STOCKHOLM, SWEDEN 2017
KTH ROYAL INSTITUTE OF TECHNOLOGY KTH CHEMICAL SCIENCE AND ENGINEERING
Evaluation of overall
environmental impacts of
alternatives for emission
control systems applied for
waste to energy process
Camilla Sundin
Bachelor of Science in
Chemical Engineering and Technology
Title: Evaluation of overall environmental impacts of
alternatives for emission control systems applied
for waste to energy process
Swedish title: Utvärdering av miljöpåverkan hos alternativa
rökgas- och kondensatreningssystem för
avfallsförbränning
Keywords: Waste to Energy, Waste Incineration,
Emission Control Systems, Environmental Impact
Evaluation
Work place: Vattenfall AB
Supervisor at
the work place: Jinying Yan
Supervisor at
KTH: Per Alvfors
Student: Camilla Sundin
Date: 2017-06-21
Examiner: Per Alvfors
incinerated to generate electricity and district heating. The waste incineration generates flue gases, and the energy in the hot flue gases is recovered by condensation. Both flue gases and the flue gas condensate are treated by emission control systems before being released into nature. The emission control system is planned to be updated with better technologies currently applied in Sweden. In this study, a comparison of the overall environmental impact of the current emission control system and the new system for emission control is performed.
Vattenfall will perform a comparative life cycle assessment, LCA, of the two emission control systems. A part of the LCA is an inventory analysis. In this study, data required for the inventory analysis will be collected and quantified. The parameters considered are emissions to air and water, consumption of chemicals, produced solid residue, and water utilization. The objectives with the planned upgrade of the emission control system, which are evaluated in this study, are to obtain a cleaner condensate stream that can be reused in the system, to reach a higher tolerance of sulphur content in the fuel, and to reach a better reduction of emissions, for future stricter regulation.
From the perspective of this study, the new system for emission control system seems to be the choice for emission control system with the least environmental impact. The results show that the reasons for upgrading the emission control system are met with the new system. The condensate is significantly cleaner with the new alternative emission control system than in the current one, the total amount of emissions decreases by 99,98 w%. The significantly smaller amount of emissions results in a condensate flow that can be reused in the system, which could save a considerable amount of raw water each year. The SO2 emissions are reduced by 99,5 w%, which show that a higher sulphur content in the fuel could be tolerated with the new emission control system. Furthermore, the total amount of emission content in the flue gas decreases with 61,9 w% with the new emission control system. The emission
parameters that are deemed likely to be more strictly regulated in coming regulations, NOx and Hg, are both significantly reduced with the new emission control system.
On the other hand, both the consumption of chemicals and the production of solid residue increases in the new emission control system, compared to the current one.
These aspects are important drawbacks with the new system, and the environmental impact of these aspects needs to be further investigated in the planned LCA.
att generera el och fjärrvärme. Avfallsförbränningen genererar rökgaser, och energin i rökgaserna utvinns genom kondensering. Rökgaserna och kondensatet renas från föroreningar innan de släpps ut i naturen. Systemet för rökgas- och kondensatrening ska uppdateras till nyare tekniker. I det här arbetet jämförs den totala miljöpåverkan av det nuvarande och det nya systemet för rökgas- och kondensatrening.
Vattenfall kommer i framtiden att utföra en jämförande livscykelanalys av de två rökgas- och kondensatreningssystemen. En del i en livscykelanalys är en
inventeringsanalys. Syftet med det här arbetet är att samla in och kvantifiera den data som behövs för inventeringsanalysen. Parametrarna som tas med i
inventeringsanalysen är utsläpp till luft och vatten, kemikalieförbrukning, restprodukter samt vattenförbrukning.
Målen med uppdateringen av rökgas- och kondensatreningssystemen är att erhålla ett renare kondensat som kan återanvändas i systemet, att kunna elda bränsle med en högre halt svavel och fortfarande hålla utsläppen under utsläppsgränserna, samt att få en bättre rening av föroreningar för att kunna möta framtida utsläppskrav. Målet med detta arbete är att utvärdera hur väl dessa aspekter möts i det nya systemet för rökgas- och kondensatrening.
Utifrån de aspekter som utvärderats i denna studie verkar det nya systemet för rökgas- och kondensatrening ha en mindre miljöpåverkan än det nuvarande. Resultaten visar att alla målen med att uppdatera rökgas- och kondensatreningssystemet nås med det nya systemet. Kondensatet blir signifikant renare med det nya systemet jämfört med det nuvarande, den totala mängden föroreningar i kondensatet minskar med 99,98 vikts%. Det innebär att kondensatet kan återanvändas i systemet, och en betydande mängd råvatten kan sparas varje år. Utsläppen av SO2 minskar med 99,5 vikts%, vilket visar att en högre svavelhalt i bränslet skulle kunna tolereras. Vidare minskar den totala mängden föroreningar i rökgaserna med 61,9 vikts%. De föroreningsparametrar som bedöms bli mer strikt reglerade inom en snar framtid, NOx och Hg, reduceras signifikant med det nya systemet för rökgas- och kondensatrening.
Däremot ökar både kemikalieförbrukningen och mängden producerad restprodukt.
Ökningarna är betydande nackdelar för det nya systemet, och miljöpåverkan av detta bör undersökas vidare i den planerade livscykelanalysen.
WtE Waste to energy, recovering the calorific potential in waste, commonly by incineration.
LCA Life cycle assessment, a method to measure the environmental impact of a product or process.
IED European industrial emissions directive.
BREF WI Best available technique reference documents on waste incineration.
BAT-AEL Best available technique associated emission levels.
SNCR Selective non-catalytic reduction, a method of reducing NOx
in flue gas.
ESP Electrostatic precipitator, a method of separating dust in flue gas.
MF Micro-filtration, a method of separating pollutions in water.
UF Ultra-filtration, a method of separating pollutions in water.
NF Nano-filtration, a method of separating pollutions in water.
RO Reversed osmosis, a method of separating pollutions in water.
TMT Trimethyltryptamine, an organic sulfide used to bind heavy metals in water.
TCDD Tetrachlorodibenzodioxin, a particularly toxic type of dioxin.
Dioxins are usually measured in TCDD – equivalents.
