the dmt multiple stage sulfurex-biogas desulphurisation process 2009.pdf

7/23/2019 The DMT multiple stage Sulfurex-biogas desulphurisation process 2009.pdf

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BIOGAS DESULPHURISATION USING THE DMT MULTIPLE STAGE SULFUREX

PROCESSAuthor: Erwin H.M. Dirkse, DMT Environmental Technology

ABSTRACT

Global rising energy costs are focusing efforts by many industrial and municipal entities to find new sourcesof energy. Generating electricity from Biogas is of course nothing new however the cost of cleaning upcertain Biogas streams with excessive levels of Hydrogen Sulphide (H2S) has previously proven to befinancially unsustainable. Rising energy costs combined with improvements and cost reductions indesulphurisation systems have increased the number of potential biogas streams that can be efficientlyand cost effectively treated to provide the end user with a lucrative additional revenue stream from theiranaerobic digestion process.

DMT a Dutch based manufacture of odour control and Biogas desulphurisation systems, is also offering theDMT Sulfurex

process in the UK. The first UK system is installed in the summer of 2006 at the Mauri plant

in Hull, Yorkshire. This system is designed to reduce the level of incoming Biogas from 20,000 ppm downto 135 ppm prior to it being fed into the CHP plant.

The DMT Sulfurex

process is a multiple stage scrubbing process based on the selective absorption of H2Sin a solution of sodium hydroxide. Numerous DMT Sulfurex

systems are operational throughout Europe.

Despite the success of the system it has been significantly improved in recent years, the key developmentsare as follows:

Improved selective absorption of H2S

Reduced consumption of sodium hydroxide

Treatment of spent lye

Regeneration of spent lye

Biogas pre-treatment with activated sludge or waste water.

Pre-treatment of Biogas using activated sludge can significantly reduce the consumption of sodiumhydroxide and thereby lower operational costs. Selective absorption reduces the amount of CO2absorbedand consequently reduces the volume of sodium hydroxide required. The second stage scrubbing processof the DMT Sulfurexsystem in which the spent lye can be used to further improve the rate of absorption.

The DMT Sulfurex

system is capable of reducing the H2S levels, up to 20,000 ppm in the incoming Biogasstream by more than 99%.

Further developments also include the potential for desulphurisation without using any chemicals at all, thissystem is known as the DMT Biosulfurex

process and utilises a purely biological approach. When a

limited amount of Oxygen is dosed into the Biogas flow Thiothrix & Thiobacillus bacteria oxidise H2S toelemental sulphur and sulphate, dependant on process conditions, or sulphuric acid. In order for thebacteria to oxidise effectively Oxygen, nutrients, and a contact surface are required. Oxygen is provided bya compressed air supply, nutrients from activated sludge and the contact surface from the media within thescrubber itself.

This paper will describe the operation and performance of both the DMT Sulfurexand the DMTBiosulfurex

systems, detailing system efficiencies, capital costs, running costs and payback periods.

KEY WORDSBiogas desulphurisation, Sulfurex, DMT, selective absorption


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INTRODUCTIONDMT Environmental Technology has developed considerable know-how and experience within the veryspecific area of desulphurisation of biogas and landfill gasses. In the mid 1980s DMT developed adesulphurisation process and registered it under the name of the DMT Sulfurex

. This process is based

upon selective absorption of H2S in a solution of sodium hydroxide. Since 1985 , DMT has been supplyingdesulphurisation systems worldwide on a variety of projects including paper mills, breweries, fertiliserplants, landfill sites, sludge-digestion facilities, waste water treatment plants and potato factories whereanaerobic treatment processes produce biogas or landfill gas containing H2S.Recently DMT has improved the DMT Sulfurex

process and adapted the process to incorporate the latest

technologies; the selectivity of H2S adsorption has been improved, the specific consumption of sodiumhydroxide has decreased, solutions are introduced to deal with the waste waterand continuous researchinvestigates new physical-chemical processes to re-use discharge water and regenerate the recirculationfluid.The latest in house development in the field of bio and landfill gas treatment is the biologicaldesulphurisation process known under the trade name of Bio-Sulfurex.

