). As 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,
3 3
considering that the density of wet 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
wastewater treatment plant: the highest values can be as high as 17 g of DM/m of sewage which means up
to 1,7 % of sludge production. At a concentration of 25 % this value increases to 6,8 %.
The quantity of any added material, especially grease, scum and screening, is limited by the capacity and the
efficiency of the gas treatment.
6.2.6 Physical consistency and others
The physical consistency of the sludge will influence the selection and design of thermal processes.
Therefore, the evaluation of specific parameters giving information on this aspect (e.g. flowability, solidity,
piling behaviour) appears useful in this designing step.
Other characteristics influencing thermal processes are particle size, bulk density and morphology.
6.3 Chemical characteristics
6.3.1 General
The main chemical characteristics to be taken into account are:
— sulphur;
— phosphorus;
— nitrogen;
— chlorine and other halogens;
— organic micro pollutants;
— trace elements (especially mercury).
The presence of the above mentioned chemicals has to be known in order to prevent or minimize toxic
emissions (gaseous, liquid, solid) from thermal processes.
Typical elemental composition of primary, secondary, mixed and digested sludge is given in Table C.1.
6.3.2 Sulphur
The sulphur content of sewage sludge ranges generally from 0,5 % to 2 % of dry matter.
In anaerobic digestion, sulphate is converted to sulphide by sulphate reducing bacteria. Some of it precipitates
with iron and other metals as insoluble sulphides, while some other is stripped as hydrogen sulphide and is
transferred to the biogas stream from which it can be removed by scrubbers. The amount of residual
sulphides in anaerobically digested sludge is proportional to the metal content in the raw sludge. If sludge is
not treated anaerobically, most of the sulphate remains in solution as such. If poly ferrous sulphate and ferric
chloride are used as inorganic conditioners in thickening and dewatering, sulphur content increases.
Sometimes, this can affect the cost of acid gas removal (e.g. in flue gas desulfurization, FGD). Because a
fraction of the sulphur is present in the oxidized sulphate form, not all of this sulphur is converted to sulphur
dioxide during combustion. Sulphur dioxide then combines with moisture, either in the waste gas treatment
system or in the atmosphere, to form sulphuric and sulphurous acids.
6.3.3 Phosphorus
Phosphorus may be present in sewage sludge in concentration ranging from 1 % to 5 % dry matter. This
concentration mainly depends on the phosphorus load in the wastewater system and on the level of
phosphorus removal accomplished in the treatment plant.
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; therefore, recovery of
2 5
phosphorus from ashes should be considered.
6.3.4 Nitrogen
Nitrogen content of sewage sludge ranges from 2 % to 12 % of dry matter; typical values are around 5 % to
7 % of volatile solids in a mixed primary and secondary sludge. Organic nitrogen can be converted during
combustion to molecular nitrogen or to NO , depending on the temperature and atmosphere inside the
x
furnace. NO formation from fuel bound nitrogen can be controlled by restricting the air flow to the minimum
x
excess above the stoichiometric requirement and by staging the air flow to the furnace (see 8.1).
6.3.5 Chlorine and other halogen
Organic and inorganic chlorine compounds play an important role in the combustion processes after tendency
of the chlorine radicals to bind active radicals, like O*, H*, OH*, RO*, thus determining a decrease in the
combustion rate with potential formation of toxic compounds. Chlorine and other halogens are also
responsible for the presence in the exhaust gases of undesirable acidic compounds inducing corrosion
problems especially at high temperatures. The presence of organic chlorine in sewage sludge is generally
negligible (less than 50 mg/kg DM) but the concentration of inorganic chlorine may exceed some units per
cent dry mass depending on the chlorine content in the sludge water and on the use of inorganic conditioners.
The agroindustry sludge, similar to sewage sludge mentioned in Directive 91/271/EC, from food and/or
beverage transformation and production, do not contain organic chlorine. As for sewage sludge, inorganic
chlorine can be present in such sludge after the use of FeCl as conditioning agent.
Bromine can exert similar effects than chlorine but the organic compounds are more easily formed and they
can also be easier destroyed at high temperatures.
