Energy Generation from Biogas Produced at STP

Gas Holders at Delawas

Excreta and wastewater sludge are resources for making biogas. Finding ways to put them to their best uses is part of developingsustainable technologies. At the same time, excreta and wastewater sludge – if not managed properly – can be dangerous to human health and the environment.

We can generate electricity at plant, run ic engines  and use as fuel by bottling  the bio gas produced at stp  during anaerobic digestion of waste water, which otherwise can be harmful for environment if allowed to escape in atmosphere. By doing this we can save electricity bill of stp there by making them sulf sustaining. By reducing ghgs we can earn carbon credits

Case studies

(1) 2Î 62.5 MLD Capacity DELWAS STP in  JAIPUR
(2) Welissa Farms — Bantayan, Cebu
(3) Anaerobic co-digestion of sewage and brewery sludge for biogas :Thammasat University, Thailand
(4) Biogas from Sewage Treatment used to Energy Generation, by a 30 kW (ISO) Micro turbine
(5) Bio Gas Production  In Indian  Perspective
(6)Power Generation  from gases at  STP(Germany)
(7)Some case studies to reduce H2s and increase CH4 contents  in Bio Gas

Power generation at a glance in world(2009)

Power generation at a glance in world(2009)

From Where Ch4 Come From………?

During Anaerobic treatment process of sewage  (carried out in the absence of O2) for the stabilization of organic materials CH4 ,CO2, NH3, H2O, H2S are end product .

Anaerobic digester microbiology

Steps In Anaerobic Digestion

1:  Extracellular hydrolysis (e.g. cellulose)

2: Fermentation leading to organic acids (VFAs), acetate, CO2 and H2 (page 7-8)

3: Fermentation leading to acetic acid (CH3COOH), H2 and O2

4: Methanogenesis leading to CH4,CO2 and H2O

Factors Affecting Ch4 Production In ASD

Ph (6.6 To 7.6) (Page 11)

Nutrients Availablity.

Temperature (35c To 55 C) (Page 10)

Solids Retention Time (Srt), (Page 8-9)

Volatile Fatty Acids (Vfa) (Page 12,18 -23)

Mixing Of Sludge

Toxic And Inhibitory Compounds ( Viz Sulphide, Light Metal Cations, Ammonia, And Heavy Metals )

PRE-TREATMENT PRIOR TO ANAEROBIC DIGESTION  

COD Balance Anaerobic Biodegradation

(During hydrolysis, cell walls are ruptured and extracellular polymeric substances are degraded resulting in the release of readily available organic material for the acidogenic micro-organisms. By means of efficient pre-treatment ,the suspended substrate can be made more accessible for the anaerobic bacteria, optimizing the methanogenic potential of the waste to be treated. Various sludge disintegration pre-treatment: include mechanical grinding, ultrasonic disintegration, chemical methods, thermal pre-treatment, enzymatic and microbial pre-treatments. )

Power Generation From Biogas Produced At Delawas Plant

• Capacity stp  :  62.5 mld  unit i.
• Amount of gas generated   from one 62.5 MLD PLANT  : 6000 M3/DAY  250 M3/hr
• Calorific value of gas  : 5000 to 5600 kcal/m3
• Total heat energy of gas   : 1300000 kcal/hr
• Total energy in kw hour  : 1509.3
• Efficiency of power  : 28 %  generation unit including engine and generator
• Electrical energy production  : 422.60 kwh
• Existing plant producing bio gas continuously and excess gas is flared.
• Data  made available  from  delawas  plant

Units required for power generation system

Gas Engine

• Gas holder
• Inter connecting piping
• Gas engines
• Gas scrubber for removal of h2 s (by naoh at delawas)
• Chiller unit  : to remove moisture from gas  coming  gas from the scrubber through the blower, by cooling the gas to low temperature  by condensing the moisture. A shell and tube type heat exchanger will be used for this purpose.

Biogas upgrading (H2S Scrubbing)

H2S Removal is needed to

• reduce air pollution As it is toxic and odorous and gives sulfur dioxide on burning
•  protect power generation equipment from corrosion
• increase safety of the operations

How to minimize H2S in bio Gas

1. By reducing H2S by Process Control

(a)  bio scrubber; bio filter; and bio trickling filter.( H2s is absorbed by Chemotrophic bacteria and elemental sulfur is produced)

2. By Scrubbing, carbon adsorption, and chemical and thermal

Oxidation from Bio Gas. Scrubing is being done at Delawas Using NaoH, H2S gets absorbed in water, The gas from the bottom of the Scrubber enters the packed Column while liquid is collected in the Tank for recalculation.  In the II stage of scrubbing, gas is scrubbed with caustic solution in a packed Column. The Column is provided with ceramic rings to have enhanced surface area for mass transfer. This is a counter current scrubber and provides high scrubbing efficiency. caustic solution ensures effective reaction of H2S and CO2 and their removal.

Biogas properties

 Selection Of Type Of Engine

Power can be generated from bio gas from following machines.

• Dual fuel engine
• Gas turbine
• Gas engine

Gas engine is most suitable and efficient for present application. Among the three types of engine gas turbine is not used for bio gas fuel since it is not suitable for low capacity. Dual fuel engine is used in remote area where normal power supply is not available or erratic. This engine can be run either with diesel or with biogas & diesel combination. Dual fuelengine needs more maintenance and diesel storage.

How  Optimise Gas Production And Optimum Use Of Energy At STPs

• By effective control of process
• Amonia stripping
• Thermal pretreatment of sludge by utilizing heat of exhaust gases
• Scum and grease can be fed in digester
• Use  thermophilic stage for more gas production but this need effective control
• Biological removal of sulfur  so that h2s production can be minimised
• In winter due to low atmospheric temperature gas generation in digester is reduced.
• Other cost effective technologies can be used to reduce hydrolysis time and over all srt of anaerobic process, minimization of toxicants and inhibition factors to increase quality and quantity of bio gas.

Anoxic decomposition 

Overall process of anoxic decomposition, showing the manner in which various groups of fermentative anaerobes cooperate in the conversion of complex organic materials ultimately to methane 1 CH42 and CO2. Acetate and H2 + CO2 from primary fermentations can be directly converted to methane, although H2 + CO2 can also be consumed by homoacetogens. But note how the  syntrophs  play a key role in anoxic decomposition by consuming highly reduced fermentation products in a secondary fermentation. By activities of the syntrophs, fatty acids and alcohols are converted to the substrates for methanogenesis and acetogenesis. This picture holds for environments in which sulfate-reducing bacteria play only a minor role, for example, in freshwater lake sediments, sewage sludge bioreactors, or the rumen. If alternative electron acceptors are abundant, as for example, sulfate in marine sediments, anaerobic respiration prevails, as syntrophs cannot compete for fatty acids/alcohols with sulfate-reducing bacteria or bacteria carrying out other forms of anaerobic respiration.