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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.


This blog post uses the terms bio-CNG and renewable natural gas (RNG) interchangeably.

Bio-CNG or bio-compressed natural gas, also known as sustainable natural gas or biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and having a methane concentration of 90% or greater. As the gas is derived from natural and renewable sources, it is also termed renewable natural gas (RNG).

Introduction

Anaerobic digestion is a prominent technology for treatment of organic 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.

For potential investors, it is essential to understand the various technologies in detail for setting up a viable RNG plant. This blog post provides details on the various steps involved in the anaerobic digestion process and the technologies available.

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 can be classified into different types based on the following parameters:

  • Operating temperature
  • Feedstock variation
  • Wet (low-solids) and Dry (high-solids)
  • Batch vs. continuous flow

Types of processes by operating temperature

Digesters are designed to run at different target temperature ranges. The temperature ranges are typically 30- 38C for mesophillic and 50-60C for thermophilic. There are different populations of anaerobic microbes that thrive in these temperature zones.

Generally, thermophilic anaerobic digestion (AD) would be used when greater pathogen kill is necessary. This temperature range produces sludge which can be directly applied to the farms without pre-treatment owing to absence of toxins. Thermophillic digesters require less time to process feedstocks but may have higher costs and be more difficult to operate.

Mesophillic digesters are commonly used and are easier to operate and maintain, but will not result in sufficient pathogen kill to produce higher quality sludge.

Types of processes by feedstock variation

Some digesters are designed to process one type of feedstock and other digesters are designed to process multiple feedstocks.  Co-digestion is often a driver for the development of anaerobic digesters.  Many feedstocks require or can benefit from pre-processing before digestion (e.g., blending, screening, thermal conditioning, etc.)

Types of processes by wet and dry solids

Wet and dry classifications of digesters refer to the moisture content of the feedstocks.  Wet digesters are more common that dry.  A wet digester or low solids AD system generally processes feedstock with less than 15 percent solids content.  The feedstocks for a wet digester are typically in slurry form and can be pumped.

A dry digester or a high solids AD system generally processes feedstock with greater than 15 percent solids content.  The feedstocks for a dry digester are often described as stackable. 

Batch vs. continuous flow

In a batch digester, feedstocks are loaded into the digester all at once.  Following loading there is a set period of time for digestion to occur.  Following this time period, the digester is manually emptied and reloaded.

In a continuous flow digester, feedstocks are constantly fed into the digester and digested material is continuously removed.

This section provides details on the process steps for a wet, mesophilic, continuous, co-digestion plant typically followed under Indian conditions.

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 1: 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 2: 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 30-38˚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 3: 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 4: 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.


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