1 Introduction ... 1
1.1 Aim ... 1
1.2 Objective ... 1
1.3 Method ... 1
2 Technical background ... 2
2.1 Life cycle assessment for processes ... 2
2.2 Waste to energy ... 3
3 Goal and scope of this study ... 6
4 Emission control systems applied for waste incineration ... 7
4.1 Emission control during combustion ... 7
4.2 Emission control systems for flue gas ... 8
4.3 Flue gas condensate treatment ... 9
5 Alternative emission control systems ... 11
5.1 Case A. Current system ... 11
5.2 Case B. Membrane based alternative... 16
6 Result and discussion - Comparison of case A and case B ... 19
6.1 Emissions ... 19
6.2 Consumption of chemicals ... 22
6.3 Solid residue ... 23
6.4 Water utilization ... 24
7 Error sources ... 25
8 Suggestions for further investigations ... 26
9 Conclusion ... 27
10 References ... 28
Appendix 1. Calculations ... 30
1
1 Introduction
Waste management in Sweden is described by a waste hierarchy. Waste prevention, reuse and material recycling are the primarily considered means in the hierarchy. (Avfall Sverige 2016) After reuse and recycling, there are two options for caretaking of waste: energy recovery and landfill (Klinghoffer, Themelis and Castaldi 2013). Energy recovery is the most suitable option for waste that cannot be recycled (Avfall Sverige 2016). Energy recovery is a way of reducing the amount of waste that needs disposal, while producing energy. The heat generated by the combustion process is utilized as steam, creating electricity, and heating up return district heating water. This process is called waste to energy, WtE. (Klinghoffer, Themelis and Castaldi 2013) Vattenfall operates a WtE cogeneration plant where three blocks incinerate household and industrial waste to generate electricity and district heating.
The incineration generates flue gases. Energy in the hot flue gases is recovered by
condensation. Both flue gases and the flue gas condensate are treated by emission control systems based on emission permits applied for WtE units in Sweden. The emission control systems for the waste incineration plant are planned to be updated with better technologies currently applied in Sweden. This study investigates the overall environmental impact of a new alternative system for emission control.
1.1 Aim
The aim of this study is to support Vattenfall in deciding, from an environmental
perspective, on the most appropriate emission control system for their waste incineration plant. Vattenfall will perform a comparative life cycle assessment, LCA, of the alternative emission control systems. A part of the LCA is an inventory analysis. In this study, data required for the inventory analysis will be collected and quantified. A further description of the part of the comparative LCA performed in this study is found in chapter 3.
1.2 Objective
The objectives with the planned upgrade of the emission control system are to:
o Avoid transfer of pollutants from flue gas to condensate, as much as possible.
o Obtain a cleaner condensate that can be reused in the system.
o Reach a higher tolerance of sulphur content in the fuel.
o A better reduction of emissions, for future stricter regulations.
The objective with this study is to evaluate if these aspects are met in the new emission control system.
1.3 Method
A literature study was performed to gain knowledge on the different available emission control techniques. To reach an understanding on the environmental impact of the current emission control system, case A, and the alternative new emission control system, case B, a comparison between the two cases were made. Aspects considered in the comparison were emissions to air and water, consumption of chemicals, produced solid residue, and water utilization. Case A is the current system for emission control applied for flue gas and flue gas condensate at the Vattenfall waste incineration plant. Case B is an alternative for a new emission control system. Data for the cases were collected via internal data systems and consultations with employees at Vattenfall.
2
2 Technical background
In this chapter, a brief introduction to life cycle assessment is given. The work on which this report is based will be used for a comparative LCA in the future, as part of an inventory analysis. The methodology for LCA is therefore presented below. Furthermore, a short description of waste management and waste to energy is given in this chapter.
2.1 Life cycle assessment for processes
Life cycle assessment, LCA, is a scientific method used for measuring the environmental impact of a product or process, during its whole life cycle. All direct and indirect
environmental impact of the product or process is assessed. (Burgess and Brennan 2001) The methodology of LCA has changed over the years, it started as a relatively undefined method mainly used by smaller companies. Nowadays, the methodology is regulated in the International Standardisation Organisation. (Burgess and Brennan 2001) The integrated approach of LCA reduces the risk of short-term solutions with little environmental
improvement, that are implemented as a reaction to a specific demand (Azapagic 1999). In figure 1, the phases of LCA are shown. The phases are described below.
Figure 1. The general phases of a life cycle assessment.
2.1.1 Methodology
During the goal and scope phase of the study, the functional unit is selected, the way to perform the LCA established, and the goal of the LCA is clearly defined. Why the LCA should be performed is decided, and how the results of the LCA should be used. Goal and scope definition is the first step of the LCA that is performed at the beginning of the study, before any data are collected.
According to what has been defined in the goal and scope phase, data for the functional unit is collected during the inventory analysis. A flow chart of the system is defined, where every unit within the functional unit is associated with its own input and output. In this manner, the ingoing and outgoing streams to the functional unit are defined and quantified.
During the impact assessment, the collected data is assessed, to understand and identify the environmental impact of the functional unit. The impact assessment is followed by the interpretation phase, where the results are interpreted and conclusions are drawn. The conclusions should refer to the goal and scope of the study. (Klöpffer and Grahl 2014)
3 2.2 Waste to energy
How to efficiently take care of waste has been a topical subject for a very long time, and is an existing concern to this day. To set standards on how waste should be taken care of, a waste hierarchy has been established, shown in figure 2.
Figure 2. Illustration of the waste hierarchy.
The purpose with the waste hierarchy is mainly to reduce the amount of landfilled waste.
The preferred method to do so is primarily by reducing the amount of produced waste. In cases where the waste is already produced, reuse and recycling are important ways to prolong the lifetime of the product or the material. (Avfall Sverige 2016) Recycling and reuse become problematic when long distance transport, or treatments with large environmental impact, are needed for the waste before utilization. Also, with recycling and reuse, the benefits of a locally available energy source are lost. (Stehlik 2016)
Considering these factors, energy recovery is the preferred choice when waste cannot be effectively reused or recycled. There are several ways to recover energy bonded in waste.
The most common method for WtE is waste incineration. Benefits of energy production through waste incineration consists of recovery of calorific potential and reduction in volume of the amount of waste that needs landfilling. (Stehlik 2016) Heat recovered in waste incineration plants is utilized for district heating, meaning that the waste incineration plant supply many households with heating. The emissions from one big plant with strict regulations is far less than if every household would have their own boiler, without emission control systems. (Vattenfall AB 2017) This is another important benefit with waste
incineration.
