Hubbry Logo
Recovery boilerRecovery boilerMain
Open search
Recovery boiler
Community hub
Recovery boiler
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Recovery boiler
Recovery boiler
Recovery boiler
View on Wikipedia
from Wikipedia

Recovery boiler is the part of kraft process of pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed wood. The black liquor is burned, generating heat, which is usually used in the process of making electricity, much as in a conventional steam power plant. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.[1]

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.

CMPC Celulosa Santa Fe Recovery boiler is one of the newest boilers in South America

Function of recovery boilers

[edit]

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

[edit]
Black liquor

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.[2] 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.[3]

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.[4]

Development of recovery boiler technology

[edit]
A nozzle used in recovery boilers for injecting black liquor

The use of kraft recovery boilers spread fast as functioning chemical recovery gave kraft pulping an economic edge over sulfite pulping.[5]

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.[6]

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

[edit]

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.[7] 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.[7]

Air system Main target But also should
1st generation Stable burning of black liquor
2nd generation High reduction Burn liquor
3rd generation Decrease sulfur emissions Burn black liquor, high reduction
4th generation Low NOx Burn black liquor, high reduction, and low sulfur emission
5th generation Decrease superheater and boiler bank fouling Burn black liquor, high reduction, and low emissions

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.[8] 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.[8] 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.[9] 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

[edit]

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.[10] 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

[edit]

Vertical air mixing was invented by Erik Uppstu.[11] 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.[12] 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.

Black liquor dry solids

[edit]
Net heating values of industrial black liquors at various concentrations

As fired black liquor is a mixture of organics, inorganics and water. Typically the amount of water is expressed as mass ratio of dried black liquor to unit of black liquor before drying. This ratio is called the black liquor dry solids.

If the black liquor dry solids is below 20% or water content in black liquor is above 80% the net heating value of black liquor is negative. This means that all heat from combustion of organics in black liquor is spent evaporating the water it contains. The higher the dry solids, the less water the black liquor contains and the hotter the adiabatic combustion temperature.

Black liquor dry solids have always been limited by the ability of available evaporation.[13] Virgin black liquor dry solids of recovery boilers is shown as a function of purchase year of that boiler.

Virgin black liquor dry solids as a function of purchase year of the recovery boiler

When looking at the virgin black liquor dry solids we note that on average dry solids has increased. This is especially true for latest very large recovery boilers. Design dry solids for green field mills have been either 80 or 85% dry solids. 80% (or before that 75%) dry solids has been in use in Asia and South America. 85% (or before that 80%) has been in use in Scandinavia and Europe.

High temperature and pressure recovery boiler

[edit]

Development of recovery boiler main steam pressure and temperature was rapid at the beginning. By 1955, not even 20 years from birth of recovery boiler highest steam pressures were 10.0 MPa and 480 °C. The pressures and temperatures used then backed downward somewhat due to safety.[14] By 1980 there were about 700 recovery boilers in the world.[9]

Development of recovery boiler pressure, temperature and capacity.

Safety

[edit]

One of the main hazards in operation of recovery boilers is the smelt-water explosion. This can happen if even a small amount of water is mixed with the solids in high temperature. Smelt-water explosion is purely a physical phenomenon. The smelt water explosion phenomena have been studied by Grace.[15] The liquid - liquid type explosion mechanism has been established as one of the main causes of recovery boiler explosions.

In the smelt water explosion even a few liters of water, when mixed with molten smelt can violently turn to steam in few tenths of a second. Char bed and water can coexist as steam blanketing reduces heat transfer. Some trigger event destroys the balance and water is evaporated quickly through direct contact with smelt. This sudden evaporation causes increase of volume and a pressure wave of some 10 000 – 100 000 Pa. The force is usually sufficient to cause all furnace walls to bend out of shape. Safety of equipment and personnel requires an immediate shutdown of the recovery boiler if there is a possibility that water has entered the furnace. All recovery boilers have to be equipped with special automatic shutdown sequence.

The other type of explosions is the combustible gases explosion. For this to happen the fuel and the air have to be mixed before the ignition. Typical conditions are either a blackout (loss of flame) without purge of furnace or continuous operation in a substoichiometric state. To detect blackout flame monitoring devices are installed, with subsequent interlocked purge and startup. Combustible gas explosions are connected with oil/gas firing in the boiler. As also continuous O2 monitoring is practiced in virtually every boiler the noncombustible gas explosions have become very rare.

Modern recovery boiler

[edit]

The modern recovery boiler is of a single drum design, with vertical steam generating bank and wide spaced superheaters. This design was first proposed by Colin MacCallum in 1973 in a proposal by Götaverken (now Metso Power inc.) for a large recovery boiler having a capacity of 4,000,000 lb of black liquor solids per day for a boiler in Skutskär, Sweden, but this design was rejected as being too advanced at that time by the prospective owner. MacCallum presented the design at BLRBAC and in a paper "The Radiant Recovery Boiler" printed in Tappi magazine in December 1980. The first boiler of this single-drum design was sold by Götaverken at Leaf River in Mississippi in 1984. The construction of the vertical steam generating bank is similar to the vertical economizer. Vertical boiler bank is easy to keep clean. The spacing between superheater panels increased and leveled off at over 300 but under 400 mm. Wide spacing in superheaters helps to minimize fouling. This arrangement, in combination with sweetwater attemperators, ensures maximum protection against corrosion. There have been numerous improvements in recovery boiler materials to limit corrosion.[16][17][18][19]

The effect of increasing dry solids concentration has had a significant effect on the main operating variables. The steam flow increases with increasing black liquor dry solids content. Increasing closure of the pulp mill means that less heat per unit of black liquor dry solids will be available in the furnace. The flue gas heat loss will decrease as the flue gas flow diminishes. Increasing black liquor dry solids is especially helpful since the recovery boiler capacity is often limited by the flue gas flow.


