BIOGAS TECHNOLOGY

Author Unknown


INTRODUCTION

    1. Background

    2. Nature has a provision for destroying and disposing off wastes and dead plants and animals. This decay or decomposition is carried out by tiny micro-organisms called bacteria. Making of farm-yard manure (FYM) and compost is also through decomposition of organic matter (OM). When a heap of vegetable or animal waste and weeds etc., die or decompose at the bottom of backwater or shallow lagoons, bubbles can be noticed rising to the surface of water. Sometimes these bubbles burn with dancing flame at dusk. This phenomenon has puzzled man for a long time. It was only during the past hundred years that Scientists unlocked this secret as the decomposition process. The gas thus produced was and is still called "Marsh Gas." The technology of harnessing this gas under artificially created conditions is known as Biogas Technology.
       

    3. Biogas Technology

    4. Biogas Technology has a very significant role to play in integrated agricultural operations, rural sanitation, large-scale dairy farms & sewage disposal etc. It is estimated that cattle dung, when passed through a Biogas unit, yields 30-40% more net energy and about 35-45% more Nitrogen in manure as compared with that obtained by burning dung cakes and ordinarily prepared compost, respectively. Besides, from a biogas plant both the products are obtained. There are about 250 million bovine (cattle and buffalo) population in India and one biogas unit for small family requires about 3-5 cattle heads, thus about 10 million family size plants fed on cattle and buffalo dung can be installed. Overall as per this estimate of NCAER, total energy produced by livestock excreta amounts to about 80% of the rural fuel requirement.

      BIOGAS PLANT

      Biogas is a mixture of a few gases, such as Methane, Carbon dioxide, Hydrogen etc., formed as a result of anaerobic digestion of organic wastes. A biogas plant is commonly described as underground masonry, well shaped fermentation tank, connected with inlet and outlet tanks and covered by an inverted floating or fixed gas storage tank.
       

    5. Process description

    6. Biogas generation is a process widely occurring in nature and can be described as a biological process in which biomass or organic matter, in the absence of Oxygen, is converted into Methane and Carbon dioxide. It is characterized by low nutrient requirement, and high degree of waste stabilization process where biogas is one of the two useful products; the other being enriched organic manure in the form of digested slurry. It is essentially a three stage process involving following reactions:

      1) Hydrolysis 2) Acid formation 3) Methane generation.

      For all practical purposes the first two steps are often defined as a single stage, i.e. hydrolysis and acid formation stages are grouped as acid formation stage. Micro-organisms taking part in this phase are termed as acid formers. As a group, these organisms are rapidly growing and are not much dependent upon surroundings. Products of first two stages serve as the raw material for the third stage where organic acids are utilized as carbon source by Methane forming micro-organisms, which are also known as Methanogens. These Methanogens are more susceptible to their surroundings. The tolerated pH range is 6.8 to 7.5 with optimum 7.0. Any departure from this range is inhibitory. Atmospheric Oxygen is extremely toxic for methanogens, as they are strict anaerobes.
       

    7. Parameters affecting anaerobic digestion

    8. There are several parameters which affect the anaerobic digestion / gas yields and they can be divided into two parts:
       

      1. Environmental factors :

      2. There are a few environmental factors which limit the reactions if they differ significantly from their optimum levels. Factors of most interest are s (a) temperature, (b) pH and (c) nutrient contents of the raw materials,
         

        1. Temperature :

        2. It is a factor which affects most small & medium size biogas installations in developing countries. There are three zones of temperature in which biogas is produced by anaerobic fermentation of organic matter, viz.: 1) Mesophillic, 2) Thermophillic and 3) Psycrophillic zones. The optimum temperature of digester slurry in Mesophillic zone is 35oC, 55oC in Thermophillic zone and 10oC in Psycrophillic zone. In different temperature zones different sets of microbes, (bacteria) especially the methanogens remain active; whereas the other two groups of microbes either remain dormant and thus more or less inactive as far as the anaerobic digestion is concerned or get killed. However, the rate of fermentation is much faster at high temperature. Most rural household biogas plants (digesters) in developing countries operate at ambient temperatures, thus digester slurry temperature is susceptible to seasonal variation but is more dependent on the ground temperature than the atmospheric temperature. As a result, gas output in winter falls by up to 50 %. Below a slurry temperature of 10oC all the reactions cease to take place but revive gradually with the rise in temperature.
           

