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Energy Recovery: Smart Management of Municipal Waste

06/23/2017

By Gonzalo González Escobar, Proposal Manager of Waste to Energy Plants at ACCIONA Industrial.

Managing the thousands of tonnes of waste that towns and cities produce every day is one of the most important challenges facing the environment.

In the case of solid waste, this is a triple challenge – firstly to reduce the quantity of this waste; secondly, to reuse the highest possible percentage of the waste produced; and lastly to recycle any waste that cannot be reused. It is known as the “3R” rule, an initiative started in Japan in 2002 and advocated by the G8 Summit in June 2004, to globally promote a society that recycles and uses resources and materials efficiently.

Landfill has been the traditional method for treating waste that was technically impossible to recycle, or for which the cost of recycling outweighed the benefits. According to Eurostat, landfill sites in Europe emit the equivalent of nearly 11 million tons of CO2 every year; they occupy large tracts of land with a significant impact on the environment and the landscape; while the biogas and the large amount of leachates they produce (liquids that filter into the subsoil and can enter surface or groundwater) pose an obvious risk of pollution. Consequently, new technologies have been developed to prevent these negative impacts, which focus on “energy recovery” plants, also known as “Waste to Energy”.

These plants thermally treat the waste, through complete oxidation, in order to release the energy contained in it, to produce electricity, heat or a combination of the two. They also allow commercially valuable chemicals to be obtained.

Complex but robust and safe plants

These types of plants comprise a series of facilities with specific functions in order to convert waste into energy via a multi-stage process:

  1. The waste arrives at the reception and storage zone where it is weighed, unloaded and stored. Since the plants operate 24 hours a day throughout the year, sufficient waste must be stored to ensure uninterrupted operations at all times.
  2. Another typical -although not always necessary- facility at an energy recovery plant is the mechanical waste classification zone, where waste can be classified by size, density or type of material.

Plants that receive a significant amount of organic waste also have a biological treatment zone. Two types of processes are generally used – composting to produce compost (organic fertiliser), and anaerobic digestion, which produces a biogas that can be used for electricity or can be washed to produce natural gas.

  1. The waste is burnt in combustion furnaces at temperatures in excess of 850º C in order to prevent dioxins forming. The by-product of this combustion is slag, which can be used as a construction material.

The heat produced by the furnaces is converted into steam in the boilers, which are especially designed to collect the ash generated by the combustion.

  1. The steam drives the turbines to produce electricity or heat.

The emissions permitted for these types of plants are the strictest in the industrial sector and they all have facilities to treat the combustion gases. Given that the trend is towards ever-stricter emissions levels, modern facilities are designed with contingent space to add further systems if future regulations require them, such as catalytic systems to reduce nitrogen oxide emissions (NOx). All of the processes at energy recovery plants are automated, especially those that control emissions.

The benefits of energy recovery

Producing energy from urban waste is not an alternative to recycling, but is a complementary process in the search for increasingly sustainable environments.

In short, it is a way of preventing landfill. Moreover, these new facilities offer major advantages compared to traditional methods of treating waste. Firstly, they use proven and wholly reliable processes. Furthermore, the waste that they process requires very little prior treatment since the systems are extremely robust and flexible. This aspect is extremely useful considering that the types of waste generated by a population can change significantly over the life of these plants (around 30 years).

Compared to landfills, which generate very little usable material, the waste to energy process recovers electricity and heat – and it does this with extremely strict emissions controls. “Waste to energy” plants also offer other environmental advantages compared to landfills. Recovering energy from waste saves on fossil fuels such as coal, gas and oil. Furthermore, in general they do not produce liquids that could filter into the subsoil, and they produce by-products such as metals and slags that can be recovered and recycled.

The unquestionable benefits of energy recovery processes based on urban waste have led to the global expansion of this approach, especially in Europe. European countries such as Denmark and Sweden have energy recovery rates above 50%; rates that are compatible with high levels of recycling. By contrast, landfill in these northern European countries does not exceed 26%.

Energy recovery in Spain.  ACCIONA’s commitment

Spain has ten energy recovery plants, which produce enough energy to supply 450,000 households. Several of these facilities were constructed by ACCIONA, such as the Mataró plant or the SOGAMA plant in the town of Cerceda, La Coruña.

The R&D+i team at ACCIONA Industrial, together with the ACCIONA Infrastructure Technology Centre and the corporate innovation team, is working on new processes and technologies that offer superior incineration performance and that are even more environmentally-friendly, such as fluidised bed gasification and energy produced from syngas (synthesis gas).

There is a clear path ahead and the possibilities for energy recovery are enormous. We still have to overcome a good number of technical difficulties to make these processes more efficient and profitable and to bring these technologies to greater maturity. This is the challenge taken up by ACCIONA, which has the necessary capacity and experience as well as core values that include a commitment to sustainable and environmentally-friendly procedures.

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