Renewable Energy Technology 3(2+1)

Lesson 12. Performance evaluation of biogas plants

Gas production

Daily gas production may be measured using gas flow meters.  Gas flow meters can be connected with the outlet of the biogas plant.  The gas outlet can be observed daily.  This can be compared with the actual capacity of the plant. 

COD destruction

There is a correlation between Chemical Oxygen Demand (COD) and methane generation in biogas plants.  For every kg of destruction of COD, about 0.3 m³ of methane can be generated.  Hence, the destruction of COD and generation of biogas in the plant can be assessed by observing the COD content of the influent and effluent and the biogas production.

COD measurement

The Chemical Oxygen Demand (COD) is a measure of oxygen equivalent of that portion of the organic matter in the sample that is susceptible to oxidation by strong chemical oxidant. This is an important and quickly measured parameter for pollution studies of streams and industrial wastewater.

Most of the organic matter is destroyed in boiling mixture of chromic and sulphuric acid. The purpose of running blank is to compensate for any error that may result because of presence of extraneous organic matter in the reagents.

Apparatus

a) COD reflux apparatus of 500 ml flat bottom flask with ground glass joints and condensers.

b) Pipette, burette, measuring cylinders etc.

Reagents Required

a) Standard  0.25  N  potassium  dichromate  (K₂Cr₂O₇) solution

Dissolve 12.259 gm of pure K₂Cr₂O₇ in distilled water and dilute to 1 litre.  Add about 120 mgs of sulphuric acid.

b) Sulphuric acid-Silver sulphate reagent

Add 5.5 gms of AgSO₄ to 1 Kg of Conc H₂SO₄. Keep the same overnight.

c) Standard 0.1 N Ferrous Ammonium Sulphate Solution Fe(NH₄)₂.(SO₄)₂.6H₂O

Dissolve 30 gm of pure salt in distilled water. Add 20 ml of conc H₂SO₄ and dilute to 1 litre.

d) Ferroin indicator

Dissolve 1.485 gm of 1-10 Phenanthraline monohydrate with 0.695 pure FeSO₄.7H₂O in distilled water. Dilute to 100 ml. This indicator is commercially available.

e) Con. H₂SO₄

Procedure

a) Take 0.4 gm of H₂SO₄ and 20 ml of sample in a reflux flask (if required dilute to suitable degree). Mix well.

b) Add 10 ml of 0.25 N, K₂Cr₂O₇.  Drop some pumice stone and slowly add 30 ml of Con H₂SO₄-AgSO₄ reagent.

c)  Mix the contents thoroughly and connect the flask to condenser. Reflux for 2 hours.

d) Cool and wash down the condensers. Dilute the mixture to 150 ml by adding distilled water.

e) Add 3 drops of Ferroin indicator and titrate with N/10 Ferrous Ammonium sulphate solution till the colour changes from green to wine red. Note the end point.

f)  Perform the same procedure with 'Blank' using distilled water instead of the sample.

Observation table

Sl.No	Sample volume, ml	ml of Ferrous Ammonium sulphate rundown
		Sample V1	Blank V2
1	20 ml
2	20 ml
3	20 ml
4	20 ml

                             

                                (V1-V2) X Normality of Ferrous Ammonium Sulphate X 8000

COD in mgs/litre = ----------------------------------------------------------------------------------

                                                             Volume of Sample

If the sample is diluted COD in mgs/litre = COD as above X dilution ratio

(For example if dilution is 10% (i.e. 10 to 100) i.e. 10 ml of sample and 90 ml of distilled water.  Then COD in mgs/litre = COD x 10 mg l^-1)

Methane content of biogas

The saccharometer is used to measure the methane content of biogas sample.  Saccharometer is filled with 10 per cent NaOH solution.  5 ml of biogas is injected into the saccharometer using syringe.  The carbon dioxide present in the biogas will be dissolved in NaOH solution.  The methane will be collected on the top of the saccharometer and the volume can be measured.

The constituents of biogas such as methane, carbondioxide etc. can also be measured using gas chromatography method.

Determination of calorific value of biogas

The continuous flow gas calorimeter comprises of a gas flow meter, a gas pressure regulator, a manometer and a combustion chamber with a burner. The gas, whose calorific value has to be determined, is passed through the gas flow meter, then through the regulator and then to the burner. The pressure is measured using the manometer and the temperature is measured at the inlet of the gas meter and also of the products of combustion

The inlet and outlet temperatures of water are measured. The calorimeter is so designed that the gases from the flame pass upwards through the central portion of the calorimeter known as combustion chamber and at the top are deflected down wards and pass through the flues, finally escape into air through the outlet. The calorimeter chamber is cooled by cooling coils. The calorimeter is enclosed in an outer container. The air gap between outer cover and calorimeter acts as insulation against radiation losses. There is an outlet to collect the condensate obtained from the cooling coils as a result of burning of H₂ in the gas to water. The condensate is also weighed.

Volume of gas used                                        =          V₁

Temperature of gas used                                 =          T₁

Gas manometer reading,                                 =          G

Barometric height,                                                   =           B

Mass of cooling water used                            =          M₁

Mass of condensate collected                         =          M₂

Cooling water temp. at inlet                           =          T_ci

Cooling water temp. at outlet                         =          T_co

Absolute pressure of gas, P₁                          =          [B + (G / 13.6)]

The volume of gas must be converted to the volume, as it would exist at 25^oC and 760 mm Hg.

Let this volume be Vo. Then

P ₁V₁ / T₁                                             =          PoVo / To

Vo = (P₁/Po) (To / T₁ ) V₁                   =

Now,   Energy transferred by gas       =          Energy gained by the cooling water

Vo x HCV                =          M₁ x (h₂- h₁)

From steam table,

h₂ = Specific enthalpy of water at T_co, J kg^-1

h₁ = Specific enthalpy of water at T_ci, J kg^-1

Hence,

HCV                                       =          kJ m^-3

Latent heat of vaporization       =          L

Therefore, LCV                      =          HCV - (M₂ x L)