        3. pH

        4. The pH range suitable for gas production is rather narrow i.e. 6.6 to 7.5. Below 6.2 it becomes toxic. pH is also controlled by natural buffering effect of NH4+ and HCO3-ions. pH falls with the production of volatile fatty acids (VFAs) but attains a more or less constant level once the reaction progress.
           

        5. Nutrient Concentration


        Biogas producing raw materials can be divided into two parts i.e. 1) Nitrogen rich and 2) Nitrogen poor. Nitrogen concentration is considered with respect to carbon contents of the raw materials and it is often depicted in terms of C to N ratio. Optimum C/N ratio is in the range of 25 to 30:1. In the case of cattle dung the problem of nutrient concentration does not exist as C/N ratio is usually around 25:1.
         

      3. 0perational Factors

      4. Operational factors contributing to the gas production process are: (a) retention time (RT) - also referred as detention or residence time, (b) slurry concentration and (c) mixing.
         

        1. Retention Time (RT)

        2. It is the period during which any organic matter is subjected to the anaerobic environment and reaction in a biogas digester. When the organic matter is fed in the form of slurry the term used is Hydraulic Retention Time (HRT); whereas if it is fed in the solid form (usually 20-30% TS), the term used is Solid Retention Time (SRT). Retention time is proportional to the temperature of the process. At 15-30oC retention period is 40-55 days, at 35-37oC, 20 days and at 55oC 6-10 days. Digester as it is equal to retention time multiplied by the volume of daily feed.
           

        3. Slurry concentration

        4. This is denoted by dry matter concentration of the inputs. The optimum level for cattle dung slurry in the range of 8 to 10% and any variations from this result in lower gas output. Mixing four parts of dung with five parts of water forms a slurry with dry matter concentration of about 9%, whereas 1 part of dung to 1 part to water would give a slurry concentration of 10%. This also affects the loading rate etc.
           

        5. Mixing & Stirring

Proper mixing of manure to form an homogenous slurry before it is fed in the digester, is an essential operation for better efficiency of biogas systems; whereas proper stirring of digester slurry ensures repeated contact of microbes with substrate and results in the utilization of total contents of the digesters. An extremely important function of stirring is the prevention of formation of scum layer on the upper surface of the digester slurry which, if formed, reduces the effective digester volume and restricts the upward flow of gas to the gas storage chamber. Mixing results in premature discharge of some of the input and a perfectly unmixed system is likely to result in better reaction rate but for the problem of scum formation.

 

BENEFITS OF USING THE BIOGAS PLANT

CLASSIFICATION OF RURAL HOUSEHOLD DIGESTER

There are three basic methods by which rural household biogas digesters in developing countries are operated in practice, namely: (i) batch, (ii) semi-continuous and (iii) semi-batch digesters.

      1. Batch digester

      2. In this process the whole digester is filled with raw materials for gas production along with some starting (seed) material. This is allowed to ferment and produce gas over a certain length of time and when gas yields become very low the digester is emptied of all the sludge which can be supplied as manure. In this system gas production begins at a low level and goes on increasing only to drop down again after reaching the peak. Because of variable gas production level, high cost and periodic emptying and filling of digesters, this process has not become popular. Examples of these digesters are small size garbage plant and crop-residues plant.
         

      3. Semi-continuous digester

      4. The rural household digesters are fed once a day and the fresh input displaces the same volume of spent materials from the digester. Every day a certain quantity of fresh inputs is fed into the digesters which is expected to remain in the digester for a prescribed retention time and produces gas over this length of time before being discharged.
         

      5. Semi-batch digester

A combination of batch-fed and semi-continuous fed digestion is known SHF digestion. Such a digestion system is used where the waste like garbage etc., which are available on daily or weekly basis but cannot be reduced to make slurry. In the semi-batch system, the animal manure can be added on a daily basis after the initial loading is done with garbage, agricultural waste, leaves, crop residence and water hyacinth etc.

SIZE SELECTION OF RURAL HOU5EHOLD BIOGAS PLANTS

Size of the rural household biogas plant to be installed should be selected on the basis of gas requirement and the livestock manure availability with the beneficiaries. Since cattle dung is the main substance for the biogas plant in rural India, the table given below shows the relationship between plant capacity, daily cattle dung requirement and gas use.

