WHY WASTE ENERGY

Waste to energy has evolved far from early waste incinerators. Now it is seen as a useful route to reclaim the energy from materials that would otherwise be forgotten in a landfill.

Burning waste is an old custom for getting rid of space demanding and smelly garbage. Unfortunately, if not cleaned efficiently, the flue gas from burning waste is both a hazard to your health as well as to the environment. The technologies used for waste incineration have gone forward from being just a way to get rid of waste to become a way to utilize the energy in the waste. Public opinion is slowly becoming more positive towards waste combustion, especially in the form of waste to energy projects instead of increased landfill disposal.

April 2005 saw the publication in the UK of a report commissioned by two organizations, the Institution of Civil Engineers and the Renewable Power Association, on the potential energy yield from waste residuals in the UK through the use of energy from waste technologies. This report proposed the potential production of energy from waste could produce around 17 per cent of total demand in the UK for electricity by 2020, and act as a significant replacement for fossil fuels.

Figure 1. Up to date flue gas cleaning techniques can cope with the differing levels of particles emitted from the wide range of waste materials used to generate energy

To put this in context, such production would provide enough electricity to power the combined populations of both Wales and Northern Ireland.

At present, energy from waste only provides approximately one per cent of all energy needs within the UK. This is among the lowest proportions in Europe and is something that undoubtedly needs to change to make best use of resources, and to meet global warming targets.

In the Nordic countries the co-combustion of municipal solid waste and biomass residues from sources such as forests and sawmills residues in combustion plants is commonplace.

This is considered a legacy of the generally high levels of public education in waste management issues, and the long history of source separation that has occurred within these countries. The need to reduce and recycle waste is also taken into consideration. The recovery of energy from waste is integrated into governmental plans, while general waste education initiatives also encourage recycling and reduction of waste.

The main objective with the Finnish national waste management strategy is to reduce waste and to improve recycling. Burning waste should only be an option to landfill disposal and the waste must always be carefully separated before combustion.

Figure 2. If combustion conditions are optimised pollutants can be minimized as early in the process as possible

Despite many preventive actions towards diminishing waste, very little has changed during the last ten years. The goal for the Finnish national waste management strategy is a 15 per cent reduction of the total amount of waste by 2010. However, since 1990 most waste sources have increased. The recycling of paper and glass is at a high level but at the same time the amount of cardboard and plastic wrapping being carelessly disregarded has increased. For these waste sources recycling is working poorly and incineration would therefore be a strong alternative to landfill disposal. This is being held back in part due to the fact that burning plastic materials is still controversial, despite its high energy content.

manage to recycle so effectively while reducing the amount of waste available by using it as fuel to create energy. The key word is efficiency.

In order to encourage the development of energy from waste a number of these countries have introduced high levels of tax on landfill sites, banned biodegradable materials from landfills and invested in educating the public through well developed communications strategies.

While, the above factors have encouraged the development of waste energy it should be noted that there were no easy wins here either, and the goodwill towards energy from waste is based almost entirely upon communication with all interested stakeholders. Lessons which have been learned include:

·                     The necessity of community involvement and education in the issues surrounding the management of waste, including realistic understanding of the limitations of all waste management options

·                     Recognition that the majority of stakeholder concerns are legitimate and require consideration

·                     Utilization of debate and provision of staff who are available to respond to local concerns

·                     Development of high emission standards and strong regulatory regimes, in some cases with publication of emissions data to name and shame

·                     National level policy to provide a framework for local decision making.

In contrast to the above lessons, consideration needs to be given to the factors considered most important to countries with a low proportion of waste energy in their generation portfolio.

These include:

·                     Lack of political will at the local, regional and national level

·                     Opposition from environmental groups and local communities, some of which is based upon political agenda and poor science

·                     Lack of widespread understanding of the technologies available, and of actual proven technologies

·                     Concerns over emissions and potential health and environmental effects

·                     Concerns over impacts on local landscapes or house prices

·                     Concerns over where such plants are located, particularly where a previous industrial use of the site has not occurred

·                     Lack of trust in the waste management sector

·                     Lack of confidence in environmental regulation

·                     Poor communication by strategy developers and waste operators with all stakeholders.

A comparison of the above points would suggest that a great deal of work is needed to assist these countries with a low proportion of waste energy to aid the develop of a realistic waste management infrastructure, particularly with respect to gaining widespread public support. In many cases such issues are being taken into account in the development of the waste management infrastructure for the next 25 to 30 years.

