Blog

Bio-CNG Production – Technology and Process Details



evnext-logo-v-smallThis post is a part of BioBiz’s Bio-CNG Perspectives.

BioBiza division of EAI, is a leading market intelligence & strategic consulting firm for the Indian bio-based sectors.


 

Introduction

Anaerobic digestion is a prominent technology for treatment of organic wet waste materials. The technology has been in existence for years and has been widely adopted at small scales to treat farm manure and sewage. However, only in the last decade, its commercial potential has been explored. While the conventional technology resulted in the generation of biogas, a product with low-calorific value and minimal applications, recent advancements have enabled the generation of RNG, a product with quality similar to natural gas and with wide applications.

However, it is to be noted that the conventional technology is still being applied at small scales – for household and community level biogas plants. With the industry marked by several technology providers with innovations across the value chain, it is critical to undertake a detailed technology evaluation study and analyse the technologies and solution providers on key parameters before selecting the right technology.

Details of biogas plant and technology

Technology process description

Biogas is commercially produced by a process called anaerobic digestion. The process involves breakdown of organic waste materials such as animal waste, food waste and industrial sludge to produce biogas and digestate. The latter is further treated to be used as a fertilizer. Anaerobic digestion process is carried out in a sealed, oxygen-free tank, also called an anaerobic digester. The biogas produced is subjected to scrubbing, upgradation and compression (200 bar pressure) processes to produce renewable natural gas (RNG).

Anaerobic digestion process involves 4 steps:

  1. Pre-processing of organic waste
  2. Anaerobic digestion for the production of biogas
  3. Biogas scrubbing and upgradation to RNG
  4. Treatment of digestate to produce manure

Pre-processing of organic waste

The organic waste procured from various feedstock sources such as commercial waste, industry waste and agro waste is subjected to pre-processing step, where (after segregation) addition of water is carried out to convert the solid waste into a slurry form. The quantity of water to be added depends on the type of waste. For a 10 TPD plant using food waste as a feedstock, the water required is in the ratio of 1:0.5 (H2O). A large portion of this water could be recycled. If one of the feedstock being used is industrial organic waste water (as part of a co-digestion plant), then the need for additional fresh water is minimal.

In the case of pre-processing of agro waste, an additional step is involved, where the agro waste is shredded and subjected to hydrolysis to release the cellulose content for digestion by the microorganisms. After hydrolysis, the agrowaste is mixed with other feedstock and sent into the hydrolysis or pre-digester tank.

RNG Feedstock pre-processing

Figure: RNG feedstock pre-processing

Raw materials and equipment used

  • Feedstock (food waste, agro waste, industrial waste)
  • Shredders
  • Hydrolysis unit (in the case of agro waste)
  • 20 MT – weighbridge
  • Conveyors to pulper
  • Mixer & pulping equipment
  • Intermediate holding tank

Production of biogas by anaerobic digestion

The following schematic describes the biogas production process from waste by anaerobic digestion.

Biogas production by anaerobic digestion

Figure: Biogas production by anaerobic digestion

Raw materials and equipment used

The raw materials and equipment required for biogas production include:

  • Biomethanation plant consisting of hydrolysis holding tanks, N2 dosing system, digester tanks, cooling tank, and sludge separator
  • Agitator
  • Sensors (Either electronic or pneumatic)
  • SCADA connected panel system

 Process description

  • After feedstock pre-processing, the feedstock is fed into the hydrolysis tank where it is allowed to get pre-digested for 2-3 days and then sent to the digester where digestion is carried out by mesophilic or thermophilic organisms. The microorganisms can be added separately or made to grow inside the digester through optimal maintenance of parameters like temperature and pH
  • The temperature is maintained at 37-39˚C at atmospheric pressure for mesophilic organisms
  • Continuous or intermittent sludge feeding and sludge withdrawal is done, with the feeding rate being 1.6 to 6.4 kg VS/m3/day
  • The digestion process to produce biogas takes place in four steps:
    • Hydrolysis
    • Acidogenesis
    • Acetogenesis
    • Methanogenesis
  • During hydrolysis the complex organic molecules are broken-down into simple sugars, amino acids, and fatty acids.
  • Acidogenesis results in further breakdown of the remaining components by acidogenic (fermentation) bacteria. Volatile fatty acids are created along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other by-products.
  • In acetogenesis, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon-dioxide and hydrogen.
  • In the last step, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water.
  • Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.
  • Unreacted materials get settled in the bottom of digester which is removed at intervals.
  • Biogas production process also produces digestate which is the material remaining after anaerobic digestion. This digestate is used as a fertilizer

Production of RNG from biogas

The following process flow diagram describes the RNG production process from biogas.

