r wise had been pos si ble. Due to trans port lim i ta tions, the ad di tional
fuel will only sup ply a mi nor part (less than a few hun dreds MW
fuel
) of the
en ergy in a plant. There are sev eral op tions: co-com bus tion with coal in
pul ver ised or fluidised bed boil ers, com bus tion on added grates in serted in
pul ver ised coal boil ers, combustors for added fuel cou pled in par al lel to the
steam cir cuit of a power plant, ex ter nal gas pro duc ers de liv er ing its gas to
re place an oil, gas or pul ver ised fuel burner. Fur ther more bio mass can be
used for reburning in or der to re duce NO emis sions or for after burning to
re duce N
2
O emis sions in fluidised bed boil ers. Com bi na tion of fu els can
give rise to pos i tive or neg a tive syn ergy ef fects, of which the best known are
the in ter ac tions be tween S, Cl, K, Al, and Si that may give rise to or pre vent
de pos its on tubes or on cat a lyst sur faces, or that may have an in flu ence on
the for ma tion of di ox ins. With better knowl edge of these ef fects the pos i tive
ones can be uti lised and the neg a tive ones can be avoided.
Introduction
Co-com bus tion is prob a bly the least com pli cated and one of the most ad van ta -
geous ways of uti lis ing bio mass and waste for re place ment of fos sil fu els for sta tion ary
en ergy con ver sion. It is there fore of in ter est to sum ma rise its pos si bil i ties and lim i ta tions.
This is the pur pose of the pres ent re port that looks upon the topic from a gen eral tech ni cal
point of view, re cog nis ing that the rea son to avoid CO
2
emis sions from fos sil fu els is not a
na tional or a lo cal is sue but a global one. Eco nomic and en ergy sys tem as pects are im por -
tant but out side of the scope of the pres ent re port that fo cuses on tech ni cal de tails.
Co-com bus tion means si mul ta neous com bus tion of two or more fu els in the
same plant for en ergy pro duc tion. Al though this mode of com bus tion has been ap plied
for many years, the in ter est has been en hanced re cently, as seen from the ris ing num ber of
sci en tific pub li ca tions in fig. 1, men tion ing co-com bus tion in their head ings.
The num ber of pub li ca tions just in di cates that co-com bus tion has been dis cov -
ered as be ing prom is ing for the use of bio mass. When the first eu pho ria has de clined, it is
likely that the num ber of pub li ca tions deal ing with the phe nom e non as such will fall, and
DOI:10.22987/TSCI0704005L
5 in ter est will turn into more spe cific top ics, like com bus tion phe nom ena and is sues re lated
to de tails of pol lu tion and op er a tion dur ing co-com bus tion.
Co-com bus tion can be car ried out in var i ous ways for var i ous pur poses. A
coarse clas si fi ca tion could be as fol lows, cov er ing new plants as well as ex ist ing ones
con verted for the pur pose:
(1) a small amount (a few percent of total fuel power) of biofuel or waste is fired together
with coal in a boiler, originally designed for coal; the purpose is to get rid of waste or
to replace coal by biomass utilisation,
(2) a small amount of fuel with a high heating value is fired together with a fuel having a
low heating value (such as sludge) that needs thermal support to attain a desired
combustion temperature, and
(3) spontaneous use of co-combustion with fuels in any ratio, depending on price,
availability and local supply conditions.
The first type is of great est sig nif i cance due to its po ten tial abil ity to re duce the
con sump tion of coal, thereby de creas ing the emis sions of green house gases. It is of in ter -
est to as sess the pos si bil ity and re li abil ity of such uti li sa tion of bio mass and waste in a
plant de signed for the base fuel (nor mally coal). Item two, ad di tion of high-value fuel to a
low-value one, or in gen eral terms, com bi na tion of any fu els with dif fer ent prop er ties,
may have use ful sec ond ary con se quences, such as re duc tion of emis sions or im prov ing
re li abil ity of op er a tion. This could en hance the in ter est for com bi na tions of fu els, be -
cause cer tain fuel con stit u ents may in flu ence each other, syn ergy ef fects may take
place, lead ing to an im prove ment of op er a tion of a boiler and to avoid ance of in con ve -
niences re lated to some fu els. This will be treated be low, but first an ac count will be given
on the tech nol ogy of co-com bus tion. Item three is more un de ter mined: it could in clude
all kinds of com bi na tions of avail able fu els, such as in dus trial re sid u als, saw dust, wood
chips, peat, petcoke etc., used for en ergy con ver sion. It de pends on lo cal gov ern men tal
re stric tions (like taxes on coal in some Scan di na vian coun tries) and on the lo cal avail abil -
ity of waste fu els. The choice of fu els is re lated to eco nomic and trans port ad van tages. In
6
THERMAL SCIENCE: Vol. 11 (2007), No. 4, pp. 5-40
Figure 1. Number of scientific
publications per year dealing with
co-combustion or co-firing as
extracted from the data bank of
ScienceFinder, January 2003 this con text, co-com bus tion has been greatly pro moted by the in tro duc tion of fluidised
bed com bus tion, a tech nique that fa cil i tates the si mul ta neous com bus tion of dif fer ent fu -
els. It is of ten re lated to waste dis posal. Coal is not nec es sar ily in volved. Ex am ples of this
third type of co-com bus tion sit u a tion have been given by Järvinen and Alakangas [1].
