BY Ankit Jain(09120008) B.Tech P&P 3rd year

Indian Institute of Technology, Roorkee

Black liquor (BL), the major by-product of the Kraft

process for the production of pulp is one of the most important industrial fuels. It consists of the remaining substances after the digestive process where the cellulose fibres have been cooked out from the wood. The black liquor contains 15–17% solids, consisting of dissolved organics from the wood and spent pulping chemicals. A typical pulp mill uses several hundred tonnes of inorganic chemicals per day. For both environmental and economic reasons, it is desirable to recover and recycle these chemicals. Black liquor has a high organic content from the dissolved lignin and carbohydrates, and in concentrated form (>60% solids) it burns in a manner similar to heavy oil.

Introduction

Black liquor incineration is carried out in a recovery boiler.

A recovery boiler serves three critical functions in the chemical process for producing wood pulp. The recovery boiler acts as a chemical reactor to produce sodium carbonate and sodium sulphide; it is a fuel combustor and steam generator; and it destroys the dissolved organic matter and thus eliminates an environmental discharge..

Black liquor burns in overlapping stages similar to other solid and liquid fuels. The four main stages are drying, pyrolysis, char burning, and smelt coalescence and reactions. The primacy of liquor burning over steam generation is one of the major differences between recovery boilers and fossil-fuel power boilers.

Black Liquor Incineration

Descriptions of the recovery processes and boiler operating strategy vary, but there are two features which are universally agreed upon:

Water from any source must not contact the char bed

Wet black liquor must not reach the char bed.

The following discussions discuss in a bit detail about the process of black liquor combustion which includes the types of burning that takes place, etc.; the chemistry of Black liquor combustion and the ash fusion characteristics. The ash fusion characteristics are important because by studying them one can know the melting temperature of the mixture of the pure compounds and hence can determine the possible temperatures where the smelt may stick to the surface and can find ways to control problems like deposition and fouling.

As a fuel black liquor is unique in that no other can match

its contents of ash and moisture. The organic material must provide sufficient heat for drying, ignition, melting, and chemical reduction process. The heating value of black liquor is governed by the presence of lignin (organic contents) having value in the range 14-16 MJ/kg solids from most of the convention pulping processes. Liquor burning takes place in a water wall furnace.

The furnace is rectangular in cross-section and extends from the hearth to the bullnose. The walls of the furnace are boiler tubes, which are part of the steam generation circuit. Heat recovery sections include a superheater, boiler bank, and economizer

Process of B.L. Incineration

In some units, water-cooled screens are used ahead of the superheater. A char pile or bed of partially burnt liquor normally forms on the hearth. Different hearth constructions are used by different boiler manufacturers, but all drain smelt out through water-cooled spouts into a dissolving tank.

Air and black liquor are introduces separately in a recovery boiler. Three levels of air entry, referred to as primary, secondary, and tertiary, are now standard on recovery boilers, although there are still some units operating with two levels of air. Primary and secondary air are introduced below the liquor guns and supply the combustion air to the bed and the lower furnace. The amount is normally slightly below the stoichiometric amount for complete combustion of black liquor.

Tertiary air is supplied above the liquor guns and is used to complete the combustion process. Thus air is an important variable in controlling the furnace operations. Usually air is preheated to a temperature of 150-200 C.

The burning of black liquor in a recovery boiler can take place in three different modes:

In-flight burning

Bed burning

Burning on the walls

Burning involves chemical reaction between oxygen in air and the black liquor, so the location of burning is strongly dependant on the air injection and liquor spray patterns.

Combustion products are carbon dioxide, water,

sodium carbonate and sodium sulfide.

The five major elements are sodium, sulfur, carbon, hydrogen and oxygen and two minor elements potassium and chlorine

Water also enters the black liquor

The four major constituents of flue gas are nitrogen, oxygen, carbon dioxide, and water vapors

Basic Burning Chemistry

Although burning of black liquor has many similarities to

the burning of other fuels, the chemistry of the process is more complex because of the chemical recovery function. In addition to the elements of carbon, hydrogen, and oxygen that are common in most fossil fuels, black liquor contains substantial quantities of alkali (sodium and potassium) and sulphur

Combustion products are not only the gases carbon dioxide and water, but also the recovered pulping chemicals, sodium carbonate and sodium sulphide. Important chemical reactions include the reduction of sulphate, formation of fume particles, and sulphur release and recapture reactions.

