#biogas scrubbing

13
1 Chapter 1: Introduction Biogas typically refers to a (biofuel) gas produced by the anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions [1]. Composition of Biogas varies based on the source used for anaerobic digestion. Table 1: Composition of Biogas Matter Raw Biogas Scrubbed Biogas Methane, CH 4 50-75% 95-98% Carbon dioxide, CO 2 25-50% 3-5% Hydrogen sulphide, H 2 S 0-3% <10ppm Advantages of Biogas: Biogas is a clean and less polluting fuel Biogas is an energy rich fuel and can be used to produce heat and power and can also be used as vehicle fuel Compared to the use of diesel for vehicles, biogas emits 80 per cent less hydrocarbons and 60 per cent less nitrogen oxides Biogas is a renewable fuel Scrubbed biogas is better than raw biogas because it has more CH 4 than raw biogas and also has lesser amounts of hazardous elements like CO 2 and H 2 S. The calorific value of scrubbed biogas (~35MJ/m 3 ) is almost equal to that of CNG (35MJ/m 3 ), whereas raw biogas has calorific value of 21MJ/m 3 . Thus scrubbing leads to a 57% increase in calorific value [1].

Upload: lai-mei-ee

Post on 26-Sep-2015

26 views

Category:

Documents


8 download

DESCRIPTION

BIOGAS

TRANSCRIPT

  • 1

    Chapter 1: Introduction

    Biogas typically refers to a (biofuel) gas produced by the anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions [1].

    Composition of Biogas varies based on the source used for anaerobic digestion. Table 1: Composition of Biogas

    Matter Raw Biogas Scrubbed Biogas

    Methane, CH4 50-75% 95-98%

    Carbon dioxide, CO2 25-50% 3-5%

    Hydrogen sulphide, H2S 0-3%

  • 2

    Chapter 2: Background Review

    Chapter 2.1: Techniques of Scrubbing

    There are several techniques being used for scrubbing biogas depending upon the type of concentration level required and also upon the limitation of temperature and pressure.

    Scrubbing of CO2

    A variety of processes are being used for removing CO2 from natural gas in petrochemical industries. Several basic mechanisms are involved to achieve selective separation of gas constituents. These may include physical or chemical absorption, adsorption on a solid surface, membrane separation, cryogenic separation and chemical conversion.

    Physical Absorption

    For biogas scrubbing, physical/chemical absorption method is generally applied, as it is effective even at low flow rates, at which the biogas plants generally operating at. Also the method is less complicated, requires lesser infrastructure and is cost effective.

    One of the easiest and cheapest methods involves the use of pressurized water as an absorbent. The raw biogas is compressed and fed into a packed bed column from the bottom; pressurized water is sprayed from the top. The absorption process is, thus a counter-current one. This dissolves CO2 as well as H2S in water, which are then collected at the bottom of the tower. The water is recycled to the first scrubbing tower. This perhaps is the simplest method for scrubbing biogas.

    After scrubbing, 5-10% of CO2 is left in biogas. This process is cost effective as the compound being consumed is pressurized water that is cheap and can be recycled easily. Currently this process is in use in the sewage sludge based plants in Sweden, France and USA [5].

    Chemical Absorption

    Chemical absorption involves the formation of reversible chemical bonds between the solute and the solvent. Regeneration of the solvent, therefore, involves breaking of these bonds and correspondingly, a relatively high energy input. Chemical solvents generally employ either aqueous solutions of amines, i.e. mono-, di- or tri-ethanolamine or aqueous solution of alkaline salts, i.e. sodium, potassium and calcium hydroxides.

    Adsorption on Solid Surface

    Adsorption involves the transfer of solute in the gas stream to the surface of a solid material, where they concentrate mainly as a result of physical or Vander wall forces. Commercial adsorbents are generally granular solids with a large surface area per unit volume. By a proper choice of adsorbent, the process can remove CO2, H2S, moisture and other impurities either selectively or simultaneously from biogas.

