This post is a part of BioBiz’s Bio-CNG Perspectives.
BioBiz, a 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
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.
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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:
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.
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.
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