Burning chemistry

In a recovery boiler, black liquor burning involves the reaction of black liquor solids with oxygen in combustion air to produce flue gases and smelt.

Four chemical steps take place during the burning of black liquor. These are:

Pyrolysis

Volatiles burning

Char burning

Inorganic oxidation

Pyrolysis is a gradual series of irreversible degradation

reactions that black liquor solids undergo as their temperature is increased.

The pyrolysis reactions produce a combustible gas and a solid carbonaceous char.

Black liquor solids pyrolysis gases + char –heat (from flue gases)

The pyrolysis gases include H2, CO, CH4, TRS,CO2,H2O, and other high molecular weight hydrocarbons and organic compounds

Char– Na2CO3+Na2S+Na2SO4 + fixed carbon

Pyrolysis

The inorganic constituents that make up the smelt

are formed by the decomposition of sodium organic compounds during pyrolysis

Greater the extent of gas production by pyrolysis, the more readily the liquor burns

Primary mechanism responsible for the release of sulfur gases within the furnace

Under certain conditions, explosive mixtures of pyrolysis gases and air can lead to recovery boiler explosions

Importance of Pyrolysis

Initiated when the temperature is about 200oC

Char – 20-25% fixed carbon, 75-80% inorganic

As much as 70% of the total sulfur in kraft black liquor has been converted to volatile gases

These do not require oxygen, only a hot environment.

Forms explosive mixtures when comes in contact with air.

This is generally the case with hot, blacked-out furncae.

Reaction conditions

Combustion of volatiles produced by pyrolysis

Rapid homogeneous gas phase reaction

Pyrolysis Gas (H2, CO, TRS, etc.) + O2 - H2O + CO2 +SO2

Sufficient air supply

Adequate mixing between combustibles and air

Temperatures (760-815oC)

Mixing is the critical factor

A combination of high injection velocities and a relatively large amount of air is used to achieve this

Volatiles Burning

Char consists of finely divided carbonaceous

material and inorganic salts

The average oxidation state of the sulfur compounds may be different

At the completion of pyrolysis, the char is 75% inorganic and 25% carbon

About 30-40% of the original oxygen demand and heating value of the black liquor solids is present in the char; the remainder is released with the volatiles

Char Burning

Compound Moles

Na2S 0.15

Na2SO4 0.15

Na2CO3 0.7

Fixed carbon 3

Bound hydrogen 1

Composition of char

Element %

Na 31.9

S 6.6

C 30.8

H 0.7

O 30

Conversion of the fixed carbon to CO and CO2 to permit

melting and coalescence of the smelt

Changes in the oxidation state of the inorganic sulfur compounds

C + 1/2O2 CO

C + O2 CO2

C + CO2 2CO

C + H2O H2 + CO

C + ½ Na2SO4 CO2 + 1/2Na2S

C + 1/4Na2SO4 CO + ¼ Na2S

Basic Steps

The conditions during char burning are such that the

predominant reactions by which carbon is burned are the reactions with molten sulfate

The sulfate and sulfide present can act as a catalyst for carbon burnup, with the sulfate reduced to sulfide by carbon and the sulfide subsequently reoxidized to sulfate by reaction with oxygen

Temperature of 982-1038oC Reaction b/w sulfate and carbon is faster than that

between sulfide and oxygen At high temperatures, the sulfide oxidation reaction is

mass controlled

Sulfate/Sulfide Cycle

Na2S + 2O2 Na2SO4 Exothermic reaction with heating value of 5550 Btu/lb of

sodium sulfide oxidized Tends to be inhibited as long as there is carbon present in

the char Occur readily when the smelt is exposed to oxygen after

the carbon is burned off Directly proportional to the rate of oxygen supplied Raises the smelt temperature A 5% decrease in reduction efficiency caused by

reoxidation can raise the smelt temperature by as much as 111oC

Inorganic Oxidation

Fume or dust is the name given to very fine particles produced

by vaporization of sodium(and potassium) in the furnace

The chemical reactions which produce sodium vapor are endothermic, and thus fume production tends to be self-limited in the absence of a heat source

The sodium carbonate dust plays an essential role in sulfur capture

Large amount of dust requires an expensive precipitator for collecting and recycling and can also contribute to fouling or plugging of the gas passages

The chlorides and potassium can form low melting eutectic mixtures and aggravate plugging or corrosion problems

Fume(dust) formation

Liquor burning Na(vapour)

2Na + 1/2O2 +CO2 Na2CO3(Fume)