  • 3

    Gas purification can also be carried out using some form of silica, alumina, activated carbon or silicates, which are also known as molecular sieves.

    Adsorption is generally accomplished at high temperature and pressure. It has a good moisture removal capacity, is simple in design and easy to operate. But it is a costly process with high pressure drops and high heat requirements.

    Other Methods

    Membrane Separation:

    The principle is that some components of the raw gas could be transported through a thin membrane (< 1 mm) while others are retained. The transportation of each component is driven by the difference in partial pressure over the membrane and is highly dependent on the permeability of the component in the membrane material. For high methane purity, permeability must be high. Solid membrane constructed from acetatecellulose polymer has permeability for CO2 and H2S up to 20 and 60 times, respectively, higher than CH4. However, a pressure of 2540bar is required for the process. This process has low reliability and high cost due to frequent replacement of expensive membrane [5].

    Cryogenic Separation

    In a cryogenic method, crude biogas is compressed to approximately 80bar. The compression is made in multiple stages with inter-cooling. The compressed gas is dried to avoid freezing during the cooling process. The biogas is cooled with chillers and heat exchangers to -45 C, condensed CO2 is removed in a separator. The CO2 is processed further to recover dissolved methane, which is recycled to the gas inlet. By this process more than 97% pure methane is obtained. No data is available on investment and operational cost. This process involves high cost because it requires generating sub-zero temperatures [5].

    Chemical Conversion

    To attain extremely high purity in the product gas, chemical conversion method can be used. It reduces the undesirable gas concentrations to trace levels. Usually the chemical conversion process is used after bulk removal has been accomplished by other methods. One such chemical conversion process is methanation, in which CO2 and H2 are catalytically converted to methane and water. Chemical conversion process is extremely expensive and is not warranted in most biogas applications.

    Due to highly exothermic nature of the methanation reactions, the removal of the heat from the methanator is a major concern in the process design. The requirement of the large amount of pure hydrogen also makes the process generally unsuitable [5].

  • 4

    Scrubbing of H2S H2S is always present in biogas, although concentrations vary with the feedstock. It has to be removed in order to avoid corrosion in compressors, gas storage tanks and engines. H2S is poisonous and corrosive as well as environmentally hazardous since it is converted to sulfur dioxide by combustion. It also contaminates the upgrading process. H2S can be removed either in the digester, from the crude biogas or in the upgrading process

    Dry Oxidation Process

    It can be used for removal of H2S from gas streams by converting it either into sulfur or oxides of sulfur. This process is used where the sulfur content of gas is relatively low and high purities are required. A small amount of oxygen (26%) is introduced in the biogas system by using an air pump. As a result, sulfide in the biogas is oxidized into sulfur and H2S concentration is lowered. 2H2S + O2 2S + 2H2O

    This is a simple and low cost process. No special chemicals or equipments are required. Depending on the temperature, the reaction time and place where the air is added, the H2S concentration can be reduced by 95% to less than 50ppm. However, care should be taken to avoid overdosing of air, as biogas in air is explosive in the range of 612%, depending on the methane content [5].

    Adsorption on Iron Oxide

    H2S reacts with iron hydro-oxides or oxides to form iron sulfide. The biogas is passed through iron oxide pellets, to remove H2S. When the pellets are completely covered with sulfur, these are removed from the tube for regeneration of sulfur. It is a simple method but during regeneration a lot of heat is released. Also the dust packing contains a toxic component and the method is sensitive to high water content of biogas.

    Wood chips covered with iron oxide have a somewhat larger surface to volume ratio than plain steel. Roughly 20g of H2S can be bound per 100g of iron oxide chips. The application of wood chips is very popular particularly in USA. It is a low cost product, however, particular care has to be taken that the temperature does not rise too high while regenerating the iron filter.

    H2S can be adsorbed on activated carbon. The sulfur containing carbon can then either be replaced with fresh activated carbon or regenerated. It is a catalytic reaction and carbon acts as a catalyst [5].

