The Use of Oxygen in Biomass and Waste-To-Energy Plants.pdf

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Venice 2010, Third International Symposium on Energy from Biomass and Waste

Proceedings Venice 2010, Third International Symposium on Energy from Biomass and WasteVenice, Italy; 8-11 November 2010

© 2010 by CISA, Environmental Sanitary Engineering Centre, Italy

THE USE OF OXYGEN IN BIOMASS ANDWASTE-TO-ENERGY PLANTS:

A FLEXIBLE AND EFFECTIVE TOOL FOREMISSION AND PROCESS CONTROL

N. CORNA*, G. BERTULESSI*

* SIAD S.p.A., Application Development Department, Bergamo, Italy

SUMMARY: Operators of waste thermal treatment and biomass-to-energy plants can achievelower pollutant emissions and increase capacity through the use of oxygen combustion

technologies. The proper selection of the oxygen injection strategy can improve unit operation,

reduce CO puff frequency and magnitude, increase throughput and allow a wider range of

materials to be treated. The decision to implement oxygen-enhanced technologies, especially in

the upgrading of existing plants, has to be made on a case-by-case basis, taking into

consideration actual operational needs, bottlenecks and available feeds.

1. INTRODUCTION

Environmental performances of biomass-to-energy plants and waste thermal treatment systems is

a “hot” and crucial issue that is gaining more and more importance both on technical and public

acceptance side. Operators of such facilities must face, from one side, strict limitations to

gaseous pollutants emission and from the other the variability of the incoming material: the

supply of high and constant quality feed is in fact a serious issue for both process stability and

emission control.

The use of state-of-the-art oxygen combustion technologies gives the operators an effective

and flexible tool able to manage most of these process problems.

This paper briefly illustrates the theoretical basis of the use of oxygen in the thermal treatment

of biomass and wastes and presents different configurations tested and actually working at

SIAD’s customers. The theoretical part will focus on the after-burning or post-combustion stageand on CO and soot formation and removal. The second part will be focused on diverse

upgrading options based on system properties and issues and shows the retrofits implemented in

existing thermal treatment plants through the use of oxygen.

The interested plants include a conventional grate system for solid fuels with underfire air and

post-combustion chamber, one hazardous solid incinerator with vertical post-combustion unit

equipped with liquid waste streams injection and rotary kilns for sludge pyrolysis.

The applications range from oxygen use in primary combustion air, to ensure maximum

flexibility to the process and lower residual carbon in the ashes, to O2 injection in secondary air,

aiming to achieve high level of gaseous pollutants removal. The use of pure oxygen lancing in






different configurations and the use of oxy-fuel burners are also shown.

Theoretical dissertation and case samples discussion highlight the potential benefits in the use

of oxygen technologies in biomass/waste-to-energy plants, but also stress the importance of

case-by-case analysis since different plant configurations, operational needs, heat loads and fluid

dynamic conditions require specific approaches and customized solutions.

1. EFFECTS OF OXYGEN ENRICHMENT ON GENERAL COMBUSTION

CHARACTERISTICS

Oxygen enrichment of air reduces the amount of nitrogen present as diluent in the reaction of

fuel and oxygen. The rate of combustion reaction usually increases significantly with oxygen

enrichment due to the higher partial pressures of both oxygen and fuel and the resulting

enhanced equilibrium temperature. This faster reaction rate contributes to most of the following

changes in the combustion characteristics:

higher flame speed;

lower ignition temperature; wider flammability range;

higher blow-off velocity gradients;

higher adiabatic flame temperature.

For incineration applications it is necessary to evaluate how these changes affect flame

stability and flame temperatures.

Figure 1. Flame temperature in relation to oxygen percentage in the combustion atmosphere






Flame stability

In general, a higher flame speed will improve flame stability and create a more intense and short

flame. It is however difficult to predict the actual effect on systems processing biomass and

wastes since flames are post mixed with various degree of turbulence.

