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
Ci a ion: Hudák, I.; Sk yja, P.;
Bojano ský, J.; Jegla, Z.; K ˇná ek, M.
The E ec o Ine Fuel Compounds
on Flame Cha ac e is ics. Ene gies
2022,15, 262. h ps://doi.o g/
10.3390/en15010262
Academic Edi o : Fla io Ca esana
Recei ed: 29 No embe 2021
Accep ed: 27 Decembe 2021
Published: 31 Decembe 2021
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Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
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A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ene gies
A icle
The E ec o Ine Fuel Compounds on Flame Cha ac e is ics
Igo Hudák1, Pa el Sk yja 1, Jiˇ íBojano ský1, Zdenˇek Jegla 1,* and Ma in K ˇná ek 2
1Ins i u e o P ocess Enginee ing, B no Uni e si y o Technology, Technická2896/2,
616 69 B no, Czech Republic; igo .hudak@ u b .cz (I.H.); [email p o ec ed].cz (P.S.);
Ji i.Bojano sky@ u b .cz (J.B.)
2E eco B no, s. .o., Hudco a 76d, 621 00 B no-Medlánky, Czech Republic; [email p o ec ed]
*Co espondence: [email p o ec ed]
Abs ac :
To desc ibe he e ec s o ine compounds in gaseous uel, expe imen s on h ee di e en
p ocess bu ne s (s aged uel bu ne , s aged ai bu ne , and low-calo i ic bu ne ) we e ca ied ou . The
es ed bu ne s a e comme cially a ailable, bu hey we e specially designed o expe imen al usage.
Tes s we e ca ied ou in he semi-indus ial bu ne es ing acili y o in es iga e he in luence o ine
gases on he lame cha ac e is ics, emissions, and hea lux o he combus ion chambe wall. Na u al
gas was used as a e e ence uel, and, du ing all es s, he mal powe o 500 kW was main ained. To
simula e he combus ion o al e na i e uels wi h lowe LHV, N
2
and CO
2
we e used as diluen s. The
ine gas in he hyd oca bon uel a ce ain condi ions can lowe NO
x
emissions (up o 80%) and
inc ease hea lux (up o 5%). Once incombus ible compounds a e p esen in he uel, he highe
amoun o uel lowing h ough nozzles a ec s he low in he combus ion chambe by inc easing
he Reynolds numbe . This can change he lame pa e n and empe a u e ield, and i can be bo h
posi i e and nega i e, depending on ac ual condi ions.
Keywo ds: low-calo i ic bu ne ; low NOxbu ne ; combus ion; ine gas; al e na i e gaseous uels
1. In oduc ion
To ul ill demands based on documen s such as he so-called g een deal o Eu ope [
1
],
i is necessa y o p o ide new design solu ions and op imize p ocess bu ne s o a ious
al e na i e uels. One o he possibili ies is he simul aneous o al e na e combus ion
o gaseous uels such as na u al gas and uels om al e na i e sou ces (biogas plan s,
py olysis plan s, algae [
2
]). Al e na i e uels p oduced, o example, in biogas s a ions and
py olysis uni s a e cha ac e ized by hei a iable composi ion. Thus, hei combus ion can
cause p oblems. Al e na i e uels wi h a lowe hea ing alue (LHV) a e usually combus ed
in bu ne s speci ically designed o he combus ion o a pa icula uel. The LHV in his
ype o uel is lowe ed by he p esence o ine gases (ni ogen, ca bon dioxide, and o he s).
To sus ain su icien ene gy powe in he u naces o boile s, i is necessa y o deli e a
highe amoun o uel in o he combus ion chambe (compa ed wi h he combus ion o
na u al gas). Usually, con en ional bu ne s ini ially designed o he combus ion o na u al
gas and uels wi h simila LHV a e no designed o be used along wi h low-calo i ic uels.
The e o e, i is usually necessa y o change he bu ne design. On he o he hand, a as
amoun o non-combus ible pa icles can be a eason o use al e na i e low-calo i ic uels
because o hei abili y o lowe empe a u e peaks and hus educe he e ec o he mal
NO
x
nuclea ion. The u iliza ion o al e na i e uels can also be a solu ion o abide by
inc easingly s ingen legisla i e limi s.
Exis ing esea ch ha deals wi h he combus ion o gaseous uels wi h ine com-
pounds (usually ca bon dioxide, ni ogen, a gon, wa e , and wa e apo ) and hei e ec s
on lame cha ac e is ics and CO and NO
x
emissions can be di ided in o wo basic ca ego ies.
The i s ca ego y is he dilu ion o he noble uel, such as me hane o na u al gas (including
esea ch wi h biogas—usually 60% ol. CH
4
, 40% ol. CO
2
), wi h he ine compound. On
Ene gies 2022,15, 262. h ps://doi.o g/10.3390/en15010262 h ps://www.mdpi.com/jou nal/ene gies
Ene gies 2022,15, 262 2 o 18
he o he hand, he second ca ego y con ains expe imen s wi h syn he ic gases (syngas) o
was e gases composed o combus ible compounds such as me hane, hyd ogen, and ca bon
monoxide and con aining incombus ible elemen s.
