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Anaerobic digestion technology has been in existence for long; however, several challenges exist in the technology, from scale operations to upgradation and waste water treatment. The following section highlights some of the key technical challenges observed in the industry.
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
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.
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.
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
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.
Managing pollution related challenges from biogas plant operations
Biogas plant operations does not significantly contribute to pollution 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 pollution problems. However, location of the plant could determine political related challenges. If the plant is close to the residential areas, the odour arising from the feedstock supply chain and storage could cause significant challenges and public outcry, thus leading to difficulties in operations of the plant. Hence, land location is critical before setting up the plant and appropriate measures should be taken for odour management. The required safety and PCB clearances should be obtained from the concerned organizations beforehand.
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Categories: Bio-CNG production process, Technology Challenges
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