THE SULFUREX

PROCESS

The selective process of desulphurisation.There are many physical, physical / chemical, chemical and biological desulphurisation processes.However, due to the capital costs, monitoring and operating costs involved most processes can only becost effective when used on large or very large gas throughputs. There are however many smallerapplications where traditional systems are just not cost effective and no business case can be made forinvestment. It is in these smaller applications that the DMT Sulfurex

Desulphurisation Processdue to its

low capital and running costshas proven to be cost effective. The DMT Sulfurex

DesulphurisationProcess has the following benefits:

simple, compact and robust construction;

low capital cost;

very flexible operating parameters;

simple low cost operation and maintenance;

low consumable usage;

no chemical waste;

economically viable for sulphur loads up to 10 25 tons a day depending on local site conditions

equipment assembled as transportable units

modular construction in optional insulated housings.

In the process the biogas is brought into intense contact with a circulating alkali liquid over an exchanger ina single stage counter current process (gas scrubber). The H2S in the gas is absorbed by the alkali liquidthrough several chemical reactions. Ultimately the hydrogen sulphide in the gas is almost totally convertedinto NaHS and sodium bicarbonate using sodium hydroxide. The process is controlled by a sophisticatedH2S-detection and sampling system for very efficient operations and low chemical consumption. Selectivedesulphurisation is possible due to the differences in the physical and chemical properties of H2S and CO2.

The course of this chemical process is further determined by chemical concentrations, pH values and thetemperature and pressure of the system. The following conditions should be taken into consideration:

the lower the final concentration of H2S in the purified gas, the higher the specific lye demand;

the higher the temperature and pressure of the system, the higher the specific lye demand;

the higher the CO2content in the raw gas, the higher the lye demand;

the longer the gas remains in the scrubber, the higher the specific lye demand.


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S p e c i fic c o n s u m p t io n c a u s t ic s o d a

0

5

1 0

1 5

0 2 5 0 0 5 0 0 0 7 5 0 0 1 0 0 0 0

H 2 S c o n c e n t ra t io n ( p p m )

kgNaOH/

kgH2S

5 0 0 m 3 / u,3 5 % C O 2 , 3 0C

5 0 0 m 3 / u,1 5 % C O 2 , 3 0C

5 0 0 m 3 / u,1 5 % C O 2 , 1 0C

Figure 1.Caustic Soda consumption based on H2S concentrations

The following design data is required in order to design a gas desulphurisation plant:

gas flow in Nm3/hour; maximum, minimum, average, current and future;

gas temperature in C. ;

gas pressure in mbar or Pa;

composition of gas ( CH4, CO2, H2S, other );

required efficiency; ppm H2S in the purified gas.

For optimal performance of the Sulfurex process the following conditions need to be considered:

fluctuations in gas flow preferably < 10 m3/hr per minute;

system pressure preferably between - 250 and + 500 mmwg; process temperature preferably between 2 and 50 C.

New developmentsDMT Environmental Technology is constantly working on further improvements of the DMT Sulfurex

and

the DMT Bio-Sulfurex

processes. The main objectives of this research and development work are asfollows:

Improvement in the selectivity of H2S absorption;

Savings on the consumption of sodium hydroxide;

Treatment of the spent lye solution if an aerobic waste water treatment is not available;

Regeneration or re-use of spent lye; studying several physical-chemical processes for the re-use orregeneration of absorption fluid;

Pre-treatment of the biogas with activated sludge or wastewater effluent.

Pre-treatment of the biogas with activated sludge or wastewater effluent.When utilising a pre-treatment stage in combination with the Sulfurexbiogas desulphurisation unitimportant savings can be achieved by reducing the consumption of sodium hydroxide. This is possible byscrubbing the biogas with activated sludge from the aeration basin or effluent from an available biologicalwastewater treatment plant.The H2S is absorbed in the water or activated sludge and micro-biologically oxidised into sulphate whenthe circulation fluid is returned to the aeration basin or the wastewater treatment plant. The efficiencydepends on the pH of the water and the amount of recirculation fluid available; at a pH level of approx. 7.5an efficiency of 75 90 % on H2S can be achieved. The pre-treatment column can be situated near theaeration basin and the chemical desulphurisation unit, placed on a skid complete with a circulation system.


