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H ig h Pe r f o r m a n ce I n d u s t r i a l Fu r n a ce ( 4 H 5 9 2 4 )

Furnace and Combustion Efficiency

Blasiak Wlodzimierz, Prof.

Weihong Yang Associate Prof.

R o y a l I n s t i t u t e o f T e ch n o l o g y

Sc h o o l o f I n d u s t r ia l En g i n e e r i n g a n d M a n a g e m e n t

D i v i si o n o f En e r g y a n d F u r n a c e T e ch n o l o g y

St o c k h o l m , Sw e d e n

w e ih o n g @m s e .k t h .s e


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Objectives

To equip the student with enough knowledge about theimportance and need of the energy conservation in industryespecially in furnaces and getting knowledge in the fields:

Evaluating the thermal performance of furnaces

Energy conservation measures in furnaces


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Lecture Contents

1. Introduction

2. Energy Balance and Efficiency

3. Measuring Method

4. Methods of Efficiency increase (Energy SavingMeasures)

Combustion Efficiency Improvement

Waste Heat Recovery Refractory and Insulation


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Introduction

Extensive experience work with furnace users has shown that

operating cost savings of 10 30 % can often be achievedWith little or no capital outlay.

Top management willingness.

Proper management system.

Applying effective management technique

The enviromental issues and legislation are already haveing asignificant impact on furnace operators and this likely toincrease CO2, CO,SO2, NOX.

Save Energy Increase Profitability Protect the environment


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Lecture Contents

1. Introduction

2. Furnace and combustion efficiency

3. Factors determining furnace efficiency

4. Measuring efficiency


Combustion Efficiency Improvement Waste Heat Recovery

Refractory and Insulation


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Energy balance is an analyses of a process in which all energy states and flows(inputs and outputs) through a predefined envelope (system boundary) arequantified. It is a tool to evaluate the furnace performance and efficiency.

1st

Law:

Assume steady state conditions in a furnace

Neglect kinetic and potential energy

For furnace, Qsys is Qsur (surface losses)

The continuity equation is

+

+++=

+++ syso

oo

sys

ii

iisys WgZV

hmdt

dEgZ

VhmQ &&&&

22

22

( ) ( ) += surooii Qhmhm &&&

= oi mm &&

EnergyBalance

exhaustlossstockFGRin QQQQQ ++=+

Qstock

Qin

QFGR

Qexhaust

Qloss


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Energy Balance

A energy balance should been obeyed . i.e.

Qin=Q

out

Qout=Q1+Q2+Q3+Q4+Q5

Q1 , Sensible heat of wast gases (upto 60-80%)

Q2, Heat losses from the furnace surface,etc

Q3, Incomplete combustion of the fuel

Q5 Efficieny heat

QinQ4, Heat required by chemical re

taking place in the charge

Sankey diagram


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Efficiencies Definition

Thermal efficiency is 100 % minus the summation ofall losses

100=in

exhaustah

Q

Q

or

Available Heat is thought of as the

total energy contained per kg (or m3)

of fuel minus the energy carried

away by the hot flue gasses exiting

through the stack, expressed as a

percentage.

100=in

lossinte

Q

QQ

100=in

stockte

Q

Q


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Lecture Contents

1. Introduction


3. Measuring Method



Waste Heat Recovery Refractory and Insulation


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Energy

Balance... Methods

There are two methods of measuring efficiency:

Input-Output method

heat loss method

100).1(100in

loss

in

lossin

teQ

Q

Q

QQ

=

=

100=in

stockte

Q

Q


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Energy

Balance... Methods

Heat loss method 100).1(100in

loss

in

lossinte

Q

Q

Q

QQ=

=

The losses measured are:

heat loss due to unburned carbon in refuse,

heat loss due to dry flue gas,

heat loss due to moisture in as fired fuel,

heat loss due to moisture from burning hydrogen, heat loss due to moisture in the air,

heat loss due to heat in the atomizing medium (steam, air),

heat loss due to formation of carbon monoxide,

heat loss due to unburned hydrogen,

heat loss due to unburned hydrocarbons,

heat loss due to surface radiation and convection,

heat losses in ash pit


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The calculation of energy balance requires the measurements of Weight offeedstock used, ms

Weight of kiln car and kiln furniture (moving parts through the system), mfur Amount of energy used (fuel flow, mf and combustion air flow, ma) Walls temperature, Tw Ambient temperature, Tamb Combustion air temperature, Ta Fuel temperature, Tf Flue gas temperature, Tg Fuel composition, molar or mass fractions (C, H, S, O2, N2, ash, moisture...etc) Flue gas composition (at least O2 or CO2)

Minimum measuring equipment required: Flue gas analyser (CO2 or O2) Flow meters, (fuel or air) Immersion temperature probe (flue gas, Combustion Air, fuel) Surface temperature probe (furnace surface walls, feedstock) Balance (stock and kiln furniture weights)

Length measaure Fuel ultimate analysis (weight basis); C, H, S, H2O, ... Etc.