The lower flammability limit (lean limit) of a fuel and air mixture is little influenced by theO2 enrichment of air. Excess oxygen in the lean limit is considered to act as a heat sink similar to

nitrogen. The upper flammability limit (rich limit), on the other hand, is extended substantially

with oxygen enrichment, as shown in Table 1. Wider flammability limits generally correlate with

greater flame stability (e.g. critical velocity limits for blow-off conditions). The change in the

upper limit allows the combustion of fuel or waste to begin even in a highly fuel-rich mixture.

Lower values of minimum ingnition energies and temperatures in pure oxygen also contribute to

enhance flame stability.

Table 1. Ignition and flammability properties of some combustible species in air and oxygen at

atmospheric pressure (adapted from NFPA, 1984)Min. IgnitionTemperature

Min. IgnitionEnergy

Flammability LimitsVol. %

Air OxygenCombustible

Air(°C)

Oxygen(°C)

AirmJ

OxygenmJ LFL UFL LFL UFL

Carbon Monoxide 609 588 - - 12.5 74 ≤ 12.5 94

Hydrogen 500 400 0.017 0.0012 4.0 75 4.0 95

Methane 537 - 0.30 0.003 5.0 15 5.1 61

Ethane 515 506 0.25 0.002 3.0 12.4 3.0 66

n-Buthane 288 278 0.25 0.009 1.8 8.4 1.8 49Benzene 560 - 0.22 - 1.3 7.9 ≤ 1.3 30

Ethylene Chloride 476 470 2.37 0.011 6.2 16 4.0 67.5

Ammonia 651 - > 1000 - 15.0 28 15.0 79

Adiabatic Flame temperature

Adiabatic flame temperature increases significantly with oxygen enrichment due to reduction of

nitrogen which acts as a diluent in combustion. The flame temperature increases by as much as

50 °C for a 1% increase in oxygen concentration for low enrichment levels (see Figure 1).It is important to recognize that the adiabatic flame temperature simply provides an upper

limit in the attainable flame temperature. A number of different oxygen usage techniques have

been developed in the industry in order to improve the stability, efficiency and control of the

combustion process in waste and biomass burning furnaces without creating higher flame

temperatures that might cause local overheating problems.

The use of high velocity oxygen jets, for example, allows to keep low flame temperatures

because aspirated combustion gases serve as a flame heat sink (see Figure 2). Even when using

oxygen enrichment of primary underfire air, the higher quantity of heat available due to the

removal of part of the nitrogen from the system, can be absorbed by other sinks as, for example,






waste with higher humidity content. The correct use of oxygen will not therefore increment the

combustion zone temperature, so that the grate will not exposed to overheating. It must be

underlined that also in furnaces with temperature limitations and already existing localized

overheated zones, the use of oxygen can be addressed to achieve better temperature uniformity

within the furnaces without increasing heat release to those zones already severely loaded.

2750

2475

2200

1925

1650

F l a m e T e m p e r a t u r e º C

Volume% of O2 in Oxidant20 40 60 80 100

FURNACE GAS

TEMPERATURE = 2400 ºF R=O

Traditional oxy-fuelburners use highflame temperaturefor welding andmelting.

←←←← Flame Temp ofNatural Gas/air mixture

R=1

R=2

R=4

R=6

R=8

SIAD advanced oxy-fuel burners reduceflame temperatureand reduce NOxemissions inindustrial furnaces.

R=Flue gasrecirculation ratio

R=1

R=2

R=4

R=6

R=8

SIAD advanced oxy-fuel burners reduceflame temperatureand reduce NOxemissions inindustrial furnaces.

R=Flue gasrecirculation ratio

Figure 2. Calculated reduction of adiabatic flame temperature of natural gas flame by

recirculation of flue gas

2. BENEFITS OF OXYGEN COMBUSTION

Once briefly mentioned the effects of pure O2 in a combustion process, it will be easier to

understand the main advantages of its use in thermal treatment of waste and biomass.

Increase in treatment capacity of the plant.

Reduction in furnace emissions.

Smaller flue gas handling equipment needed.

Higher flexibility in the plant management and in the feed quality and composition.

Reduced fuel consumption in case of low calorific fuels.

Improved process stability and control.

