Presented by Kimmo Peltonen - WCBLRBAC.org · We accept the challenge! Kiln Ring Formation– a review of what (we think) we know Presented by Kimmo Peltonen Placeholder for picture(s);

We accept the challenge! www.andritz.com

Kiln Ring Formation– a review of what (we think) we know Presented by Kimmo Peltonen

Placeholder for picture(s); the gray area must be filled COMPLETELY

with one or more individual picture(s)

Kiln Ringing

2

Kiln Ringing

3 Kiln Multi Fuel Burner 12-10-09

What we want to see…

Kiln Ringing


What we hope NOT to see…

Kiln Ringing


Calcining Reaction:

CaCO3+ heat (1286 btu/lb CaO at 1650°F) <==> CaO + CO2

•Lime Mud (CaCO3) enters the kiln at 20% to 30% moisture.

•The moisture is driven off and the mud is preheated to reaction temperature using the residual heat in the flue gases.

•The Calcining reaction begins when the mud temperature reaches 1400°F. The reaction proceeds well only after reaching 1800°F . Temperature is controlled by burning fuel and controlling draft.

•The completeness of the reaction is a function of the retention time and temperature profile in the kiln.

Purpose of a Lime Kiln

Kiln Ringing

Kiln Chemistry

Main: CaCO3+ heat (1286 btu/lb CaO at 1650°F) <==> CaO + CO2

CaCO3 CaO CaO

Soft Burned

S.G = 1.5 (50% void)

Highly Reactive

Lime Mud

S.G.= 2.71

Hard Burned S.G = 3.4 (no void)

Unreactive

Kiln Ringing


Kiln Ringing


Kiln Ringing


Kiln Ringing

Sodium Vaporization , SO2 and Lime Kiln Ring Formation

Impact on Kiln Ringing

Kiln Ringing

Sodium Enrichment in Lime Kiln

Mud Lime

Burner

Dust

Na

Chains

Na

Na Na Na

Temperature

Na2O + CO2 Na2CO3

High

Low

Na2CO3 + H2O 2 NaOH + CO2

High

Low

Kiln Ringing


Fume Formation in a Kiln

- Na2SO4 is least volatile form of sodium alkali

- Na2CO3 and NaOH are more volatile

- As Na alkalis volatilize, they form either fume particles which increase PM or form sticky molten phases which increase ring formation

Kiln Ringing

- If ratio Na2/S>1 then more volatile Na compounds form Na2CO3,NaOH forming mid-zone rings

- If ratio Na2/S<1 then you form “sticky” CaSO4 mist forming hot-end rings (Sulphation)

CEMENT KILN RINGING (same reactions will occur in lime reburning kiln) Rings consist of 3 types: sulphur rings, spurrite rings, and alkali rings

Na2/S Ratio Effect on Kiln Ring Formation

Kiln Ringing Cement Kiln Rings consist of 3 types: sulphur rings, spurrite rings, and alkali

rings Sulphur induced rings are formed when the molal Na Alkali to S ratio is less

than 0.8 In such cases the SO3 level is high The SO3 reacts with the CaO to form CaSO4 (sulphation) which coats the

surface of the particles If the kiln is operating under slightly reducing conditions (such as inside the

flame plume), more volatile and lower melting point sulphur salts will form increasing ring formation The molten salts coat the lime particles and dust forcing it the stick to the kiln

wall Dusty kilns will accelerate this mechanism


Kiln Ringing Rings consist of 3 types: sulphur rings, spurrite rings, and alkali rings

Carbonate, or Spurrite rings are formed when the molal Na Alkali to S ratio is

more than 1 In such cases volatile Na compounds level is higher The CO2 in the fluegas reacts with the free CaO to form C2S-CaCO3

(Spurrite) via re-carbonation which forms hard, layered rings Spurrite rings form whenever partial pressure of CO2 above the bed of

material is high to reverse the calcining reaction


CaO CaO

M (Ca, Na)+CO3 CO2

Kiln Ringing Rings consist of 3 types: sulphur rings, spurrite rings, and alkali rings

The third type of ring occurs whenever the Na to S molal ratio is above 1.2 In such kilns, typically low melting potassium salts provide the “binder” for the

lime particles traveling through kiln. Chlorine can accelerate this. Through a “freeze and thaw” mechanism due to changing temperature

profiles, rings of this type can be very aggressive


Kiln Ringing


Vaporization/Condensation of Na in kilns - Hot end rings have less and less Na in the

ring as you move down the kiln

- Na is vaporizing from the hot end of the kiln and then re-condensing

- This molten phase increases stickiness of the dust particles allowing them to sinter