Energy

Balance... Measurement


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Energy Balance... Combustion Analysis

The following parameters should be determined in order to calculate theflue gases losses and therefore the Energy balance:

1. Theoretical Air (TA): This is the minimum amount of air that supplies sufficientoxygen for the complete combustion of all the fuel

(kg air)

The following equation can be used in general:

2222 76,376,3 NCONOC +++

44.1176,3 2222 =

+=

CC

NNOO

MN

MNMNTA

(%S),))/-(%O(%H,(%C),TA 324822534511 ++=

13


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2. % Excess air in flue gas (EA): An additional quantity is required to achieve

3. The actual air fuel ratio (AF): is the actual total air supplied for 1 kg of fuel

(kg air)

4. Total flue gas: (kg flue gases)

5. Act. H2O in combustion air = (kg vapor)

6. if O2 rather than CO2 is measured

%1001-22

.

=

Act

MAX

CO

COEA

+= 1100

EATAAF

)1( += AFTFG

AFSH

MaxCOCO 2

21

%O21%2

=

Energy Balance... Combustion Analysis


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Energy Balance

... Energy Input to thesystem

HHVmQ ff = &

=f

R

T

T

ffsenf dtCpmQ &

,

=a

R

T

T

aasena dtCpmQ &,

)( ,, Rinssistock TTCmQ = &


=a

R

T

T

OHfO, airH dtCpSHAFmQ 22 &

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Inputs:

4) Sensible heat in moisture

contained in combustion air

3) Sensible heat (physical

enthalpy) in air

2) Sensible heat (physicalenthalpy) in fuel

1) Chemical enthalpy in fuel

5) Energy contained in stock


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Outputs:

1) Energy contained in feedstockuseful energy

2) Physical enthalpy in exhaust gasloses

i. Sensible heat in flue gases

ii. Sensible heat due to moisture in air

iii. Latent heat due to H2 in fuel

iv. Latent heat due to water in fuel

Energy Balance... Energy Output from the system


)TC(TmQ Rs,outsstock,o = &

=g

R

T

T

iifg,sens dtCpx(TFG)mQ &

=g

R

T

T

OHfO, airH dtCpSHAFmQ 22 &

fgfHg, Latent h(%H),mQ = 0902 &,

fgfOHLatentg hO)(%HmQ = 22 &

,,


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Propane

0%

10%

20%

30%

40%

50%

60%

70%

0 2 4 6 8 10 12

Oxygen in flue gas, on dry basis (%)

Energylossesinfluegas

(%)

Tg= 800oC

400oC

600oC

200oC

6

8

10

12

14

0 2 4 6 8 10 12

O2% in flue gases, dry basis

CO2%

Can be determined by onlyknowing

Tg and CO2 or O2 in flue gases Flue gases losses increases with

Flue gas temperature Tg

Excess air, O2%

Flue Gases Losses

CO2 Max for

Propane is 13,8%


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Flue gases losses due to H2in Fuel.

Heat loss due to burning hydrogen

in fuel can be a high portion in fluegases

This amount of energy might not

appear in the analysis of energy

balance if the lower heating value

were chosen.

However they exist in flue gasesand can be recovered.

Heat loss due to burning hydrogen in fuel

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Energy BalanceBalance Sheet ...

Specific energy consumption = 0,0185(kg fuel oil/ kg Product)

Useful energy () = 32,4 %

A Balance sheet shows all possible

energy conservation measuresFlue gases heat recovery

Surface losses from furnace walls

Off-spec products

T/hr MWH % T/hr MWH %

1 Feedstock 30 0,200 3,3% 1 Product 27 2,192 35,7%2 Fuel /enthapy of comb 0,5 5,833 95,1% 2 Off-Spec Product 3 0,325 5,3%

3 Fuel/sensible 0,009 0,1% 3 Flue gas/ sensible 8,5 2,361 38,5%

4 Combustion air 8 0,089 1,4% 4 Flue gas/ latent 0,9 0,611 10,0%

5 Surface losses 0,642 10,5%

6,131 6,131TotalTotal

OutputsInput

Surface

losses

11%Product

36%

Flue gas/

sensible

38%

Off-Spec

Product

5%

Flue gas/

latent

10%


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Lecture Contents

1. Introduction


3. Measuring Method



Waste Heat Recovery

Refractory and Insulation


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Factors determiningFurnace Efficiency

Excess air

Air infiltration

Stack loss

Combustion losses

Qstock

Qin

QFGR

Qexhaust

Qloss


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Efficient combustion requires the correctair/fuel ratio and adequate mixing

High excess air levels result in Dilution of the flue gases due to increasing of the

total air supplied.