2.2.1 Fuel
The fuel incinerated at the Vattenfall waste incineration plant consists of 50 % municipal solid waste and 50 % industrial waste. The main share of the waste originates from nearby municipalities, while some is imported from the United Kingdom, Ireland, Norway, and Åland. For 2015, the amount of imported waste was 29 %. (Vattenfall Värme Uppsala 2015)
2.2.2 Boiler
The Vattenfall waste incineration plant consists of two grate boilers. The combustion air is drawn from the waste bunker to induce a negative draft, to ensure no odor escapes the bunker. (Jung 2010) (Vattenfall Värme Uppsala 2015)
4 2.2.3 Condensation
The flue gases resulting from the combustion have a steam content of about 15 %. The condensation energy can be utilized by condensing the moisture in the flue gases. There are two main principles of utilizing the condensation energy: by direct condensation or by indirect condensation. With direct condensation, the flue gases are cooled by spraying water over the flue gases. In indirect condensation, the flue gases are cooled via heat exchangers. (Persson 2005) The heat recovered with condensation is utilized by heating return district heating water (Pöyrö Sweden AB 2016). The return district heating water have an inlet temperature of 45 – 55 °C. (Stenqvist 2012) The hot flue gases have an inlet temperature of about 140 °C (Pöyrö Sweden AB 2016). The bigger temperature difference there is between the two medias inlet temperature, the more energy is recovered in the condensation process. The condensation results in a water stream that needs to go through a condensate treatment before being let out to recipient or reused in the system.
2.2.4 Pollutants
As with all incineration processes, waste incineration result in flue gases containing pollutants. The most important pollutants resulting from waste incineration are acid components, metals, and dioxins.
2.2.4.1 Acid components
Acid components in the flue gas consists mainly of nitrogen oxides NOx (x=1,2), SO2, and HCl (Vehlow 2015). Acid components contributes to acidification and overfertilization of the environment, and are therefore important to reduce before letting the flue gas out to the atmosphere (Larsson 2016).
2.2.4.2 Metals
Waste is the fuel for incineration with the highest content of heavy metals, compared to other commonly incinerated fuels such as wood and peat (Vattenfall Värme Uppsala 2015) (Persson 2005). The metals that are subject to emission requirements are Sb, As, Pb, Cr, Co, Cu, Mn, Ni, and V (Pöyrö Sweden AB 2016). Several of these metals have carcinogenic or other toxic effects on humans (Kyrklund 2016). Most of the metals are evaporated during the combustion and end up in the flue gas. At the outlet of the boiler, where the
temperature drops, all metals except mercury condense onto particulate matter.
(Christensen 2011) 2.2.4.3 Dioxins
Dioxins are chlorine containing organic compounds. They are among the most carcinogenic substances known, and are largely toxic for both human and nature. Dioxins are formed during incineration when chlorine is present. (Al-Hanbali 2017) Part of the dioxins are absorbed onto particulate matter in the flue gas (Vehlow 2015).
5 2.2.5 Regulations
In Sweden, waste incineration is mainly regulated in regulation (SFS 2013:253) on waste incineration. The regulation state, among other things, emission limits for air and water. The regulation on waste incineration is harmonized from the European Industrial Emissions Directive, IED (2010/75/EU).
Furthermore, on an EU basis, emission limits for waste incineration are stated in the best available technique reference documents on waste incineration, BREF WI.When IED was harmonized into national legislation, it also gave stricter meaning to the best available technique associated emission levels, BAT-AELs, in the BREF WI document. Prior to this, the BAT-AELs in the BREF WI document was only used as reference when setting emission limits.
In addition to emission limit values in the Swedish regulation on waste incineration, each waste incineration plant in Sweden is given their own emission limits, in an environmental permit. These are decided partially on environmental quality standards for the specific geographic location of the plant. That generally means, if there are a lot of other
environmentally affecting activities in the area, the emission limits will be stricter than if the plant is in an area with few other environmentally affecting activities. However, the
emission limits set for each plant can never exceed the emission limits set in the Swedish regulation on waste incineration.
During 2017, a new BREF document on waste incineration will be released. When the new BREF WI document is published and entered force, the BAT-AELs will be applied on top of all other emission limits already in place via the Swedish regulations and the plant’s
environmental permit. Consequently, the 2017 BREF document can affect existing waste incineration plants, either with lower emission limits than their previously set individual limits, or with new requirements on measurement intervals. In these cases, the plant has four years to adapt to the new terms. There are two emission parameters deemed specifically likely to be more strictly regulated in the 2017 BREF document: NOx and Hg.
(Sahlén 2017)
6
3 Goal and scope of this study
The goals with the part of the LCA performed in this study are to:
o Give an overview of the data for emissions to air and water for case A and case B.
State the data of emissions to air and water that are currently not available. The available data should be presented as concentrations and in g/ton steam, e.g.
emissions related to the steam production in the boiler.
o Compare the data on emissions with current emission limits.
o Give an overview of the flows of chemicals, solid residues, and water.
The functional unit is the emission control system for flue gas and flue gas condensate. The functional unit is indicated in the illustration of the Vattenfall waste incineration plant below, see figure 3.
Figure 3. The Vattenfall waste incineration plant with the system boundary indicating the functional unit of the system (Pöyrö Sweden AB 2016).
For secrecy reasons, no specific data on the cases will be presented in this official report.
Instead, a percentage comparison between case A and case B will be presented.
7
4 Emission control systems applied for waste incineration
The techniques for emission control of flue gas and flue gas condensate that are relevant for case A and case B are described in this chapter.
4.1 Emission control during combustion
The condition in the boiler is relevant to the formation of some pollutants, and chemical dosage can be used for abatement.
4.1.1 NOx reduction
NOx that forms during waste incineration originates from nitrogen compounds in the waste.
Thermal NOx is almost completely non-existent, since the temperature for waste incineration, typically 900 – 1 000 °C, is lower than that where thermal NOx is formed.
(Vehlow 2015)
Primary NOx reduction refers to the prevention of NOx formation by modifying the
conditions in the boiler. NOx formation depends greatly on the nitrogen content in the fuel, but, among other things, temperature, type of boiler, and oxygen surplus also affects the NOx formation. A measure of preventing NOx formation is recirculating the flue gases, and thereby lowering the temperature in the boiler.