Since the erection of green field waste treatment facilities in EU is extremely rare due to

environmental concerns and public acceptance, high investment costs and complex

authorizations needed, the possibility for an increase in the capacity of an existing plant without

major process modifications and significant investment costs is a topic of growing interest.

Oxygen combustion technologies have been successfully used for increase in capacity and

productivity in a broad range of industrial furnaces. While the enhanced heat transfer

mechanisms typical of oxygen combustion are of little importance for waste treatment furnaces,

this type of plants can benefit from the reduced specific flue gas volumes so that the throughput

can be significantly increased. The extent of improvements possible for a particular incinerator

Furnace gastemperature 1315°C






depends on the nature of the plant limitations, the most common of which are usually related to

the air blower capacity, the gas residence time and the size of the flue gas cleaning system.

Oxygen enhanced combustion is very effective in overcoming these limits due to the reduction

of the volume of oxidant and flue gas for the same fuel input to the furnace. As can be noticed

from Figure 3, in methane combustion a 3% oxygen enrichment of the combustion air results in

a 12% reduction of the flue gas volume. This reduction can result in significant savings also indownsizing the air pollution control devices.

The dust carryover problem another common process limitation, is related to particle size,

characteristic of particulate matter and fluid dynamics within the furnace. Although the

improvement due to oxygen usage varies according to the injection strategy applied, the lower

superficial gas velocity due to reduced volumes inside the furnace is beneficial in reducing the

dust carryover. According to Stenburg et al. (1966), the particulate emission for solid waste

burning systems is essentially dependent on underfire air rate. In particular they report the

release rate to be directly proportional to u0.543, where u = underfire air velocity. According to the

previous empirical relation, a 3% enrichment of underfire air would result in a 7% less

particulate emission.

Flue gases from CH4 combustion

The effect of O2 enrichment

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

20% 40% 60% 80% 100%

O2 in the oxidizer

V o l u m e F r a c t i o n i n F l u

e G a s

2

3

4

5

6

7

8

9

10

11

12

F l u e G a s V o l u m e [ N m

3 F L U E

/ N m

3 C H 4

]

O2

Flue Gas Vol.

H2O

CO2

N2

Figure 3. Effect of O2 enrichment on flue gas volume and composition

(combustion of CH4, air/fuel ratio = 11.5, O2/fuel ratio = 2.0)

Where the main limitation of the plant is the heat exchange capacity of the steam production

section, a detailed study must be undertaken in order to ensure that no overheating can occur in

the system, while, if the bottleneck is the solid handling capacity of the pretreatment and feeding

section, mechanical modifications must be made before oxygen usage can be useful.






When an increase in throughput is not requested, the use of oxygen can be useful in order to

lower pollutants emissions as well as to achieve higher treatment flexibility, especially in terms

of feed quality, humidity and LHV. It is in fact not uncommon that Biomass-To-Energy plants,

originally designed to treat substances with defined and specific chemical composition, need,

after some years, to deal with different feedstocks because of biomass availability, cost

opportunities or authorizations. Similar problems must be faced by waste thermal treatmentfacilities, where the properties of incoming waste is variable and the pressure exerted by public

opinion is much stronger.

These unpredictable variations in fuel composition together with the intrinsic complexity of

the process often result in sudden variations of the conditions inside the reaction chamber and in

the formation of a wide range of pollutants. Due to the possibility to remove diluent nitrogen,

locally increase temperatures and lower flue gas volumes, the use of oxygen can be a very

effective tool to overcome this type of operational problems.

Another major issue is the variability of the water content of the feed due to seasonal

variations, weather conditions, collection methods and storage facilities available. An increase in

the feed humidity will necessary result in a decrease of plant capacity, waste residence time and

furnace temperatures with a consequent increase of residual carbon in ashes. The use of oxygen

enrichment of underfire air allows better ash burnout since less heat is carried away by the

reduced volume of combustion gases and more energy is released to the load. This additional

heat release improves the burning process by shortening the area needed for material drying and

improving the combustion process.