Reduction in flue gas temperature.

Reduction in heat transfer rate.

Increasing in flue gas losses.

Reduction in combustion efficiency.

Low excess air operation can cause unburned hydrocarbons to discharged

leads to fuel wastage,

reduce throughput,

poor product quality,

excessive emissions and/or structure damage to the furnace.

Increase combustionefficiency

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Many factors may cause undesirabledeviations:

Burner wear Hysteresis in control system

Variation in fuel properties

Variation in combustion air temperature

Variation in furnace pressure

Measures Modern automatic control (A/F, Pfur)

Routing eficiency monitoring (CO2%, Tg)

Regular burner and controls maintenance

Pay back could be immediate

Increase combustion

efficiency

Rule of thumb: 10% reduction of EA 1% increase in eficiency24


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How to recover the energy from waste gas :

1) Reduce the final exhaust flue gas temperature

Recuperation, made of metallic or ceramic elements, heat

recover efficient 10-50%, preheated air temperatureMax.800 K..

Regeneration, made of honeycomb, heat recover efficient

80-90%, preheat air temperature, 1500 K. (HTAC)

2) Reduce waste gases mass, mwaste

Rich oxygen combustion

Pure oxygen combustion (OXY-FUEL)

Saving Energ from Stock gas

( )= iiexhaust hmQ

&


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Saving Energ from Stock gas

Available heat vs exhaust gas temperature for C3H8 combustion at 2% oxygen

concentration in exhaust gases


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Oxyfuel Combustion

O24%

N2

9%

H2O41%

CO246%

N275%

O2

4%

H2O10%

C O2

11%

Air/Fuel Oxygen/Fuel

AGA

3.4 MW 2.0 MW

576 nm3/h wet

337 nm3/h dry4132 nm3/h wet

3725 nm3/h dry


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0,6%

0,8%

1,0%

1,2%

1,4%

1,6%

0% 50% 100% 150%

Excess air level

Energy

savingsevery20oC

reductioninfluegases

Waste Heat Recovery

For Propane, energy saving by 20C

recovery from flue gases isapproximately ranging between0,7% and 1,5% depending on thelevel of excess air in thecombustion process

Rule of thumb: 20C reduction of Tg 1% increase in efficiency

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Waste Heat Recovery Up to 50% energy saving is possible, payback (1 5 years).

Economy: The retrofit modification is more applicable to large,continuous furnaces and least applicable to smaller, intermittent

ones. The heat sink can be to the furnace itself by raising the

combustion air temperature.1- flue gas recuperation

2- self recuperative burners

3- flue gas regeneration4- stock recuperation

Lower Tg will reduce problems after chimneyX higher combustion air higher flame temperature increase

Nox.

Rule of thumb: 20C reduction of Tg 1% increase in efficiency

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Recovery heat can also be transferred foruse in other processes:1- drying

2- space heating

3- process steam

4- steam for power generation

(waste heat boiler)

Heat available and heat required should

match in quantities,

temperature

timing

Waste Heat Recovery

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Regenerative Burners

Uses a short-term cyclic heat storage device asthe means of achieving waste heat recovery.

Efficiency ~ 6595%

Combustion air preheated to very hightemperatures, up to 50 degrees belowthe furnace operating temperature.

! NOx emission can be very high.?!! Flame temperature is very high.?!

(But can be reduced if the right combustiontechnology is used)

Example of the ceramic regenerators


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Operation of two pairs of compactregenerative burners


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Stock Recuperation

Incoming feedstock can be

preheated using waste heatcontained in flue gases as auseful means of heat recovery.

Easily applied to continuousfurnaces

No problems in matching the

timing Applicable to intermittent

furnaces.

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Refractory and Insulation Consideration for selecting refractory:

1. Maximum operating temperature

2. Efectiveness as insulator:

3. Thermal mass (LTM); Ceramic fibber,specially for intermittent furnaces

4. Corrosion resistance

5. Errosion resistance

6. Coefficient of expansion

7. Longevity: replacement is expensive.

They are often made of layers

Monitoring the surface temperature of thefurnace from time to time.

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Improving FurnaceYield

Furnace yield: quantity of usable product per unit feedstock.Increase furnace yield Increase throughput and Minimize waste

There is often a trade off between throughput and product quality

Throughput is limited byLoading and unloading

Rate of heat transfer

Upstream and downstream processes

Measures to increase furnace yield:

Optimise the furnace operation for maximum yield furnace. Consider otherprocesses when scheduling furnace operation.

Feedstock must be on-spec to prevent wasted firing.

Ensure the correct temperature profile and furnace atmosphere.

Continually monitor the yield by weighing the feedstock and product.

Monitor the key variables that affect throughput and product quality

Use effective Quality Assurance techniques, e.g. hot inspection.

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Thank You For Your Attention !

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