- The liquid Na chemically binds the dust particles together to harden the ring via a sulphation reaction to form CaSO4

- Sulfur is coming from NCG and sulfur

containing fuels

- Rings consistently form where the S/Ca mass ratio peak

- Burning of sulfur containing fuels and NCG is the primary source of this type ringing

Kiln Ringing

Kiln Chemistry

Main: CaCO3+ heat (1286 btu/lb CaO at 1650°F) <==> CaO + CO2

CaCO3 CaO CaO

Soft Burned

S.G = 1.5 (50% void)

Highly Reactive

Lime Mud

S.G.= 2.71

Hard Burned S.G = 3.4 (no void)

Unreactive

Kiln Ringing

Hard v.s. Soft Burned Lime

Kiln Ringing

Lime Porosity and Slaking Rate

Kiln Ringing


Why does fresh lime cause problems? Basically, fresh lime makes lime mud particles that are smaller than reburned lime which do not settle and filter as well due to fresh lime’s higher reactivity (porosity). theoretical answer : fresh lime is much more reactive than reburned lime which means it slakes quickly (converting CaO to Ca(OH)2) due to its porosity. Reburned lime is made from lime mud that has been thru the caustic plant and has residual soda in it. The soda in lime mud helps sinter a glass like coating around the lime mud pebbles which reduces porosity and slows the diffusion of the water in green liquor into lime particles and thus slows the slaking reaction down. Fresh lime is made from limestone rock which has virtually no soda. Fresh lime is very porous and soaks up water like a sponge and converts to Ca(OH)2 very quickly and creates very small Ca(OH)2 particles in an emulsion. This makes the slaking reaction in lime diffusion limited. The causticizing reaction of converting Ca(OH)2 to CaCO3 (lime mud) is a slower reaction which is concentration limited. If the lime slakes faster than can be converted (causticized) into mud particles, then the lime mud particles formed will be much smaller and will not settle as well and will not filter or drain as well. If the Ca(OH)2 can be causticized (converted to mud) while it is still on the surface of the CaO particle as Ca(OH)2 is just being formed, then the mud particle formed will be larger which settles faster and filters better.

Kiln Ringing

NON-PROCESS

ELEMENTS AND THEIR EFFECTS IN THE KILN AND

CAUSTIC PLANT

Kiln Ringing


Main impurity is Na2SO4 Comes in 3 types; Water-Soluble Water-Insoluble and Guarded Guarded Na acts like water soluble when heated above 400 deC As mud moves through kiln, Na becomes a mix of of Na2CO3 and Na2SO4 which in the presence of other impurities in the lime mud, melts at ~800C (1470F). This melt temp is about the same as the calcining temp in the kiln. This is were ring starts to form

Kiln Ringing

Undesired Kiln Reactions (ringing and ball formation)

Formation of Low Temperature melting compounds: Compound Softening Point (°C) Na2CO3 - CaCO3 783 K2CO3 - CaCO3 750 Na2CO3 -K2CO3 - CaCO3 693 Na2CO3 - NaCl - CaCO3 649 Na2O - SiO2 - CaO 725 K2O - SiO2 - CaO 730 CaSO4 - Na2SO4 900

CaCO3 decomposes at 850 °C to 1250 °C which is higher than the melting

temperatures of the above compounds thus causing rings and balls in the kiln.

Kiln Ringing


- S absorption Eff = S absorbed by lime/ S input

- Eff. decreases as S input to kiln increases

- Results in increased SO2 emissions

- SO2 reacts with Na first then Ca forming CaSO4 on surface of particle

- CaSO4 coating prevents further reaction resulting in excess SO. Leaving with fluegas

Kiln Ringing

Sodium Vaporization , SO2 and Lime Kiln Ring Formation

Impact on Kiln Ringing

Kiln Ringing


Sulphur absorbed by Lime

CaSO4 Formation: SO2 + CaO + 1/2O2 CaSO4

Na2SO4 Formation (occurs first): Na2CO3 + SO2 + 1/2O2 Na2SO4 + CO2 2NaOH + SO2 + 1/2O2 Na2SO4 + H2O