A modern recovery boiler consists of heat transfer surfaces made of steel tube; furnace-1, superheaters-2, boiler generating bank-3 and economizers-4. The steam drum-5 design is of single-drum type. The air and black liquor are introduced through primary and secondary air ports-6, liquor guns-7 and tertiary air ports-8. The combustion residue, smelt exits through smelt spouts-9 to the dissolving tank-10.

The nominal furnace loading has increased during the last ten years and will continue to increase.[20] Changes in air design have increased furnace temperatures.[21][22][23][24] This has enabled a significant increase in hearth solids loading (HSL) with only a modest design increase in hearth heat release rate (HHRR). The average flue gas flow decreases as less water vapor is present. So the vertical flue gas velocities can be reduced even with increasing temperatures in lower furnace.

The most marked change has been the adoption of single drum construction. This change has been partly affected by the more reliable water quality control. The advantages of a single drum boiler compared to a bi drum are the improved safety and availability. Single drum boilers can be built to higher pressures and bigger capacities. Savings can be achieved with decreased erection time. There is less tube joints in the single drum construction so drums with improved startup curves can be built.

The construction of the vertical steam generating bank is similar to the vertical economizer, which based on experience is very easy to keep clean.[25] Vertical flue gas flow path improves the cleanability with high dust loading.[26] To minimize the risk for plugging and maximize the efficiency of cleaning both the generating bank and the economizers are arranged on generous side spacing. Plugging of a two drum boiler bank is often caused by the tight spacing between the tubes.

The spacing between superheater panels has increased. All superheaters are now wide spaced to minimize fouling. This arrangement, in combination with sweetwater attemperators, ensures maximum protection against corrosion. With wide spacing plugging of the superheaters becomes less likely, the deposit cleaning is easier and the sootblowing steam consumption is lower. Increased number of superheaters facilitates the control of superheater outlet steam temperature especially during start ups.

The lower loops of hottest superheaters can be made of austenitic material, with better corrosion resistance. The steam velocity in the hottest superheater tubes is high, decreasing the tube surface temperature. Low tube surface temperatures are essential to prevent superheater corrosion. A high steam side pressure loss over the hot superheaters ensures uniform steam flow in tube elements.

Future prospects

[edit]

Recovery boilers have been the preferred mode of kraft mill chemical recovery since the 1930s and the process has been improved considerably since the first generation. There have been attempts to replace the Tomlinson recovery boiler with recovery systems yielding higher efficiency. The most promising candidate appears to be gasification,[27][28] where Chemrec's technology for entrained flow gasification of black liquor could prove to be a strong contender.[29]

Even if new technology is able to compete with traditional recovery boiler technology the transition will most likely be gradual. First, manufacturers of recovery boilers such as Metso, Andritz and Mitsubishi, can be expected to continue development of their products. Second, Tomlinson recovery boilers have a long life span, often around 40 years, and will probably not be replaced until the end of their economic lifetime, and may in the meantime be upgraded at intervals of 10 – 15 years.

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Recovery boiler

View on Grokipedia
from Grokipedia
A recovery boiler is a specialized industrial boiler integral to the in the , designed to combust —a viscous of wood pulping containing organic and inorganic components—to recover essential pulping chemicals such as (NaOH) and (Na₂S) while simultaneously generating high-pressure for production. This dual-purpose operation enables the of inorganic chemicals with over 90% reduction efficiency and produces approximately 3.5 kg of per kg of solids, making it a cornerstone of sustainable pulping by minimizing and providing equivalent to processing 1.3 billion tons of weak annually worldwide. The kraft recovery boiler's development began in the early , with significant advancements in leading to modern designs that operate at steam pressures up to 110 bars and temperatures reaching 510°C, allowing for larger capacities such as 5,000 tons per day of dry solids. Key components include the furnace for controlled , superheaters for , and a smelt dissolving tank where the recovered inorganic chemicals form a molten "smelt" that is subsequently processed into for reuse in pulping. Operation involves spraying concentrated (typically 65-80% solids) into an oxygen-deficient lower furnace, where it undergoes , devolatilization, char burning, and smelt formation, followed by recovery in the upper furnace to produce that can generate 25-35 MW of in a typical 1,000 tons per day . Beyond chemical and energy recovery, the boiler plays a pivotal role in environmental and , as solids retain about half the energy content of the original wood chips, enabling mills to produce 700 million tons of steam globally each year and supporting the industry's shift toward utilization. However, challenges such as emissions control (e.g., reducing CO and through optimized air mixing) and maintenance of refractory materials at high temperatures around 950°C highlight ongoing innovations, including modeling to enhance uniformity and . Overall, recovery boilers underscore the kraft process's closed-cycle nature, with causticizing efficiencies of 80-83%, ensuring the pulp industry's viability by efficient recycling of pulping chemicals.

Function and Operation

Role in the Kraft Process

In the , emerges as a major byproduct from the digestion of wood chips, where and hemicelluloses are dissolved alongside the spent cooking chemicals, primarily (NaOH) and (Na₂S). This viscous, dark liquid serves as both a waste stream and a valuable resource, encapsulating the organic residues from wood breakdown and the recyclable inorganic salts essential for the pulping cycle. The recovery boiler plays a pivotal role in reclaiming these inorganics through controlled of the , converting the organic components to heat while reducing the sodium and compounds to a molten smelt of (Na₂CO₃) and (Na₂S). This smelt is dissolved to form green liquor, which undergoes causticizing to regenerate —the active pulping agent—for reuse in the digester, achieving a chemical recovery efficiency of 95-98% and minimizing the need for fresh chemical inputs. Simultaneously, the boiler harnesses the 's inherent heating value of 12-15 MJ/kg dry solids to generate high-pressure , which drives turbines for production and supplies process heat throughout the mill, enabling modern kraft facilities to attain up to 240% energy self-sufficiency by offsetting all internal demands and exporting surplus power. Prior to firing, the is concentrated in multiple-effect evaporators from its initial 15% solids content to 65-90% solids, ensuring efficient and linking seamlessly to the downstream causticizing for chemical . Globally, over 1,000 recovery boilers operate, processing approximately 200 million tons of dry solids annually to sustain this integrated cycle.