Secondary NOx reduction refers to abatement of already formed NOx. This can be done via selective non-catalytic reduction, SNCR, where ammonia or urea is added to the boiler to reduce NOx to N2. The most prominent reduction reactions are shown below.
4 𝑁𝑂 + 4 𝑁𝐻3+ 𝑂2 → 4 𝑁2+ 6 𝐻2𝑂 6 𝑁𝑂2+ 8 𝑁𝐻3 → 7 𝑁2+ 12 𝐻2𝑂
The critical parameter for the SNCR method is temperature. If the temperature where the reducing agent is added into the boiler is too low, the NOx reduction moves along very slowly. This results in high NOx content in the flue gas, and a high content of unreacted NH3. If the reducing agent is added into the boiler at a high temperature, it reacts to form NOx and N2O, resulting in a higher content of pollutants in the flue gas rather than lowering it.
(Persson 2005)
The varying temperature of the boiler impacts the need of chemical dosing. Furthermore, not all the added reducing agent, i.e. ammonia or urea, will be available to reduce the NOx. This could be due to uneven distribution of reducing agent in the boiler, or that the reducing agent is added at a temperature high enough for it to take place in other reactions. To reach a satisfactory NOx reduction via SNCR, the reducing agent therefore must be added to the boiler in quite a big surplus. The overdosing results in a varying amount of unreacted NH3, that leaves the boiler with the flue gases. This is called NH3 slip, which needs to be abated downstream in the emission control system. (Goldschmidt, Wiig and Lord 2015) SNCR reduces the amount of NOx in the flue gas by 50 – 60 % (Persson 2005).
8 4.1.2 Dioxin prevention
To avoid dioxin forming in the boiler, temperature and residence time are important factors (Persson 2005). The largest amount of dioxin formation takes place within the temperature range of 200 – 450 °C. Keeping the temperature consistently over 450 °C, the residence time over two seconds, and having an oxygen surplus of at least 6 % in the boiler, keeps the dioxin formation at a minimum. (Cheng and Yuanan 2010)
4.2 Emission control systems for flue gas
When the flue gas leaves the boiler, it needs to be reduced from dust, acid gas components, dioxins, and metals, before it can be let out into the air.
4.2.1 Electrostatic precipitator
ESPs are commonly chosen dust separators, since they are efficient and give a low pressure drop. An ESP removes dust, and thereby condensed heavy metals and dioxins, from the flue gas via electrostatic force. In the ESP, there are two parallel metal plates with emission electrode wire in between. By corona discharge from the wire, electrons collide with the dust particles in the slowly flowing flue gas. The dust particles get a negative charge and precipitate on to the metal plates. The particles, often referred to as fly ashes, are removed from the metal plates either by mechanical banging, called dry ESP, or by hosing, called wet ESP. (Persson 2005) (Vehlow 2015)
The ESP is usually placed in the beginning of the flue gas treatment, since fly ashes are preferably collected before any wet separation methods. ESP can also manage higher temperatures in the flue gas, meaning that a placement just downstream of the boiler is possible. (Persson 2005)
Since heavy metals and dioxins are condensed onto the fine particulate matter, it is
important that the dust removal system is efficient for small particles. ESPs have over 99,5
% removal efficiency for particles of 10 µm. To avoid further dioxin formation, the temperature of the flue gas in the ESP should be kept below 200 °C. (Vehlow 2015) 4.2.2 Baghouse filter
Baghouse filters are used to separate dust, dioxins, heavy metals, and acid components in the flue gas. Baghouse filters are made up of several fabric filter bags, that the flue gas flows through. The process builds up a cake of dust on the outside of the filter bags, which
increase the dust separation largely. This means that a baghouse filter varies in separation efficiency, depending on how long it has been in operation. When the pressure drop over the filter becomes too big, the dust is collected from the filters by mechanical vibration or blowing. (Persson 2005) (Vehlow 2015)
To reduce the risk of hot particles burning holes in the fabric filter, the baghouse filter is typically placed further downstream of the boiler than the ESP. Moreover, baghouse filters are even more efficient than ESP for separation of fine particulate matter, making the placement after ESP logical. (Persson 2005) (Vehlow 2015)
9 4.2.3 Scrubber
A scrubber can be used for separation of dust and acid components in the flue gas. The most common scrubber type is the spray scrubber, where a liquid is sprayed out over the flue gas. Different acid components need different conditions to absorb into the liquid.
Therefore, the scrubbing can be done in two steps: an acid step where mainly NH3 is absorbed, and a neutral, or slightly acidic, step where SO2 is absorbed. (Persson 2005) (Vehlow 2015)
The absorption process depends greatly on the contact surface between the flue gas and the liquid. Therefore, the liquid is typically sprayed over the flue gas to create tiny drops resulting in large amount of contact surfaces. Another way to create contact surface in a scrubber is by using filling bodies. (McCabe, Smith and Harriott 2005)
This separation of acid components also takes place in the condensation of the flue gases, where pollutants dissolve into the condensed liquid (Pöyrö Sweden AB 2016).
4.3 Flue gas condensate treatment
When the condensate leaves the condenser, it needs to be abated from suspended particles, acid components, metals, and dioxins before it can be led out to recipient or reused.
4.3.1 Conventional condensate treatment
A conventional condensate treatment is built up of basins with different purposes. In the first basin, the pH of the condensate is raised to around 8,5, by adding NaOH or Ca(OH)2. At the higher pH, metals in the condensate are precipitated according to the reaction formula shown below.
𝑀𝑒2++ 2 𝑂𝐻− ⇋ 𝑀𝑒(𝑂𝐻)2 (𝑠)
In the next basin a flocculation chemical, normally a polymer, is added to the condensate.
This is done to separate very small suspended particles from the condensate stream. Also, the precipitated metal hydroxide particles are small, and the flocculation chemical ensures their sedimentation. Accordingly, the next basin in the condensate treatment is the
sedimentation. Sedimentation is where a flow of water is slowed down to a rate where particles in the water can no longer stay floating. The difference in density is utilized to separate suspended and precipitated particles from the condensate stream.
The condensate stream is hereafter led through polishing steps, to further reduce the number of particles in the condensate stream. The sand filter is commonly used as a polishing step after sedimentation. The sand filter can be followed by an activated carbon filter where further removal of particles takes place, before the stream is let out to the recipient or reused in the system. (Persson 2005)
4.3.2 Membrane based condensate treatment
Membrane filtration is based on semi permeable membranes, that lets water pass but stops pollutions from getting through. Essentially, it is a way to concentrate the pollutions in a much smaller amount of liquid. The cleaner stream that goes through the membrane is
10 called permeat, and the concentrated polluted stream is called concentrate, or more
commonly, reject.