    Liquid Phase Oxidation Process

    This process is primarily used for the treatment of gases containing relatively low concentration of H2S. It may be either: (a) physical absorption or (b) chemical absorption.

    In physical absorption process the H2S can be absorbed by the solvents. One of the solvent is water. But the consumption of water is very high for absorption of small amount of H2S. If some chemicals like NaOH are added to water, the absorption process is enhanced. But it forms sodium sulfide or sodium hydrosulfide, which is not regenerated and poses problems of disposal.

    Chemical absorption of H2S can take place with iron salt solutions like iron chloride. This method is extremely effective in reducing high H2S levels. The process is based on the formation of insoluble precipitates. FeCl3 can be added directly to the digester slurry. In small anaerobic digester system, this process is most suitable. All other methods of H2S removal are suitable and economically viable for large-scale digesters. By this method the final removal of H2S is about 10ppm [5].

  • 5

    Chapter 2.2: Scrubbing Techniques in Use A number of gas upgrading technologies have been developed for the treatment of natural gas, town gas, sewage gas, landfill gas etc. However, not all of them are recommended for the application with biogas because of price or environmental concerns. Water Scrubbing:

    Water scrubbing is used to remove carbon dioxide but also hydrogen sulphides from biogas since these gases are more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurized and fed to the bottom of a packed column where water is fed on the top and so the absorption process is operated counter-currently.

    Water scrubbing can also be used for selective removal of hydrogen sulphide since hydrogen sulphide is more soluble than carbon dioxide in water. The water which exits the column with absorbed carbon dioxide and/or hydrogen sulphide can be regenerated and recirculated back to the absorption column. The regeneration is made by de-pressurising or by stripping with air in a similar column. Stripping with air is not recommended when high levels of hydrogen sulphide are handled since the water will soon be contaminated with elementary sulphur which causes operational problems. The most cost efficient method is not to recirculate the water if cheap water can be used, for example, outlet water from a sewage treatment plant [4].

    Figure 1: Schematic flow sheet for water absorption with recirculation for removal of carbon dioxide or hydrogen sulphide from biogas [3]

    Carbon Molecular Sieving: Molecular sieves are excellent products to separate specifically a number of different gaseous

    compounds in biogas. Thereby the molecules are usually loosely adsorbed in the cavities of the carbon sieve but not irreversibly bound. The selectivity of adsorption is achieved by different mesh sizes and/or application of different gas pressures.

    When the pressure is released the compounds extracted from the biogas are desorbed. The process is therefore often called pressure swing adsorption (PSA). To enrich methane from biogas

  • 6

    the molecular sieve is applied which is produced from coke rich in pores in the micrometer range. The pores are then further reduced by cracking of the hydrocarbons.

    In order to reduce the energy consumption for gas compression, a series of vessels are linked together. The gas pressure released from one vessel is subsequently used by the others. Usually four vessels in a row are used filled with molecular sieve which removes at the same time CO2 and water vapour. After removal of hydrogen sulphide, i.e. using activated carbon and water condensation in a cooler at 4C, the biogas flows at a pressure of 6bar into the adsorption unit. The first column cleans the raw gas at 6bar to an upgraded biogas with a vapour pressure of less than 10ppm H2O and a methane content of 96 % or more.

    In the second column the pressure of 6bar is first released to approx. 3bar by pressure communication with column 4, which was previously degassed by a slight vacuum. In a second step the pressure is then reduced to atmospheric pressure. The released gas flows back to the digester in order to recover the methane. The third column is evacuated from 1 bar to 0.1bar. The desorbed gas consists predominantly of carbon dioxide but also some methane and is therefore normally released to the environment. In order to reduce methane losses the system can be designed with recirculation of the desorbed gases.

