Co-processing of Poultry Biowaste in MSW-to-Energy Kilns: Technical and Financial Feasibility of Wet Flue-Gas Desulfurization Sorbent Isolation in Varanasi, India

Power plants and waste-to-energy facilities face a constant, two-pronged challenge: they must burn fuel to generate electricity while simultaneously scrubbing the toxic sulfur dioxide gas from their exhaust before it escapes into the atmosphere. To clean this gas, these facilities traditionally rely on wet scrubbers, massive towers that spray a slurry of water and crushed limestone to absorb the acid. However, the limestone itself is a problem. It must be dug up from the earth in destructive open-pit mines, a process that scars the landscape, destroys habitats, and releases significant carbon emissions during transport. At the same time, rapidly growing cities in India generate mountains of organic waste, including tons of chicken eggshells that are usually dumped in landfills where they rot, smell, and pollute the soil. The question researchers asked was whether these two separate problems could be solved by one simple solution: could the waste eggshells replace the mined limestone?

A researcher led by V S Narasimha Sai Sangapu of NTPC Limited investigated this possibility specifically for the Varanasi district in India. They focused on the NTPC Waste-to-Energy facility and the nearby Vindhyachal desulfurization plant. Their study confirms that eggshells are chemically almost identical to high-grade limestone, composed of roughly ninety-five percent calcium carbonate. Yet, using them directly is impossible because raw eggshells are coated with a tough, water-repelling membrane. If thrown into the water-based scrubber system, this membrane prevents the shells from mixing properly, creating a thick, sticky sludge that clogs pipes and destroys the pumps. The researcher found that a simple thermal treatment solves this. By heating the eggshells to five hundred degrees Celsius, the organic membrane burns away completely, leaving behind a brittle, water-loving powder that behaves exactly like standard limestone.

The study maps out a complete, closed-loop system to make this work in a real-world setting. First, the researcher estimated that the Varanasi district, fueled by its large local population and millions of annual tourists, generates between three point three and five point one metric tons of raw eggshell waste every single day. This waste is collected from commercial kitchens and street vendors. Instead of sending it to a landfill, the shells are co-roasted alongside municipal solid waste in the plant's reactors. This heating process not only removes the problematic membrane but also carbonizes the organic matter, turning it into a fuel source known as green charcoal.

The researcher then designed a mechanical system to separate the two resulting materials. As the hot mixture exits the reactor, it passes through a series of rollers and vibrating screens. Because the roasted eggshells have become brittle and the charcoal remains dense and tough, the machinery can easily crush the shells into a fine dust while leaving the charcoal chunks intact. A final air-current system blows the light eggshell dust into a collection chamber for the scrubbers, while the heavier charcoal is routed to the plant's fuel silos. This separation is crucial; it ensures that the fuel used to power the plant is clean, while the dust is pure enough to be used as a chemical scrubbing agent.

When this roasted eggshell dust is introduced into the wet scrubber, it reacts with the sulfur dioxide in the exhaust gas. The chemical reaction neutralizes the acid and produces a solid byproduct called gypsum, which is a high-purity material that can be sold for use in construction and agriculture. The study calculates that this system could capture between one point eight six and two point eight eight metric tons of sulfur dioxide emissions daily, producing over six metric tons of marketable gypsum.

The financial analysis suggests this is not just an environmental idea but a profitable business model. The researcher calculated that using processed eggshells costs significantly less than buying and transporting mined limestone, especially when factoring in subsidies for diverting waste from landfills and the revenue from selling the gypsum. Over a fifteen-year period, the project is projected to pay for its initial equipment costs in just over three years. The study indicates a strong return on investment, with the project remaining profitable even if the collection of eggshells is less efficient than expected. The model shows that the system can withstand fluctuations in fuel prices and supply issues, provided that at least half of the available eggshell waste is collected.

Beyond the numbers, the study highlights the broader environmental impact. By using eggshells, the plant avoids the need to quarry up to one thousand three hundred and fifty tons of natural limestone every year, preserving local ecosystems and reducing the carbon footprint associated with mining and transport. It also eliminates the environmental hazards of rotting eggshells in landfills, such as the release of foul-smelling gases and the contamination of soil. The research concludes that this approach offers a viable, financially sound path for turning a common waste product into a critical industrial resource, creating a circular economy where waste from the city helps power and clean the industry.