Recovery boiler

Recovery boilers are also used in the (less common) sulfite process of wood pulping; this article deals only with recovery boiler use in the kraft process.

Function of recovery boilers

Concentrated black liquor contains organic dissolved wood residue in addition to sodium sulfate from the cooking chemicals added at the digester. Combustion of the organic portion of chemicals produces heat. In the recovery boiler, heat is used to produce high pressure steam, which is used to generate electricity in a turbine. The turbine exhaust, low pressure steam is used for process heating.

Combustion of black liquor in the recovery boiler furnace needs to be controlled carefully. High concentration of sulfur requires optimum process conditions to avoid production of sulfur dioxide and reduced sulfur gas emissions. In addition to environmentally clean combustion, reduction of inorganic sulfur must be achieved in the char bed.

Several processes occur in the recovery boiler:

• Combustion of organic material in black liquor to generate heat.
• Reduction of inorganic sulfur compounds to sodium sulfide, which exits at the bottom as smelt
• Production of molten inorganic flow of mainly sodium carbonate and sodium sulfide, which is later recycled to the digester after being re-dissolved
• Recovery of inorganic dust from flue gas to save chemicals
• Production of sodium fume to capture combustion residue of released sulfur compounds

First recovery boilers

Some features of the original recovery boiler have remained unchanged to this day. It was the first recovery equipment type where all processes occurred in a single vessel. The drying, combustion and subsequent reactions of black liquor all occur inside a cooled furnace. This is the main idea in Tomlinson's work.

Secondly the combustion is aided by spraying the black liquor into small droplets. Controlling process by directing spray proved easy. Spraying was used in early rotary furnaces and with some success adapted to stationary furnace by H. K. Moore. Thirdly one can control the char bed by having primary air level at char bed surface and more levels above. Multiple-level air system was introduced by C. L. Wagner.

Recovery boilers also improved the smelt removal. It is removed directly from the furnace through smelt spouts into a dissolving tank. Some of the first recovery units employed the use of Cottrell's electrostatic precipitator for dust recovery.

Babcock & Wilcox was founded in 1867 and gained early fame with its water tube boilers. The company built and put into service the first black liquor recovery boiler in the world in 1929. This was soon followed by a unit with completely water cooled furnace at Windsor Mills in 1934. After reverberatory and rotating furnaces the recovery boiler was on its way.

The second early pioneer, Combustion Engineering (now GE) based its recovery boiler design on the work of William M. Cary, who in 1926 designed three furnaces to operate with direct liquor spraying and on work by Adolph W. Waern and his recovery units.

Recovery boilers were soon licensed and produced in Scandinavia and Japan. These boilers were built by local manufacturers from drawings and with instructions of licensors. One of the early Scandinavian Tomlinson units employed an 8.0 m high furnace that had 2.8×4.1 m furnace bottom which expanded to 4.0×4.1 m at superheater entrance.

This unit stopped production for every weekend. In the beginning economizers had to be water washed twice every day, but after installation of shot sootblowing in the late 1940s the economizers could be cleaned at the regular weekend stop.

The construction utilized was very successful. One of the early Scandinavian boilers 160 t/day at Korsnäs, operated still almost 50 years later.

Development of recovery boiler technology

The use of kraft recovery boilers spread fast as functioning chemical recovery gave kraft pulping an economic edge over sulfite pulping.

The first recovery boilers had horizontal evaporator surfaces, followed by superheaters and more evaporation surfaces. These boilers resembled the state-of-the-art boilers of some 30 years earlier. This trend has continued until today. Since a halt in the production line will cost a lot of money the adopted technology in recovery boilers tends to be conservative.

The first recovery boilers had severe problems with fouling.

Tube spacing wide enough for normal operation of a coal-fired boiler had to be wider for recovery boilers. This gave satisfactory performance of about a week before a water wash. Mechanical sootblowers were also quickly adopted. To control chemical losses and lower the cost of purchased chemicals electrostatic precipitators were added. Lowering dust losses in flue gases has more than 60 years of practice.

One should also note square headers in the 1940 recovery boiler. The air levels in recovery boilers soon standardized to two: a primary air level at the char bed level and a secondary above the liquor guns.

In the first tens of years, the furnace lining was of refractory brick. The flow of smelt on the walls causes extensive replacement and soon designs that eliminated the use of bricks were developed.

Improving air systems

To achieve solid operation and low emissions the recovery boiler air system needs to be properly designed. Air system development continues and has been continuing as long as recovery boilers have existed. As soon as the target set for the air system has been met new targets are given. Currently the new air systems have achieved low NOx, but are still working on lowering fouling. Table 1 visualizes the development of air systems.

Table 1: Development of air systems.

The first generation air system in the 1940s and 1950s consisted of a two level arrangement; primary air for maintaining the reduction zone and secondary air below the liquor guns for final oxidation. The recovery boiler size was 100 – 300 TDS (tons of dry solids) per day. and black liquor concentration 45 – 55%. Frequently to sustain combustion auxiliary fuel needed to be fired. Primary air was 60 – 70% of total air with secondary the rest. In all levels openings were small and design velocities were 40 – 45 m/s. Both air levels were operated at 150 °C. Liquor gun or guns were oscillating. Main problems were high carryover, plugging and low reduction. But the function, combustion of black liquor, could be filled.

The second generation air system targeted high reduction. In 1954 CE moved their secondary air from about 1 m below the liquor guns to about 2 m above them. The air ratios and temperatures remained the same, but to increase mixing 50 m/s secondary air velocities were used. CE changed their frontwall/backwall secondary to tangential firing at that time. In tangential air system the air nozzles are in the furnace corners. The preferred method is to create a swirl of almost the total furnace width. In large units the swirl caused left and right imbalances. This kind of air system with increased dry solids managed to increase lower furnace temperatures and achieve reasonable reduction. B&W had already adopted the three-level air feeding by then.

Third generation air system was the three level air. In Europe the use of three levels of air feeding with primary and secondary below the liquor guns started about 1980. At the same time stationary firing gained ground. Use of about 50% secondary seemed to give hot and stable lower furnace. Higher black liquor solids 65 – 70% started to be in use. Hotter lower furnace and improved reduction were reported. With three level air and higher dry solids the sulfur emissions could be kept in place.

Fourth generation air systems are the multilevel air and the vertical air. As the feed of black liquor dry solids to the recovery boiler have increased, achieving low sulfur emissions is not anymore the target of the air system. Instead low NOx and low carryover are the new targets.

Multilevel air

The three-level air system was a significant improvement, but better results were required. Use of CFD models offered a new insight of air system workings. The first to develop a new air system was Kvaerner (Tampella) with their 1990 multilevel secondary air in Kemi, Finland, which was later adapted to a string of large recovery boilers. Kvaerner also patented the four level air system, where additional air level is added above the tertiary air level. This enables significant NOx reduction.

Vertical air

Vertical air mixing was invented by Erik Uppstu. His idea is to turn traditional vertical mixing to horizontal mixing. Closely spaced jets will form a flat plane. In traditional boilers this plane has been formed by secondary air. By placing the planes to 2/3 or 3/4 arrangement improved mixing results. Vertical air has a potential to reduce NOx as staging air helps in decreasing emissions. In vertical air mixing, primary air supply is arranged conventionally. Rest of the air ports are placed on interlacing 2/3 or 3/4 arrangement.