    The product gas of column 1 is monitored continuously for CH4 by an infrared analyser. If the required Wobbe index is not maintained the gas flows back to PSA. If the methane content is high enough, the gas is either introduced into the natural gas net or compressed in a 3 stage compressor up to 250bar. Continuous monitoring of a small-scale installation (26m3/hr) demonstrated excellent results of gas cleaning, energy efficiency and cost.[3]

    Figure 2: Schematic flow sheet for upgrading of biogas to vehicle fuel standards with carbon molecular sieves [3]

    Membrane Separation:

    There are two basic systems of gas purification with membranes: a high pressure gas separation with gas phases on both sides of the membrane and a low-pressure gas liquid absorption separation where a liquid absorbs the molecules diffusing through the membrane.

  • 7

    High Pressure Gas Separation: Pressurized gas (36bar) is first cleaned over for example an activated carbon bed to remove

    (halogenated) hydrocarbons and hydrogen sulphide from the raw gas as well as oil vapour from the compressors. The carbon bed is followed by a particle filter and a heater. The membranes made of acetate-cellulose separate small polar molecules such as carbon dioxide, moisture and the remaining hydrogen sulphide. These membranes are not effective in separating nitrogen from methane.

    The raw gas is upgraded in 3 stages to a clean gas with 96 % methane or more. The waste gas from the first two stages is recycled and the methane can be recovered. The waste gas from stage 3 (and in part of stage 2) is flared or used in a steam boiler as it still contains 10 to 20 % methane.

    First experiences have shown that the membranes can last up to 3 years which is comparable to the lifetime of membranes for natural gas purification, a primary market for membrane technology, which last typically two to five years. After 1 years permeability has decreased by 30 % due to compaction. The clean gas is further compressed up to 3.600 psi (250bar) and stored in steel cylinders in capacities of 276m3 divided in high, medium and low pressure banks. The membranes are very specific for given molecules, i.e. H2S and CO2 are separated in different modules. The utilization of hollow-fiber membranes allows the construction of very compact modules working in cross flow [3].

    Gas Liquid Absorption Membrane:

    Gas-liquid absorption using membranes is a separation technique which was developed for biogas upgrading only recently. The essential element is a microporous hydrophobic membrane separating the gaseous from the liquid phase. The molecules from the gas stream, flowing in one direction, which are able to diffuse through the membrane, will be absorbed on the other side by the liquid flowing in counter current. The absorption membranes work at approx. atmospheric pressure (1bar) which allows low-cost construction. The removal of gaseous components is very efficient. At a temperature of 25 to 35C the H2S concentration in the raw gas of 2% is reduced to less than 250ppm. The absorbent is either Coral or NaOH.

    H2S saturated NaOH can be used in water treatment to remove heavy metals. The H2S in Coral can be removed by heating. The concentrated H2S is fed into a Claus reaction or oxidised to elementary sulphur. The Coral solution can then be recycled. CO2 is removed by an amine solution. The biogas is upgraded very efficiently from 55% CH4 (43% CO2) to more than 96% CH4. The amine solution is regenerated by heating. The CO2 released is pure and can be sold for industrial applications [3].

    Adsorption Using Iron Oxide and Activated Carbon

    Hydrogen sulphide reacts easily with iron hydroxides or oxides to iron sulphide. The reaction is slightly endothermic; a temperature minimum of approximately 12C is therefore required to provide the necessary energy. The reaction is optimal between 25 and 50C. Since the reaction with iron oxide needs water the biogas should not be too dry. However, condensation should be avoided because the iron oxide material (pellets, grains etc.) will stick together with water which reduces the reactive surface.

    The iron sulphides formed can be oxidised with air, i. e. the iron oxide is recovered. The product is again iron oxide or hydroxide and elementary sulphur. The process is highly exothermic, i.e. a lot of heat is released during regeneration. Therefore, there is always a chance that the mass is self-ignited. The elementary sulphur formed remains on the surface and covers the active iron oxide surface. After a number of cycles depending on the hydrogen sulphide concentration the iron oxide or hydroxide bed has to be exchanged. Usually an installation has two reaction beds. While the first is desulphurising the biogas, the second is regenerated with air.

  • 8

    The desulphurisation process works with plain oil free steel wool covered with rust. However, the binding capacity for sulphide is relatively low due to the low surface area.