Size No

Plant capacity m3

Plant capacity (ft3)

Daily dung Required (kg)

Approximate No. of cattle

No. of family members

1

1

(35)

25

2 - 3

3 - 4

2

2

(70)

50

4 - 6

5 - 8

3

3

(105)

75

7 - 9

9 - 12

4

4

(140)

100

10 - 12

13 - 17

5

6

(210)

150

12 - 20

18 - 25

POPULAR DESIGNS OF BIOGAS PLANT MODELS

There are three popular Indian designs of biogas plants namely: KVIC, Janata and Deenbandhu Biogas plants. For construction of KVIC & Janata model plants - Indian Standard IS:9478-1986 released by Bureau of Indian Standards should be followed. Brief description of the three models is given below.

    1. KVIC plant

    2. It was in or around the year 1945 that Scientists at Indian Agricultural Research Institute (IARI), New Delhi worked on semi-continuous flow digesters and in the year 1961 Khedi and Village Industries Commission (KVIC) patented a design which is being popularized by various agencies in many countries. This design consists of a deep well shaped underground digester connected with inlet and outlet pipes at its bottom, which are separated by a partition wall dividing the 3/4th of the total height into two parts. A mild steel gas storage drum is inverted over the slurry which goes up and down around a guide pipe with the accumulation and withdrawal of gas. Now FRP and ferro-cement gas holders are also being used in the KVIC plant.
       

    3. Janata plant

    4. The Janata model is a fixed roof biogas plant which was developed by PRAD in 1978. This is also a semi-continuous flow plant. The main feature of the Janata design is that the digester and gas holder are part of a composite unit made of bricks and cement masonry. It has a cylindrical digester with dome shaped roof and large inlet and outlet tanks on two sides. It requires shuttering for making the dome shaped roof and a skilled & trained master mason is a must for the construction of a Janata Biogas plant. This plant costs about 20-30% less than the KVIC floating drum type plant.
       

    5. Deenbandhu plant

As a result of concerted efforts to reduce the cost of biogas plants, AFPRO designed and developed a new low cost fixed roof biogas plant which has been named Deenbandhu Biogas Plant (DHP). The reduction in cost of DHP have been brought about without adversely effecting the efficiency of Biogas plants. After intensive trials and testing under controlled conditions and field applications, designs of DHP have been standardized for family size installations. The designs of Deenbandhu biogas plants have been approved by the Department of Non-Conventional Energy Sources (DPJES), Govt. of India for extension under the National Project on Biogas Development (NPHD). Deenbandhu biogas plants are built with locally available building materials such as bricks, cement and sand. Unlike Janata biogas plants, for constructing plants of this design no shuttering is required for making the dome shaped roof. This also results in less labour and time required for completing the construction. Details of constructional methodology and other aspects related to Deenbandhu biogas plants can be obtained from "A Manual on Deenbandhu Biogas Plants" prepared by AFPRO and published by and available from Tata McGraw-Hill,Y Publishing Co. Ltd., New Delhi.

 

COMPARISION AMONG FAMILIY SIZE BIOGAS PLANTS

#

KVIC

Janata

Deenbandhu

1.

The digester of this plant is a deep well shaped masonry structure. In plants above 3m3 there is a partition in the middle of the digester.

Digester of this plant is a shallow well shaped masonry structure. No partition wall is provided.

Digester is made of segments of two spheres one each for the top and bottom.

2.

Gas holder is generally made of mild steel. It is inverted into the digester and goes up and down with formation and: utilization of gas,

Gas holder is integral part of the masonry structure of the plant, Slurry from the gas storage portion is displaced out of the digester with the formation of gas and comes back when it is used.

The structure described above includes digester and the gas storage chamber, Gas is stored in the same way as in the case of Janata plants.

3.

The gas is available at a constant pressure of about 10cm of water.

Gas pressure varies from 0 to 90 cm of water.

Gas pressure varies from 0 to 75 cm of water.

4.

Inlet and outlet connections are provided through A,C pipes

Inlet and outlet tanks are. large masonry structures designed to store the slurry displaced out of the digester.

Inlet connection is through AC pipe. 0utlet tank is large masonry designed to store displaced slurry.

5.

The volume of the gas holder governs gas storage capacity of the plant.

.it is the combined volumes of inlet and outlet chambers, (portions of inlet and outlet tanks above the second step.)