Combustion Process Stages

The combustion of in a recovery boiler proceeds through four distinct stages: , devolatilization, char combustion, and smelt formation. These stages involve sequential chemical and physical transformations of the sprayed droplets, enabling both energy generation and chemical recovery within the . The process is influenced by the droplet size and initial conditions, with typically entering at 60-80% solids content to optimize efficiency. In the drying stage, water evaporates from the black liquor droplets as they are exposed to furnace heat, causing initial swelling of up to 1.5 times the original . This occurs at surface temperatures of 100-200°C and lasts 1-2 seconds for typical 2-3 mm droplets, with the process driven by convective and radiative from the surrounding gases. The removes bound and free water, preparing the droplet for subsequent without significant . Devolatilization follows as the organics in the dried droplet pyrolyze, releasing combustible volatiles such as CO, H₂, and CH₄, which ignite to form a visible . This stage takes place at 400-600°C, leading to rapid swelling—up to three times the diameter and 30 times the volume—due to gas evolution within the droplet structure, and completes in less than 0.5 seconds. The breaks down lignin-derived organics into gases and tars, contributing to the boiler's heat release while establishing a reducing environment. During char , the remaining carbonaceous char burns with oxygen, primarily forming CO₂ through the reaction: C+O2→CO2\mathrm{C + O_2 \rightarrow CO_2} This exothermic oxidation occurs at 800-1000°C over a slower duration of 0.5-1 second, shrinking the particle as carbon is consumed. The stage is mass transfer-limited, influenced by oxygen availability, and heat is transferred mainly via (up to 80 kW/m² in the furnace) and , with the total heat release reaching approximately 145 MW in a typical 2000 tons dry solids per day . Smelt formation marks the final stage, where inorganic components melt into a molten mixture of and at 700-900°C. Key to chemical recovery is the sulfate reduction reaction in the char bed: Na2SO4+2C→Na2S+2CO2\mathrm{Na_2SO_4 + 2C \rightarrow Na_2S + 2CO_2} The resulting smelt, tapped through floor spouts, contains recovered cooking chemicals. The char bed at the furnace bottom maintains a reducing zone essential for this reduction, preventing excessive smelt dissolution into the bed and ensuring efficient drainage. Black liquor is introduced via liquor guns that spray droplets of 2-3 mm diameter at velocities of 100-120 m/s, forming a wide sheet for uniform distribution. The firing rate is controlled to balance bed growth and prevent blackout, with higher solids content enhancing stage efficiency but requiring precise adjustments.

Historical Development

Early Recovery Systems

Before the development of dedicated recovery boilers, chemical recovery in the during the late 19th and early 20th centuries relied on inefficient methods such as flame ovens and rotary kilns, where was hand-filled, dried using wood gases, scraped, and reduced in smelt pots. These systems, including the S-S rotary oven introduced in 1912 at the Skutskär mill in , achieved only up to 50% dry solids concentration and recovered less than 60% of the pulping chemicals, resulting in significant losses primarily due to incomplete sodium and recapture, high , and operational constraints like and low reduction rates. processes, integral to regenerating caustic chemicals, further exacerbated inefficiencies by limiting overall recovery capacity to 70–75 tons of dry solids per day and contributing to substantial environmental emissions, including 25–35% of mill and notable releases from incomplete reactions. The invention of the recovery boiler in the late 1920s marked a pivotal advancement, credited to G.H. Tomlinson, whose design integrated with chemical recovery in a single unit, patented by in 1929. The world's first kraft recovery boiler, a Tomlinson-type unit, was installed that year at the Canada Paper Company's Windsor Mills in , though it featured refractory-lined furnace walls prone to rapid deterioration. This initial design prioritized chemical reclamation over energy production, operating at low solids content of 40–50% and generating at pressures below 2.5 MPa and temperatures under 300°C. A milestone came in 1934 with the introduction of water-cooled furnace walls in a unit at Windsor Mills in , , replacing linings to mitigate , enhance , and allow safer operation at higher temperatures while maintaining the small-scale capacity of approximately 20 tons of dry solids per day. Early recovery boilers faced numerous operational challenges that limited their reliability and scale, including capacities under 100 tons of dry solids per day, necessitating multiple small units per mill to meet production needs. Low solids firing led to frequent plugging in and systems, causing weekly shutdowns in some installations, such as those in during the 1930s and 1940s. Explosions were a recurring due to smelt-water reactions in the corrosive lower furnace environment, compounded by inadequate air distribution and tube degradation from sulfidation. Despite these issues, the focus remained on recovering inorganic chemicals for reuse in the kraft cycle, with steam generation as a secondary benefit, as evidenced by early units like the 160 tons per day at Korsnäs in in 1943. These foundational designs laid the groundwork for later improvements in efficiency and safety.