    Wood chips covered with iron oxide have a somewhat larger surface to volume ratio than plain steel. Their surface to weight ratio is excellent thanks to the low density of wood. Roughly 20 grams of hydrogen sulphide can be bound per 100 grams of iron oxide chips.

    The application of wood chips is very popular particularly in the USA. It is a low cost product, however, particular care has to be taken that the temperature does not rise too high while regenerating the iron filter.

    The highest surface to volume ratios are achieved with pellets made of red mud, a waste product from aluminum roduction. However, their density is much higher than that of the wood chips. At hydrogen sulphide concentrations between 1.000ppm and 4.000ppm totally 50 grams can be loaded on 100 grams of pellets. Most of the German and Swiss sewage treatment plants without dosing of iron chloride are equipped with an iron oxide pellet installation.

    With PSA systems H2S usually is removed by activated carbon doted with potassium iodide (KI). Like in biological filters in presence of air which is added to the biogas, the hydrogen sulfide is catalytically converted to elementary sulphur and water. The sulphur is adsorbed by the activated carbon. The reaction works best at a pressure of 7 to 8 bar and a temperature of 50 to 70C. The gas temperature is easy to achieve through the heat formed during compression. Usually, the carbon filling is adjusted to an operation time of 4.000 to 8.000 hours. If a continuous process is required the system consists of two vessels. At H2S concentrations above 3.000ppm the process is designed as a regenerative system [3].

  • 9

    Chapter 3: Proposed Design

    Figure 3: Proposed design of scrubber

    Assumptions:

    Meshed sheet is assumed as a plane cylinder i.e. we are assuming that pressure will drop continuously along the length, which has to be corrected using experimental results.

    Laminar flow in the tubular region which is found correct after calculation.

    Repeating pattern of corrugation with a fixed height and pitch as shown in Fig 4

    Stripped water has no H2S or CO2

    Efficiency of both blower and pump were taken to be 80% each.

    Design parameters:

    Dimension of Absorber

    Outer Diameter (OD) = 500mm

    Inner Diameter (ID) = 500/3mm

    Flow rate of biogas = 10,800 m3/day

    Absorbing Solvent = Water (Fresh + Stripped)

    Consumption of a given chemical in biogas= tD . NR . Gma1. x

    Where,

    Run Duration = tD

    No. of runs =NR

    Gma1 = mass flow rate of biogas

    x = % of the respective constituent of biogas

  • 10

    Area of cross-section of absorption = 0.117 m2

    Hydraulic Diameter= 4Acs/L

    Where Acs=Area of cross section

    L= Wetted perimeter

    Calculation of biogas exposed surface:

    R - Radius of the outer cylinder r- radius of the shaft

    Angle subtended by wetted portion at the centre

    From Fig. 3

    = - 2

    Also, sin = r/R => = 2 sin-1(r/R)

    Area under water = (Area of sector subtending angle at the centre) (Area of two s)

    = ( /2 )(R2) (R cos )r Substituting and , we get

    Wetted Area = R2/2 R2 sin-1(r/R) r (R2 - r2 )

    Taking R = 250 mm and r = 250/3mm

    Wetted Area = 57,293.11 mm2

    Cross Section exposed to Biogas (Acs) = Total Cross Section Wetted Area

    = 174,532.93 57,293.11 mm2

    = 117,239.82 mm2 = 0.117 m2

    Calculation of wetted perimeter (P)

    Figure 4: Meshed sheet

  • 11

    Length of one section of curve y = sin ( x/5) from 0 to 10 (i.e. for pitch p)

    210

    0

    1 dyl dxdx

    = + (Formula for length of any curve for given interval)

    102

    0

    1 (2.5cos )l dx= + Or l = 0.021 m

    If Number of such meshed sheets are x then

    R r = x (h+2) => 5003( 2)

    xh

    =+

    Or x = 24

    Diameter after ith sheet

    Di = ID + i (h+2)