Kiln Ringing

Mud Impurities

LIME MUD IMPURITIES Magnesium (MgO) 0.75% (max) Silica (SiO2) 0.5% (max) Aluminum (Al2O3) 0.5% (max) Total 2.0% (max)

Free lime (CaO) 1.0% (max)

Kiln Ringing Effect of Lime Impurities on Settling Rate

Kiln Ringing Specific Surface of Mud

0102030405060708090

0 10 20 30 40

BET m2/g

DS

% ds. %Log. (ds. %)

LIME MUD SPECIFIC SURFACE AREA

Kiln Ringing

Lime Mud Solids vs Lime Mud Impurities

666870727476

0 50 100 150 200 250

Impurities (X100) Example: 100=1%

Lim

e M

ud

% S

olid

s

Total Impurities (X100)

MgO Impurities

Kiln Ringing Silica Content

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10 12

Silica %

Dry

Sol

ids

%

Kiln Ringing


How does burner operation impact ringing? - Excessively short flames will produce

high heat flux in the first 4 diameters increasing molten phase reactions in the hot end

- Excessively long flames will potentially

overheat the inlet of the chain section producing molten phases in the chain section inlet (balls, chain coming out,etc)

Kiln Ringing


Recirculation in a Confined Jet

Kiln Ringing


Jet Behavior Jet will entrain fluid based on relative momentum of jet and surrounding

stream- similar to a steam ejector Source of entrainable outside fluid is limited in a confined system like a kiln Source of entrained gas originates from the edge of the jet further

downstream—this flow of gas back toward the nozzle is called recirculation. Ratio between the primary and secondary jet momentum is a measure of the

aerodynamic mixing of the streams (Craya-Curtet) If ratio is high (typically >5), then recirculation is high Some recirculation is desired for stability. Flame width and length will increase due to increased CO2 concentration

around flame Excessive recirculation can damage brick–low recirculation rates cause loss

of control/stability Recirculation of combustion products lower Adiabatic Flame Temp but

increases combustion completeness NOTE!: C-C does not account for swirl

Kiln Ringing

OIL (Methanol, Tall Oil, Coke) More Luminous higher ratio of LHV to HHV ratio than gas. Higher AFT Better (faster) heat transfer Less Flue Gas generated Higher heat flux on refractory Difficult to handle

GAS (CH4,Producer,Biogas,H2) Easy to handle Generates slightly more flue gas volume per BTU Lower AFT Higher Feed-End Temperature More susceptible to aerodynamic flow problems Lower kiln capacity

Fuel Selection in Kilns

Kiln Ringing


Oil Flame v.s. Gas Flame

Kiln Ringing NCG Incineration

Kiln Ringing

39

NCG Gun

Kiln Ringing NCG Incineration Design NCG burner (injector) to maintain sufficient velocity to prevent flame

propagating “backwards” into piping. Typical safety margin : design velocities for 50-100X turbulent flame speed.

(Turbulent flame speed and laminar flame speed are both theoretical calculations) For typical concentrated NCG preferred velocity is >20m/sec, and for SOG,

30m/sec. Actual design velocities are often adjusted to minimize disturbance to main flame

and to maintain allowable pressure drop to the NCG delivery system. Cooling air (outer annulus) is generally supplied via primary air fan (600cfm) or a

separate small blower Generally- increasing the angle of NCG jet intersection with main flame lower

refractory damage and increases mixing but lowers flame temperature significantly. Closer to parallel is best compromise Kiln operation always a compromise when trying to operate as an incinerator Excess air levels have to be run higher than optimum Refractory lifetime reduced

40

Kiln Ringing

. Methanol and Turpentine Should be Used as Auxiliary Fuel Only For methanol, we recommend limiting Methanol to 15 to 20% of heat

input with natural gas as main fuel and 10 to 15% with oil as main fuel. Maximum amount of turpentine we recommend is 5% of total heat input.

Pressure atomization is used for both. Air/Steam assisted atomization has been used as well

Often causes ring formation in kiln (primarily due to turpentine) Me OH contains 2.5% N. This 5 times more N than in Oil therefore

burning Me OH Increases NOx formation Contains S compounds that are potentially noxious.

Kiln Ringing

So….what do we really know?