Major Technological Advancements

In the 1950s and 1960s, recovery boiler technology shifted toward higher operating pressures exceeding 6.5 MPa and steam temperatures around 400°C to enhance energy recovery from black liquor combustion. This period also saw the introduction of composite tubing, featuring AISI 304L stainless steel cladding over carbon steel bases, which improved corrosion resistance in the harsh, sulfide-rich environment of the boiler. A notable milestone was the delivery of the first ANDRITZ recovery boiler in 1952 to the Lohja mill in Finland, with a capacity of 110 tons of dry solids per day (tds/d), marking an early step in commercial scaling. During the and , advancements focused on safety and throughput, including the adoption of single-drum designs starting in 1984, which simplified construction and reduced explosion risks compared to multi-drum configurations. solids content increased to 70-75%, enabling more efficient and higher generation at pressures up to 80 bar and temperatures of 480°C. Boiler capacities grew to around 2000 tds/d by the late , supporting larger pulp mills without proportional increases in footprint. The and brought further refinements in fuel preparation and materials, with high-solids firing at 80% dry content becoming standard, which minimized water evaporation needs and reduced (SO2) emissions to near-zero levels by promoting complete reduction in the furnace. The largest units reached 4450 tds/d by 2004, exemplifying scaled designs for modern kraft mills. Advanced alloys, such as TP347HFG , were widely adopted to withstand elevated temperatures and corrosive gases, extending component life. In the 2010s, the push for mega-boilers culminated in units like the 12,000 tds/d installation at PT OKI Pulp & Paper Mill in Indonesia in 2016, the world's largest at the time, optimizing chemical recovery and power generation. A milestone was the startup in 2021 of a 13,000 tds/d boiler at the Bracell mill in Lençóis Paulista, São Paulo State, Brazil, which became the world's largest recovery boiler at the time, reflecting ongoing capacity expansion. Digital controls integrating Internet of Things (IoT) sensors and artificial intelligence (AI) algorithms emerged for real-time optimization of combustion and heat transfer, while improved heat recovery designs doubled power output relative to earlier generations. Overall, these advancements drove a dramatic increase in recovery boiler capacity from under 400 tds/d in the to over 10,000 tds/d by the 2010s, accompanied by energy efficiency gains from approximately 60% to 90% through better control and materials.

Design and Components

Black Liquor Properties and Preparation

Black liquor, the spent cooking liquor from the kraft pulping process, is a complex mixture essential for and chemical in recovery boilers. On a dry solids basis, it typically comprises 45-55% organic components, primarily (accounting for 30-40% of the organics) and reduced sugars or carbohydrates, alongside 45-55% inorganic salts such as sodium (around 20%), compounds (2-4%), and (1-2%). These inorganics originate from the pulping chemicals, while the organics derive from the degraded wood components, making black liquor a renewable with significant calorific potential. Key physical and chemical properties of influence its handling and combustion efficiency. The higher heating value (HHV) on a dry basis ranges from 12 to 15 MJ/kg, though this effective value decreases with moisture content; at 65% , the liquor delivers approximately 12-15 MJ/kg overall due to the water fraction. Rheologically, exhibits pseudoplastic behavior, with increasing sharply with concentration—typically 100-1000 cP at 80% at standard temperatures (around 100-120°C)—which can hinder pumping and spraying. is mitigated through , such as direct contact with hot gases at 300-400°C in , reducing it by breaking down organic polymers and improving flow characteristics. -derived tends to be more viscous than hardwood-derived due to higher content, while levels (2-4% of dry ) show minor variations, with liquors often containing slightly higher effective from extractives. Preparation of black liquor for boiler firing involves concentrating the weak (initially 15-20% solids) through multi-stage to achieve suitable solids content. Multiple-effect , often using falling-film or rising-film designs, progressively increase solids to 65-80% by utilizing economy ratios up to 5:1, removing efficiently while minimizing use. For final concentration to 80-90% solids, direct-contact expose the to hot boiler flue gases, further and recovering , though this step requires careful control to avoid . The prepared is then sprayed into the boiler via liquor guns, which must be cleaned every 4-8 hours to prevent plugging from dried residues, ensuring consistent firing. Higher solids content in significantly enhances performance by maximizing and minimizing excess . Firing at up to 90% solids can boost production by approximately 7% compared to 65-70% solids, as less is lost to vaporizing , while also reducing emissions through improved completeness and lower volumes. This impact is quantified by the input : Q=m⋅HHV⋅(1−f)Q = m \cdot \text{HHV} \cdot (1 - f) where QQ is the heat release, mm is the mass flow rate of , HHV is the higher heating value on a dry basis, and ff is the fraction (solids fraction = 1−f1 - f). Thus, increasing the solids fraction directly amplifies QQ, supporting higher throughput and in the recovery process.

Key Structural Components

The furnace serves as the primary in , typically measuring 25-35 in with a base area of approximately 100-200 m² to accommodate the of solids at rates up to 5000 tons per day. It features walls constructed from water-cooled tubes that form a gas-tight enclosure, preventing leakage while absorbing radiant heat, and includes a refractory-lined base to withstand the high-temperature char bed formed during . The furnace operates under a slight negative pressure, ranging from -0.5 to -2.5 kPa, to ensure safe containment of gases and minimize explosion risks. Superheaters are positioned above the furnace to further elevate the temperature of saturated steam, producing superheated steam at 480-515°C for efficient power generation or process use, with a typical projected heat transfer area of about 3 m² per ton of dry solids per day. These components utilize wide-spaced tube arrangements made from corrosion-resistant alloyed steels, such as Sanicro 28, to handle the aggressive flue gas environment while maximizing heat absorption. However, superheaters are susceptible to fouling from ash deposits, which can reduce efficiency and necessitate periodic cleaning. The boiler bank, located downstream of the superheaters, consists of convection tube sections with spacing between tubes of 164-230 mm, where saturated steam is generated as flue gases transfer sensible heat to boiling water. Following the boiler bank, economizers feature closer tube spacing of 110-180 mm and preheat incoming feedwater to 250-300°C, enhancing overall by recovering residual heat from the flue gases before they exit to emission control systems. Modern recovery boilers typically employ a single , 10-15 meters in length, positioned at the top of the boiler bank to separate steam from water via natural circulation, with capacities supporting steam production rates of 200-800 tons per hour depending on boiler size. At the furnace base, 4-8 cooled smelt spouts per boiler direct the discharge of molten inorganic smelt at temperatures of 700-900°C into a dissolving below, where the smelt is quenched and dissolved in to form green liquor for chemical recovery. These spouts are water-cooled to prevent structural degradation and are designed for safe, controlled flow to avoid blockages. To combat in high-risk areas like the lower furnace and floor tubes, composite tubes are widely used, featuring an inner layer of 304L bonded to an outer sheath for structural integrity and resistance to the corrosive environment. Routine thickness monitoring of these tubes occurs every six months to detect or cracking early and ensure operational .