    Now for ith sheet, perimeter of ith sheet = number of loops of pitch p * pitch p

    i id n p =

    ii

    dnp

    =

    Total wetted perimeter L = ni (2 l +2p)

    Or 0

    (2 2 ) ( 2)x

    n

    l pL ID n hp

    =

    += + +

    Or L = 112.288 m

    So wetted area Aw = L*1 (as we are assuming length of the sheet to be 1m)

    Hydraulic Diameter Dh = 4Acs/L

    Or Dh = 4.23 mm

  • 12

    Chapter 3: Conclusion After entering all the mass transfer, pressure drop, area and wetted perimeter calculation

    equations etc in the MathCAD program (see Appendix) some initial results have been calculated.

    For our proposed design, with Inner diameter of wheel/Diameter of the shaft (ID) =500/3mm and Outer diameter of wheel (OD) =500mm. The corrugated sheets disc if dipped in to water up to 1/3rd of its height, the area exposed to biogas (Acs) turns out to be 0.117 m2. Since we are using meshed sheets as shown in figure 4, this entire area wont be available for mass transfer. Thus on calculating the actual area involved in mass transfer, we get the wetted area density as Aw = 112.288 m2/m3.

    In order to get desired concentration output of. CH4 = 95-98%, CO2 = 3-5%, H2S < 10ppm we require some minimum flow rate of water for the given flow rate of biogas (10,800 m3/day). This is calculated from the mass transfer equations and found to be 889,729.92m3/day which is around 80 times the flow-rate of biogas. Also the minimum contact area required to get this mass transfer done was 0.205 m2.

    On the other hand we propose to use atmospheric air which is available cheaply to strip the rich solution of its CO2 and H2S content. Working of stripper is the same as that of absorber, the only difference being, that in absorber, the components of biogas were getting absorbed in water while in stripper, components from water were getting absorbed in air. We assumed the composition at the outlet of stripper in order to get minimum flow rate of air required. Here we assumed that CO2 and H2S both are absent from stripped water. This stripped water is then re circulated in the system. So we got the minimum flow rate of air required (28.95 mol/sec), for a given flow of water which is equal to outlet flow rate of water.

    To get the pressure drop across the tube, in order to get blower power, we approximate the meshed sheet tube as a plain tube. We get the hydraulic diameter for the given flow rate as 4.23mm and Reynolds number as 641.591. Since its a tubular flow, knowing the friction factor, we can get the pressure drop as 23.482bar and from that we get the blower power as 458.626kW. Similarly we get the pressure drop for stripper as 60.278 bar and blower power required as 2.59 * 103 kW.

    As we know the contact area required and area density, we can then get length of the tube required. From this we can decide how many wheels in series we would require, depending on the wheel size constraints.

    What needs to be done?

    In the end we were able to design a basic working model for the biogas scrubber. The next stage requires iterating the values obtained from the stripper and the absorber and using them as initial conditions. These results have not been optimized as we have assumed some initial parameters here, so we need to calculate those parameters at the outlet (of each stripper and absorber) from the results obtained from the assumed case and then recalculate them on the basis of these new parameters. After a few iterations, the values obtained, would start converging, leading us to an optimized result. Also makeup/re-circulated water has not considered at this stage of the design process.

  • 13

    References 1. www.wikipedia.com

    2. www.sciencedirect.com

    3. Biogas up gradation and utilization, Task 24 : Energy from biological conversion of organic waste by IEA Bioenergy

    4. O. Jnsson , M. Persson , Biogas as transportation fuel, Swedish Gas Centre

    5. S.S. Kapdi, V.K. Vijay, S.K. Rajesh, Rajendra Prasad, Biogas scrubbing, compression and storage:perspective and prospectus in Indian context, Renewable Energy xx(2004)

    6. M. Gatrell, Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas

    7. Treybal, R.E., Mass-Transfer operations 3rd ed. ISE.

    8. International critical tables of numerical data, physics, chemistry and technology ed. by E.W. Washburn.