Introduction:
Landfills emit by-products like methane, dioxins and leachate (a toxic liquid that is formed when waste breaks down in the landfill and filters through waste), which, when left untreated, can leach into the soil, contaminating water sources, plants and even food.
Waste-to-Energy (WtE) technologies that process non-renewable waste can reduce environmental and health damages, all the while generating sustainable energy.
Types of waste:
At present, waste is classified as the one of the following:
- Municipal solid waste (MSW)
- Process waste
- Medical waste
- Agricultural Waste
The World Energy Council (2016) reports that according to the current rate of waste generation, global waste is estimated to reach 6 million tonnes/day by 2025.
So, increasing utility scale WtE plants using MSW or agricultural waste would be a constructive way to deal with waste, as evident from figure 1.
Processing methods:
WtE technology is an energy recovery process that converts chemicals from waste residues into practical forms of energy like electricity, heat or steam. As of now, thermal conversion techniques lead the market among WtE technologies.
MSW has really low calorific value and directly incinerating it will not generate adequate thermal energy. So, pre-treating MSW into refuse derived fuel (RDF) is more effective.
There are various methods to process waste and are classified as follows in figure 2:
Incineration is considered when the calorific value of the input feed is at least 7MJ/Kg. When there are no complex collection techniques or when the water content is higher, biochemical methods need to be taken into account. Conversion of thermal energy from incineration can be used to drive a steam turbine for electricity, but only with 15-27% efficiency. Gasification produces syngas that is easily combustible in a gas turbine/engine to generate electricity, which is about 30% efficient.
Anaerobic Digestion (AD) uses more agricultural waste and synthesizes waste that has higher water content through a series of microbial processes to generate biogas. Heat generation can be increased by 90% when syngas/methane is combusted in a cement kiln. Combined heat and power (CHP) WtE plants can have an efficiency of 40% if utilized suitably. Countries that have cold weather always have a high demand for heat and so this can be partially supplied through combined heat and power district heating.
Current utility scale plants around the world:
Figure 3 below indicates that combustion/incineration is still the major type of technology used, while gasification and AD are still in the minority.
Below are some of the utility scale plants that are coming up or are already online globally:
China: China has incineration plants based on circulating fluidized bed (CFB). They have around 28 operational CFB plants of which the recent build was in 2012 handling more than 800 tonnes/day. A new plant is underway to be built in Shenzhen that can handle around 5000 metric tonnes/day, while claiming to become the largest facility for WtE in the world.
Abu Dhabi and Sharjah: An 8.2 billion USD plant was commissioned in 2012 to be built in Abu Dhabi. As there are already good incineration technologies at hand, Abu Dhabi and Sharjah have taken a new direction to treat waste through a combination of gasification and pyrolysis.
Europe: There are several operational RDF plants in Italy, Denmark and France. An incinerator was built in 2013 at Naples, Italy that can manage 650,000 tonnes/year. Sweden and Denmark on account of having colder weather, have a number of CHP WtE plants like Aros, Vartan, Herning etc. generating more than 100 kWe of energy. Germany and Sweden, who are the forerunners of WtE, are also known to import deficit waste from neighboring countries!
Turkey: A micro-scale KI energy plant in Turkey treats biomass, generating around 300 KWth and 50 KWe with fixed bed gasification using pyrolysis.
UK: A gasification plant Energos (part of ENER-G) in Manchester, UK provides an economic substitute to the large combustion technique of WtE. This facility can treat MSW, industrial and commercial waste with a handling capacity of up to 78,000 tonnes/year.
USA: Novo Energy is a WtE small scale utility plant that runs versions in four states and uses a combustion technology, processing up to 66,000 tonnes/year. A mobile gasification system based in Massachusetts from IST energy converts around 200 lb of dry waste per hour.
Japan: Japan has the most modern types of thermal treatment plants that processes around 39 million tonnes/year.
Canada: The oldest plants use incineration technology and they were improved to use plasma gasification plants from Plasma Energy Group and Nevitus Plasma Inc. Recently built facilities like Nexterra Systems Corp. and Enerkem have used gasification conversion.
Australia: A plasma gasification plant by Phoenix Energy Australia Pty Ltd. is in the initial stages of commissioning in Kwinana, Australia.
India: Out of the 14 commissioned plants, only 4 plants (Jindal Ecopoils Management Company PVT ltd, Organic Waste Recycling Systems Pvt, Rochem and Shalivahana (MSW) Green Energy Ltd) are in operation in different states (which uses RDF or dry AD technology). However, dry AD technology seems to be more efficient and 4 more have been commissioned recently using dry AD.
Upcoming technologies:
There a few upcoming new WtE technologies like Hydrothermal Carbonisation (HTC) that fast-tracks the slow process of geothermal conversion of wet waste with an acid catalyst at high pressure and heat to simulate the production of ‘hydro-char’ that has properties similar to fossil fuels. The main advantages of this to AD is the lower processing time and similar operating conditions needed to generate the same amount of energy.
Dendro Liquid Energy (DLE) is a nearly ‘zero-waste’ WtE innovation from Germany. It is said to be four times more efficient than AD and costs less.
Conclusion:
Europe is said to be the largest market for WtE technologies (47.6%), while Japan dominates 60% of Asia-Pacific WtE market for incineration. However, China has been growing their capacity since 2011. It is said that biological WtE will grow at an average rate of 9.7% as it becomes more commercially feasible. Despite all the advancements, WtE technologies have a long way to go in terms of being on par with conventional energy sources.