Air Systems and Controls

Recovery boilers employ sophisticated multilevel air delivery systems to ensure complete of while minimizing emissions and optimizing chemical reduction. These systems introduce air at multiple elevations within the furnace to stage the process, promoting efficient burning of char, volatiles, and residual gases. Primary air, typically comprising 25-60% of the total , is directed at the char level to maintain a that facilitates the recovery of cooking chemicals. Secondary air, injected above the char , targets the volatile gases released during devolatilization, while tertiary air enters higher in the furnace to complete oxidation of unburned species. The total is approximately 3-4 kg per kg of dry solids, balancing stoichiometric requirements with excess oxygen for stable operation. Vertical air firing configurations, featuring air ports at multiple heights such as 4-6 levels, enable staged combustion that enhances mixing and reduces peak flame temperatures. This approach significantly lowers nitrogen oxide (NOx) formation by limiting oxygen availability in early combustion stages. Automated controls, including adjustable dampers on air ports, maintain precise flow distribution and furnace draft. Oxygen sensors monitor flue gas composition, typically targeting 3-6% excess oxygen to prevent incomplete combustion while avoiding excessive air that could cool the furnace. Flue gas recirculation may be incorporated to further dilute oxygen and temper temperatures, and liquor guns oscillate to ensure even black liquor distribution across the furnace cross-section, promoting uniform combustion. The emission impacts of these air systems are notable, with staged air delivery achieving low levels of 0.8-1.8 kg per air-dried ton (ADt) of pulp through improved mixing and . Total reduced sulfur (TRS) emissions approach zero when air mixing ensures thorough oxidation of sulfur compounds. Advancements in air systems include preheating combustion air to 200-300°C using dedicated air heaters, which boosts stability and efficiency without increasing fan power demands. (CFD) modeling is widely used to optimize port design and air jet trajectories, tailoring systems to specific geometries and characteristics.

Advanced Recovery Boilers

High Temperature and Pressure Designs

High temperature and pressure designs in recovery boilers represent a significant evolution aimed at enhancing and power generation within the kraft pulping process. These designs operate at elevated steam parameters to improve thermodynamic , allowing for greater production and electricity output from combustion. Modern units typically generate at pressures up to 10.3 MPa (1500 psi) and temperatures of 515°C, enabling higher cycle efficiencies compared to earlier low-pressure configurations. Furnace exit gas temperatures in these advanced boilers range from 1000°C to 1200°C, facilitating more complete combustion and heat transfer while necessitating robust materials to withstand thermal stresses. To accommodate such conditions, design modifications include thicker furnace walls, often 50-100 mm in effective thickness for pressure containment and corrosion resistance, and the use of advanced alloys like TP347HFG stainless steel for superheater tubes and Alloy 625 for critical high-temperature components. Additionally, single-drum configurations have become standard for high-pressure stability, eliminating mud drums and incorporating external downcomers to reduce corrosion exposure and improve overall structural integrity. These adaptations yield substantial benefits, including power generation that can meet 20-30% of a pulp mill's electrical needs through integrated turbines, and overall efficiencies of 85-90%, partly achieved by optimized tube spacing that minimizes fouling and maintains rates. However, operating at these extremes introduces challenges, such as accelerated corrosion via sulfidation from sulfur-rich flue gases, which demands specialized coatings and monitoring. Stability also requires black liquor solids content exceeding 80% to prevent instability and ensure safe smelt formation. Key milestones in this development include the installation of the first 10 MPa recovery boiler in 1955, marking the shift toward high-pressure operation for improved . By the 2000s, units operating at 105 bar and 525°C became common, further boosting power output and setting the stage for contemporary supercritical designs.

Safety Features and Risk Management

Recovery boilers, critical to the kraft pulping , face significant hazards due to the high-temperature of , which produces molten smelt containing sodium and compounds. The primary risk is smelt-water explosions, occurring when water contacts the molten smelt at temperatures around 1700–1800°F (930–980°C), leading to rapid generation and a that can cause severe structural damage to the furnace. These explosions account for approximately 75% of all recovery boiler incidents, with tube leaks responsible for over half of such events, as water from boiler mixes with smelt. Other major risks include combustible gas buildups, such as (H₂) and (CO) from incomplete or auxiliary fuel systems, which can ignite and propagate explosions, and corrosion-induced failures like floor tube cracking, which compromise the boiler's integrity and allow water ingress. To mitigate these dangers, recovery boilers incorporate multiple engineered safety features. Automatic shutdown systems, governed by interlocks, trigger an Emergency Shutdown Procedure (ESP) in response to critical conditions such as low drum water levels or high furnace pressure exceeding 80% of the design pressure, halting and auxiliary fuel firing while minimizing air supply. Rapid drain systems are integral to the ESP, evacuating to a safe level of about 8 feet within approximately 20 minutes to prevent water from reaching the smelt bed during emergencies. Flame scanners monitor burners to detect and prevent ignition of unburned gases, reducing the risk of combustible gas explosions from auxiliary fuels or gases. Operational protocols further enhance . Furnace pressure is maintained below that of the surrounding boiler house, typically at -2.5 kPa or less relative to , to contain any potential explosions and prevent air ingress that could buildups. Smelt spouts are protected with jackets or cooling systems operating at low (using 140–150°F inlet , not exceeding 180°F outlet) to avoid leaks that could introduce to the smelt. Operator training adheres to standards outlined in NFPA 85, emphasizing recognition of tube leaks, interlock functions, and emergency responses, with regular programs ensuring compliance and hazard awareness. Historically, recovery boiler incidents peaked in the mid-20th century, with explosion rates of about 4 per year in the –1970s, declining to 2 per year in the 1980s and 1 per year in the due to advancements like single-drum designs and improved interlocks, resulting in roughly 20–30 major s from 1930 to 2000 across ; over 150 smelt-water incidents have been documented overall. As of 2022, BLRBAC sessions reported no smelt-water s, indicating sustained low incident rates. Post-1980s improvements reduced explosion frequency by over 90%, and in the 2020s, emphasis has shifted to to further minimize risks. Ongoing monitoring includes ultrasonic thickness measurements for detecting in tubes and floors, allowing early identification of thinning walls, and gas analyzers to detect combustible gases or low black liquor solids (below 58%) for proactive intervention.

