The use of alternative fuels in the cement industry by xld14276


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Morten Kyhnau Hansen FLSmidth Alternative Fuels, Denmark

Possibilities for the use of alternative fuels in
the cement industry
In this paper, FLSmidth – recent winner of the Global Fuels Award 2008 for Most Innovative
Technology – presents an overview of the technology and services it provides that make it possible
for cement producers to efficiently replace fossil fuels by alternative fuels.

T    he use of alternative fuels in the cement industry
     has increased dramatically in the past decade, with
different developments achieved in different parts of the
                                                               • Oil sludge from refineries;
                                                               • Municipal solid waste, either raw or sorted and re-
                                                                   fined fractions such as refuse derived fuel (RDF);
world. However, a strong global trend is evident, which        • Meat and bone meal.
is expected to continue at an even faster pace in the          Biofuels are fuels that are derived from forestry and
coming decades.                                                agricultural waste. The types are many and will reflect
    Fuel changes in the cement industry are nothing            the local agriculture and forestry. Examples of biofuels
new. The main fuels used have changed substantially            include:
during the past century. Oil has to a large extent been        • Wood chips;
replaced by coal, of which a large fraction lately has         • Straw;
been replaced by petcoke (and natural gas). In some            • Rice husk;
countries alternative fuels are the principal fuel used        • Coffee husk;
in the cement industry. A good example is that of Ger-         • Bio oils (e.g. palm oil);
many (Figure 1).                                               • Ethanol produced from biomass (bioethanol);
    The use of alternative fuels has become more es-           • Chicken manure;
tablished and accepted by the industry as a whole. In          • Residues from palm oil production.
continuation of this development there has been a              This list is not complete and one main characteristic of
desire from cement producers for a more complete               alternative fuels is that the availability very much de-
scope of supply of equipment and services related to           pends on local conditions. There is not a world market
alternative fuels. To address these needs FLSmidth has         for alternative fuels as the volume specific value is low   Figure 1: Usage of
established a group solely dedicated to this endeavour         and does not make transportation over long distances        alternative fuels in
                                                                                                                           the whole German
– FLSmidth Alternative Fuels. In addition to supply-           feasible. This means that the availability of alternative     cement industry
ing market leading machines, FLSmidth is now able to           fuels in terms of quality, quantity and price must be es-     from 1999-2005.
take responsibility for complete alternative fuel project      tablished individually for each cement producer.
implementation – as well as for any form of technical
service required.

What are alternative fuels?                                      50,0
The term alternative fuels is not strictly defined, but in
this context refers to fuels that differ from today’s stand-
ard. This also means that the term is dynamic; what are
today’s alternative fuels will become standard fuels in
the future. Thus in today’s cement industry context, al-         30,0
ternative fuels include all fuels other than gas, oil, coal
and petcoke.
   Generally alternative fuels can be divided into two           20,0
main groups: Waste fuels and biofuels. Waste fuels are
fuels derived from household waste and industrial waste
and include:                                                     10,0
• Plastics;
• Paper;
• Used tyres;
                                                                         1999 2000 2001 2002 2003 2004 2005
• Sewage sludge;