Upgradation of biogas to bio-CNG

Figure: Upgradation of biogas to bio-CNG

Raw materials and equipment used

The raw materials and equipment required for RNG production include:

  • Biogas mixture from anaerobic digestion
  • Pipelines for transport
  • Sensors (Either electronic or pneumatic)
  • SCADA connected panel system
  • Gas conditioning and up gradation plant consisting of moisture separator, H2S scrubber, PSA units, pumps and associated piping & electrical
  • RNG compressor and accessories

Process description

  • The biogas produced by anaerobic digestion is a mixture of gases and trace elements – methane, CO2, hydrogen, nitrogen, hydrogen sulphide, ammonia – of which only CO2 and CH4 are the desirable products.
  • In order to obtain methane in its commercially usable form, it is necessary to subject the biogas to scrubbing and upgradation.
  • The biogas mixture is scrubbed using H2S scrubbers and moisture trapping units to remove undesirable components such as H2S. The biogas is then subjected to upgradation where CO2 is separated from methane using PSA or membrane type purification technologies.
  • The pure methane (92-97%) is compressed at 200 bar pressure and sent to the bottling unit, where it is filled in cylinders and sealed.
  • This is then packed in cascades and sent for storage and sale.
  • The CO2 obtained is either released or purified, bottled and sold in a similar manner like RNG

Production of fertiliser/manure from the digestate

The following process flow diagram describes the manure production process from the waste/residue generated from the biogas production process.

Production of digestate

Figure: Production of digestate

Raw materials and equipment used

  • Aeration tower
  • Solid and liquid separator
  • Reactor
  • Entry pump
  • Forced-draught fan
  • Storage tank
  • Mating filling lines
  • Extruder/screw press

Process description

  • The digestate obtained is fermented in an aeration tank either by inducing conditions for the existence of anaerobic microorganisms or by externally adding lab-inoculated microorganisms into the fermenter
  • The fermented slurry is separated by a solid-liquid separation apparatus, and part of the liquid portion is reacted with humic acid, where other fertiliser elements are added and this forms a liquid fertiliser, which is then packaged and sold. The remaining water is recycled.
  • The solid part that is separated is sent to the extruder or a screw press and the fibrous material is extruded to produce solid manure, which is packaged and sold in a similar manner as that of the liquid fertilizer

Effluent treatment & water recovery unit

Biogas production process requires the use of large amounts of water of which a larger portion is to be recycled. The recycling process occurs aerobically. COD and BOD are reduced by ultra filtration methods. The residual gas in the effluent is removed through anaerobic filter. Assuming a 10 TPD plant requires 25,000 litres of water, 5000 litres remains unrecoverable. Of the remaining quantity, a small portion is used as a fertilizer, rest is recycled.


evnext-logo-v-small

Know more on how BioBiz can assist your business in your strategy for the bio-based businesses, Here



Categories: Anaerobic digestion, Bio-CNG /renewable natural gas, Bio-CNG production process, Waste to Energy

Author - Narasimhan Santhanam


Thank you for reading this post – hope it was helpful. You can connect with me at narsi@eai.in. My detailed profile here and my LinkedIn profile here

Related Posts


About EAI and BioBiz for Bio-CNG

BioBiz for bio-CNG, EAI's specialized consulting division for bio-CNG, provides assistance in market research, strategy consulting, technology vendor/supplier finder assistance and more for the complete bio-CNG value chain – feedstock sourcing, logistics, pre-processing techniques, anaerobic digestion, biogas scrubbing, biogas upgradation, end user markets for bio-CNG and digestate, and more. Send a note to consult@eai.in if you wish to take BioBiz’s assistance for the Indian bio-CNG market.

About EAI & EV Next

EV Next, EAI's specialized consulting division, provides assistance in market research, strategy consulting, technology vendor/supplier finder assistance and more for the complete EV value chain - Li-ion & Lead Acid Battery Cells & Battery Packs, BMS, Battery Manufacturing equipment, EV charging station, battery swapping station, Li-ion & Lead Acid battery recycling, and for a range of electric vehicles - e-bikes, electric bicycles, electric scooters, electric motorbikes, electric autos, e-autos, e-rickshaws, electric trucks, electric LCVs, electric buses and more. Send a note to consult@eai.in if you wish to take EV Next assistance for the Indian electric vehicle market.



Devising Your EV Strategy: White Paper from EAI.


Leave a Comment ( 0 ) →



Leave a Comment

X

Name

Email

Mobile

Company Name

X

Name(*)

Email(*)

Mobile

Company Name

X

Name(*)

Email(*)

Mobile

Company Name

X

Name(*)

Email(*)

Mobile

Company Name

//