It is the volume of outlet displacement chamber

6.

The floating mild steel gas holder need regular maintenance to prevent corrosion. It has short life.

There is no moving pert and hence no recurring expenditure. It also has e long working life.

There is no moving part and hence no recurring expenditure, It also has a long working life.

7.

Installation cost is very high, A 2 m3 plant costs over Rs. 6900.00.

It is cheaper than the KVlC type plants, A 2 m3 plant costs about Rs.5400.00.

It is much cheaper then KVIC and Janate type plants. A 2 m3 'plant of this design costs Rs 4000.00.

8.

Digester can be constructed locally the gas holder needs sophisticated workshop facilities.

A trained mason using locally available materials can build entire plant.

Entire plant can be built by a trained. mason using locally available materials.

Cost comparison of 55 days HRT plant of KVIC, Janata & Deenbandhu models based on estimation of average Cost of building materials, labour and mesons as on January 1990 from AFPRO records.

 

PIPELINE FOR BIOGAS PLANTS

Employing correct size pipeline for transporting biogas from plants to the points of use is very crucial from the point of view of efficiency of gas utilization and the cost of installation.

The gas distribution pipeline has bean designed and recommended pipe sizes for different combinations of flow rates and distances between gas production and utilisation points are given in Table 1. These recommendations are made for galvanised iron pipe.

    1. Laying the gas distribution pipeline

Like no uniform design can be prepared for suiting all thplants to the points of use is very crucial from the point of view of efficiency of gas utilization and the cost of installation.

The gas distribution pipeline has bean designed and recommended pipe sizes for different combinations of flow rates and distances between gas production and utilisation points are given in Table 1. These recommendations are made for galvanised iron pipe.

      1. Laying the gas distribution pipeline

Like no uniform design can be prepared for suiting all the biogas installations, there is no laid down procedure for laying of gas pipeline for all biogas facilities.

Pipeline may have to be above or below the ground or it may be partly above and partly below the ground, while a properly laid underground pipeline would require less maintenance, it may get corroded faster at some places whereas in other places corrosion of above ground pipeline may be more rapid.

Employing high density polyethylene pipe enables us to overlook the problem of corrosion and in this case underground pipe may be preferred over the above ground pipe.

Various factors which need to be adhered to at the time of pipe laying are:
 

          1. Pipe and fittings to be used for laying gas distribution system must be of best quality. It is important from safety point of view and needs to be paid more attention for in-the-house connections. Extra emphasis must be given to the selection of valves to be employed.
          2. All underground pipes should be coated with protective paints to avoid corrosion. Underground pipes should be about 1 foot below the ground level.
          3. As far as possible only gradual bends (not sharp elbows) should be used for 90o turns in the pipeline, This reduces pressure drop.
          4. Only gate valves, plug valves and ball valves should be used for gas pipeline to minimize pressure loss during flow of gas through the valves.
          5. For connecting the burners with gas pipeline use of transparent polyethylene tubes should be avoided and only neoprene rubber tube should be used.
          6. Biogas is saturated with water vapour and slight fall in temperature causes its condensation in the pipeline. Therefore, adequate arrangements to remove the condensate must be made at the time of pipe laying. All the pipes must have some gradient and at all the low points water removers should be installed. Water accumulation in pipe results in drop in pressure which causes reduction in flow rate. The water remover can be of two basic types :
            1. manually operated water remover

            2. A schematic diagram of this type of water remover is depicted in Fig. l. It is a 'T' joint at the lowest point of a certain section of gas pipeline. The vertical branch of the 'T' is kept in a perpendicular downward direction and it is connected to one foot long piece of pipe. The other end of this pipe is either plugged or fitted with a valve. The condensate in the pipeline will flow into this pipe and will be drained off manually at an interval of a week or ten days or as guided by experience.

              Fig. 1 : Schematic diagram of water remover
               

            3. Automatic water removal siphon

 
In this type of water removers the vertical branch of 'T' joint should be at least of 1" (25mm) diameter. It is connected to a 'U' shaped assembly as shown in Fig. 2.

