The realization that the “waste to energy” was more than a slogan came to me when, during a half year residence in Vienna, I visited the Spittelau incinerator. This plant is one of the oldest in Europe, set up in 1971, and processes around 250,000 Tonnes of Vienna’s household waste every year. Around 50 per cent of the annual energy production comes from the incineration of biogenic or renewable resources. It has a pivotal contribution to Vienna’s waste management system, producing 60 GWh of electricity and 500 GWh of heating and warm water for 60,000 households.
The Municipal incinerator plants rarely become architectural icons. Friedensreich Hundertwasser rebuilt it to his unique design in the 80s after a fire destroyed the original structure built in 1960. The facility is one of Vienna’s signature landmarks, with the huge golden globe on the chimney providing a unique cityscape.
Increased consumption of natural resources comes from increasing urbanization, affluence, and industrialization, resulting in the concurrent rise in waste generation. Municipal Solid Waste (MSW) includes household garbage and rubbish, street sweeping, construction, and demolition debris (1). India generated 53 million Tonnes of waste in 2019. By the end of this year, the quantity will double. By the mid-century, it will be a gigantic 500 MT. Waste generation grows faster than the population. The massive challenge of growing quantities and changing composition patterns exists. Most of the waste in Indian is biodegradable (40–70 % of the total). Non-biodegradable fraction, like construction waste and yard trimmings, is 20–40 % in 2018. Ghazipur in Delhi, Deonar in Mumbai, Pirana in Ahmedabad, Dhapa in Kolkata are legacy waste dumps that have acquired global notoriety.
What is the nature of this waste? From the waste to energy context, the Carbon content is essential. A Tonne of Municipal Waste has about 1/3 Calorific value of coal: in the range of 1400–2200 Kcal/Kg for Indian Towns and Cities. It can produce 600 kWh of electricity. The garbage from European countries and the US has a calorific value ranging between 1,900–3800 kcal/kg. A matter of concern in the WtoE context is the water content, which is close to 50%, which is terrible for energy extraction.
Pumping excess air in incineration can improve energy extraction. The waste is the fuel. Excess means above the stoichiometric requirement. Burning takes place in a furnace grate with the required quantity of air. Air flow from the bottom makes it a fluidized bed furnace. The heat produces steam, which drives a turbine to generate electricity. Unfortunately, the bottom ash is highly contaminated. The flue gas contains fly ash, steam, CO2, and NOx, which needs to be cleaned up for release into the atmosphere.
Using the waste as fuel as well as feedstock can improve the conversion efficiency. In this process called gasification, air quantity is below stoichiometric levels. Just enough to keep the fire glowing. We do this with coconut shell to carbonize it by covering it with sackcloth while burning. In the gasification process, the MSW is a feedstock for a high-temperature chemical conversion process. In the gasifier, the MSW as feedstock breaks down into simple molecules. The gaseous products formed during the Pyrolysis of organic waste are H, CO, CO2, CH4, higher hydrocarbons, soot, and some oily products. The gas mixture of H2 and CO is Syngas, which has a heating value of one-quarter to one-third of natural gas. Syngas can be combusted to produce steam to run turbines. The flue gas is cleansed of NOx and released into the air. The remnant in the furnace is slag, which is a building material.
An even more efficient process called Pyrolysis uses the fuel purely as feedstock. No air enters the process. So, the heating must be external. Here also, the chemical energy in the form of energy gases is released.
These processes have different operating temperatures. For example, combustion is done at reasonably low temperatures of 800 deg, whereas Pyrolysis happens at very high temperatures of about 2000 deg.
The combustion process produces flue gas which contains gaseous and particulate contaminants and is hence highly polluting. Electrostatic precipitators and cyclones can clean the particulates. In addition, selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) help clean NOx. Through the injection of ammonia into the flue gas at a temperature around 850–950°C. A good plant keeps all pollutants within limits.
Advanced Gasification systems use a Plasma Torch to heat the waste. Electrical Arcs convert electricity into a hot gas. Plasma Torches can produce plasma plumes at ~ 10,000 Degree K. The vital quality of this flame is that it can be chemically inert, non-oxidizing.
Prevalent waste processing options globally show that Japan has more than 1000 plants, followed by China with close to 300. Scandinavian countries, Europe and the US also have such plants.
What is the Indian situation? A Task Force on “Waste to Energy” has submitted its report in 2014, highlighting the need for an integrated approach towards MSW management. Reduction and segregation of waste at source and efficient utilization of various recoverable from the trash recommended. The report recommends building waste processing facilities considering the quantity and quality of waste. The financial viability of the processing technology also needs consideration. Government has big plans to set up WtE plants across the country (2). For instance, Niti Aayog, the planning organization, proposes generating 511 MW from WTE plants by 2018–19 under the Swachh Bharat Mission (SBM). A Waste to Energy Corporation to execute these plans under the PPP model is also under consideration. NTPC, the biggest public sector power generation company invited plans to set up 100 WTE plants in the country.
97 WTE plants have been built in the country Since 1987 of which 7 shut down. Apart from. The plants’ inability to handle mixed solid waste and the high cost of electricity generation seem to the critical reasons for closure.
The major affordability factor is the Capital cost. MNRE offers financial incentives by way of interest subsidy to reduce the interest rate to 7.5 per cent (2). Tipping fee to the Concessionaire. Land at a nominal fee and long-term lease. Concessional custom duty on imported parts helps. These subsidies/incentives take care of about 35 per cent of the project cost.
Yet, electricity produced from these plants is quite expensive. Compared to Rs 3–4 per kWh from coal and solar plants, WTE plants charge about Rs 7/kWh.
The sustainability of Waste to Energy processes will depend on how it becomes part of a circular economy concept, where recycling is a significant component. Advanced techniques like gasification should have priority over burning, which should be the last option.
References
1. (Srinivasarao Meka, International Journal of Civil Engineering and Technology 5(2):1–8, 2014) 2. https://cdn.cseindia.org/docs/aad2019/swati-AAD.pdf