Kiln ringing mechanisms are not fully understood. The chemistry can be very complex and will vary from kiln to kiln However, the basic reactions are understood – we think…:

Sodium compounds vaporize from the lime bed in the burning zone Some of the Na will condense on itself (making a fume particle) and

some will stick to the lime Molten phase Na can act as a “glue” which will cause lime particles to

stick together and to the walls of the kiln. This Na combines with CO anion

Re-carbonation and Sulphation reactions will harden the agglomeration of lime being held together by the alkali “glue”

Cycles of freezing and thawing because of temperature fluctuations will deposit new overlapping layers of material in the same basic area causing a large, structurally stronger ring formation

When the rate of erosion no longer equals the rate of deposition, we have a problem 42 Kiln Multi Fuel Burner 12-10-09

Kiln Ringing

So…. can we prevent ringing?

It is not possible to completely eliminate ringing. It is a fundamental side effect of lime reburning kiln operation However, keeping the kiln operation stable is the best way to prevent the buildup of the

ring to the point where it interferes with the kiln operation Constant lime mud feed Minimize sheet drops (use a Continuous Precoat Renewing system) Make Small operating changes in kiln to maintain steady state Installing a Flash Dryer, Dust Cyclone will eliminate or reduce impact of lime mud

feed fluctuation Correct burner operation/design to optimize heat release in burning zone Good lime mud washing will remove as much alkali (Na) as possible reducing the

amount of Na in the vaporization/condensation loop. Scrubbing S from NCG stream (caustic/WL scrubbers) will reduce the amount of S

entering the kiln burning zone. Install good droplet separators in NCG system to eliminate condensate from NCG injection Proper atomization of oil/tall oil/methanol etc is required to prevent droplets directly

impacting bed 43 Kiln Multi Fuel Burner 12-10-09

Kiln Ringing So…. can we prevent ringing?

FUTURE work…. we are significantly improving the causticizing operation and lime mud washing

(pressure disc filtration of white liquor, disc filtration of lime mud, advanced controls,etc). Improvements in the modern Flash dryer systems allow very precise control of kiln

temperature zones minimizing the “freeze and thaw” The levels of washable Na alkali are significantly reduced. This leaves only the “bound”

or “guarded” Na. Unfortunately, this Na is released in the burning zone. We can scrub much of the S from the NCG stream. However, the S entering with the

fuel is still a problem. Fuel oil, tall oil, methanol,pet coke all contain significant levels of sulfur Improved green liquor clarification and filtration removes significant NPE from the kiln

reducing the levels of adverse low melting point compounds that accelerate ringing/balling This leaves us with controlling the ratio of Na and S in the kiln to minimize the amount

of Na volatilization and maintaining the kiln chemistry in the most non-ringing state


Kiln Ringing

SUL-SUPREME 70%

Burning Emulsified Sulfur to Reduce Sodium in the Lime Kiln at Weyerhaeuser Albany (RIP…)

Kiln Ringing


. The maximum or ultimate kiln capacity is limited in this case by the velocity of flue gases in the feed end of the kiln immediately in front of the kiln neck. The reason for this is that re-entrainment of dried mud in the kiln and feed housing becomes the eventual controlling variable in limiting further production/feed rate increases. The velocity at which we see this limit in Andritz LimeFlash flash dryer applications is ~7.5 m/sec. This will allow a maximum production rate (assuming a LimeFlash conversion) of 500 short ton/day. A balance calculation at this production rate is included (see appendix). (It is important to note that the velocity limit for other kiln designs is significantly lower. For example, a standard kiln with chain section has a velocity limit in the chain section of approximately half of what is possible in a LimeFlash kiln due to higher levels of entrainment of lime mud as well as temperature limitations in the chain section.) The velocity limit for the existing Cloquet kiln configuration should be the approximately the same as the LimeFlash system. However, the circulating flue gas system approach used by the FLS design also has a temperature limit which will reach an operating maximum before the velocity effects predominate. The effect of this is that the Cloquet kiln currently is reaching temperatures in the feed end (“smokebox”) where the lime mud is softening or melting and sticking to the hot surfaces in the smokebox causing pluggage. Further increases in production rate over current rates will be quickly limited by the acceleration of this pluggage behaviour.

Documents

Presented by Kimmo Peltonen - WCBLRBAC.org · We accept the challenge! Kiln Ring Formation– a review of what (we think) we know Presented by Kimmo Peltonen Placeholder for picture(s);