6.3.6 Organic micro pollutants
Although the presence of biopersistent organic micro pollutants (such as chlorinated hydrocarbons, phenols
and polyphenols, polychlorinated biphenyls (PCB), pesticides and polycyclic aromatic hydrocarbons (PAH)
and pharmaceuticals) in sewage sludge may be in some cases noticeable, they generally do not pose relevant
problems in thermal processing.
Formation of dioxins can be a serious problem depending on the gas treatment and the temperature of the
incineration. Dioxins can be formed again (de novo synthesis) during the gas treatment, especially in the
range of temperature 200 °C to 600 °C, for sludge with a high content in organochlorine compounds, this can
be avoid by a rapid quench of the exhaust gas. Significant formation of particularly stable compounds has
been evidenced in oxygen-deficient environments.
6.3.7 Trace elements
The presence of trace element, such as mercury, arsenic, lead, cadmium and zinc, in sewage sludge shall be
considered for their tendency to be transferred in the gaseous phase. Except for mercury, they may be
concentrated in fly ashes collected in bag and electrofilters. Mercury generally escapes with flue gases but
can be condensed in scrubbers or captured by activated carbon filters.
Trace elements are generally present in sewage sludge at variable concentrations depending on the
proportion of industrial effluents in the wastewater. Table C.2 shows the typical concentration range of trace
elements.
7 Thermal processes fundamentals
7.1 Incineration
Incineration (or combustion) is an oxidation reaction carried out at high temperature which makes it possible to
reduce both the mass and volume of the materials being treated by reducing them to ash, while taking
advantage of their inherent energy potential. The contained water, converted into vapour, and the organic
matter converted into combustion gases are discharged into the atmosphere after treatment.
The reaction of oxygen with carbon, hydrogen and sulphur yields energy and products of combustion, namely,
carbon dioxide (CO ), water (H O) and sulphur dioxide (SO ). Organic nitrogen is preferentially converted to
2 2 2
nitrogen gas but a certain amount (2 % to 7 %) can also be further oxidized to nitrogen oxide (NO).
The nitrogen in the air is also candidate to be converted to oxides of nitrogen (NO ). This phenomenon begins
x
to be noticeable at temperatures higher than 1 100 °C and increases with any further increase of temperature.
The reactions taking place are:
C + O → CO + 393.77J
2 2
C H + (x + y/4) O → xCO + y/2 H O
x y 2 2 2
Incineration produces a waste gas composed primarily of carbon dioxide (CO ) and water (H O). Other air
2 2
emissions include nitrogen oxides, sulphur oxides, etc. The inorganic content of the waste is converted to ash.
In the case of lack of oxygen (generally referred as starved-air combustion) the reactions are characterized as
incomplete combustion ones, where the produced CO reacts with C that has not been consumed yet and is
converted to carbon monoxide (CO) at higher temperatures:
C + CO + 172.58J → 2CO
The temperature achieved in the combustion process will depend on the balance between the energy inputs
and the energy outputs. Nevertheless, concerning European legislation it shall be ensured that the
temperature is higher than 850 °C during a 2 s time period.
All oxidizing combustion reactions require some excess air to ensure that the reaction proceeds rapidly to
completion. Air required for combustion is mainly a function of time of stay, temperature and turbulence,
commonly referred to as the “3Ts of combustion”. Generally as turbulence is maximized, 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 % (1,2 to 1,3 times the stoichiometric amount of air) excess air while less efficient furnaces, like multiple
hearth and rotary kiln furnaces, need 100 % to 125 % (2,00 to 2,25 times the stoichiometric amount of air)
excess air at least. As excess air quenches the combustion temperature it is desirable to minimize 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.
The influence of cake concentration on fuel consumption, air requirements and flue gas production in
incineration of sewage sludge is known. It has been shown that fuel consumption, air and flue gas production
may be expressed as a linear function of cake concentration, with line slopes changing at the two points
identified by the operating modes of (i) minimum concentration for autogenous combustion in the furnace, and
(ii) minimum concentration for no air requirements in the afterbur
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