Oxygen can be used only when considered necessary: regardless the type of installation

required (enrichment of primary or secondary air, lancing, oxy-fuel burners, etc.) the oxygen

control and distribution system allows operators to choose between fully automatic, variable

rates injection strategies based on external inputs, as for example the real-time flue gas analysis,

to simpler manual controlled systems that can be started and stopped directly by the operator on

the actual need.After a detailed study of the furnace temperatures and fluid dynamics and of the current

operating conditions, the correct and customized oxygen injection technology can dramatically

increase the waste blend flexibility and improve the performance of the plant due to enhanced

fame stability and longer residence times as well as higher control of process temperature and

excess oxygen.

3. POLLUTANT CONTROL

Organic products of incomplete combustion

Waste thermal treatment is a really complex process and incinerators are not ideal combustors.

Variations in the feed input, temperature perturbations, gases preferential paths, solid waste

stratification and different volatilization temperatures and rates, often result in local zones of

deficient oxygen where decomposition of the organic constituents proceeds through pyrolytic

processes and a number of organic volatile compounds are originated. If these pyrolysis products

do not undergo through adequate post-combustion residence times, temperatures, turbulent

mixing and sufficient oxygen supply, some organic combustible micropollutants will escape

from the system and be emitted in the atmosphere.

As previously illustrated, the use of oxygen enhanced combustion can effectively overcome

all the mentioned limitations, increasing residence time with the reduction of the flue gas






volumes, raising temperatures in colder furnace zones and promoting more efficient mixing

between oxidant and combustible particles through localized injection strategies and higher

partial O2 pressures.

Carbon Monoxide (CO)

Carbon Monoxide is the major compound formed during the pyrolysis and gasification phases

through which biomass and waste undergo in a thermal treatment process. When, downside the

CO formation, the turbulence is not sufficient, temperatures are not high enough or oxygen is not

available in adequate quantities, significant CO emissions will be measured at the stack. Local

non homogenous composition or unwanted stratification of the charge and variable composition

of the material, can also lead to instantaneous and isolated CO emissions ( puffs).

In order to ensure the more complete as possible removal of CO from flue gases, the kinetic

of CO oxidation must be quickly reviewed. The kinetic of CO reaction to CO2 (Hottel, 1965;

Morgan, 1967; Dekker, 2002) is generally given by:

DC

O H

B

CO

A

OCO

RT

P f f f

RT

ba

dt

df

⋅=−

22exp

Where

f CO , f O2 , f H2O = molar fraction of carbon monoxide, oxygen and water vapor

t = time

T = temperature

P = pressure

R = ideal gas constant

t = time

A, B, C, D, a, b = constants varying according to the different formulations

While authors agree on B = 1 and C = 0.5, values of A range between 0.3 and 0.5.

It can be noticed that the process is strongly dependent on temperature and, to a lesser extent,

on partial pressure of oxygen and water vapor. Oxygen enrichment of secondary or tertiary air or

pure oxygen injection through lances will have the consequence of increasing all these three

parameters governing the CO oxidation rate. The reduction of the total volume will also allows

higher residence times, with consequently higher removal rates.

We can consider, as an example of a mixture with high CO content, the gas composition

resulting from the pyrolysis of dried sludge from one of the case samples illustrated in the

following section (see Table 2). To evaluate the effect of oxygen usage on CO oxidation

kinetics, the rate constant, in presence of three different streams of oxidant, i.e. air, 3% oxygen

enriched air and pure oxygen can be calculated based on the above model.

If we consider the same temperature for the three mixtures between the combustible gas and the

oxidant, the reaction rate of CO oxidation when 3% oxygen enriched air is used is between

12.0% and 12.5% higher with respect to air. In case of pure oxygen, the CO conversion for the

mixture is from 8.6 to 10.8 times faster than in presence of air.






Table 2. Example of gas mixture with high CO content

Species Gas composition

mole mass

H2 26.95 % 2.82 %

CH4 24.79 % 24.79 %

C2H6 2.76 % 4.31 %

C2H4 4.59 % 6.68 %

C3H6 0.96 % 2.10 %

CO 30.46 % 30.46 %

CO2 7.66 % 7.66 %

H2O 1.82 % 1.70 %

Soot

A number of approaches have been developed for soot burnout modeling and different

formulations have been proposed in literature, with increasing detail level and different scope of

application.

