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 is a prominent technology and has been in existence for a long time. The technology has been implemented at small-medium scales with select feedstocks such as food waste, cow dung, sewage, select industrial wastes, etc. The biogas produced was used as a cooking fuel, heating and electricity generation. However, with technology advancements and the introduction of upgradation techniques, it is possible to implement plants at varying scales and also upgrade biogas to a higher quality product called bio-CNG. Yet, challenges exist in the technology which affect the efficiency and operation of a RNG plant.
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For an entrepreneur to set up large scale plants, it is critical to identify the key challenges in the anaerobic digestion technology and come up with suitable solutions to operate a viable plant. This blog post highlights the key challenges in the RNG technology.
Major challenges in the RNG technology
1. Challenges for scale operations & maintenance of digesters
The ideal scale for a renewable natural gas production plant is 100 TPD. Beyond this, challenges exist in the operations across the value chain as well as maintenance of digesters. The following are the key challenges during scale operations and maintenance.
- Feedstock – Considering the plant size and quantum of wastes generated by different feedstock sources, it becomes difficult in sourcing a huge quantum of feedstock for plant size higher than 100 TPD. Even when we consider the press mud (150 TPD available) generated from the sugar mills, large storage area is required to store the press mud for a period of 6 months, which adds to the cost
- Logistics – Optimal handling and monitoring of logistics operations including feedstock accessibility becomes a challenge at higher scales owing to increased cost and processes involved
- Technology operations – At higher scales, while automation of the process could ease the monitoring, some of the processes like pre-processing and/or segregation (if necessary) could be a challenge. Further, while most of the equipment is modular in nature, digester is single equipment and at higher scales, in case of any emergency, the entire feedstock needs to be drained before repairing and then re-fed. This becomes a challenge in terms of waste disposal, additional feedstock requirements and manpower involved. Choice of technology also determines the problem of scum formation during operations, leading to frequent failure of digester and high opex. Clogging of pipelines could pose a challenge in maintenance.
- Wastewater treatment – The digestate generated from the biogas plant comprises both solid and liquid fraction. While the solid fraction can be separated, dried and sold as compost, liquid digestate has a poor market acceptance. Even if small quantities can be enriched and sold to farmers, the remaining water needs to be treated aerobically and recycled. Increase in plant size leads to generation of large quantity of water generated, thus making the maintenance operations challenging
2. Challenges in pre-treatment of raw biogas (for instance removal of impurities)
Raw biogas contains methane, carbon-di-oxide, nitrogen and hydrogen sulphide. The composition of biogas depends on the feedstock used. In the pre-treatment stage, H2S and N2 can be removed using biological and chemical scrubbing techniques. Removal of H2S is essential as its presence is corrosive to steel in the reactors and gas engines.
While challenges in the pre-treatment of raw biogas are low compared to other processes in the value chain, choice of technology determines the process efficiency. For instance, in the case of chemical scrubbers for H2S removal, the zeolite being used needs replacement every 3-4 years.
3. Challenges in CO2 separation
CO2 present in the biogas can be removed by various upgradation techniques such as PSA, membrane and water scrubbing. As in the case of biogas cleaning, choice of technology determines the process efficiency. For instance, membrane technology results in improved methane yield compared to PSA, while the latter technology causes reduction in the yield after a few years of its application. CO2 removal is also a costly process. As CO2 from biogas is not being commercialized, application of a costly process becomes a challenge.
4. Challenges in maintaining efficiency
Maintaining efficiency of the process across the value chain is essential for a RNG project. Key challenges in maintaining efficiency of the process include
- Choice of feedstock: Feedstock to be used can vary significantly both in physical and chemical compositions on a daily basis. For instance, if the feedstock is food waste, its pH and other chemical parameters could vary on an everyday basis depending on the types of ingredients added to it. In such cases, detailed analysis of the feedstock composition and their impact on the efficiency and yield of the process is critical. However, such challenge may not exist with industry or agro waste as the manufacturing process in these cases do not differ significantly on a daily basis
- Segregation and pre-processing: Proper segregation of feedstock being used, especially food waste is essential to maintain the efficiency of the plant. For large scale RNG plants, unless 100% source segregation is ensured, efficiency cannot be maintained. Similar to segregation, pre-processing of wastes, especially agro waste is essential to ensure higher yield.
- Continuous operation and maintenance: Biogas plants should be properly maintained throughout the year with careful monitoring of the operations and ensuring proper feeding on a daily basis. Lack of feedstock and halt of the digester operations frequently could affect the efficiency of the plant significantly.
- Digester technology: Choice of technology plays a major role in determining the efficiency of the plant. Unless, detailed evaluation of the various technologies is not carried out, the efficiency of the process can significantly get affected. For instance, agitation is a must for biogas plants to ensure proper conversion of waste and improve the yield. Stirrer mechanism is possible only with fixed type digesters and not floating digesters. If this provision is not available, it can lead to scum formation and increase the need for frequent cleaning and maintenance, while also decreasing the efficiency
- Upgradation system: Similar to digester technology, choice of upgradation system, PSA, membrane technologies determine the extent of challenges involved in maintaining the efficiency of the process
5. Challenges in wastewater treatment
Anaerobic digestion process utilizes large quantities of water, a major portion of which needs to be properly treated and recycled. Key challenges for setting up a proper wastewater treatment include higher economics for infrastructure and high power consumption. The wastewater treatment system should have an aerobic plant to reduce COD and BOD, a UV plant (which costs around Rs. 3 crores) and an anaerobic filter to remove excess gas in the liquid. During the process, some water remains unrecoverable. For example, if suppose a 10 TPD plant requires 25,000 litres of water, after wastewater treatment, about 5000 litres of water remains unrecoverable.
6. Managing pollution related challenges from biogas plant operations
Major pollution which could be created from the biogas plant operations is water or land pollution owing to the discharge of waste water. While the wastewater generated can be utilized in two ways – small quantity as digestate and rest of the water recycled, the quantum of wastewater generated will be huge, hence some quantity may need to be discharged. While discharging this wastewater, unless adequate effluent treatment methods are not carried out, the water released might cause land or water pollution (depend on the plant location) owing to the presence of microbes, toxic compounds, nitrogen and more.
When we consider air pollution, biogas operations do not significantly contribute to this as during the operations, only CO2 is released which is considered carbon neutral owing to its source being organic matter. The other impurities such as H2S are also reduced to their elemental forms, thus reducing related pollution challenges.
From the above challenges, it could be observed that
- Plant scale plays a major role in successfully implementing an RNG project
- Choice of technology from feedstock pre-processing to biogas upgradation and wastewater treatment is critical in determining the operational efficiency and yield of the product
- Proper operations and maintenance (O&M) is necessary for continuous and efficient functioning of a RNG plant.
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