There are four different pressure driven membrane processes: micro-filtration, ultra- filtration, nano-filtration, and reversed osmosis. The principle for the first three processes are as follows. A liquid with a certain concentration of a substance is in contact with a liquid, with another concentration of the same substance, through a semi permeable membrane.
The membrane does not allow the substance to flow through it. Nature then strives to even out the concentration difference in the two liquids, so that the more concentrated liquid is diluted and both liquids approach the same concentration. This goes on until the osmotic pressure difference is reached. In reversed osmosis, the side of the membrane where the liquid is more concentrated is applied with a pressure. In this case the liquid diffuses from the more concentrated liquid to the less concentrated one. Through this, one stream with pure water and one stream with the concentrated substance is obtained. Another
difference with reversed osmosis is that it does not work through filtration, but rather with diffusion. A comparison of the different membrane based processes is presented in table 1.
Table 1. Comparison of membrane processes. (Persson 2005)
Membrane process Diameter separated particles [µm]
Pressure need [MPa]
Means of separation Micro-filtration, MF 0,2 - 10 0,01 – 0,1 Filtration
Ultra-filtration, UF 0,001 - 0,1 0,2 – 1,5 Filtration Nano-filtration, NF 0,001 - 0,01 2 – 4 Filtration + diffusion Reversed osmosis, RO 0,0001 - 0,002 2 – 10 Diffusion
Membranes easily get clogged and need cleaning or replacement. Therefore, a reversed osmosis membrane is usually preceded with membranes that separate larger particles than what is separated in the reversed osmosis. Even though this precaution is applied, the membranes will need the occasional cleaning, and eventually replacement. (Persson 2005)
11
5 Alternative emission control systems
In this report, the current emission control system is compared to an alternative new emission control system. The current emission control system is presented as case A, and the new alternative emission control system is presented as case B.
SNCR is used in the boilers to reduce the amount of NOx in the flue gas. Also, flue gas recirculation is done to the boilers to reduce the amount of formed NOx. When the flue gas flows leave the respective boilers, they go through an ESP each before they are combined to a single flue gas flow. The emission control system thus far will remain unchanged, and is applicable for both case A and case B.
5.1 Case A. Current system
Case A is the current emission control system of the waste incineration plant operated by Vattenfall.
5.1.1 Emission control system flue gas
After fly ashes are removed by ESPs, flue gases from the two blocks are put into one flue gas stream, which is treated by flue gas condensation and further flue gas treatment. In the flue gas condensation, about 2/3 of the water content in the flue gases is condensed out. The condensing also works as an emission control step for the flue gas, since some components in the flue gas is dissolved into the condensate. Mainly HCl is separated through the
condensation. After the condensation, the flue gas is led to a baghouse filter, where further separation of fly ashes and other pollutants take place. The cleaner flue gas flow is then led to the stack. A schematic illustration of the emission control system is shown in figure 4.
(Pöyrö Sweden AB 2016)
Figure 4. Simplified illustration of emission control system for flue gas, case A.
Before the flue gas flow leaves the system through the stack, it undergoes measurements of the emission content.
12 5.1.2 Flue gas condensate treatment
When the condensate flow leaves the condenser, it is taken to another building for
conventional condensate treatment. The first step in the treatment is the addition of CaCO3
to neutralize acidic components in the condensate. After this, Ca(OH)2 is added to the condensate to raise the pH to 8,5. TMT, which is an organic sulfide, is then added to the condensate to bind heavy metals. In the next step, a polymeric flocculation chemical is added. In a sedimentation basin, the sludge is separated from the condensate. To further reduce the suspended particles and pollutants in the condensate, it is led through a sand filter followed by an activated carbon filter. After the emission control system, the
condensate is led to the recipient. A schematic illustration of the emission control system is shown in figure 5. (Pöyrö Sweden AB 2016)
Figure 5. Simplified illustration of flue gas condensate treatment, case A.
Before the condensate is led out to recipient, measurements of the emission content are performed.
13 5.1.3 Inventory analysis case A
In this chapter, the input and output for each component in case A is illustrated. The consumption of chemicals in case A is illustrated in figure 6.
Figure 6. Chemical dosage case A.
The production of solid residue in case A is illustrated in figure 7.
Figure 7. Solid residues produced in case A.
The water balance in case A is illustrated in figure 8.
Figure 8. Water balance in case A.
14 5.1.4 Regulations
The emission limits set for the Vattenfall waste incineration plant is generally set lower than the highest limits in the 2006 BREF document. Current limits for emissions to air are
presented in table 2.
Table 2. Limits for emissions to air. *Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V.
Parameter BREF WI 2006
Case A Unit
Dust 1 to 5 3 mg/Nm3 dry, 11% O2
SO2 1 to 40 20 mg/Nm3 dry, 11% O2
NOx 120 to 180 150 mg/Nm3 dry, 11% O2
HCl 1 to 8 3 mg/Nm3 dry, 11% O2
NH3 <10 3 mg/Nm3 dry, 11% O2
HF <1 0,1 mg/Nm3 dry, 11% O2
N2O - 5 mg/Nm3 dry, 11% O2
Hg <50 20 µg/Nm3 dry, 11% O2
Cd + Tl 5 to 50 20 µg/Nm3 dry, 11% O2
Sb, …, V* 5 to 500 100 µg/Nm3 dry, 11% O2
TCDD 0,01 to 0,1 0,05 ng/Nm3 dry, 11% O2
Current limits for emissions to water are presented in table 3.
Table 3. Limits for emissions to water. * Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V.
