Currently, the majority of biodegradable waste (green waste, food waste, paper/card waste) we generate is sent to landfill for final disposal, which in turn, is having a negative effect on our environment. This is because when sent to landfill, biodegradable waste is broken down by bacteria and releases methane into the atmosphere, a harmful greenhouse gas that contributes to climate change.

Demanding targets have been set by the EU Landfill Directive (1999/31/EC) in order to reduce the amount of biodegradable waste being sent to landfill. These targets are: by 2010, to reduce the amount of landfilled biodegradable municipal waste to 75% of that arising in 1995; by 2013, to reduce the amount to 50% of that arising in 1995; and by 2020, to reduce the amount to 35% of that arising in 1995 (DEFRA).

Gasification and Pyrolysis are alternative waste treatment technologies that use high temperatures to treat the biodegradable fraction of municipal solid waste (BFMSW), as well as plastics. Commonly referred to as Advanced Thermal Treatment (ATT), these technologies are designed to recover energy (in the form of heat, electricity or fuel) and can contribute to the diversion of biodegradable municipal waste (BMW) from landfill (DEFRA). Ultimately, the aim of gasification and pyrolysis as alternative waste treatment technologies is to reduce the impacts residual waste has on the environment when sent to landfill. This article will take a closer look at both processes, as well as taking into consideration their potential advantages and disadvantages.

Pyrolysis and Gasification Processes

Pyrolysis is the thermal degradation of a substance in the absence of oxygen (DEFRA). This process uses temperatures between 300°C and 850°C in order to break down waste materials. Gasification is a partial oxidation process whereby a carbon source is broken down into syngas. This means that oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur (DEFRA). This process uses temperatures between 750°C and 1200°C in order to break down waste materials.

The first stage of the two processes (gasification and pyrolysis) involves the preparation/pre-treatment of waste. This stage is important as gasification and pyrolysis both focus on treating the biodegradable fraction of municipal waste (green waste, food waste, paper/card waste) as well as plastics. Therefore, it is important to remove non combustible materials and recyclables, (typically metals and glass) prior to the primary treatment stages of the two processes. Shredding waste materials into smaller particle sizes may also occur in order to facilitate pyrolysis and gasification.

Following this initial preparation stage of the two processes, waste materials are then placed into reactors where the primary treatment stage takes place. Such reactors used include: rotating kilns, heated tubes (gasification), fluidised beds, and fixed beds (pyrolysis). This process involves waste being heated to a high temperature in a low oxygen atmosphere. This results in the production of gas, oils, and char (ash).

The next stage of the two processes is called the scrubbing stage. This involves the cleaning, or ‘scrubbing’ of the gas produced in the primary treatment stage in order to remove particulates and hydrocarbons. This process produces clean gas that can be utilised to generate electricity and heat in combined heat and power (CHP) plants.

Potential Advantages

• Advanced Thermal Treatment (ATT) technologies (Gasification, Pyrolysis) are able to recover much more value from waste compared to mass burn incineration.
• Advanced Thermal Treatment (ATT) technologies are much more flexible than mass burn incinerators. This is because ATT plants are modular and are made up of a number of small units that can be added to, or taken away at any time. This flexibility allows ATT plants to either increase or reduce their size depending on the amounts of waste they receive.
• Syngas, a by product of ATT (Gasification, Pyrolysis) can be used to generate energy much more efficiently than mass burn incineration. Syngas can be converted into energy through use of a gas engine, whereas incineration can only generate energy through use of steam turbines which are less efficient.
• Advanced Thermal Treatment (ATT) technologies comply with the EU Waste Incineration Directive (2000/76/EC) as they achieve low emission levels.
• Advanced Thermal Treatment (ATT) technologies are easy to build.
• Advanced Thermal Treatment (ATT) technologies reduce the biodegradable content of residual waste reducing the production of landfill gases when sent to landfill.

Potential Disadvantages

• Unless they only deal with truly residual waste (what is left once maximum recycling and composting has happened), Advanced Thermal Treatment (ATT) technologies will undermine recycling and composting (FOE).
• Advanced Thermal Treatment (ATT) technologies need to use pre-sorted or processed waste as feedstock.
• Advanced Thermal Treatment (ATT) technologies have much higher capital and operational costs compared to mass burn incineration.
• Increased technical experience is required to operate Advanced Thermal Treatment (ATT) technologies compared to mass burn incineration.

If you would like to reduce the amount of waste you are sending to landfill then why not try a home composting system for your food and organic waste? Or make sure what rubbish you do have is in recycled bin bags!