In others, an ostrich mentality exists which does not take into account the potential opportunities related to the utilization of energy recovery techniques.

The benefits of generating energy from waste are wide ranging. It can be utilized as a complement to recycling and material recovery rather than as a competitor as targets can be met and exceeded with respect to landfill obligations whilst still providing sufficient feedstock for smaller scale energy for waste plants.

Also, waste to energy plants have the ability to effectively deal with waste residues after recyclable materials have been recovered. The current favouring of Mechanical biological treatment (MBT) options within municipal waste strategies provides an ideal base for the introduction of energy from waste later in the strategy period; many have viewed MBT as a final waste management solution rather than as an ultimate producer of refuse derived fuel.

A further benefit is that energy production is a rational and positive diversion of waste from landfill, and as technology advances occur and improve in terms of economic viability, it should be considered as an effective enhancement of the overall waste strategy. Current concerns over energy production from traditional fuels in the future may provide a further driver for development.

Finally, energy from waste is an effective stabilization of the waste, reducing overall environmental burden and reducing the volume of waste for final disposal to landfill.

The two most commonly used techniques for incinerating waste are fluidized bed boilers and grate boilers. The advantage of the fluidized bed boiler is that the fuel (i.e. the waste) has to be very well prepared before incineration. When using a grate boiler a real mix of waste can be burnt, which from an environmental point of view is a waste of resources. The fluidized bed boiler works hand in hand with the common strategy for source separation and recycling, as only waste that is left after these processes have been finalized will be used for incineration.

Finland has a local excess supply of district heating, and this is mainly the result of the fact that when waste is burned heat is the primary product, with some electricity as the by-product. By carefully choosing the incineration technique you can raise the level of electricity produced. When using fluidized bed boilers you can raise the efficiency by several per cent in comparison to other techniques due to higher steam parameters.

On the other hand it can be debated whether grate boilers can utilize a bigger fraction of the waste and thus reach higher total utilization.

Due to enter into force at the start of 2006, a new EU directive for flue gas cleaning from waste incineration plants will set limits for dust particles, nitrogen oxides, sulphur oxides, heavy metals and dioxides. A large proportion of waste incineration will therefore have to include extensive flue gas cleaning.

The different levels of particles and other contraries required can easily be met by up to date flue gas cleaning techniques. When using contemporary flue gas cleaning equipment a waste to energy plant emits less pollutant than an ordinary power plant burning heavy fuel oil.

Waste incineration in all Swedish plants is combined with energy production. The resulting energy is used for district heating in most cases. The systems for flue gas condensation, which are in use in several locations, aim to increase energy recovery without increasing fuel consumption.

More than 99.5 per cent of the original amount of burnt waste oxidizes into carbon dioxide and water. The quantity of non-oxidized gases which leaves the furnace along with the flue gas mainly consist of carbon monoxide and the same type of light hydrocarbons that occur with all incineration.

The incineration conditions have a greater influence on many of the other organic micro-compounds, such as, polycyclic aromatic hydrocarbons (PAH). The chlorinated hydrocarbons are an important pollutant group that need to be specially considered when constructing waste combustion plants.

The three factors to be considered when limiting the production of chlorinated aromatics during waste incineration are the chlorine content during incineration, the combustion efficiency and the energy density in the furnace.

Measures to reduce the production of chlorinated aromatics are the same for incineration in grate fired and fluidized bed combustion. The homogenous composition of the fuel, the particle size and heating value are recognized factors, which are of great importance to the possibilities of carrying out good combustion in FCB boilers.

By optimizing combustion conditions, pollutants are minimized as early in the process as possible. The ideal and best is of course to minimize the pollutants in the waste to start with.

Flue gas cleaning is playing an important part of the cost in a waste combustion plant. By combining the flue gas cleaning with condensation of the flue gases, considerably increased energy recovery can be achieved.

In Sweden, for example, releases with the flue gas during the incineration of municipal waste with both dry and wet cleaning systems can be limited, so that the limits specified in EU directives are fulfilled.

It can be concluded that several well proven technical solutions exist for efficient and environmentally feasible waste combustion. But the key to successfully implementing waste to energy projects is clearly on the political level.

Both governments and supervision authorities need to have a clear framework in place, and this must be connected to an efficient permitting procedure. Careful planning and communication from developers regarding waste to energy projects and continuous information to all interest groups at the early stages of a project will diminish the risk of possible misunderstandings from the public.

Power Engineering International June, 2005
Author(s) :   Staffan Asplund   Karl Hyland