Environmental and Modern Aspects

Emission Control and Regulations

Recovery boilers in the kraft pulping process emit several key pollutants, including (SO₂) at levels typically ranging from 0.1 to 1.5 kg per air-dried ton (ADt) of pulp, though best available techniques () can reduce this to 0.05–0.5 kg/ADt when operating with solids content exceeding 80%. emissions generally fall between 0.8 and 1.8 kg/ADt, primarily as formed during high-temperature combustion. Total reduced sulfur (TRS) compounds, such as (H₂S), are emitted at near-zero levels when solids exceed 75%, due to efficient oxidation in the furnace. Dust or particulate matter emissions range from 0.05 to 0.3 kg/ADt, consisting mainly of inorganic salts like . Emission controls focus on post- and process optimizations to meet stringent standards. Electrostatic precipitators (ESPs) are widely employed for removal, achieving over 99% efficiency by charging and collecting fine particulates from flue gases. reductions are accomplished through air staging, which involves injecting primary and secondary air at optimized levels to create fuel-rich zones that minimize thermal formation, often reducing emissions by 20-30%. High solids content inherently lowers SO₂ and TRS by promoting complete and chemical reduction, with additional scrubbing systems capturing residual sulfur compounds. Regulatory frameworks worldwide mandate low emission levels from recovery boilers to protect air quality. Under the Industrial Emissions Directive 2010/75/, SO₂ limits for processes are set at less than 150 mg per megajoule (MJ) of net input, with BAT-associated emission levels (BAT-AELs) for recovery boilers specifying 5-25 mg/Nm³ (at 6% O₂) as annual averages. In the United States, the Agency's New Source Performance Standards (NSPS) under 40 CFR Part 60 Subpart BB cap TRS at 5 ppmv for kraft recovery furnaces equipped with direct-contact evaporators. emissions from recovery boilers are regulated under National Emission Standards for Hazardous Air Pollutants (NESHAP, 40 CFR Part 63 Subpart MM) and state-specific limits, rather than a uniform federal NSPS value. The 2017 BAT conclusions for the pulp and sector remain in effect as of 2025, with broader policies in both regions aligning with net-zero goals through enhanced monitoring and efficiency measures. Compliance is ensured through continuous emission monitoring systems (CEMS), which provide real-time data on SO₂, , TRS, and dust concentrations, flow rates, and opacity, as required by EPA and directives for major sources. Ash further minimizes waste, as fly ash from ESPs is returned to the liquor cycle, recovering sodium and compounds and reducing solid waste by up to 50%. The closed-loop chemical recovery enabled by recovery boilers significantly reduces overall mill , over 90% of pulping chemicals and thereby decreasing discharge by approximately 90% compared to non-recovery processes.

Recent Innovations and Efficiency Improvements

In the 2020s, digitalization has transformed recovery boiler operations through the integration of AI and IoT technologies for and process optimization. Advanced control systems, such as ANDRITZ's Metris platform, enable real-time and automated adjustments to firing rates, minimizing by detecting potential issues early and improving overall reliability. Similarly, AI-driven solutions from providers like Aperio validate data to prevent upsets, reducing unplanned shutdowns caused by sensor failures in high-temperature environments. These tools have been adopted in modern installations, with IoT sensors facilitating up to 20% reductions in operational disruptions through proactive alerts. Advancements in have enhanced component durability, particularly in corrosive zones like . Nickel-based alloys, such as Alloy 825 and Alloy 625, are increasingly used for tubing due to their superior resistance to and sulfidation in high-temperature, reducing gas environments. These alloys allow for higher steam parameters without frequent replacements, extending service life in boilers operating above 480°C. While for custom components like liquor ports remains exploratory, composite tube designs combining stainless steels with carbon bases continue to mitigate thermal fatigue. Efficiency improvements stem from innovations in evaporation and cleaning processes. High-solids evaporators now concentrate to 80-85% dry solids, boosting generation by approximately 7% compared to traditional 65-70% systems and achieving chemical recovery rates up to 97%. Automated sootblowing systems, like Valmet's Sootblowing Optimizer, use data analytics to target selectively, reducing consumption for cleaning and maintaining by minimizing losses from ash deposits. Hybrid configurations incorporating condensing back-end further elevate performance, recovering low-grade heat to generate 1-6% additional while displacing low-pressure demands. Post-2020 capacity expansions have scaled recovery boilers to support larger pulp mills, with units like Nippon Paper Industries' boiler ordered in 2025 handling 2,200 tons of dry solids per day and designed to contribute significantly to a mill's power needs through optimized . Expansions at facilities such as the Äänekoski bioproduct mill have integrated higher-throughput designs, enhancing processing without proportional increases in footprint. By 2025, recent designs emphasize , including co-firing capabilities to reduce CO2 emissions by blending agro-residue liquors with for cleaner combustion. TAPPI studies highlight these updates, such as advanced back-end recovery, as key to achieving overall efficiencies nearing 95% in integrated systems.

