Bio-CNG Production – Optimal Feedstock Sources

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).


Feedstock is considered to be the most critical component when implementing a renewable natural gas project. It plays a major role in determining the viability of the project. Anaerobic digestion is well suited for organic wet wastes. A wide range of organic wastes are generated on a daily basis – from households, industrial, commercial sectors and agricultural fields. These wastes differ widely in their physical and chemical characteristics – pH, calorific value, biochemical composition and more which determine their feasibility for use in an anaerobic digester.

Not all organic wastes can be considered potential feedstocks for anaerobic digestion. Optimal feedstock should have higher moisture content, higher calorific value, should possess less complex chemical structures to enhance the digestion process and more.  It is hence imperative for prospective investors to have a detailed understanding of the various feedstock sources available and analyse their potential for use in an anaerobic digester.  

This blog post provides details on potential feedstock sources for bio-CNG production and their characteristics.

Classification of feedstock sources for bio-CNG production

Feedstock procurement with improved quality is essential for efficient operations of anaerobic digestion plants. Anaerobic digestion is suited to wet organic waste materials such as grass clippings, paddy straw, leftover food, sewage, animal waste and more. In addition, large quantities of organic wastes in both solid and liquid forms are generated by the industrial sectors such as breweries, dairy, sugar mills, distilleries, food-processing industries, tanneries, and paper & pulp industries which can be considered potential feedstocks. Poultry and livestock manure are also considered to be potential feedstocks for RNG production.

Feedstock sources for renewable natural gas production can broadly be classified into three types:

Bio-cng image feedstock

Figure 1: Classification of feedstock sources

Chemical composition of feedstock sources

A detailed understanding of the chemical composition of any feedstock is essential to analyse its feasibility for use in an anaerobic digester. The following table lists the different types of residues under the above categories and their chemical composition.

Waste Lignin (%) Hemicellulose (%) Cellulose (%) Starch (%)
Commercial waste
Food waste Traces Traces 18.30 (w/w) 12.54 (w/w)
Fruit and vegetable waste 5 75 9 No data
Fish waste No data No data No data No data
Industrial waste
Cow dung 14 19 26 No data
Poultry litter 1.07-2.16 1.89-2.77 2.26-3.62 No data
Animal litter 3-13 16-21 11-25 No data
Press mud 9.3 10 11.4 No data
Spent grain 7 30 13 10-12
Agricultural waste
Paddy straw 4-10 20-30 33-40 Not present
Sweet sorghum stalk 5-8 20-28 34-46 Not present
Corn cobs 9 42 39 Not present
Napier grass 10-30 20-40 30-50 Not present
Maize straw 12-20 20-35 30-50 Not present
Wheat straw 15 20-30 35-45 Not present
Cotton stalk 21 77.5 47 Not present
Bamboo 22-26 15-26 57-63 Not present
Coconut shells 36 26 34 Not present
Nut shells 26-30 15-29 38-40 Not present

Table 1: Chemical composition of various feedstocks 

Source: EAI analysis

Note: Not present: Starch content is not found in agricultural residues.

It can be observed that feedstock rich in starch and cellulose content are potential candidates for renewable natural gas production. In the case of lignocellulosic agricultural feedstocks, those containing lignin content in the range of 4-22% are potential feedstock sources for the production of RNG. However, separate pre-treatment techniques such as hydrolysis is required before adding them in the digester.

Feedstocks with lignin content greater than 25% may not be suitable for use in a digester.

Based on the above table and an understanding of commercial biogas plants, it is observed that at both decentralized and centralized facilities, the following feedstocks are being widely used for biogas and RNG production and have been tested for their potential.

Bio-CNG - list of feedstocks


Figure 2: Commercial sources of feedstock for bio-CNG production

It is to be noted that while different types of agro wastes are available in India with chemical composition suitable for anaerobic digestion, only paddy straw and Napier grass are being commercially tried out for bio-CNG production. This is owing to the still evolving technology for pre-processing of agro residues. Thus, the choice of feedstock for a viable anaerobic digestion plant depends on the availability, accessibility, cost of procurement, logistics, biogas yield, extent of pre-processing required and more.

Characteristics of different feedstocks

Characteristics or quality of feedstock can be studied from the following two parameters to understand their feasibility for use in a digester.

  • Calorific value – Calorific value is the energy contained in a fuel or food, determined by measuring the heat produced by the complete combustion of a specified quantity of the given fuel. It is expressed in joules per kilogram. In the case of anaerobic digestion, calorific value is the yield of biogas per ton of feedstock.
  • Moisture content – Moisture content of a feedstock plays a major role in anaerobic digestion projects. As wet organic waste is the preferred feedstock, the wastes containing moisture content of 70% and more could be ideal. In the case of dry feedstock, either pre-processing needs to be followed to make the inoculum in the form of slurry or a mixed feedstock containing dry waste and wet waste is suggested for ease of bacteria to digest the dry feedstock.

The following tables provide data on the biogas yield and moisture content of optimal feedstocks

Calorific value of feedstocks

Feedstock Total solids (%) Volatile solids (%) Biogas yield (m3/ton)
Commercial wastes
Food waste 21.0 90.0 80-100
Vegetable market waste  20.0 90.0 110
Fish waste 23.0 63.0 55-60
Industrial wastes
Natural dung
Cow dung 18 75 55-60
Poultry droppings 42 78 70-75
Slaughterhouse waste
Slaughterhouse waste 20 98 160
Industrial agricultural waste
Press mud (Sugar Industry) 25 85 90-100
Whey (Dairy Waste) 6 85 88
Fruit juice concentrate 40 89 306
Spent grains (Brewery) 50 85 120
Distillery spent wash 600 T – feedstock 120
Livestock manure 21 85 44
Paper mill sludge No data No data No data
Agro waste
Sweet Sorghum 30 98 135
Wheat straw 91.7 85 103
Maize straw 64 98 135
Paddy straw 58 76 200

Table 2: Calorific value of feedstocks

Source: EAI analysis

Moisture content of different feedstocks

Feedstock Moisture content (%)
Commercial wastes
Food waste 70
Fruit and vegetable waste 80-89
Fish waste 75-77
Industrial wastes
Poultry litter 70
Press mud 71
Cow dung 80-90
Spent grain 80-85
Food and beverage industry waste Mostly in liquid form with some organic solids
Livestock dung 80-90
Paper mill primary sludge 65
Slaughterhouse waste No data
Dairy waste Sludge form
Distillery spent wash Sludge form
Agro wastes
Sweet Sorghum 76.2 g / 100 g ± 2.1
Paddy Straw Uncompressed rice straw – 15-18%
Maize Straw Twice that of the grain during harvesting
Wheat Straw 7.79%

Table 3: Moisture content of feedstocks

Source: EAI analysis

It could be observed that feedstock with higher moisture content and calorific value are potential feedstocks for anaerobic digestion. 


Thus, choice of feedstock for a bio-CNG plant primarily depends on its chemical composition, calorific content and moisture content.


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

Categories: Bio-CNG /renewable natural gas, Bio-CNG Feedstock, Optimal feedstock sources

Author - Narasimhan Santhanam

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

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