Parameter BREF WI 2006
Case A Unit
Susp 10 to 45 20 mg/l
pH 6,5 to 11 7 to 9
Hg 0,001 to 0,03 0,002 mg/l
Cd 0,01 to 0,05 0,003 mg/l
Tl 0,01 to 0,05 0,01 mg/l
As 0,01 to 0,15 0,01 mg/l
Pb 0,01 to 0,10 0,01 mg/l
Cr 0,01 to 0,5 0,01 mg/l
Cu 0,01 to 0,5 0,05 mg/l
Ni 0,01 to 0,5 0,01 mg/l
Zn 0,01 to 0,10 0,2 mg/l
Sb 0,005 to 0,85 0,01 mg/l
Co 0,005 to 0,05 0,01 mg/l
TCDD 0,01 to 0,1 0,1 mg/l
Sb, ..., V* - 0,01 mg/l
15 5.1.5 Reasons to upgrade the system
One of the main reasons to upgrade the system is the location of the baghouse filter. Its current location, after the condensation, results in a high content of suspended particles and other pollutants in the condensate. Instead of collecting the main part of the pollutants as early as possible in the emission control system, the pollutants are transferred from the flue gas to the condensate before collection. The current location of the baghouse filter results in a condensate stream with too high pollution content to go through a membrane based condensate treatment.
A high content of pollution in the input to the condensate treatment results in an output that is too polluted to reuse in the system. If a cleaner condensate flow can be attained, the water consumption of the system can drop considerably. For example, the cleaner
condensate can be used as feedwater for the boilers, as make-up water for district heating, etcetera. This could significantly lower the consumption of raw water.
Another reason to upgrade the emission control system is to get a better tolerance of sulphur components, in case of an increase of sulphur content in the waste used as fuel.
Finally, an important aspect is of course an improvement of the emission reduction.
Emission limits will surely be stricter in future, and preceding the restrictions could only be counted for as an advantage. Also, any reduction in amount of pollutants reaching the environment is indeed an important gain.
16 5.2 Case B. Membrane based alternative
Case B is the alternative new emission control system.
5.2.1 Background information
In this report, two waste incineration plants are used as reference for case B. The data for case B, that is used for comparison with data for case A, is a merge of data from these two plants. They have similar emission control systems as case B, although not exactly the same.
5.2.2 Emission control system flue gas
In case B, after the ESPs and flue gas flow is put to one, it is led to a baghouse filter.
Activated carbon and CaO are added to the baghouse filter to ensure sufficient separation of dioxins and acid components. After this, the flue gases are led to a two-step scrubber. In the first step, HCl in the flue gas assure a low enough pH for a good separation of NH3. In the next step, NaOH is added to reach a neutral or slightly acidic pH, to enable a good
separation of SO2. After the scrubber, the flue gases are led to the condenser. Here, about 2/3 of the water content in the flue gas is condensed out. The condensate is cleaner than in case A, but will still have some of the pollutants in the flue gas dissolved into the
condensate. Further separation is therefore done in the condensation step. The cleaner flue gas flow is then led to the stack. A schematic illustration of the emission control system is shown in figure 9. (Pöyrö Sweden AB 2016)
Figure 9. Simplified illustration of emission control system for flue gas, case B.
17 5.2.3 Flue gas condensate treatment
In case B, the condensate stream to begin with is significantly cleaner than in case A. The condensate is led through a micro-filtration and an ultra-filtration as a pretreatment.
Following this, is a two-step reversed osmosis. Thereafter, the condensate stream goes through a CO2 degassing process. After this treatment, the condensate is very clean and can be reused in the system or led out to recipient. A schematic illustration of the emission control system is shown in figure 10. (Pöyrö Sweden AB 2016)
Figure 10. Simplified illustration of flue gas condensate treatment, case B.
18 5.2.4 Inventory analysis case B
In this chapter, the input and output for each component in case B is illustrated. The consumption of chemicals in case B is illustrated in figure 11.
Figure 11. Chemical dosage in case B.
The production of solid residue in case B is illustrated in figure 12.
Figure 12. Solid residue produced in case B.
The water balance in case B is illustrated in figure 13.
Figure 13. Water balance in case B.
19
6 Result and discussion - Comparison of case A and case B
In this chapter, the chosen parameters: emissions to air and water, consumption of chemicals, produced solid residue, and water utilization, for case A and case B are compared. Some calculations are done on the data presented in this report. These are presented in appendix 1.
6.1 Emissions
For the comparisons below, the yearly amounts of emissions are compared. Since the plants where data have been collected differ in operating hours, fuel load, etcetera, the
concentrations in flue gas and condensate are recalculated to the same basis, the operation conditions for the Vattenfall waste incineration plant, as shown in appendix 1.
Some parameters presented in the emission limits for case A, in chapter 5.1.5, are missing in the comparison of case A and case B below. The reason is either missing data for both or one of the cases, making a comparison impossible.
The percentage change for each parameter shown in this chapter, is reported as the decrease or increase of the parameter, that case B would give compared to case A.
6.1.1 Emissions to air
The particularly important aspects in emissions to air, acid components, metals, and dioxins, are interesting to compare, see table 4. For acid components, NOx, HCl and SO2 are
considered. For metals, Hg, Cd, Tl, As, Pb, Cr, Co, Cu, Ni, and V are considered.
Table 4. Difference in specific groups of emissions to air, case B compared to case A.
Parameter Difference Unit
Acid components -75,3 w%
Metals +1 119 w%
Dioxins -35,0 w%
The decrease in acid components can be attributed to the separated quench. The decrease in dioxins is due to activated carbon dosage in the baghouse filter. The increase in metal emissions have no obvious explanation. The increase could be due to difference in the waste used as fuel in the two plants compared. This is something that needs to be investigated further.
20 The annual amounts in mass unit of each emission parameter have been compared, see table 5.
Table 5. Difference in amount of emissions to air, case B compared to case A.
*As, Pb, Cr, Co, Cu, Ni, V.
Parameter Difference Unit
Dust +46,9 w%
SO2 -99,5 w%
HCl +43,5 w%
NH3 -63,5 w%
Hg -65,7 w%
Cd + Tl +337 w%
Heavy metals* +1 316 w%
Dioxins -35,0 w%
If the dust parameter would be measured before condensation, it would show a decrease in case B compared to case A, instead of the increase shown in table 5. In case B, more dust is removed from the flue gases before the condensation takes place, meaning that less dust ends up as suspended particles in the condensate. This is obvious in table 7. After the condensation, the difference in amount of dust in the flue gas in case A and case B is not significant. Moreover, the amount of emitted dust is so small, that an increase by 46,9 w%
corresponds to a very small amount. In both cases, the dust concentration in the flue gas is still far below the emission limits.