Future Prospects

Alternative Technologies

One prominent alternative to traditional recovery boilers is black liquor gasification (BLG), which converts black liquor into syngas primarily composed of hydrogen (H₂) and carbon monoxide (CO) for use in chemical production or power generation, thereby avoiding the formation of molten smelt associated with boiler combustion. The Chemrec process, developed in Sweden since the 1960s, represents one of the most advanced BLG technologies at pilot scale, employing entrained-flow gasification at high temperatures above the melting point of inorganic salts to produce clean syngas suitable for downstream applications like methanol or dimethyl ether (DME) synthesis. Key pilots include Chemrec's pressurized oxygen-blown unit in Piteå, operational in the 2000s with a capacity of 20 tons per day of black liquor solids, yielding about 3 MWth of syngas, and an integrated demonstration at Domsjö Fabriker that achieved over 4 tons of DME per day in 2012 before project withdrawal. As of 2025, BLG technologies like Chemrec remain at pilot or demonstration scale, with limited commercialization due to economic barriers. BLG offers several advantages over recovery boilers, including significantly lower emissions of nitrogen oxides (NOx) and sulfur dioxide (SO₂) due to the oxygen-blown process and clean output, as well as enhanced chemical recovery rates approaching 99% for pulping chemicals like sodium and compounds. It also enables flexible integration with biorefineries for production, potentially boosting overall energy efficiency to 85-90% when combined with (IGCC) systems, compared to 65-70% in setups. Despite these benefits, BLG faces challenges such as high capital expenditures, estimated at over $200 million for chemical recovery components alone in mid-scale plants, and scaling difficulties for facilities below 1,000 tons of dry solids per day, limiting adoption to larger mills. Pilots like Domsjö have demonstrated around 50% in integrated fuel production, underscoring the need for further optimization to match boiler efficiencies. Other alternatives include , which processes in a bed of inert material for controlled combustion and chemical recovery, offering potential reductions in through lower temperatures but requiring sub-stoichiometric conditions to generate low-calorific gas. Direct causticization bypasses by reacting in with agents like to directly form , simplifying recovery and reducing energy use, though it demands precise control to achieve high conversion rates above 90%. Given the typical 40-year service life of recovery boilers, transitions to alternatives are gradual, with hybrid systems combining boilers and gasifiers projected to emerge by 2030 for phased integration and risk mitigation.

Sustainability Challenges

Recovery boilers in the face significant challenges related to goals, primarily due to CO2 emissions generated during startup and auxiliary operations that rely on for ignition and steam generation. While the primary fuel, , is a renewable byproduct that enables near CO2-neutral operation during normal , these intermittent fossil fuel uses contribute to the sector's overall footprint. Emerging 2025 regulations and industry commitments are intensifying pressure for reductions, with the American Forest & Paper Association setting a target of 50% GHG emissions cuts by 2030 from 2005 levels across Scope 1 and 2 sources, including recovery processes. Carbon capture technologies, such as bioenergy with (BECCS), are positioned as key pathways, potentially enabling negative emissions by capturing up to 200–400 million metric tons of CO2 annually from flue gases by 2050. Economically, retrofitting recovery boilers for advanced emissions controls poses substantial barriers, with costs for replacing or upgrading these large-scale units often exceeding $100 million per due to the need for specialized corrosion-resistant materials and compliance upgrades. Volatile prices further exacerbate challenges by impacting the profitability of excess power sales from , as mills with flexible recovery operations must navigate fluctuating electricity markets to optimize revenue from . Technically, the aging fleet— with a age exceeding 25 years for U.S. recovery —requires frequent maintenance and increases vulnerability to and efficiency losses, compounded by global disruptions for high-alloy steels essential for tubes and components. Regulatory and social pressures add layers of complexity, as 2025 EPA standards enforce tighter deadlines for hazardous air pollutant reductions from industrial boilers, including recovery units, amid broader pushes for zero-emission operations. General OSHA updates in 2025, including increased penalties and protocols, may heighten compliance costs for industrial boiler operations. Community concerns over odors and localized emissions from recovery processes, often sulfur-based and linked to combustion, have prompted investigations and abatement demands near pulp mills, as seen in cases where strong sulfur-like smells affected resident health. Despite these obstacles, pathways toward exist through lifecycle assessments demonstrating that recovery in boilers yields 80% to 97% lower GHG impacts compared to alternatives like landfilling waste, by avoiding and enabling . Ongoing R&D explores use in boiler startups or co-firing to further decarbonize operations and enhance flexibility. However, 2025 DOE funding cuts to initiatives pose challenges to advancing these technologies.

