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

Anaerobic digestion technology has been in existence for a long time. However, the technology has predominantly been applicable for small-medium scale plants – family type biogas plants being implemented by residential sectors and collocated plants being implemented by industrial sectors. The biogas produced from these plants have been utilized for cooking, heating and electricity generation applications respectively. In addition, most of these plants relied on only one feedstock (e.g. poultry litter) or sometimes a combination of two feedstocks – food waste and cow dung.

Only in the last few years has the technology seen many innovations across the value chain, including development of a new product with higher market value called bio-CNG. Unlike biogas, bio-CNG can be used as a fuel in various applications as a potential replacement for natural gas and other conventional fossil fuels.

This blog post provides details on various innovations happening in the RNG value chain on key parameters. An understanding of these latest technology trends would assist an entrepreneur in setting up a viable, cost-effective bio-CNG plant.

Technology trends by parameters

The following are some of the latest trends in the technology across the value chain: 

1. Efficiency

Efficiency of the process is a key parameter when considering waste to energy projects. In the case of RNG production, the following are some of the emerging trends in the technology to enhance the efficiency of the process. 

• Multi-feedstock

Dependence of a large scale bio-CNG plant on a single feedstock has its own constraints – waste from commercial establishments have challenges with quality and accessibility, industrial wastes have challenges with uncertainty in long term supplies and agro wastes have constraints with collection of feedstock. Hence, co-digestion plants using multiple feedstocks are suggested which would enhance the process efficiency – both in terms of viability of the project as well as product yield. Optimal carbon:nitrogen (C:N) ratio should be maintained to avoid challenges with higher nitrogen content.

• Feedstock pre-treatment

Feedstock pre-treatment is a critical step in improving process efficiency in terms of yield and economics. While commercial and industrial wastes have minimal pre-treatment processes – shredding and addition of water, agro waste requires an additional pre-treatment step such as enzymatic or any other process to remove the lignin content. Some of the pre-treatment processes such as enzymatic are expensive which results in an increase in economics. However, latest technology trends such as novel microbes and hydrolysis processes are cost effective as well as result in an increased biogas yield which significantly enhance the efficiency of the process.

• Digester technology

Anaerobic digester technology is primarily classified into fixed and floating type digesters. Fixed type digesters with continuous stirring mechanism ensure higher efficiency compared to floating type due to the absence of agitator and hence formation of scum in the latter. Stirring mechanism ensures higher efficiency of the process.

• Technology innovations

While several conventional anaerobic digester technologies exist – Continuous stirred tank reactor (CSTR), KVIC models and more, recent innovations in technologies such as plug and play digester models ensure improved efficiency and automation of the process compared to conventional technologies

• Choice of upgradation system

Upgradation system is used to concentrate methane in a biogas to natural gas standards. The systems are used to remove hydrogen sulphide, carbon dioxide, water and other contaminants. While several conventional upgradation systems exist – water scrubbing, PSA, etc, membrane type upgradation systems have recently entered the market which, although expensive, enhance the process efficiency and yield.

2. Modularity

Modularity of a plant is essential to ease the implementation of the plant, scale up and maintenance of key components. Several containerized models, plug and play digester models, fabric based and fibre-reinforced plastic based digesters have entered the market that have improved the modularity of the structure significantly.

3. Scale

In India, small-scale biogas digesters are commonly seen both in distributed and captive models. While, this may suit for biogas alone, for RNG, a minimum scale of 20 TPD is required as scales lower than this have challenges with installing advanced purification techniques, owing to increased economics at lower scales. In addition, the economics of the project is also poor with the return on investments being not so attractive for a project developer.

4. Mixed feedstock

Co-digestion leading to increased efficiency: Co-digestion mechanisms are being extensively practiced in most industries in order to increase the efficiency, as dependence on a single feedstock will cause technology issues in the case of agro-waste and accessibility issues in the case of food waste. Similarly, when using feedstock like poultry litter, an increased quantity of the feedstock leads to increased generation of ammonia content in the gas which requires expensive cleaning processes. Ammonia scrubbing technology costs around Rs. 4-5 crores. Use of feedstock like poultry litter, with other carbon rich feedstock reduces the challenges involved and increases process efficiency and yield.

With technology trends, it is possible to co-digest agro residues like paddy straw along with press mud, food waste, cow dung, poultry litter, industrial wastes such as dairy, food and beverage and more. Optimal required ratio should be calculated depending on the biogas yield.  

5. Gas purification units

Novel scrubbing techniques: While PSA seems to be an ideal technology for scrubbing and purification, advanced purification techniques such as fibre membrane technology have been developed to enhance the methane content to 97%. Not only is the yield increased, the cost involved in maintenance of PSA gets reduced. In the case of PSA, continuous operation causes a reduction in the yield of biogas.

The fibre membranes for biogas upgrading are made of materials that are permeable to carbon dioxide, water and ammonia.  Hydrogen sulphide and oxygen permeate through the membrane to some extent while nitrogen and methane only pass to a very low extent. Usually membranes are in the form of hollow fibers bundled together. The technology is a recent innovation promising higher efficiencies at lower costs.  

6. Economics

• Feedstock pre-treatment: Agro residue requires extensive pre-treatment before digestion owing to the presence of lignin content. Some of the traditional pre-treatment techniques such as enzymatic are very expensive and requires maintenance of the operation. However, latest technology options such as novel bacteria, hydrolysis techniques, and co-digestion methods have resulted in a reduction in the cost involved for pre-treatment step.
• Digester technology: Conventional integrated digester models made of concrete and steel have challenges in maintenance and relocation of the plant. However, modern digesters such as fabric based digesters, containerized, modular and plug-and-play models help in improving the efficiency of overall operations and maintenance including economics.
• Novel upgradation techniques: Innovations in the upgradation techniques for RNG production such as membrane technology improve the economics by reducing the need for frequent maintenance as well as enhancing product yield
• Diverse end products and markets: While RNG is a well known fuel in the automotive sector, other end use markets exist for RNG such as industrial and commercial segments. These segments improve the economics of the project owing to higher price of end product compared to that supplied for the transport sector. In addition to RNG, by-products CO2 and digestate are also being focused on by the industry stakeholders to enhance its quality and make it a saleable product, thus improving the economics

In addition to the trends in the core technology, efficiency across the value chain including plant location, feedstock supply chain and logistics help in improving the economics significantly.  

7. Low maintenance

Conventional digester technologies require manpower attention for overall process maintenance. Further, the integrated nature of the digesters and the materials involved such as steel or concrete increase the maintenance process owing to leakage or corrosion, thereby increasing the cost. However, with technology advancements, the entire process has been automated and modular designs have entered the markets which ease the process of maintenance. Further, innovations in the type of materials used for digesters such as fabric based biogas plants reduce the need for process and equipment maintenance significantly.  

8. Odour management

Odour management is critical as it determines successful operation of biogas plants without any political issues. Several technologies are available in the market to effectively manage the odour emanating during the operations. Some of the trends include maintaining negative air for the feedstock storage area and other proper ventilation mechanisms. It is to be noted that in the case of appropriate operation, a biogas plant does not release any bad odour into the environment. Hydrogen sulphide produced in the course of decomposition is converted to odourless elementary sulphur biologically or chemically in a closed space.  

9. Gas storage

Gas storage systems are used to compensate imbalances between gas production and gas utilization. Gas storage systems for biogas applications are limited to storing the gas produced over several hours, because the storage of gas in larger quantities is too costly. On‐site gas can be stored by means of a flexible rubber dome attached to the digesters, by using external gas bag storage, or in pressurized tanks. The capacity of the gas storage has so far been designed for short‐term flexibility making allowances for fluctuations in gas production or gas utilization. The gas is primarily stored in internal gas storages of the biogas plants.

Recent technology trends have resulted in the development of double membrane biogas storage systems to enhance the efficiency of the storage process. It’s mounted tightly on the roof of the digester and separated sphere on surface of land. The biogas holder system has a double membrane structure. The external cover dome is made of PVC and produced with special additives that are ultraviolet and precipitation resistant. Biogas double membranes are designed and cut using machines with numerical program control. Welding is performed using high-frequency current welding. This gives considerable advantages in quality if to be compared with membranes that are handmade and glued or welded by heating elements.

Anaerobic digester technology has thus seen several innovations in the last few years which assists project developers to implement a scalable, viable bio-CNG plant with improved process efficiency at a lesser cost.

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