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                           Why use alternative fuels?                                1000
                           Cement producers find the use of alternative                                         B ulky waste
                           fuels desirable for two main reasons:                                                  & biomass
                           • To lower or eliminate the fuel costs;
                           • To reduce greenhouse gas emissions (mainly
                                                                                                    Shr edded waste
                               CO2).                                                                & biomass
Figure 3: Particle         The two greatest variable production costs for
sizes and burnout          cement producers have been fuel and power
times for different                                                                              Pulver ised
                           consumption. This makes reductions in fuel
fuels used in the
                           cost very attractive. Reduction of greenhouse
cement industry.                                                                          10
                           gas emissions is also relevant because the ce-
                           ment industry accounts for approximately 5%
                           of CO2 emissions globally. For society the use of
                           waste derived fuels in the cement industry rep-
                           resents an environmentally and economically                      1
                           attractive means of waste disposal.
                                                                                            0.001     0.01        0.1        1         10        100
                                                                                                              D iameter (mm )
                           Technical challenges
                           Alternative fuels have different properties than
                           the fuels they replace and the use of alternative fuels will     2. The diffusion of O2 limits the combustion of alter-
                           therefore introduce challenges for the design and op-               native fuel rates to a much larger extent than fossil
                           eration of the fuel system and indeed the whole cement              fuels;
                           plant. Figure 2 presents an overview of the many aspects         3. Temperature does not have a strong effect on burn-
                           to be coordinated when firing alternative fuels on a ce-            out of alternative fuels;
                           ment plant. The technical challenges can be divided into         4. Particle size is not simply related to sieve residue for
                           five categories:                                                    alternative fuels;
                           • Proper burnout of the fuel;                                    5. The heating value of many alternative fuels is lower
                           • Maintaining clinker chemistry;                                    than for the standard fuels.
                           • Maintaining proper control of the process;                     Figure 3 shows an overview of particle sizes and burn-
                           • Maintaining the production capacity;                           out times for different fuels used in the cement industry.
                           • Keeping emissions within requirements                          Cement plants have traditionally been designed for
                                                                                            burning pulverised fuels with particle sizes generally
Figure 2: Illustration
                           Proper burnout                                                   smaller than 0.1mm, which generally burn out within
of the many details        There are several characteristic differences between the         10s of particle residence time. Alternative fuels – in
to be coordinated          combustion behaviour of solid alternative fuels and              shredded or bulk form – generally have much larger
when firing                solid fossil fuels.                                              particle sizes, which result in substantially increased
alternative fuels
on a cement plant.
                           1. The pyrolysis rate typically has a great influence on         times required to burn them out. Unless specific design
                              alternative fuel burnout, whereas its influence typi-         considerations are taken, operational difficulties are to
                              cally is insignificant for fossil fuels;                      be expected. Thus in order to ensure proper burnout of
                                                                                                               the fuel it is necessary to evaluate the
                                                                                                               process carefully and perhaps to make
                                                                                                               modifications to the calciner or other
       R ec eiving,                                                                                            components.
       s torage &                  Overall aspects:
      pretreatment                 • Control/regulation                                                       Clinker chemistry
                                   • Clinker production, heat consumption, availability                       Alternative fuels frequently contain
                                   • Chemistry                                                                impurities such as phosphates, chlo-
                                                                                                              rine, heavy metals, etc. The clinker
            C onveyal                                                     E mis s ions                        chemistry is mainly affected by minor
                                                                                                              components, e.g. Zn or P from tyre-
                                                                                                              derived fuels or meat and bone meal
                                                                                                              respectively. Other alternative fuel
                         Dos age                C alc iner
                                                                                                              impurities, for example coarse quartz
                                                                                                              particles, may also interfere with sat-
                                                HOT DIS C
                                                                                                              isfactory clinker formation. Practical
                                                                                                              experiences show that the use of al-
                            K iln burner
                                                                                                              ternative fuels rarely affect the clinker
                                                                                                              chemistry, but it is an issue that must
                                            B ac k-end                                                        be addressed in the design phase of a
                                                 &                                                            project.
                                                                    B ypas s

30       globalfuelsMAGAZINEMay 2008
Process control
Using alternative fuels leads to
challenges in the control system
that are due to the different physi-
cal and chemical properties of the
fuels. As with standard fuels, it is
vital to have accurate control over
the feeding rate of the fuel. This
can be more difficult with alterna-
tive fuels due to the fuels being less
homogenous than standard fuels.
Thus the handling and feeding sys-
tems must be carefully designed
to deliver the fuel as mixed and
homogenised at possible in order
to allow for a good control of the
feeding rate.