The increase in HCl emissions is due to keeping the HCl levels high enough to ensure a low pH in the acid scrubber step. The acid scrubber step separates NH3 from the flue gases. As is apparent in table 5, the HCl helps reduce the NH3 by 63,5 w%, but increases the HCl output by 43,5 w%. However, the amount of HCl emitted to air is so small, that an annual increase in 43,5 w% is very little. The HCl concentration is still far below the emission limits.
The total amount of emissions to air have been related to the steam production in the boiler. Case B decreases the emissions to air in [g/ton steam] by 40,0 w%, compared to case A.
The overall difference of emissions to air is a reduction of 61,9 w% if the emission control system for the Vattenfall waste incineration plant is changed from case A to case B. The impact on the total amount of emissions to air is illustrated in figure 14.
21
Figure 14. Comparison of the total amounts of emission to air resulting from case A and case B.
6.1.2 Emissions to water
The particularly important aspects in emissions to water, metals, and dioxins, are interesting to compare, see table 6. For metals, Hg, Cd, Tl, As, Pb, Cr, Cu, Ni, Zn, and Co are considered.
Table 6. Difference in specific groups of emissions to water, case B compared to case A.
Parameter Difference Unit
Metals -95,7 w%
Dioxins -99,99 w%
The decrease in metal and dioxin emissions to water is mostly due to the cleaner inlet condensate, but could partly be accounted to the more efficient condensate treatment in case B.
The annual amounts in mass unit of each emission parameter have been compared, see table 7. A decrease of a parameter with more than 99,99 % is shown as 100 % in table 7.
Table 7. Difference in amount of emissions to water, case B compared to case A.
*As, Pb, Cr, Co, Cu, Ni.
Parameter Difference Unit
Susp -57,1 w%
Hg -100 w%
Cd -100 w%
Tl -90,5 w%
As -100 w%
Pb -100 w%
Cr -47,6 w%
Cu -70,7 w%
Ni -95,2 w%
Zn -95,9 w%
Co -100 w%
Dioxins -99,99 w%
Heavy metals** -100 w%
NH3 -100 w%
Total amount of emissions to air [kg/year]
Case A Case B
22 All parameters for emission to water decrease in case B compared to case A. This is probably mainly due to that the condensate is a lot cleaner going into the condensate treatment.
Some of the decrease could be attributed to a more efficient condensate treatment in case B.
NH4 for the respective cases have been considered, even though it is not currently
regulated. The reason is that NH4 is an interesting parameter from an environmental point of view, since it contributes to overfertilization.
The total amount of emissions to air have been related to the steam production in the boiler. Case B decreases the emissions to water in [g/ton steam] by 92,3 w%, compared to case A.
The overall difference of emissions to water is a reduction of 99,98 w% if the emission control system for the Vattenfall waste incineration plant is changed from the current case A to case B. The impact on the total amount of emissions to water is illustrated below in figure 15.
Figure 15. Comparison of the total amounts of emission to water resulting from case A and case B.
6.2 Consumption of chemicals
Since the consumption of chemicals depends largely on the amount of emissions in the raw flue gas, it is important to determine that the concentrations of emissions in the raw flue gas in two plants compared are largely the same. The amounts of acid components in the flue gas before the emission control system was therefore compared for case A and case B.
The difference was under 30 %, and was deemed sufficiently similar for a comparison.
Urea dosage in the boilers is not considered for this comparison, since the dosage is expected to remain unchanged.
For case A, activated carbon, CaCO3, Ca(OH)2, TMT and polymer are considered. For case B, activated carbon, CaO and NaOH are considered. Case B has a 162 w% increase in
consumption of chemicals compared to case A, illustrated in figure 16.
Total amount of emissions to water [g/year]
Case A Case B
23
Figure 16. Amount of chemical dosage in case A compared to case B.
6.3 Solid residue
There are three types of solid residue that are mainly concerned in waste incineration:
bottom ashes, fly ashes and sludge. Bottom ashes are collected in the boiler, fly ashes during emission control for flue gases, and sludge during emission control for condensate.
Since the conditions in the boiler will remain the same even if the emission control system is upgraded to case B, bottom ashes are not included in this comparison.
How big of an environmental impact the solid residues have, depends on how the residues are taken care of. The separated fly ashes contain heavy metals and dioxins and are not suitable for reuse, instead they are landfilled as hazardous waste. The sludge collected from the condensate treatment also contain heavy metals, and is landfilled as hazardous waste.
(Vattenfall Värme Uppsala 2015) Thus, fly ashes and sludge from the condensate treatment are treated with the least preferred method according to the waste hierarchy.
In case B, the condensate treatment does not cause any sludge. However, the reject flows from the membrane based treatment, which are concentrated streams of pollutions, needs disposal. In case B, the reject flows are either destroyed in the boiler or used to humidify the absorbent dosed to the baghouse filter. The reject flows are not considered in the
comparison of the solid residues. However, they might increase the amount of bottom ashes or fly ashes in the flue gas. Through this, the reject flows from the membrane treatment could be a contributing factor in the solid residues in case B.
The amount of fuel incinerated, chemical dosage and amount of separated emissions are significant factors regarding how much solid residue an emission control system cause. The chemicals dosed into the system and the pollutants separated from flue gas and flue gas condensate must be collected somehow. Therefore, it is logical that an emission control system that separates more emissions and has a larger consumption of chemicals will result in larger amounts of solid residue. This is evident in figure 18.
Total chemical dosage [ton chemical/ton fuel]
Case A Case B
24
Figure 17. Total amount of solid residue in case A compared to case B.
Case B cause 194 w% more solid residue than case A. In case A, fly ashes and sludge are considered, in case B only fly ashes are considered.
The dosage of CaO to the baghouse filter is an interesting aspect for the generated amount of solid residue. The large amount of CaO dosed to the baghouse filter, generates a large amount of solid residue.
6.4 Water utilization
In case A, the condensate is too polluted to be reused in the system. All condensate is therefore let out to recipient, and the water used in the system is raw water.
In case B, the rejection from the acid step in the scrubber is destroyed in the boiler. This results in a smaller condensate flow to the condensate treatment. The reject flows from the MF-filter and UF-filter are used in the baghouse filter. The reject flow from the RO are taken to the acid step in the scrubber, and then also to the boiler. The permeat flow from the last step in the condensate treatment is very clean, with emission concentrations well below any emission limits. This permeat flow can be reused in the system. (Pöyrö Sweden AB 2016) This results in a considerable amount of raw water that could be saved each year.