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Optimising the use of the spent lye.The DMT Sulfurex

process is based upon the following chemical reactions:

H2S + NaOH (lye) NaHS (Sodium hydrogen sulphide) + H2O (1) and

CO2+ 2NaOH Na2CO3(Sodium Carbonate) + H2O (2)

Because of the selectivity in the process relatively less carbon dioxide is absorbed, nevertheless thepresence of CO2 will be apparent in the amount of sodium hydroxide that is required. Important savingscan be achieved via the following reaction:

Na2CO3+ H2S NaHS + NaHCO3(Sodium hydrogen carbonate)(3)

This requires an extra absorption column in which the spent lye according to reaction (1) and (2) can beused for the reaction (3).The pH level of the spent lye will then be reduced to 9-10 through the formation ofsodium bicarbonate instead of 10-11.5 without the re-use of the sodium carbonate. Theoretically it ispossible to save up to 60% of the lye consumption; in practice a lot of different process details are involvedand the likely savings will probably be more like 25-50 %.

Chiller

NaOH

Water supply

Second

stage

Biogas inlet

Ventilation

Inertgas out

Firststage

Drain

Biogas outlet

Inertgas out

Inertgas in Inertgas in

Two stage combined biogas drying and -desulphurization plant

Clean air

Biogas sample take-off system

Analizer

M3

M1 M2 M1A M2AM4 M5

V1

V2

Process Diagram

Figure 2. Typical P & I D for a two stage Sulfurex system.

Cooling and drying the biogas.

It is advisable to have the biogas dried before it enters the CHP engine because of potential for condensateformation, consequently DMT have developed a very compact vertical biogas drying system, combining theremoval of water and H2S in one process. The biogas conditioner is based on direct contact betweenbiogas and the scrubber fluid, within a packed column. The scrubber fluid is cooled over an integrated platetype heat exchanger running at less than 5 C. prior to it entering the top of the scrubber column. The waterin the biogas (RH of 100 %) will condenses on contact with the surface of the packing material. Thecondensate and scrubber fluid is collected in the circulation basin of the scrubber. Surplus fluid is drainedout of the tank. The treated biogas passes through a demister to remove droplets and is heated by anintegrated gas / gas heat exchanger at the top of the scrubber column or by a separate heater fed withengine cooling water at +/- 90 C. The removal efficiency of these type of plants for sulphur is very high;the relative humidity of the biogas is reduced from 100 % to 60 %. As a result the reliability of the gasengine or steam turbine can be increased considerably. Consequently down time and maintenance costcan be significantly reduced.


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Running costsRunning costs will vary according to make up of the Biogas, various examples are shown in Fig 3, buttypically for a 500 m

3/hr biogas plant running at 5000 ppm H2S and a target level of 200 ppm H2S the

running costs will be less than 0.02 per m3biogas treated. (Based on depreciation over 10 years and

interest at 5%)

BIOLOGICAL DESULPHURISATION THE DMT BIOSULFUREX

It is also possible to remove H2S form Biogas or landfill gas using a biological system. The DMT Bio-Sulfurex

system is a very cost effective method and yet still provides efficient removal of H2S. The

process is fully biological, requiring no (or in some cases minimal) chemicals or external utilities.When a limited quantity of air is added to the biogas, specific aerobic bacteria such as Thiothrix andThiobacillus will oxidize the H2S into elemental sulphur and/or depending on the environmental conditionsinto sulphuric acid. These bacteria require:

Oxygen

Nutrients and trace elements

Growth area

OperationThe biogas is fed into a vertical column with packing media in a counter current pattern. The packing mediaprovides sufficient contact surface area for the gas flow, nutrients and oxygen: it is also used as a carrierfor the bacteria. Oxygen is added to the gas in the form of compressed air. The compressed air is broughtinto the column and when it moves up through the column, the bacterium treats the gas flow. An automaticcontrol system adjusts the amount of air flow according to the actual requirement which in turn is correlatedwith the biogas flow. The amount of oxygen required depends on the amount of air and the oxygen alreadypresent in the flow. The nutrients are sprayed on top of the medial and refreshed automatically as required.Nutrients and trace elements can usually be sourced from the digested substrate or other natural sources(e.g. landfill leachate) and need not to be added artificially.

Advantages of biological desulphurisation using the Bio-Sulfurex process.

High efficiency > 95% reductions in of H2S for incoming biogas with up to 1 vol. % (10.000 ppm H2S).

Almost no utilities necessary.