                                                                                                                                     Figure 4: Illustration
     Calciner type             SLC               SLC-D             ILC low-NOx
                                                                            O                 O                    O
                                                                                     ILC low NOx (high T) ILC low NOx with Hotdisc    of the great effect of
                                                                                                                                       calciner design and
     NOx (hv, coal)
      O                        850                830                  380                   330                     410              of fuel properties on
     NOx (petcoke)
      O                       1600                870                 1180                   720                    1780               NOx emission levels
                                                                                                                                     (mg NO2/Nm3 @ 10%

                                                                                                                                     Figure 5: Illustration
                                                                                                                                         of the significant
                                                                                                                                         effect of calciner
                                                                                                                                          design and fuel
                                                                                                                                        properties on NOx
                                                                                                                                          emission levels.

Production capacity                                           filter dust. If an appropriate bleed of Hg from the system
For existing plants designed to operate on fossil fuels,      is not applied, e.g. filter dust removal during raw mill
switching to alternative fuels may cause production ca-       processing, the average gaseous Hg emission level will
pacity to be limited by capacity on ID fan. This is mainly    equal the input level. Reduction of Hg emissions can
due to the increased amount of flue gas and pressure loss     also be achieved using activated carbon.
that are to be expected when switching from fossil to             Alternative fuels can contain higher amounts of
alternative fuels. The increase in flue gas flow using al-    nitrogen, which can lead to higher NOx emissions. How-
ternative fuels is mainly a consequence of the increased      ever, NOx can be reduced by process design. FLSmidth
moisture content of the fuels. In addition, plants using      offers calciner solutions that are specifically designed for
alternative fuels typically also employ a bypass to com-      low NOx emissions. NOx can also be actively reduced by
pensate for increased chlorine inputs, which will also        injecting NH3 in the flue gas, hereby using a Selective
increase the flue gas flow. It is therefore important that    Non-Catalytic Reduction, (SNCR), process.
a thorough process analysis of the system is performed            Alternative fuels have different combustion charac-
to avoid loss of production capacity. Whether it is a new     teristics to fossil fuels. This feature will influence the
plant or a retrofit, FLSmidth can perform this analysis,      CO formation mechanism and hence emission levels.
and suggestions to system modifications are made to           Theoretical models and plant measurements have shown
ensure that the alternative fuels utilisation will not have   that CO oxidation rates under conditions pertinent to
a severe impact on production capacity.                       preheaters drop drastically with temperature. At tem-
                                                              peratures pertinent to calciners, the CO oxidation rate is
Emissions                                                     relatively good. However with decreasing temperatures
When using alternative fuels, CO (carbon monox-               the CO oxidation rate drops – and below 650°C almost
ide) NOx, and Hg emissions, are among the issues to           no CO oxidation takes place. It is therefore important
be dealt with carefully. Alternative fuels may contain        that the calciner is designed appropriately to reduce CO
higher amounts of Hg than fossil fuels, e.g. municipal        before it escapes to the preheater and further that the
solid waste and sewage sludge. Hg is very volatile and        feeding of the fuel is well controlled.
only a negligible amount of the input is found in the
clinker. Consequently, Hg is ultimately concentrated          Financial possibilities
in the colder parts of the system, e.g. raw meal silo and     The use of alternative fuels offers cement producers new