Water consumption should be kept at a minimum for economic reasons as well as environmental reasons. Since all raw water used in the process goes through municipal wastewater treatment plants before it is distributed to the incineration plant, each cubic meter of raw water used is entailed with a prize. Herein lies an economic driving force to reduce the water consumption. For environmental reasons, a smaller water output from the system results in a smaller output of pollutants. The concentration of pollution could
increase with a smaller water output, but the specific amount will decrease. (Persson 2005)
Total amount of solid residue [ton residue/ton fuel]
Case A Case B
25
7 Error sources
For some of the emission parameters, the water content in the flue gas where they are measured is not stated. Whether the concentration refers to a dry flue gas flow or the total flue gas flow, is an important factor to consider. The water content in the flue gas is a dilution factor, more water in the flue gas means a more diluted emission. When the flue gas is not stated dry or total, it is assumed to refer to the total flue gas flow.
Moreover, the total flue gas flow is calculated from average operation conditions at the Vattenfall waste incineration plant. These conditions differ depending on outside
temperature, fuel load, etc. Consequently, it would be more accurate to only use emission concentrations in dry flue gas.
Some parameters for emissions to air does not have a stated oxygen content in the flue gas.
The oxygen content in the flue gas is also a dilution factor, more oxygen in the flue gas means more air in the flue gas diluting the emission. For the parameters where
concentration is given but the oxygen content is unknown, the oxygen content is assumed to be 11 v%, which is the common oxygen surplus for waste incineration. If the oxygen content was in fact 9 v%, that would mean a decrease in emission concentration by 21 w%.
If the oxygen concentration was in fact 6 v%, the emission concentration decreases by 50 w%. This in an important error source in the comparisons in this report.
Furthermore, calculations are made on most of the initial data. The calculations include assumptions and simplifications, which could constitute in errors.
26
8 Suggestions for further investigations
As stated before, there will be a comparative LCA performed on the emission control systems presented in this report. Consequently, the overall environmental impact of the emission control systems will be further investigated. However, in this chapter, the aspects that need further investigation that have come up during this study will be presented.
The water and oxygen content of each concentration of emission parameter should be confirmed. These parameters could have significant impact on the results in this study, if they are assumed to incorrect values.
The role CaO plays in the emission control system in case B needs to be evaluated. The large amount of CaO dosage comes with a considerable environmental impact, bearing in mind the production and transportation, and the increased amount of solid residue that it contributes to. Therefore, the contribution that CaO makes to the reduced amount of emissions in case B needs to be determined.
The reason for the increased amount of heavy metals in the flue gas in case B needs to be determined.
Another aspect that could be worth investigating, is putting an economizer or similar before the ESPs, to cool the flue gases to a temperature under 200 °C. Today, the flue gases have a temperature about 260 °C in the ESPs (Pöyrö Sweden AB 2016). Cooling the flue gases before the ESPs could reduce the risk of forming additional dioxins downstream of the boiler (Cheng and Yuanan 2010).
27
9 Conclusion
From the perspective of this study, case B seems to be the choice for emission control system with the least environmental impact.
The results show that the reasons for upgrading the emission control system are met in case B. The condensate is significantly cleaner in case B than in case A, the total amount of emissions decreases by 99,98 w% in case B. The significantly smaller amount of emissions results in a condensate flow that can be reused in the system, which saves a considerable amount of raw water input each year. Through this, the wastewater discharge is significantly reduced, and the water utilization is improved in case B. The SO2 emissions are reduced by 99,5 w%, which show that a higher sulphur content in the fuel could be tolerated in case B.
Furthermore, the total amount of emission content in the flue gas decreases with 61,9 w%
in case B. The emission parameters that are deemed likely to be more strictly regulated in the coming 2017 BREF, NOx and Hg, are both significantly reduced in case B compared to case A. NH3 emissions to air are reduced by 63,5 w%.
On the other hand, both the consumption of chemicals and the production of solid residue increases in case B compared to case A. These aspects are important drawbacks of the emission control system in case B, and the environmental impact of these aspects needs to be further investigated in the planned LCA.
28
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30
Appendix 1. Calculations
Calculations made on the data presented in the report.
Recalculation of oxygen surplus in flue gas
Reporting the oxygen surplus in the flue gas is a way of controlling dilution of
concentrations. A larger oxygen surplus in the flue gas makes for a lower concentration of a component in the flue gas. Standard amount of oxygen surplus for waste incineration is 11
%. Therefore, in cases where the concentrations were given in other oxygen surpluses than 11 %, the concentration was recalculated to 11 % oxygen surplus. The formula for this is shown below. (Naturvårdsverket 2004)
𝑥 = (21 − 𝑎) ∗ 𝑦 (21 − 𝑏) x – concentration of component at 11 % oxygen surplus y – measured concentration
a – 11 % oxygen surplus
b – oxygen surplus of measured concentration Total flue gas amount
Based on information on the annual dry flue gas amount and the water content of the flue gas, shown in table 8, the total flue gas flow at the Vattenfall waste incineration plan can be calculated.
Table 8. Input to calculate total flue gas amount.
Parameter Data Unit
Water in flue gas 6,18 v%
Dry flue gas amount 1 255 000 000 Nm3 dry gas/year
6,18 v% water in the flue gas means 93,82 v% dry gas in the flue gas. Using this, the total amount of flue gas can be calculated as below.
1 255 ∗ 106 [𝑁𝑚3 𝑑𝑟𝑦 𝑔𝑎𝑠
𝑦𝑒𝑎𝑟 ] ∗ 1
0,9382 [𝑁𝑚3 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠
𝑁𝑚3 𝑑𝑟𝑦 𝑔𝑎𝑠] = 1 337 ∗ 106 [𝑁𝑚3 𝑓𝑙𝑢𝑒 𝑔𝑎𝑠 𝑦𝑒𝑎𝑟 ] Basis for calculated annual amounts
For case B, the annual amounts from the reference plants are not representable for the Vattenfall waste incineration plant. Therefore, the concentrations are recalculated into annual amounts based on the operation conditions the Vattenfall waste incineration plant.
The basis for the calculations are shown in table 9.
Table 9. Basis for calculated annual amounts case B.
Parameter Data Unit
Operating hours 7 188 h/year
Dry flue gas amount 1 255 000 000 Nm3 dry gas/year Total flue gas amount 1 337 000 000 Nm3/year
Condensate amount 129 400 m3/year