Since an in depth description of the process is beyond the scope of this work, we will briefly

present the base of a widely accepted approach and show the effect that the correct and aware

use of oxygen can have on the removal of soot particles that can be formed in poor combustion

zones of the furnace.

The soot particles burnout process, as a first approximation, can be modeled studying the

kinetics of heterogeneous combustion of small char particles in air and oxygen.In a carbon-oxygen system, the rate of weight loss of carbon per unit external surface area Rt is

given by the so called “resistance equation”:

+=

d sOt K K P R

1111

2

( ) 75.01T

d C K s ⋅⋅=

−⋅= RT

E A K d exp

Where

t R = rate of carbon consumption scm

g ⋅2

s K = surface reaction rate coefficientatm scm

g ⋅⋅2

d K = diffusion coefficientatm scm

g ⋅⋅2






0.0000001

0.000001

0.00001

0.0001

0.001

0.01

800 900 1000 1100 1200 1300 1400 1500

T [K]

R t [ g / c

m 2

s ]

Figure 4. Rate of carbon consumption for a small char particle as a function of temperature

K s represents the influence of chemical kinetics on the overall reaction rate, while K d symbolizes

the limitation to the process due to diffusion resistance. When K s << K d it means that the soot

burnout is limited by the kinetics of the surface reaction, while with K s >> K d the process is

constrained by the oxygen transfer rate from the boundary layer to the surface where the reaction

is almost immediate.

The relative importance of reaction kinetics vs. oxygen diffusion in soot burnout depends on

soot particle diameter and operating temperature (see Figure 5).

0.001

0.01

0.1

1

10

100

1000

10000

100000

0.001 0.01 0.1 1 10 100 1000

Soot particle diameter (µµµµm)

K s ,

K t

K s

K t

Figure 5. Kinetic and diffusion coefficient as a function of soot particle diameter

For diameters typical of soot particles the process is by far constrained by the surface chemistry,

while the burnout of residual particles generated from the combustion of liquid wastes made up

of long-chain organic molecules is kinetically quite different from soot burnout, mainly because

the residual particles are orders of magnitude larger than soot and the rate is controlled by

boundary layer diffusion.

As can be noticed from the above formulations, the rate of soot burnout, in a temperature






range representative of incineration processes, can be expressed by the Arrhenius equation; the

reaction is therefore strongly influenced by reaction temperature and proportional to O2 partial

pressure.

When trying to face issues related to soot emissions, the first actions to be taken should be

addressed to improve the combustion process itself: soot emission, being generally a result of

poor combustion, is in fact often associated to other uncombusted pollutants emission (CO,VOCs, dioxins, etc.). When no further improvements can be done in tuning waste or biomass

burning process, the solution is to improve the soot burnout process. The use of pure oxygen

injection in the colder zones of the furnace will raise the temperature of the area preventing the

flue gas cooling, increase the O2 partial pressure and promote good mixing between soot

particles and oxygen, thus dramatically decreasing the time required for the complete oxidation.

4. RECENT APPLICATION OF PURE OXYGEN IN WASTE THERMAL PLANTS

Case History 1

Case sample 1 is related to the upgrade of a municipal waste incinerator in Northern Italy. The

plant comprises two continuous fed lines, having moving grates, with a nominal treatment

capacity of 100 t/day each. The combustion chambers consist of four mechanical grate sections

providing continuous variable speed movement through oleodynamic actuator. The waste supply

takes place through refuse charging hoppers terminating in vertical throats that act also as plugs

for waste gases. The incineration capacity is regulated through the combustion air and the pusher

movement.

Underfire air is supplied through four plenums from an air duct. Combustion gases flow

countercurrent to the waste feed and are conveyed to the post-combustion chamber where

secondary air is injected; the chamber is equipped with auxiliary natural gas burners which

automatically start whenever the temperature falls down minimum value, according the EUcodes.

Gases from post combustion chamber pass through a steam boiler producing high pressure

superheated steam for electric energy generation and low pressure steam used for district

heating.

The plant, originally designed to treat MSW having different composition and source with

respect to the actual plant feed, was suffering two major operational problems.

The first was related to the quality of the incoming waste, which, due to the collection and

storage procedures, could have high and extremely variable water content. It resulted in the

worsening of burning conditions inside the chambers, with an increase of the grate fraction

where drying and preheating phase took place and a consequent reduction of gasification and

combustion zones. As a result, high residual carbon values were frequently found in the ashes.

Increases of underfire air to solve this problem was not possible, since blowers were at full

capacity and the higher volumes of primary air would have resulted in smaller residence times in

the post-combustion chamber, with an increase in pollutants emission and higher loads to the

waste gas treatment unit.

The second issue was related sudden and unpredictable CO puffs releases. The operators, in

the attempt to maintain the desired treating capacity with a feed at low LHV and high humidity,

had to face a less complete combustion in the primary chamber so that the post-combustion

section became overloaded with puff emissions high in carbon monoxide. The objective of our

activity has been therefore to increase, from one side, the actual plant treatment capacity and






remove, from the other, CO puffs.

After a detailed analysis of the operating parameters in the combustion and post-combustion

chambers, a proposal based on two solutions was defined and finally applied.

The primary activity was to “improve” the underfire air system through oxygen enrichment.

The higher humidity of the waste would have required a longer grate in order to achieve a

complete char burnout from the ashes since the “drying” zone was too wide and the gasificationand combustion zones resulted insufficient. The most effective solution has been the oxygen

enrichment of combustion air, creating the same effect of a longer gasification and combustion

section, allowing a more intense and reactive reaction. In order to optimize the pure oxygen

usage, it is sometimes useful to enrich only the portion of underfire air that is really participating

in combustion, avoiding the enrichment of the air delivered to the first portion of the grate which

is mostly used for drying the incoming material. In this case, since the four air distribution

sections are fed through the same air duct without the possibility of independent regulation and

because the grate surface is relatively limited, the enrichment has been done on all the

combustion air. A simple oxygen distributing pipe, or “sparger” (see Figure 6), was therefore

placed in the main air duct.

Figure 6. CFD simulation of Oxygen mixing through a sparger in combustion air duct

An additional improvement was obtained through the positioning of oxygen lances in the post-

combustion chamber. The air enrichment at the grate level has been designed at low levels

(2.0%) so that the residual carbon could be decreased at the desired content without any concern

for the grate integrity. To eliminate CO puffs, the injection of pure oxygen jets, just at post-

combustion chamber inlet, was a very effective solution. One high velocity oxygen lance has

been therefore placed concentrically inside each secondary air distribution pipe (see Figure 7) so

that a turbulent mixing region with elevated oxygen partial pressure is created at the inlet of the

post-combustion chamber.

Since CO emission puffs in waste combustion units occur as sudden releases from the






combustion process, it is not straightforward to design a system that, in order to minimize

operating costs, will intervene only when necessary. An O2 injection strategy starting only when

stack CO emissions will overcome a defined threshold is likely to be both inefficient and

ineffective. The analysis of the available data carried out in conjunction with the plant

technicians and operators, together with the prescriptions from local authorities, made possible to

identify the free O2 level at the post combustion chamber exit as the best parameter to bemonitored in order to prevent CO releases. An automatic system based on the input of an oxygen

analyzer placed downstream the post-combustion chamber was therefore chosen for the control

of the secondary air oxygen injection, while the primary undergrate combustion air enrichment

control was left at operators’ decision, enabling them to determine, mainly through the

evaluation of the incoming material, the optimal oxygen usage.

After an initial test phase where the main process parameters have been fine tuned, the results

clearly indicated the full achievement of the intervention targets and the installation has been

permanently implemented allowing an increment in treatment capacity of 10% – 20% together

with the almost complete removal of CO emission limits overcoming.

Secondary air

distribution pipe

P u r e

o x y

g e n l a n

c e s

Figure 7. Pure O2 injection lances placement in secondary air distribution system

Case History 2

The following example presents the upgrading of a thermal treatment plant based on

pyrolysis/gasification in different stages in rotary kilns. The purpose of the plant is the thermal

treatment of sludges and the production of inert material to be used for aggregates production for

the construction industry.