References

  1. https://www.tappi.org/content/events/08kros/manuscripts/1-1.pdf
  2. https://www.babcock.com/home/about/resources/learning-center/a-numerical-modeling-example
  3. https://www.netl.doe.gov/research/Coal/energy-systems/gasification/gasifipedia/blackliquor
  4. https://www.sciencedirect.com/topics/engineering/recovery-boiler
  5. https://www.tappi.org/content/Events/19PEERS/19PEE59.pdf
  6. https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1744&context=etd
  7. https://lutpub.lut.fi/bitstream/handle/10024/111915/KRBFull.pdf?sequence=2&isAllowed=y
  8. https://www.tappi.org/content/events/08kros/manuscripts/4-2.pdf
  9. https://ifrf.net/research/handbook/how-does-black-liquor-burn/
  10. https://www.babcock.com/assets/PDF-Downloads/Steam-Generation/E101-3194-PR-Boilers-Babcock-Wilcox.pdf
  11. https://lutpub.lut.fi/bitstream/handle/10024/104722/Recovery%2520Boiler%2520History%2520and%2520Future%2520Vakkilainen.pdf?isAllowed=y&sequence=2
  12. https://p2infohouse.org/ref/29/28987.pdf
  13. https://www.valmet.com/globalassets/media/downloads/white-papers/power-and-recovery/evaporator_recovery_boiler_energy_whitepaper.pdf
  14. https://www.researchgate.net/profile/Esa_Vakkilainen/publication/279871546_Kraft_recovery_boilers_-_Principles_and_practice/links/561a6dba08ae6d1730898b68/Kraft-recovery-boilers-Principles-and-practice.pdf
  15. https://www.sciencedirect.com/science/article/abs/pii/S0308016120301381
  16. https://www.andritz.com/spectrum-en/latest-issues/issue-37/oki
  17. https://www.andritz.com/spectrum-en/magnum-sized-recovery-boilers
  18. https://www.ieabioenergy.com/wp-content/uploads/2013/10/Black-Liquor-Gasification-summary-and-conclusions1.pdf
  19. https://ifrf.net/research/handbook/what-are-the-properties-of-black-liquor/
  20. https://www.tappi.org/content/events/08kros/manuscripts/3-1.pdf
  21. https://www.tappi.org/content/events/08kros/handouts/3-1.pdf
  22. https://www.epa.gov/sites/default/files/2020-10/documents/c10s02.pdf
  23. https://ippta.co/wp-content/uploads/2022/08/Dr.-Dixit-1.pdf
  24. https://lutpub.lut.fi/bitstream/handle/10024/111915/KRBFull.pdf?sequence=2
  25. https://www.researchgate.net/publication/241793129_Recovery_boiler_superheater_corrosion_field_study
  26. https://www.valmet.com/globalassets/media/downloads/white-papers/power-and-recovery/xxl_size_recovery_boilers_whitepaper.pdf
  27. https://www.tappi.org/content/Events/19PEERS/19PEE44.pdf
  28. https://www.tappi.org/content/events/08kros/handouts/5-4.pdf
  29. https://patents.google.com/patent/US5715763A/en
  30. https://www.tappi.org/content/events/08kros/handouts/4-5.pdf
  31. https://www.sciencedirect.com/science/article/pii/S1359431122009693
  32. https://studylib.net/doc/10399821/no-emissions-from-kraft-pulp-mills
  33. https://www.nationalboard.org/index.aspx?pageID=164&ID=231
  34. https://ippta.co/wp-content/uploads/2022/08/P.S.Ravishangar-1.pdf
  35. https://www.tappi.org/content/events/08kros/manuscripts/4-1.pdf
  36. https://www.nipponsteel.com/product/catalog_download/pdf/P008en.pdf
  37. https://www.babcock.com/home/products/chemical-process-recovery-pr-boilers
  38. https://www.valmet.com/globalassets/media/downloads/white-papers/power-and-recovery/high_power_generation_recovery_boilers_whitepaper.pdf
  39. https://ippta.co/wp-content/uploads/2021/01/2013_Issue_1_IPPTA_Articel_13.pdf
  40. https://www.sciencedirect.com/science/article/pii/S2352550921003122
  41. https://www.tappi.org/content/events/08kros/manuscripts/4-6.pdf
  42. https://blrbac.net/wp-content/uploads/2020/10/recommended-good-practice-safe-firing-of-black-liquor-in-blrbs-april-2016.pdf
  43. https://www.nfpa.org/product/nfpa-85-code/p0085code
  44. https://blrbac.net/wp-content/uploads/2022/07/meeting-minutes-s2022-final.pdf
  45. https://eippcb.jrc.ec.europa.eu/sites/default/files/2019-11/PP_revised_BREF_2015.pdf
  46. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/subpart-BB
  47. https://www.epa.gov/stationary-sources-air-pollution/chemical-recovery-combustion-sources-kraft-soda-sulfite-and
  48. https://www.andritz.com/pulp-and-paper-en/pulp-production/automation-and-digitalization-pulp-en/automation-digitalization-solutions-recovery-boiler
  49. https://aperio.ai/case-study-pulp-and-paper-boiler-recovery/
  50. https://www.linkedin.com/pulse/recovery-boilers-market-analyzing-key-trends-industry-drivers-4khdf
  51. https://nickelinstitute.org/media/8da582ed07d9a7b/nickelpub11025_pulppaperbiomass_rev2022.pdf
  52. https://www.researchgate.net/figure/High-dry-solids-evaporators-Courtesy-of-Andritz_fig2_267565045
  53. https://www.valmet.com/automation/applications/process-optimization/pulp/recovery-boiler-sootblowing-optimizer/
  54. https://www.tappi.org/publications-standards/tappi-journal/home/2023/mar/recovery-boiler-back-end-heat-recovery-tappi-journal-march-2023/
  55. https://www.pulpapernews.com/20250902/16965/nippon-paper-industries-invests-chemical-recovery-boiler
  56. https://www.valmet.com/insights/articles/pulp/chemical-pulping/the-next-generation-bioproduct-mill/
  57. https://www.researchgate.net/publication/395591118_Recent_Developments_in_Recovery_Boiler_Design_and_Operation
  58. https://www.ieabioenergy.com/wp-content/uploads/2022/01/Gasification_case_story_01.pdf
  59. https://www.sciencedirect.com/science/article/abs/pii/S0960852410008242
  60. https://www.researchgate.net/publication/288538740_A_cost-benefit_analysis_of_black_liquor_gasification_combined_cycle_systems
  61. https://onlinelibrary.wiley.com/doi/abs/10.1002/cjce.5450650515
  62. https://pubs.acs.org/doi/abs/10.1021/ie0342457
  63. https://paper360.tappi.org/2018/12/14/are-north-american-kraft-pulp-mills-at-risk/
  64. https://www.afandpa.org/news/2023/reducing-greenhouse-gas-emissions-paper-manufacturing
  65. https://carbongeocapture.com/manage-carbon-emissions/pulp-and-paper/
  66. https://www.bangordailynews.com/2013/12/13/news/recovery-boilers-can-cost-100-million-to-replace-expert-says/
  67. https://www.sciencedirect.com/science/article/pii/S2949839224000804
  68. https://downloads.regulations.gov/EPA-R09-OAR-2022-0420-0003/attachment_11.pdf
  69. https://www.theseus.fi/bitstream/10024/857181/2/Heiskanen_Taneli.pdf
  70. https://nbwinc.com/blog/july-2025-industrial-boiler-industry-recap/
  71. https://www.osha.gov/news/newsreleases/osha-national-news-release/20250714
  72. https://www.atsdr.cdc.gov/HAC/pha/PortTownsendPaperCorp/Port-Townsend-HC-508.pdf
  73. https://www.ieabioenergy.com/wp-content/uploads/2013/10/Using-a-LCA-approach-to-estimate-the-net-GHG-emissions-of-bioenergy.pdf
  74. https://nrri.umn.edu/news/doe-82m-industry-innovation
  75. https://nbwinc.com/blog/summer-2025-global-developments-in-industrial-boiler-technology/
Add your contribution
Related Hubs
User Avatar
No comments yet.