No use of chemicals.

No waste flow requiring treatment or transportation.

Safe process. All equipment & instrumentation Ex-proof.

Compressed air supply linked to the biogas flow and oxygen demand.

Low capital and running costs (see table 3).

SAPPI PLANT, LANAKEN, BELGIUM.South African Pulp and Paper Industries (SAPPI) paper mill in Lanaken, Belgium took delivery of a DMT

Sulfurex

process in 2004. The SAPPI mill is very large and has its own anaerobic wastewater treatmentfacility producing biogas. Originally the biogas was used to fire a boiler. However SAPPI had two importantreasons for investing in a biogas desulphurisation plant: Firstly they were investing in a new CHP plant andsecondly they had to comply with new SO2emission standards.SAPPIs design data is as follows:

Biogas flow : 200 Nm3/ hour

Biogas volume : 1.750.000 m3/ year

Biogas composition : 78 % methane20 % carbon dioxide

H2S concentration : max. 20.000 ppmH2S out of the process : max. 200 ppmEfficiency of H2S removal : > 99 %

Biogas Temperature : 35 CHumidity : saturated


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Because of the very high levels of H2S a physical / chemical process was preferred to a biological process;therefore several Sulfurexprocess options were discussed. Table 2 shows the various options and theirrespective running costs. SAPPI selected the most extensive option: a three stage process with anactivated sludge absorber as pre-treatment step followed by a two stage Sulfurex process including

cooling and drying of the biogas.

Process descriptionThe SAPPI system consists of three scrubbing towers in which the four main process stages take place.The first stage is an activated sludge absorber in which the biogas is pre-treated for initial H 2S removal.The soda utilization tower, in which soda formed in the third stage is re-circulated, is used for further H 2Sremoval. The third stage is a caustic scrubbing tower in which the caustic is dosed and used to remove thefinal concentrations of H2S. The soda formed is transported to the second stage. The fourth stage processis the cooling and drying of the biogas which is integrated within the total system.

Stage 1 - Activated sludge absorber, water scrubber, first biogas treatment stage.The pre-cooled biogas enters the activated sludge absorber in a counter flow pattern. The absorberconsists of a scrubber with a special type of contact media, water-distribution system and a demister. In the

process activated sludge is used with a sludge concentration of 7 10 gram per litre.The pH value of the effluent is very important in relation to the scrubbing efficiency of the activated sludgeabsorber; therefore a pH measurement system is installed for adjusting the circulation flow over thescrubber. The following processes take place in the absorber:

2 H2S(g)+ H2O(l)H2S(aq)+ HS-(aq) + H3O

+(aq) (1)

2 CO2(g)+ OH-(aq) CO2(aq) + HCO3

-(aq) (2)

(1) Dissolution of H2S in water with a specific pH.(2) Dissolution of CO2in water.

Except for the reactions mentioned above various other chemical reactions of sulphide compounds alsooccur, depending on the composition of the activated sludge. By scrubbing the biogas the pH value of the

activated sludge decreases; the higher the pH value the better the scrubbing process operates.

Stage 2- Sodautilisation stage, soda scrubber.In the second step the biogas flows through the scrubbing tower in a counter flow pattern; before passingto stage 3. A soda solution is sprayed over the scrubbing tower column. In the circulation system there is aheat exchanger as the lower temperatures provide enhanced scrubbing and the biogas is therefore cooledfrom approximately 30C to 8C.The following reactions will take place in the second stage:

2 H2S(g)+ H2O(l)H2S(aq)+ HS-(aq)+ H3O

+(aq) (3)

H2S(aq)+ Na2CO3(aq)NaHS(aq)+ NaHCO3(aq) (4)CO2(g)+ Na2CO3(aq)+ H2O(l) 2 NaHCO3-(aq) (5)

(3) Dissolution of H2S in water(4) Chemical reaction between H2S and soda.(5) Dissolution and reaction of CO2in water by soda