The plant was designed to treat about 15 t/hour of sludge at 25% dry solid content and was

composed of the following main processes: sludge drying, 2-stage pyrolysis, sintering, off-gases






post-combustion and flue gases cleaning and steam production.

Sludge is fed to a two-stage drier to increase the dry solid content up to 90%. In the original

configuration the dried sludge was supplied into 2 pyrolysis reactors, working in parallel,

indirectly heated by hot gases generated downstream in the process.

From the reactors the resulting material entered into the second pyrolysis/gasification stage.

The process was carried out in a rotary kiln, directly heated by combustion gases generated in aseparate gas generator.

The process unit, at the time of the intervention, was suffering some operational problems

especially related to the two indirect heated medium-temperature pyrolysis reactors. A successful

application of oxygen made possible to overcome the limits of the process and simplify

significantly the plant configuration.

In order to overcome the limitations of the two indirect pyrolysis reactors, where limited

intervention was possible, it was proposed the customer to bypass them, providing all the energy

needed for pyrolysis in a rotary kiln. The kiln has been equipped with an oxy-fuel burner

specifically designed for processing fine materials.

The burner has been placed at the kiln material inlet, firing co-current the material path. In

this way it was possible both to capitalize the existing configuration with flue gas extraction at

kiln inlet and to maximize the efficiency of the energy transfer process thanks to a longer gas

path inside the furnace promoted by the U-shape flame typical of this design. The burner has a

firing rate of 3 MW, allowing the maximum achievable heat transfer to the material bed; the

balance between natural gas and oxygen supplied to the burner is kept at the stoichiometric

value, so that the atmosphere inside the furnace is maintained neutral and non-oxidizing. The use

of an oxy-fuel burner consents also to extract a pyrolysis gas with a considerable higher LHV

than the one achievable with air-fuel flue gases. The gases pass through the existing cyclone and

are directly fed to the post-combustion chamber.

Thanks to the high achieved discharge temperature of the material, about 600 ÷ 700 °C, it has

been possible to reach, within one single short rotary kiln, almost the complete volatilization ofthe organics present in the dried sludge; the residual carbon content from this stage lies between

7% and 15%.

A second rotary kiln, 6 m long, equipped with two oxygen – natural gas burners and an

oxygen lance is used to remove the residual carbon and sinter the final product.

The material discharged from the first kiln, due to the existing conveying system

configuration, experience a sensible temperature loss before being charged to the second kiln so

that the charge temperature falls down to values close to 400 °C.

The purpose of the second kiln, operating at about 950 °C, is to lower under 3% the residual

carbon content and perform a partial sintering of the material so that it can be easily used, after

mixing with other additives, as inert in the construction industry. Since the material temperature

at kiln inlet is about 400 °C, it is necessary to provide a definite amount of energy just in the first part of the 6 m kiln to raise the temperatures, while at about 800 °C, when the carbon burning is

fast, it is crucial the availability of free oxygen to complete the combustion reaction. The choice

has therefore been the installation of two high impulse oxy-fuel burners at the kiln inlet, 500 and

300 kW, providing a very short and intense flame (see Figure 8).

At the same time, an oxygen injection lance, placed closed to the charge, generates an

atmosphere with a high O2 partial pressure able to burn the residual carbon to the required level.

Kiln inlet presents also an open section, 300 mm diameter, which promotes a determined air

input ensuring enough turbulence level inside the kiln.

The discharged material from the second rotary kiln is rapidly quenched in water in order to






fix the heavy metals impeding re-oxidizing conditions.

The successful application of oxygen in the different sections of the plant allows today a more

steady operation with higher control on process variables and a more efficient energy and plant

utilization.

Figure 8. Short and intense flame of the high-impulse Oxygen-Natural Gas burner

Case History 3

Case history 3 illustrates the application of oxygen to an incinerator of industrial toxic and

dangerous waste both in solid and liquid form.

The incineration unit is located inside a chemical factory; the plant has the authorization to burn hazardous waste both in-house generated and on behalf of third parties.

