Recovery Boiler

The thick liquor from the evaporation plant is burnt in the recovery boiler. This process has two primary objectives:

  • To generate steam that will be used in the by burning dissolved wood substance
  • To recover cooking chemicals

During combustion, the reaction between lignin and cooking chemicals are broken down so that further treatment converts the chemicals back into new cooking liquor. The inorganic chemicals are then converted into a smelt and collected at the bottom of the furnace. It contains among others, sodium carbonate (Na2CO3) and sodium sulphide (Na2SO3). The smelt then flows out and is dissolved in a liquor from the lime mud washer, which is known as weak liquor. After the smelting process the liquor is then called green liquor.

The steam produced in the recovery boiler is of great economic benefit to the pulp mill. It provides enough heat to meet the demands on the pulping process and more often, there is sufficient steam to produce electricity using a steam turbine. In newer and larger pulp mills, the surplus electricity is sold on the open market. As the source material (i.e. wood) is renewable, this is considered “green energy”.

Click the image to enlarge.

Some examples of NAF valves successfully used in this process

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