Na2CO3 + SO2 +1/2O2 Na2SO4(Fume) + CO2

Nacl (Liquid) Nacl (vapour) – heat

NaOH (liquid) NaOH (Vapour) – heat

2NaOH + CO2 Na2CO3(Fume) + H2O

Direct contact between molten smelt and metal

Corrosive attack by elemental sulfur generated by Na2S-Co2 or H2S-O2 combinations

Attack by sulfate, Na2SO4; acid sulfate, NaHSO4; and pyrosulfate, Na2S2O7

Acidic corrosion below the dew point of gases

Attack by condensed NaOH vapor on the cold side of nonmembrane wall units, in air port openings, and in other locations

Corrosion Reactions

Fe + S ↔ FeS

H2S +O2 ↔ ½ S+ ½ SO2 + H2O

or

Na2S +2CO2 ↔ S + Na2CO3 + CO

Corrosion rate on carbon steel is low at temperatures below 310 oC, but increases rapidly above this temperature

Corrosion from this mechanism can be controlled by keeping metal temperatures low

Sulfidation corrosion occurs at about 482 oC

For this reason composite or chromized tubes are often used for corrosion protection in the lower furnace

Sodium sulfate can become aggressive at metal temperatures above 550 oC

Chlorides or the combination of chlorides and potassium can lead to increased attack on superheaters by lowering the melting point of the slag and decreasing or eliminating the protective frozen layer

When flue gas is cooled below the due point, components such as SO2,SO3,HCl can for corrosive acids in the cold end of system including ducting, I.D. fan, precipitator.

A quote from Combustion: Fossil Power sums up the

difficulties with solid combustion by products:” Without ash, all furnaces could easily be designed on the basis of heat transfer only”

Inorganic minerals contained within fuel do not burn with the combustion process and either transport out of the boiler or deposit on boiler internals. Deposition may consist of molten and partially-molten compounds buildups on furnace tubes(slagging) or ash accumulation on convective pass super heater and reheater tubes(fouling). The volume and complexity of ash are obviously greatest in a black liquor fired unit and in large measure plants are designed around ash characteristics and removal requirements.

Ash Fusion Characteristics

Pure compounds are rarely found in deposits

Mixtures do not have a fixed melting point

Mixtures can form eutectics that can melt at much lower temperature than any of the pure compounds

Liquidus temperature is the temperature at which crystallization just begins for a given composition

The eutectic temperature is the temperature at which final solidification of the melt takes place

Characteristics of Ash

The system forms a low melting eutectic point at 40 mole% Na2S with a melting point of 763OC whereas the melting point of pure components, Na2S & Na2CO3 are 1170OC and 851OC respectively.

Part of the sodium in a smelt consisting of 66.3%

Na2CO3, 20.8% Na2S, and 12.9%Nacl was replaced by potassium(all in mole%).

The liquidus temperature decreased gradually with increased potassium content

Eutectic temperature dropped to 525OC at 5% potassium replacement and then rose gradually.

There is a maximum melting point drop of 61OC at 5% potassium replacement.

Effect of potassium

Effect of potassium

66.3% Na2CO3 20.8% Na2S 12.9%Nacl

Data on the viscosities of the various molten salts is

read from the plots of viscosity vs temperature.

They are observed to be curves which have a negative slope.

At any given temperature the viscosity of the smelt decreases with increasing sulphidities lowers the melting temperature for sulphidity less than 40%.

The viscosities of carbonate- sulphide are strong function of temperature.

Viscosity

Temperature The following figure indicates lower furnace equilibria composition for a pure kraft black liquor as a function of temperature at a fixed air fuel

ratio & sulphur to sodium molar ratio.

Excess Air vs O2 The following figure reflects the

relationship between excess air and O2 in the flue gas. It is a direct

relationship between the excess air and the O2 content of the flue gas, which is not very sensitive to the

liquor elemental analysis. The relationship can be used to estimate the excess air in an

operating unit.

As per my understanding the higher the ash fusion temperatures better will be its performance in terms of fouling and slagging in power plant boiler tubes. High alkali containing fly ashes generally do have low ash fusion temperatures. Indonesian coal which is largely used in power plants. Indonesian coal ash is generally alkali rich thereby reducing ash fusion temperature. Effect of low ash fusion temperature on pyro processing process is not yet comprehensively investigated and documented

Chemical recovery in the alkaline pulping process by Robert P.Green & Gerald Hough

HRDP on chemical recovery

Volume 5, alkaline pulping by M.J. Kocurek

References

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