Fig. 2 : Automatic water removal siphon
 

Height of the free arm of the U tube, (marked H) should be at lest 100cm for Deenbandhu plants, 110cm for Janata biogas plants and 20cm for KVIC type biogas plants. The upper end of free arm of 'U' should be a little below the gas pipeline. A bend facing downwards is also provided on top of the free arm of 'U' for draining out the condensate. The 'U' tube will always be kept filled with water which can be ensured by periodic checks. When some condensate flows into the fixed arm of the 'U', equal quantity of water from the 'U' will be discharged through the bend fitted to the free arm.
 

          1. The whole gas distribution system should be divided in a few sections so that anyone of them can be isolated from rest of the pipeline if it were to be repaired. This can be done by providing UNIONS at points where bends have been employed.
          2. Above ground pipe should be only along the walls and not hanging free. It should be hooked all along the walls especially on both sides of valves with the help of clamps at every two meters or so and no pipe should sag at any point. There should be a continuous slope in the directions of water remover.
          3. Gas cock in the houses should be out of the reach of children.
          4. At the time of installation whole pipeline should be tested for any leakage at a pressure of 1 kg/cm2, if possible.
          5. Burners should be connected in such a way that gas taps are in the front so that to operate the burner the user does not have to take her/his hand over the burner.
          6. Sketch of sample layout for pipeline from biogas plant to house is shown in Fig. 3. Normally, at least one water remover for 100 m pipe length should be installed. Details of in-the-house connections are not shown in the figure as it will vary from house to house. However, all the points mentioned above must be kept in mind while laying pipeline in the house.

Fig.3 : Sketch of sample layout of pipeline from biogas plant to the house
 

UTILIZATION OF BIOGAS

Biogas is a very clean fuel, which can be used for cooking, lighting and generating motive power. Gas required for different uses is as follows:

      1. Biogas requirement for cooking is 8 to 10 cft (0,25 to 0.3 m3) gas per head per day. Standard biogas stoves consume 16 cft (0.425 m3) gas per hour.
      2. Biogas lamp consumes 4-5 cft (0.15 m3) gas per hour.
      3. Dual fuel (diesel & Biogas) engines consume 15-16 cft (0.425 m3) gas per hour per hp.

 
UTILIZATION OF BIOGAS PLANT EFFLUENT

The digested slurry (dung and water mixture) available from biogas plants is highly nutritious organic manure. To derive maximum benefit from Biogas plants it is necessary to use this manure efficiently. One of the ways - which is most common and recommended - is to have manure flowing into the pits covered by a layer of wastes from the cattle shed, household end the farm. The sizes and their numbers along with details of costs for different capacities of Biogas plants are given below:

Table-2

DETAILDS 0F COMPOST PITS

 

Plant size

1 m3

2 m3

3 m3

4 m3

6 m3

No. of Pits

2

2

2

2

2

Size (in meters)

1.5x1x1

1.5x2x1

1.5x3x1

2x2x1

2x3x1

 

 

 

 

 

 


 

COST OF COMPOST PITS

 

Plant size

 

1 m3

2 m3

3 m3

4 m3

6 m3

 

 

 

 

 

 

 

 

 

 

 

 

Material

Rate

Qty

Cost

Qty

Cost

Qty

Cost

Qty

Cost

Qty

Cost

 

 

 

 

 

 

 

 

 

 

 

 

Brick

450/thou

800 

360 

1000

450 

1200 

540

1800 

810 

2200 

990

Cement

65/kg

130 

130 

130 

195

4

260

Sand

2.5/cft

15

37.50

15

37.50

15

37.50

20

50

30

75

Labour

15/day

60 

90

120 

12

180 

16 

240

Masons

35/day

70

70

105

140

175

Labour

15/day

30 

45 

5

75 

105 

10 

150

 

 

 

 

 

 

 

 

 

 

 

 

Total 

 

 

700

 

825

 

1025

 

1500

 

1900


 

Quantity Of Fresh Manure Available And Gas Produced From Different Feed-Stock

 

animal source

fresh green dung

moisture

gas yield / kg

yield / animal / day

 

 

 

 

 

 

 

 

 

 

 

 

kg

%

Cu.m

Lts

Cft.

Cu.m

Lts

Cft.