Stage 3 -Caustic stage, caustic scrubber.In the third stage the biogas from stage 2 enters the tower in a counter flow pattern. The medium that issprayed over the column consists of a caustic solution which is formed by addition of a caustic solution intothe circulation flow. Make-up water is also added. In this way the total concentration of caustic addedshould not be more than 3-4 %, because of the danger of clogging the system (crystallisation of sodiumbicarbonate ).In the water circulation system is a heat exchanger, which cools the circulation water from approximately14 to 4 degrees C. The lower temperatures in the scrubbing column, enhances the reaction. The caustic isadded through a dosing system. The volume of caustic is adjusted by increasing or decreasing the pump

frequency. This is controlled by a DMT purpose designed H2S sampling and take-off system. The waste


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water drains via an overflow system back into the second stage scrubber. The following reactions willmainly take place in the third stage:

3 H2S(g)+ 3OH-(aq)H2S(aq)+ HS

-(aq) + S

2-(aq)+ 3H2O(l) (6)

2H2S(aq)HS-

(aq)+ S2-

(aq)+ 3H2O(aq) (7)3 CO2(g)+ 3 OH

-(aq) CO2(aq) + HCO3

-(aq)+ CO3

2-(aq)+ H2O(l) (8)

(6) Chemical reaction between H2S and soda.(7) Dissolution of H2S in water(8) Dissolution and reaction of CO2in water by caustic

DMT bases its guarantees on caustic consumption using in house developed design programs andprocess calculations; checked and adjusted with field data from operational systems. Biogas emanatingfrom processes for which DMT has no previous experience or data may have significantly differentcompositions should these compounds have a catalytic effect on the absorption of carbon dioxide, thespecific caustic consumption could be significantly higher than those where the Biogas composition isknown. Potential unknown compounds could be alcohols, aldehydes, fatty acids and similar structures. To

ascertain if these compounds are present the biogas should be sampled using gas chromatographyanalysis. If alcohols and or acids are present in the biogas in concentrations of 50 100 ppm, caustic sodaconsumption will increase by 1 3 % when compared to normal process conditions. If alcohols and oracids are present in the biogas in concentrations of 2 %, caustic soda consumption will increase toapproximately 50 % when compared to normal process conditions

Cooling and drying of the biogas.Scrubbing of H2S is more efficient at lower process temperatures that consequently results in lower causticconsumption. Reheating the biogas at the outlet reduces the relative humidity of the Biogas to 60%. Thecooling system begins with a chiller that cools a mixture of water and glycol from approximately 6C to 2C.The water/glycol mixture is circulated by a pump. The cooling fluid is fed into the third stage scrubber andincreases in temperature, within the scrubber, from approximately 2C to 4C. At the same time thecirculation fluid has the cooling potential to reduce the temperature from 14C to 4C.

Secondly the cooling fluid enters the second stage. The cooling fluid in this section has a temperature gainfrom approximately 4C to 6C. At the same time the circulation fluid cools from approximately 12C to 7C.Optimisation of the cooling system according to the actual process conditions can be achieved by partiallybypassing or putting the heat exchangers in parallel..The final stage of the cooling and drying system is the gas/gas heat exchanger in the outlet of the biogasscrubber. The treated cold biogas is reheated with the untreated warm biogas. The untreated gas will coolfrom approximately 30C to 27C and the treated gas will rise in temperature approximately 12C from12C to 24C.

Waste water dischargeThe spent lye discharged from the desulphurisation process needs to be disposed of. The waste waterconsists of concentrations of 4 8 % by weight NaHS and NaHCO3, furthermore there are traces ofNa2CO3and Na2S at a pH value of 9.5 to 11. Usually this stream is fed into the aeration stage of a waste

water treatment plant where the NaHS can be rapidly oxidised microbiologically into sulphate. If there is noaeration stage available, waste water can alternatively be subjected to random oxidation. Extendedaeration results in NaHS being oxidised into sulphur, sulphate and poly-sulphur compounds.

Make-up water.Make-up water is required in order to dilute the sodium hydroxide that is fed in (25 45 % solution) in orderto keep the compounds created soluble and to carry off heat from the chemical reactions taking place inthe process.The make-up water should be free of hardness (zero) and iron (< 0.1 mg/l), as soda lye plus hard waterproduces lime and hydrogen sulphide, plus iron produces black sulphide of iron, which may deposit in thegas scrubber. Consequently, DMT stipulate the use of softened and de-ironed water and consequentlyinclude a water treatment unit to provide de-ironing and softening. In the event that deposits still occur, theycan easily be dissolved by acidifying the gas scrubber. The materials of construction (PVC, PP, HDPE,

stainless steel) are acid-resistant.