Solid waste is burnt in two static batch furnaces where primary air is supplied under the fixed

grate and no secondary air injection is used. Combustion gases from the two static furnaces go to

a vertical, top fired post-combustion chamber; the burner, located in the upper part of the unit, is

designed to burn high-calorific liquid wastes. Secondary air is and combustion gases from the

two solid waste furnaces are injected separately about 1 m below the liquid waste burner.

The system was designed to treat approximately 500 kg/h of solids and 150 kg/h of liquid

waste. Solid waste consists mainly of paperboard boxes, packaging, pharmaceutical and color

industry waste, while injected liquids are generally spent oils and solvents.

The main problem that the operators of the unit were facing was related to the worsening of

the combustion conditions inside the vertical post-combustion chamber after each charging ofthe two solid waste furnaces. Once the batch charge was introduced, the resulting combustion

gases, at relatively low temperature, immediately reached the post-combustion chamber causing

a drop in temperatures and free O2; as a consequence the CO content measured in the resulting

gases showed a rapid increase.

To simultaneously reduce the volume of gases to the post-combustion chamber and increase

their temperature, pure oxygen lancing in the two batch furnaces has been applied. One lance has

been placed inside the charging door of each furnace, injecting oxygen close to the material bed.

Once the batch has been charged, oxygen lancing starts and primary air is substantially lowered.

This allows, from one side, an important reduction of the volume and an increase of the








The possibility to use oxygen gave also the system the flexibility to be operated at different

loads of solid wastes, oils and waste waters. If an increase in the solid treatment capacity is

needed, the oxygen lancing through the charging door can be increased, while higher injection of

both waste waters and oils can be achieved varying the amount of oxygen in the post-combustion

chamber.

5. CONCLUSIONS

A number of field installations have demonstrated the significant advantages of oxygen usage

in the field of waste thermal treatment and biomass combustion. Common difficulties sometimes

associated with conventional oxygen combustion, such as local overheating and high NOx

emissions, are no cause for concern when the correct oxygen technology is applied.

CO puff frequency and magnitude can be reduced, and residual values of carbon in the ashes

can be decreased to the required level.

Oxygen combustion technology can improve the operation and increase the capacity of the plant; materials with lower quality can also be treated without negatively affecting the process

parameters.

While the general principles discussed would apply to most situations, the best method to

employ oxygen will vary on case-by-case basis: a detailed analysis of the process parameters and

economics is therefore needed before the selection of the appropriate technology.

REFERENCES

Hottel, H. C., Williams, G. C., Nerheim, N. M., and Schneider, G. (1965). Combustion of

Carbon Monoxide and Propane. 10th International Symposium on Combustion, Combustion

Institute, Pittsburgh, PA, pp. 111–121.

McGowan, T.F. (1988). The use oxygen in industrial incinerators. Proceedings of the

International Conference on Incineration of hazardous, Radioactive and Mixed Wastes by the

University of California at Irvine, San Francisco, California.

Min-Da Ho (1993). Recent advances in oxygen combustion technology for hazardous waste

incineration. 3rd International Conference on Waste Management in the Chemical and

Petrochemical Industries.

Morgan, A. C. (1967). Combustion of Methane in a Jet Mixed Reactor, D.Sci. thesis, M.I.T.,

Cambridge, Massachusetts.

Murzyn P. J., Min-Da Ho, Romano L. (1992). Oxygen enhanced hazardous waste incineration: a

detailed discussion of the technology and economics. Thermal Treatment and Air Pollution

Control, ppb 416-418.

Niessen W. R. (2003). Combustion and Incineration Processes. Third Edition, Marcel Dekker

Inc.

Stenburg, R. L., Horsley, R. R., Herrick, R. A., Rose, A. H. Jr. (1966). Effects of Design and

Fuel Moisture on Incinerator Effluents. JAPCA, 10:114–120.

Zabetakis, M. G. (1976). Flammability characteristics of combustible gases and vapors. Bulletin

627, Bureau of Mines, U.S. Dept. of Interior, Washington D.C.

Documents

The Use of Oxygen in Biomass and Waste-To-Energy Plants.pdf