 

 

 

 

 

 

 

 

 

 

 

a

b

c

d

e

f

g

h

i

 

 

 

 

 

 

 

 

 

 

1

Cattle

 

 

 

 

 

 

 

 

 

- Large

15

80-85 

0.04

40

1.4

0.60

600

21.0

 

- Medium

10

80-85 

0.04

40

1.4

0.40

400

14.0

 

- Small

8

80-85 

0.04

40

1.4

0.32

320

11.2

 

- Calf

4

80-90 

0.04

40

1.4

0.16

160

5.6

 

 

 

 

 

 

 

 

 

 

2

Buffalo

 

 

 

 

 

 

 

 

 

- Large

20

80-85 

0.04

40

1.4

0.80

800

28.0

 

- Medium

15

80-85 

0.04

40

1.4

0.60

600

21.0

 

- Small

10

80-85 

0.04

40

1.4

0.40

400

14.0

 

- Calf

5

80-90 

0.04

40

1.4

0.20

200

7.0

 

 

 

 

 

 

 

 

 

 

3

Pig

 

 

 

 

 

 

 

 

 

- Large

2.0

75-80 

0.07

70

2.5

0.14

140

5.0

 

- Medium

1.5

75-80 

0.07

70

2.5

0.10

100

3.7

 

- Small

1.0

75-80 

0.07

70

2.5

0.07

70

2.5

 

 

 

 

 

 

 

 

 

 

4

Poultry

 

 

 

 

 

 

 

 

 

- Large

0.15

70-80 

0.06

60

2.1

0.009

9

0.32

 

- Medium

0.10

70-80 

0.06

60

2.1

0.006

6

0.21

 

- Small

0.05

70-80 

0.06

60

2.1

0.003

3

0.11

 

 

 

 

 

 

 

 

 

 

5

Goat/Sheep

 

 

 

 

 

 

 

 

 

- Large

5.0

75-80 

0.05

50

1.75

0.25

250

8.8

 

- Medium

2.0

75-80 

0.05

50

1.75

0.10

100

3.5

 

- Small

1.0

75-80 

0.05

50

1.75

0.05

250

2.5

 

 

 

 

 

 

 

 

 

 

6

Duck

0.15

70-80 

0.05

50

1.75

0.008

8

0.26

 

 

 

 

 

 

 

 

 

 

7

Pigeon

0.05

70-80 

0.05

50

1.75

0.003

3

0.11

 

 

 

 

 

 

 

 

 

 

8

Horse

15

80-85 

0.04

40

1.4

0.60

600

21.0

 

 

 

 

 

 

 

 

 

 

9

Camel

20

70-85 

0.03

30

1.05

0.60

600

21.0

 

 

 

 

 

 

 

 

 

 

10

Elephant

40

70-85 

0.02

20

0.7

0.80

800

28.0

 

 

 

 

 

 

 

 

 

 

11

Human

 

 

 

 

 

 

 

 

 

- Adult

0.4

75-80 

0.07

70

2.5

0.028

20

1.0

 

- Child

0.2

75-90 

0.07

70

2.5

0.014

14

0.5
 
DO's and DONT's :

DO's

DONT' s

DIMENSIONS of GAS HOLDER

 

 

 

 

 

 

 

 

Capacity m3

2

2

3

4

6

8

10

Diameter (cm)

110

125

150

165

200

225

260

Height (cm)

100

100

100

100

100

125

125


 
 
 
 

FIG.4 GAS HOLDER

FIG.5 FLOATING DRUM BIOGAS PLANT ( 1, 2 & 3m2 Above )

FIG. 6: FLOATING DRUM BIOGAS PLANT (4m2 & Above)
 
 
 

TABLE 2 DIMENSIONS OF FLOATING DRUM TYPE BIO-GAS PLANTS

( Clause 7.1 and Fig. 4 and 5 ) All dimensions in centimetres.

                                            Plant Capacity - 30 Days Retention Period

Dimensions       1          2         3         4         6         8       10

A                     120     135     160     180     220     240     275

                    157     187     202     212     212     242     232

                    170       95     110     135     136     125     115

                    112       70       70       70       70       70       70
 

                            Plant Capacity - 40 Days Retention Period

Dimensions       1          2         3         4         6         8       10

A                     120     135     160     180     220     240     275

B                     177     257     277     292     292     332     317

C                     170     165     185     200     200     200     200

D                     112     110     100       90       90       90     120
 

                            Plant Capacity - 55 Days Retention Period

Dimensions       1          2         3         4         6         8       10

A                     120     135     160     180     220     240     275

B                     227     327     377     427     427     477     477

C                     170     220     270     320     320     345     345

D                     112     175     180     180     210     210     205

Mike Witherden