Recovery

According to the document, how many times can cellulose fiber be recycled?

Approximately 7 times

What two materials are burned in a Kraft mill to generate steam and electricity?

Bark and Black Liquor

What are the two main cooking chemicals recovered in white liquor?

NaOH (Sodium Hydroxide) and Na₂S (Sodium Sulfide)

What is a "blackout" in a recovery boiler?

"An unstable condition where all or a portion of the char bed stops burning and combustion above the bed also ceases."

What is a common cause of blackouts in recovery boilers?

Low black liquor solids (typically < 65%)

What is the most reliable method for testing strong black liquor solids?

The TAPPI sand method (sample is diluted, poured into sand, and dried in an oven)

What does BLRBAC stand for and why was it formed?

"Black Liquor Recovery Boiler Advisory Committee – formed to reduce the risk of recovery boiler explosions"

What percentage of recovery boiler explosions are smelt-water explosions?

Three quarters (75%)

What is a tell-tale sign that a recovery boiler is starting to plug with ash?

An increase in ID fan speed over time

What two reactions occur in the recausticizing area?

Slaking (CaO + H₂O → Ca(OH)₂ + heat) and Causticizing (conversion of Na₂CO₃ to NaOH)

What is Causticizing Efficiency (CE) a measure of?

"The percentage of Na₂CO₃ that has been converted to NaOH – indicates completeness of the reaction"

What is the purpose of the lime kiln?

To convert lime mud (CaCO₃) back to reburned/hot lime (CaO) via the calcining reaction

What is the calcining reaction?

CaCO₃ + Heat → CaO + CO₂

What percentage of leaks in recovery boilers are found by operators on their rounds (per a 2017 BLRBAC report)?

81%

What is the sticky temperature (TSTK)?

"The temperature at which 15-20% of ash is in liquid phase – affects boiler plugging potential"

What happens during an Emergency Shutdown Procedure (ESP) for a recovery boiler?

Alarms clear personnel, fuel/black liquor flow is diverted, feedwater is stopped, and water is drained to 8 ft above the furnace floor

What are three types of explosions or hazards mentioned for recovery boilers?

1) Combustible gas explosions, 2) Smelt-water explosions, 3) Dissolving tank explosions

Why is falling film evaporation generally better than rising film?

Falling film has the benefit of gravity and performs better at lower delta T

What is the typical solids range for weak black liquor (WBL) leaving brownstock washers?

13-17% solids

What solids range is required to burn black liquor in the recovery boiler?

60-80% solids

What is the difference between Evaporator Capacity and Evaporator Steam Economy?

"Capacity = rate of water removal. Economy = amount of water removed per unit of steam applied"

What is a key safety risk when collecting black liquor samples?

It can flash when you open a valve. Use sample coolers. Also, H₂S (Hydrogen Sulfide) is present.

What are potential causes of high % Delta T in an evaporator effect?

Scale/fouling/pluggage, improper NCG venting, improper condensate removal, or liquor side flooding

What is a key safety rule when walking through recausticizing areas?

Never assume a puddle or drip is water – assume it is caustic

What is over-limiting in recausticizing and what does it cause?

Trying to push causticizing efficiency too high, resulting in unreacted Ca(OH)₂ – causes blinding of white liquor filters and slower settling rates

What unusual safety tool is mentioned for lime kiln operation?

A shotgun (used to break up large agglomerations of lime)

What are three general "nuggets of wisdom" from the document?

1) Treat operators with respect and listen to them, 2) Always notify operators when you are in the process area, 3) When it comes to instrumentation, trust but verify

How many recovery boilers were in the US as of 2022?

129 recovery boilers across 89 mills