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Dilution effects of water free sodium hydroxide

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130140

150

160

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Weight-% NaOH

Endtemperature,degreesCelsius

0 C

15 C

30 C

45 C

Boiling point curve

Coagulation point curve

Figure 3.The effects of diluting pure sodium hydroxide.

Caustic soda.The DMT Sulfurex

Desulphurisation Process requires chemically pure sodium hydroxide in order to avoid

clogging problems. Another important consideration is the selection of the correct concentration. Figure 3shows the consequences of different concentrations of sodium hydroxide.

H2S detection and sample system.A very important component in the DMT Sulfurex

Desulphurisation Process is the H2S detection and

sample system. The total desulphurisation process is controlled by this unique and sophisticated systemfor very efficient operations and low chemical consumption. The design is specially made by DMT forefficient operation in gas scrubbers. Besides the necessary sample conditioning, the sample preparationcontains a dilution step. The dilution step makes it possible to measure wet gas in high ranges up to 500 1.000 ppm. The sensor itself operates in a lower measurement range but by dilution of the gas and heatingof the cabinet, water condensation is prevented. The sample flows via a fast loop back into the process.The transport of the gas is handled by a heavy duty membrane pump. If instrument air is not available, thedilution of the sample can be achieved with ambient air from the second head of the membrane pump.Dilution of the sample has the extra benefit of low H2S emissions (max. 10 ppm). The air is also used, as asafety measure, to purge the small cabinet to prevent high concentrations of dangerous gases. The designof the system allows the use in control loops as the response time is short. The system was installed in azone 1 gas group IIC T4 area.

Measuring principle of the H2S sensor.

The sample gas diffuses through a hydrophobic, gas permeable membrane into the electrolyte of thesensor. Located inside the sensor are measurement, reference and counter electrode components. Apotentiometer circuitry enforces a constant bias voltage between the measurement and referenceelectrodes. The bias voltage, the electrolyte and the electrode material are selected in a way that the targetgas electro-chemically reacts with the measurement electrode. This reaction generates a current flowthrough the sensor which is proportional to the gas concentration.

To compensate for temperature effects, a temperature sensor (NTC) is located directly inside the sensorcell. The effects of temperature can therefore be compensated according to the actual sensor temperature.

Materials & installation of the unitMaterials used in the plant at SAPPI are as follows:-

all components were constructed from corrosion resistant materials such as plastics and/or

stainless steel;


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the gas scrubbers were constructed as compact packaged units and then installed in a modular,insulated housing system for easy transportation and erection on site;

The system was fully automated and controlled via a PLC control panel with optional remotetelemetry system

Scope of SupplyThe scope of supply for the SAPPI Sulfurex desulphurisation plant included the design manufacturesupply installation and commissioning of the gas scrubber(s), instrumentation system, control system,chemical storage tanks, make-up water system and water treatment unit, discharge of bleed-off,interconnecting piping and cabling, modular insulated housing incl. ventilation, heating and CH4/ H2Sdetection;

CONCLUSIONSThe DMT Sulfurex

desulphurisation plant installed at the SAPPI Plant has proven to be a robust and

reliable system for conditioning the biogas prior to it being fed to the CHP plant. The combination ofphysical, biological and chemical processes provides optimum performance, reliability and low runningcosts. Consequently maintenance and operational costs for the CHP plant since the installation of the

system have been significantly reduced of the engine over the last period were minimized and a very shortpay back period has been achieved

The DMT Sulfurex

desulphurisation plant has met its design targets: system efficiency was in accordancewith the process guarantees and process calculations that were made using the DMT developed designprogram, caustic consumption was within the limits of the design process calculations with the scrubbersoverall performance being excellent. The efficiency of the pre-treatment stage by the activated sludgescrubber was better than expected: calculated 75% removal had been expected at the design stage but>85% removal was actually achieved. This is an interesting development in terms of lower causticconsumption in the chemical stage of the plant.

During commissioning some problems were experienced when the contact media was in danger ofbecoming clogged with calcium and bio sludge however this was quickly resolved by adjusting water levels

and cleaning.

Running costs of the DMT Sulfurex

desulphurisation plant were in line with those predicted at the designstage, they were even better than originally predicted because of the higher efficiency of the pre-treatmentof the biogas by the activated sludge scrubber. The running costs of this plant are more comparable tothose of a purely biological plant.

