Team:Uppsala/Power Plant
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Visting the Power Plant
Ten years ago, a directive about deponing of waste was settled. The directive involved more strict requirements to fulfill and higher fees to pay before being able to depone waste in Europe. This resulted in a reduced amount of degradable waste being deponed. Some of the Swedish power plants, or incineration plants, have been developing effective burning systems in order to generate energy from waste. Due to cheap waste and the efficiency of the systems these incineration plants become profitable, in such extent that waste is being imported to produce energy. Whether this import of waste is good or bad is debated in media and among politicians and researchers. The import of waste results in lower energy costs for the Swedish population but the increased burning of waste may however lead to increased concentrations of toxic chemicals from the burnt waste on a local level.
Because PAHs are formed during incomplete combustion there is a risk that the smoke and ashes from an incineration plant could contain traces of these compounds. The Uppsala iGEM team wanted to further analyze the amount of the carcinogenic and teratogenic pollutants polycyclic aromatic hydrocarbons among the incinerated waste, however this analyze was not possible to carry out because of lack of time. We also wanted to examine how and where the Decyclifier could be incorporated in the industrial process of a combined heat and power plant (CHP) in order to reduce the concentration of the toxic PAHs in effluents from incineration plants. We hope that our bacteria could be able to help cleaning the waste products. In Uppsala there is a big CHP where first and foremost waste is incinerated, but also peat and oil is used as fuel during colder winter months. We were very curious about how a CHP works and above all how the waste from the burning process, such as ashes and smoke, is taken care of. To find out more about this, a group from our team went on a guided tour at Vattenfall, Uppsala's combined heat and power plant. The tour was held by Bo Wohrne, nuclear power engineer and employee at Uppsala Vattenfall.
During the tour we got to see all the steps in the process, from the arrival of the waste and peat to the disposal of the ashes. This included looking into a 1000°C hot furnace, seeing a giant claw picking up loads of waste and throw it into the incinerator and seeing a big pile of ash, containing loads of strange metal objects. More details about what we learned at the CHP can be read below.
The burning process
As mentioned above, at Uppsala´s power plant both household waste and special waste like drugs, customs seizures, coffins etc. are burnt. A lot of the waste is imported, for example from Norway and Great Britain, countries that normally put a lot of their waste on landfills. In this way these countries get rid of their waste without having to take care of it and therefore the power plant that handles the waste gets paid to do so.
An overview of the process can be seen in Figure 1. The first step in the combustion process is to feed the waste to the waste incinerator, which is done by a big grapple claw maneuvered manually. This has to be done in a regular pace to keep the conditions in the incinerator constant. Around 55 ton of waste can be burnt every hour at Uppsala’s power plant. In the incinerator the temperature is kept constant at around 1000 °C. It is important to keep the temperature above 850°C all the time to make sure that the combustion is as complete as possible and thus prevent the formation of unwanted byproducts.
During combustion a lot of smoke and ashes, containing hazardous gases and particles, are produced. The smoke and fly ashes are captured at the top of the incinerator. Ashes consisting of heavy particles, especially a lot of scrap metal, are collected at the bottom of the pan and treated separately. This is not considered hazardous waste and the metal is recycled. The rest is cinder gravel, normally sent to landfills.
The particles in the smoke and fly ashes are captured by electro-filters which refines the fly ashes through a membrane filter-electrophoresis combined process. These filters can reduce air contaminants with up to 90%. The particles and chemicals that remain in the smoke after this step passes through a scrubber, where the smoke is washed with different chemicals in three different filtering systems. In the first step HCl is removed. The pH can be lowered by solving additional HCl to precipitate ammonium and heavy metals, if the ash contains further heavy metals. In the next scrubber the smoke is neutralized and that makes SO2 precipitate. During the third step the moist in the smoke is condensed and heavy metals are collected in this condensed liquid. All the ashes collected in the cleaning steps above is hazardous waste. Out of this waste a type of concrete is made to stabilize the hazardous chemicals and then the concrete is put on landfills. The condensed liquid is led to a wastewater treatment plant. Here the chemicals are precipitated and collected when they settle. The water is also filtered through sand- and carbon filters and treated with calcium oxide to collect heavy metals. The cleaned water is drinkable after this treatment.
After the condensation step the smoke is reheated and led through a textile filter where activated carbon is added to bind acidic gases and organic compounds. This is sent back to the incinerator to be burnt once more. (Other systems have the cleaning steps in different order which means that the organic compounds are not sent back to the incinerator but instead sent to a landfill with the ashes.)
Last the smoke passes a catalyst where nitrogen oxides are reduced. The remaining smoke is let out through the chimney and the emissions are controlled continuously to make sure that the rate of contaminants in the smoke are kept below the threshold values. During the combustion process and also the cleaning of the ashes and smoke, heat and steam is produced. The heat is used as district heating and the steam is both sent to the turbine for production of electricity and to industries that use steam in their production.
To see the implementation of our system, click here.