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Thermal

Oil, gas, and coal are also used to make electricity in New Brunswick. Generating stations, such as Belledune and Coleson Cove that use one or more of these fuels, are known as thermal stations. NB Power has some of the most advanced thermal stations in North America. In fact, all of our thermal generating stations are equipped with environmental protection equipment. Visit our system map for an overview of our stations and what protections are in place.

All thermal stations work basically the same way: Burning the fuel produces heat. The heat is transferred to water, which is then turned into steam.

In 2007, our Belledune Generating Station won an award for best performer by a large coal-fired plant for the previous five-year period. Here is an example of a cross section of the Belledune station:

 

Coal is transported by the conveyor system from the active stockpile in the dome through a crusher house located along the conveyor route. The crushers reduce large lumps of coal to a maximum allowable size. Crushed coal is moved to coal bunkers(1) inside the boiler house from where it is fed to the pulverizing mills(2) to be ground into a fine powder. The powdered coal is mixed with preheated air and blown into the combustion chamber(3) where it is burned.

The combustion chamber is lined with boiler tubes(4) through which water circulates. The burning coal heats the circulating water in the boiler tubes and turns it into steam which rises to the steam drum(5) at the top of the boiler. The steam then enters the primary superheater(6) and then the secondary superheater(7) where heat from the combustion gases is used to increase the temperature to 540°C. At this temperature, the high-pressure steam is piped to the turbine(8) where jets direct it against the turbine blades, causing the turbine to spin. The spinning turbine is connected to the generator(9) which also turns and produces electricity.

As the high-pressure steam passes through the turbine, the useful energy is extracted and it cools. To reuse this highly purified, low-pressure steam, it must be changed back into water by cooling it in a condenser(10). The condenser is a network of tubes containing cold water pumped from the Bay of Chaleur. As the steam passes around the condenser tubes, it cools and changes back into water. The cooling water, which does not mix with water in the steam system, is then returned to the Bay of Chaleur 10°C warmer than it was when it was pumped into the system.

The water condensed from the steam is now ready to be heated again. First, it is pumped through low-pressure stage heaters(11) and then a de-aerator(12) where steam is used to heat the water and remove any trapped air. Boiler feed pumps(13) send the water to the high-pressure stage heaters(14) and then to an economizer(15) where flue gases are used to preheat the water before it enters the steam drum as hot water. The water from the steam drum(5) is returned to the boiler tubes(4) to be heated into steam for another cycle through the turbine.

After the flue gases have given up heat to superheat steam and preheat water, they are used a third time in the air heaters(16). Air heaters take heat from the flu gases and transfer it to the air used for combustion. A portion of the heated air goes to the pulverizing mills(2) to help transport coal to the main burner zone. Another portion of the air is redirected through specially designed windboxes or nozzles(17) that are mounted above the main burner zone where they provide overfire air to complete the combustion of the coal. This staging technique reduces significantly the amount of nitrogen oxide emissions in the flue gases.

After most of the usable heat has been extracted, the flue gases, including suspended particulates, are exhausted into electrostatic precipitators(18). Particulates or "fly ash" are removed from the exhaust gases and transported to the fly ash handling and storage facility(20). The gases are pumped by induced draft fans(19) into the flu gas desulphurization (FGD) system(21).

The exhaust gases enter the desulphurization tower where they are sprayed with a mixture of limestone and water. The chemical reaction created by spraying the limestone and water slurry into the path of the gases, neutralizes most of the sulphur dioxide gases. The flue gas, with most of the pollutants removed, is then discharged high into the atmosphere through the 168 meter chimney(24).

The limestone slurry residue which settles at the bottom of the flue gas desulphurization unit, goes through a process of dewatering(22) and is then transported to the gypsum stackout area(23). From here the gypsum is taken to an environmentally controlled landfill where it is available for use in the production of gypsum products.