                                                                                                      globalfuelsMAGAZINEMay 2008                       31
                                                                                   possibilities for lowering       kiln system. This may also simply be provided for as a
                                                                                   production costs via re-         future option.
                                                                                   duced fuel costs. In some
                                                                                   cases the use of alternative     Storage, handling and feeding systems
                                                                                   fuels can even be a new          Storage and extraction systems are also supplied by FLS-
                                                                                   source of revenue. In many       midth. KOCH, an FLSmidth group company, supplies
                                                                                   countries such as Germany,       silo extraction systems specifically designed to eliminate
                                                                                   cement plants are getting        bridging phenomena, thereby ensuring reliable fuel
                                                                                   paid for receiving waste         feed. FLSmidth group company Pfister supplies accu-
                                                                                   fuels. The reduced emis-         rate dosage systems for solid alternative fuel (Figure 8).
                                                                                   sion of CO2 as a result of       Both pneumatic and mechanical conveying systems are
                                                                                   using alternative fuels can,     supplied. For example KOCH supplies tube conveyors,
                                                                                   in some cases, be converted      which provides a flexible solution for reliable mechanical
                                                                                   into revenue by selling          conveying of solid alternative fuel (Figure 9). FLSmidth
                                                                                   carbon credits. One way of       Automation supplies the multi fuel controller, which is
                                                                                   achieving this is to establish   very valuable for ensuring stable operation when firing
                                                                                   a project under the Clean        a number of alternative fuels simultaneously.
                                                                                   Development Mechanism
                                                                                   (CDM) which is a part of         Conclusions
                                                                                   the Kyoto protocol.              The trends in the cement industry and in society in
                                                                                                                    general show that alternative fuel is an important fuel
                                                                                   FLSmidth technology              presently and its usage in the global cement industry
                                                                                   for alternative fuels            will increase rapidly. This is because fuel costs can be
                  Figure 6: Duoflex                 FLSmidth offers many products suited for handling of            substantially reduced and because its use can reduce
                  kiln burner for firing            firing alternative fuels in kiln systems for cement pro-        greenhouse gas emissions from the cement industry.
                  liquid and solid                  duction.                                                           Alternative fuels differ from fossil fuels in many as-
                  alternative fuels in
                  the rotary kiln.                                                                                  pects. Therefore, these differences ideally should be dealt
                                                     Calciner solutions                                             with at the design stage. This is relevant for the design
                                                     Calciner solutions should be designed to suit proc-            of specific sub-processes
                                                     ess and plant specific considerations, as well as to the       as well as for the process
                                                     properties of the intended fuels. Figure 4 summarises          as a whole. Substantial
                                                     the main categories of calciner solution supplied by           advantages are linked
                  Figure 7: The                      FLSmidth. Figure 5 illustrates the significant effect that     to considering linkages
                  Hotdisc, used to fire              calciner design and fuel properties have on NOx emis-          between all aspects of
                  bulky waste and
                  biomass to                         sion levels.                                                   the clinker production
                  substitute calciner                      Complete kiln systems can be designed for using          process – including han-
                  fuel.                              alternative fuels as the principal fuel. Alternatively – if    dling of alternative fuel
                                                                                    large amounts of alterna-       prior to being admitted         Figure 8:
                                                                      Waste inlet                                                                   Pfister’s rotor-
                                                 Hot tertiary air                   tive fuels are not to be used   to the process. FLSmidth
                                                                                                                                                    weigh feeder for
                                                                                    immediately – design con-       addresses all aspects of
                                                                                                                                                    accurate dosage
                                                                                    siderations can be made         optimal alternative fuel        of solid
                                                                    Raw meal        facilitating future usage       use on cement plants.           alternative fuels.
                                    Cold raw                        inlet           of alternative fuel. A Du-
                                    meal inlet
                                                                                    oflex kiln burner (shown
                                                                                    in Figure 6) provides for
                                                                                    firing of both liquid and
                                                                                    solid alternative fuels in
                                                                                    the rotary kiln. A Hotdisc
                                                                   Burning material allows for bulky waste and
                                                                    Rotating disc   biomass to substitute for
                                                                                    calciner fuel (Figure 7). The
                                                                                    Hotdisc may be installed
Kiln riser duct
                                                                                    immediately, or simply be
                                                                                                                     Figure 9: KOCH tube convey-
                                                                                    prepared for alternative
                                                                                                                     ance giving flexible and reliable
                                                                                    fuel as a future option. An      mechanical conveyance of solid
                                                                                    ILC calciner is installed        alternative fuel.
                                                                                    with provision for extend-
                                                        Scraper                     ing the residence time in
                    Kiln gases                                                      the future is shown. A by-
                                                                                    pass is included to alleviate
                                                                                    the chlorine loading of the

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