Advantages and disadvantages
Co-com bus tion has a num ber of po ten tial ad van tages. A brief list could be as
fol lows:
reduction of CO
2
emissions from fossil fuels,
increased use of local fuels,
conversion of biomass and waste fuels with high efficiency and under controlled
environmental conditions,
seasonal variations that are inherent in some biofuels can be handled because the ratio
of added to base fuel can easily be changed down from its maximum value,
less complicated than alternative conversion methods for biofuels and, hence,
potentially economically advantageous,
the amount of additional fuel employed can be adjusted to the availability of biofuels
and wastes within a reasonable transport distance from the conversion plant, and
possible positive synergy effects between different fuels can be utilised.
Dis ad van tages can also be sus pected to oc cur:
the cost of some additional equipment or treatment processes has to be considered,
the threat of harmful influence on the plant, caused by the additional fuel,
possible negative synergy effects if the additional fuel has extreme properties (some
wastes) or if the combination of fuels is unfortunate, and
lack of experience, as reflected from two of the above items.
Methods
Any type of boiler may be used for co-com bus tion; boil ers for pul ver ised fuel or
for coarsely sized fuel in fluidised or fixed beds, prob a bly fir ing coal as the main fuel. The
boiler is the heat source in a util ity plant for power pro duc tion or an in dus trial or dis trict
heat ing plant. In a util ity boiler, the mi nor amount of co-fuel added to the main fuel is
treated in a highly ef fi cient en vi ron ment of a boiler with high steam data (such as typ i cal for
a util ity boiler). In the third group men tioned above, co-com bus tion ap proaches waste in -
cin er a tion in small (less than a few hun dreds MW
t
) boil ers, and ef fi ciency is op ti mised,
emphasising the re li abil ity of op er a tion with dif fi cult fu els and not nec es sar ily the ef fi -
ciency of elec tric ity pro duc tion. The boiler may even be a dis trict-heat ing boiler; a hot-wa -
ter boiler or a steam boiler for co-gen er a tion of elec tric power. In the lat ter one the steam
data are im por tant for the amount of power pro duced; the steam tem per a ture is a crit i cal pa -
ram e ter, since the sur face of the superheater may be come cov ered by de pos its from the ad -
7
Leckner, B.: Co-Combustion A Summary of Technology di tional fuel and sub se quently cor roded. Thorson [2] has clas si fied steam data used in this
group of ap pli ca tion (the third case of co-com bus tion, men tioned above), tab. 1.
Table 1. Typical maximum recommended steam data for co-combustion boilers [2]
Type of com bus ti ble
Superheater lo cated in
flue gas path, [°C]
Superheater lo cated in the re turn
leg from cy clone in CFB, [°C]
Conventional waste fuels
405
465
Problematic fuels in
co-combustion
460
520
Conventional bio fuels,
such as wood waste
480-500
540
Conventional bio fuels,
co-fired with peat
540
565
The ta ble shows a ben e fi cial in flu ence of peat on the pro pen sity of de posit for -
ma tion on tubes, as re flected by the ex pe ri ence of boiler de sign ers. Coal, and prob a bly
also sul phur, would have a sim i lar im pact as peat. The data in tab. 1 are lower than those
aimed at in mod ern util ity boil ers fired with fos sil fu els.
Fig ure 2 de picts four con ceiv able groups of ar range ment for co-com bus tion. In
ar range ment (a) the ad di tional fuel is sim ply added to a boiler de signed for the base fuel,
usu ally coal. This is the most con ve nient method, which can be used in con nec tion to
both fluidised bed and pul ver ised fuel boil ers. As men tioned, fluidised bed com bus tion
(FBC) is quite suit able for the pur pose be cause of its fuel flex i bil ity, whereas the pul ver -
ised coal (PC) combustor re quires a well de fined fuel size dis tri bu tion. The ex pe ri ence
shows that only mi nor quan ti ties of ad di tional fu els (a few per cent of the fuel power) can
8
THERMAL SCIENCE: