General
Functions of a recovery boiler
Recovery of sodium and sulfur
Production of steam

General

Sodium compounds used in the cooking of sulfate pulp are a significant cost item. Even in the early days of pulping, the sodium compounds were recovered from the black liquor and recycled back into the process.

Black liquor is formed when white liquor used in sulfate pulping reacts with lignin and other ingredients in the wood. Black liquor is actually black in color and includes wood lignin, sodium compounds, and sulfur–based chemicals used in the cooking process.

Black liquor is separated from pulp during the post-cook washing stages . First, the liquor is pumped from the washers to the evaporation plant for the removal of excess water and then onwards to the recovery boiler to be burned .

Functions of a recovery boiler

A recovery boiler has two different functions :

1. recovering chemicals used in the cooking process, and
2. harnessing the heat resulting from the recovery process and converting it into steam energy .

The pulping chemicals sulfur and sodium are released from black liquor in the recovery boiler. They are recovered as suitable compounds for further treatment in the chemical circulation loop. At the same time, heat is released when the organic compounds in the black liquor are combusted. This heat is used to produce steam. So, the recovery boiler also functions as a steam boiler.

This double function of the recovery boiler requires a sophisticated structure and makes the operation of the boiler considerably more demanding than power plant boilers using “normal” fuels.

Recovery of sodium and sulfur

In the recovery boiler, sulfur within the black liquor is reduced to sodium sulfide . A part of the sulfur compound does not reduce. The degree of reduction from sodium sulfate into sodium sulphide (reduction degree) is an important measure of the recovery boiler's performance. The sodium that remains in the black liquor will create sodium carbonate when it burns and reacts with carbon dioxide.

The chemical smelt that flows from the furnace through smelt spouts includes sodium sulphite, sodium carbonate, and sodium sulfate. The chemical smelt is dissolved into weak wash, forming green liquor . The green liquor is fed into the causticizing plant to be processed further into white liquor for the cooking process.

Production of steam

In the cooking process, wood fibers are separated from each other for further processing by dissolving the lignin between fibers. The dissolved lignin and other dissolved organic and inorganic matter from cooking chemicals (black liquor) are separated from the pulp during washing. The dry solids content of separated black liquor is increased by removing water from the black liquor in the evaporation process. Sodium and sulfur are recovered from the black liquor by burning the black liquor in recovery boiler. This chemical reaction and burning of organic materials releases a considerable amount of heat energy. The heat is recovered by transferring it through water-filled tubes in walls of the recovery boiler. The water vaporizes into steam and electricity is produced from steam with a turbine. In addition, some of the steam can be used in different stages of the process (e.g. heating the cooking process and sootblowing in the recovery boiler itself).

Actually, the two different functions of a recovery boiler, chemical recovery and the production of electricity and steam, are totally separate processes. The chemicals could be separated without recovering the heat that is generated during burning. The water/steam system of the recovery boiler, which consists of a feed water tank, feed water preheaters, boiler banks, superheaters, steam drum and boiler wall pipes, actually exists only for producing steam and it has no particular significance in the separation of the chemicals. Electricity is generated from the high-pressure steam with turbine .