The pay back period of the DMT Sulfurex

desulphurisation plant was less than two years which comparesvery favourably to conventional biogas desulphurisation processes that utilise a single stage scrubber withsodium hydroxide, iron oxide and activated carbon.

Figures 11 to 15 shows the consequences of sulphur damage to the gas engines, in terms of themaintenance schedules

Table 1. Running costs for the DMT Sulfurex

system

Capacity plant (500 m3gas/ hr), efficiency 200ppm H2S, pressure 50mbar

5000 ppm H2S,30 C,35 vol % CO2

5000 ppm H2S,30 C,15vol % CO2

5000 ppm H2S,10C,35 vol % CO2

10.000ppmH2S, 30 C,35 vol % CO2

Costs of water 7.464 4.270 6.113 10.917Costs of caustic soda 49.632 28.416 40.599 72.680Costs of energy 525 525 2.223 525Depreciation *) 20.361 20.361 23.500 21.361Maintenance 2.000 2.000 3.000 2.000Operation 4.160 4.160 4.160 4.160

Total cost in / year 84.000 60.000 79.595 112.000Total cost per m3 biogas 0,0192 0,0137 0,0182 0,0256


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Table 2.Running costs for the proposed options at the SAPPI plant (/pa)

One stage DMT

Sulfurex

Two stage DMT

Sulfurex

Two stage DMT

Sulfurex

incl.cooling / drying

Two stage

DMTSulfurex

incl. cooling /drying pre-treatment withactivated sludgeabsorber

Costs of water 4.000 5.000 5.000 5.000Costs of causticsoda

65.000 32.500 25.000 8.000

Costs of energy 525 725 2.000 5.000Depreciation *) 12.500 15.000 18.000 21.000Maintenance 2.000 3.000 4.000 5.000Operation 4.160 4.160 5.000 7.500

Totalcost in /year

88.185 60.385 59.000 51.500

Total cost per m3biogas

0,050 0,035 0,034 0,029

Table 3.Running costs for The DMT Bio-Sulfurex

process for varying parameters

Capacity plant (m3 gas /hr), efficiency 200 ppm

H2S , pressure 50 mbar

500 m3/hr,

5000 ppm H2S,

30 C,35 vol % CO2

500 m3/hr,

5000 ppm H2S,

30 C,15vol % CO2

500 m3/hr,

5000 ppm H2S,

10C,35 vol % CO2

500 m3/hr,

10.000ppm

H2S, 30 C,35 vol % CO2

Costs of water *) 8.570 8.570 9.176 14.823Costs of energy 412 412 2.129 653Costs of nutrients **) 1250 1250 1250 2500Depreciation ***) 25.791 25.791 28.000 32.578Maintenance 5.000 5.000 6.000 5.000Operation 10.400 10.400 10.400 10.400

Total cost in 51.000 51.000 56.955 66.000Total cost per m3 biogas 0,0118 0,0118 0,0130 0,0151

*) Costs of water can be 0,= if effluent out of WWTP is being used.

**) Costs of nutrients can be 0,= if effluent out of WWTP is being used...***) At depreciation of 10 years and an interest of 5 %.


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Housing

Gas

Diffusion membrane

ElectrolyteReference electrode

Reference electrode

Measuring electrode Temperature sensor

Bias voltage

operational amplifier

Measuring resistor

-U

signal

Figure 4.General arrangement of Hydrogen Sulphide measurement system

Figure 5.DMT BIOSULFUREXbiological biogas desulphurisation plant


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Figure 6.DMT two stage DMT Sulfurex

process, Mauri plant in Hull, Yorkshire, UK.

Figure 7.Interior of a H2S detection and sampling unit


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Figure 8.DMT three stage DMT Sulfurex

process, SAPPI plant in Lanaken, Belgium.

Figure 9.DMT BIOSULFUREX

biological biogas desulphurisation plant


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Figure 10. Illustration of a DMT Sulfurex plant


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Figure 11. Effect of sulphur on gas engine maintenance schedule 1

Figure 12.Effect of sulphur on gas engine maintenance schedule 2


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the dmt multiple stage sulfurex-biogas desulphurisation process 2009.pdf

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