Black Liquor Recovery Boilers - An Introduction
Factory Mutual Research Corp.
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)
The sodium sulfate or kraft pulping process was patented in 1884 and accounts for over 80% of all pulp produced in North America. This process would not be economically viable without the black liquor recovery boiler (BLRB). The purpose of the BLRB is to reclaim the spent pulping chemicals in the "black liquor" by the process of combustion and to capture the heating value as steam to generate electricity and supply process steam demands.
The design of the BLRB is attributed to G.H. Tomlinson and the first unit having water walls was constructed in 1934 by Babcock and Wilcox. A contemporary BLRB is similar to a large industrial watertube boiler and may be either two drum design (popular for operating pressures less than 900 psi) or single drum (popular for pressures above 900 psi). The single drum design has the advantages of removing the drum from the corrosive flue gases and eliminating potential leakage from rolled tube joints. The combustion gases are highly corrosive, particularly at temperatures necessary for operation above 900 psi. Above 900 psi, special corrosion protection (such as composite tubes, chromizing, metal spray coating and stainless weld overlay) is provided for furnace tubes. The furnace of a BLRB is generally taller than a utility or industrial watertube boiler of similar steam generating capacity, due to the sticky nature of the combustion gases.
The pulp production capacity of a mill determines the size of a BLRB. For each ton of pulp, about 3,000 lbs of dry solids are generated. A typical small BLRB will process 750,000 lbs/day dry solids and be rated for 300 psi MAWP and 140,000 lbs/hr steam. A typical large BLRB will process 6,000,000 lbs/day dry solids and be rated for 1500 psi MAWP and 850,000 lbs/hr steam. Replacement values are on the order of $18,000,000 for the smaller BLRB and $40,000,000 for the larger BLRB. The daily revenue generated by a mill could be $200,000 for the smaller and $1,000,000 for the larger BLRB. These high values require exceptional attention to the operation and maintenance of BLRBs to assure optimum return on investment.
BLRBs have auxiliary fuel (gas or oil) burners located near the floor to raise the temperature for initiating combustion of the black liquor and to stabilize combustion if upsets occur. Auxiliary burners may also be provided higher in the furnace to supplement heat input during times of limited liquor availability or other upsets. The potential for fuel gas explosions in BLRBs is similar to that of any large watertube boiler, and explosion experience has been greatly improved by application of fuel combustion control systems. Auxiliary fuel combustion control systems equivalent to the requirements of NFPA 8502, standard for the prevention of furnace explosions/implosions in multiple burner boilers, are recommended. Generally, auxiliary fuel is shut off once firing of black liquor has become self-sustaining and stabilized.
Similar to any boiler, dry firing is a potential risk. Application of low-water protection schemes has greatly reduced the incidence of BLRB dry firing. Provision of either two fully independent low-water fuel cutoff devices, or a system providing similar reliability, is typically recommended.
Combustion of the black liquor reduces the sulfur compounds to sulfide and recovers the inorganic chemicals in a molten form. Black liquor, with at least 58% solids, is sprayed into the furnace. The remaining water is vaporized and partial combustion occurs during travel to the furnace floor. Combustion air is limited at the floor to promote a reducing atmosphere. The partially combusted liquor accumulates as a porous char bed. Molten smelt accumulates on the floor (protected from overheating by refractory or by solidified smelt) to a level of several inches before flowing out water-cooled smelt spouts to the smelt dissolving tank.
It is the pool of molten smelt, at a temperature of 1700°F - 1800°F (925°C - 980°C) that presents the unique BLRB hazard. As with any high-temperature molten material, mixing water can result in an expanding vapor explosion. The rapid vaporization of one pound of water can release as much energy as a half- pound of TNT. A smelt-water explosion generates a shock wave that causes severe local damage when the wave contacts the furnace. Although much study has been done, the only known method to avoid a smelt-water explosion is to prevent smelt-water contact. A challenging task in a water-cooled vessel! In addition to boiler water, several other water sources exist. If the water content of the black liquor exceeds 42%, an explosion can result. Condensate may enter through a faulty sootblower system or a faulty steam coil air heater. Water can be introduced from external water wash hoses, from the black liquor piping water wash system or from faulty liquor heat exchangers.
The Black Liquor Recovery Boiler Advisory Committee (BLRBAC) has developed a recommended system to avoid or mitigate smelt-water explosions. This system is known as the Emergency Shutdown Procedure or ESP. If water is known or suspected to be entering a BLRB furnace, the operator is trained to actuate the ESP which simultaneously sounds an emergency evacuation alarm, stops all fuel flow to the furnace, minimizes combustion air to the char bed, shuts off all water sources to the furnace and opens a group of "rapid drain" valves that drain all but the last 8 feet of water from the BLRB. This 8 feet of water is intended to prevent overheating of the floor and lower wall tubes. The drain is completed in about 20 minutes, and the superheater vent valve is then opened to more rapidly relieve the pressure (stored energy) remaining in the BLRB.
BLRBAC has been notified of over 150 BLRB explosions in North America. In most incidents, BLRB damage required days or even weeks to repair. In the worst incidents, damage was extensive, taking months to repair, and injury and death of operating personnel has resulted. The primary source of water entering a BLRB furnace is from boiler pressure part leaks. In addition to semi-annual or annual fireside inspections, annual inspection of pressure parts is recommended to locate existing leaks and identify potential leaks so proactive measures can be taken to prevent smelt-water contact. During this annual inspection it is also recommended all safety systems be tested - ESP, low-water protection, black liquor and auxiliary fuel firing systems. Also recommended is periodic determination that operators continue to know not only how to operate the BLRB on a daily basis, but also to recognize various upset conditions, know what corrective actions are appropriate, be able to recognize when water is entering the furnace and how to actuate the ESP system.
With some 280 BLRBs currently operating in North America and new ones under construction, the challenge of safe operation will be with us for years to come.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.