Se e al esea ch g oups ha e in es iga ed he combus ion o dilu ed gaseous uels
wi h hyd oca bons as he main combus ible componen . Fo example, Li e al. [
3
] conduc ed
expe imen s and nume ical simula ions o add ni ogen, ca bon dioxide, a gon, and wa e
apo in o me hane. The addi ion o s eam o he me hane lame was also examined
by Zhao e al. [
4
]. The in luence o supe hea ed wa e apo and CO
2
on me hane/ai
p emixed lames was expe imen ally in es iga ed by Kobayashi e al. [
5
,
6
]. To summa ize,
dilu ion by wa e and wa e apo , once added o he lame, signi ican ly lowe s NO
x
emissions in he lue gas. Mo eo e , a e s eam addi ion, a change was obse ed in he
decomposi ion o me hane in o OH adicals. Fu he mo e, he s eam added in o he lame
esul ed in a dec ease in he OH concen a ion in he lame; hus, a he same ime, i was
possible o obse e a dec ease in CH adicals, which esul ed in he supp ession o p omp
NO
x
. I was also obse ed ha he e ec o dilu ion by CO
2
is highe han dilu ion by
wa e , i.e., when ecycled lue gas is used, CO
2
is p edominan . Mo eo e , Pa k e al. [
7
]
e ealed ha he addi ion o H2O signi ican ly in luenced NO nuclea ion.
Mo e s udies ega ding he educ ion o NO
x
emissions whe e ine gases such as CO
2
and N
2
we e added o he uel we e conduc ed by Gla bo g e al. [
8
], Rang azi e al. [
9
],
De i e al. [
10
] (combus ion o biogas), and Sal ado e al. [
11
] (addi ion o N
2
). These
expe imen s e ealed ha he eac ion o CO
2
wi h hyd oca bons could con ibu e o
CO o ma ion, which can lead o nea -bu ne co osion and slagging. Mo eo e , i was
ound ha A dilu ion is less e ec i e han N
2
dilu ion ega ding educing NO
x
o ma ion.
Reduc ion o NO
x
can be achie ed by inc easing he ai su plus ( educ ion up o 30%), bu
when he ai is ully subs i u ed wi h ni ogen, he educ ion can be 60%. Fu he mo e,
du ing expe imen s wi h biogas, Dai e al. [
12
] e ealed ha a highe concen a ion o
CO
2
and lowe uel empe a u e c ea ed uns able condi ions o combus ion. Fu he mo e,
Hin on e al. [
13
] ound ou ha he highe p essu e o biogas led o a dec ease in bu ning
eloci y; howe e , a highe biogas empe a u e had he opposi e e ec .
Fo example, he combus ion o syngas and i s dilu ion was examined by
Chun e al. [14]
.
Among he esul s desc ibed abo e, i was disco e ed ha an inc ease in N adicals leads
o mo e signi ican NO
x
p oduc ion. Kied zynska e al. [
15
] used CFD o analyze he
co-combus ion o na u al gas and syngas in an unmodi ied bu ne . I was ound ha
i is possible o add only a small amoun o syngas (up o 10% o he mal sha e) be o e
eaching he bu ne limi . Fu he mo e, in e changeabili y be ween na u al gas and o he
enewable gases was s udied by Maznoy e al. [
16
]. The esea ch was mainly ocused on
s abili y limi s, adia ion e iciency, and CO/NO
x
emissions, which we e s udied du ing
he expe imen s. The addi ion o CO
2
educed NO
x
emissions bu o med a colde lame,
which esul ed in a CO emission inc ease. I was con i med ha uel in e changeabili y
is possible and can inc ease he adia ion e iciency in adial bu ne s. Dai e al. [
12
] also
examined lame s abili y, whe e i was disco e ed ha a lowe a io o me hane in uel
mo i a ed he lame li ing, while, on he o he hand, yellow ipping was mo i a ed by a
highe me hane con en . Flame s abili y was also pa ly in es iga ed by Song e al. [
17
]. A
as amoun o an ine compound can nega i ely a ec lame s abili y, and when a speci ic
limi is eached, he lame can ex inguish.
F om he e iew, i is possible o see he e ec on mul iple pa ame e s once an ine
compound is p esen in he gaseous uel. Howe e , mos s udies ha e been ca ied ou in
labo a o y condi ions, o hei esul s we e acqui ed by compu e simula ion, which used da a
om measu emen s on small-scale de ices. The au ho s o his s udy ca ied ou expe imen s
on a semi-indus ial acili y ha had simila pa ame e s o some o he smalle u naces. The
main con ibu ion o his s udy o he p e ious esea ch is he desc ip ion o he changes
in he hea lux, bu also he cla i ica ion and alida ion o p io esul s ega ding he NO
x
emissions and especially in lame empe a u es, which a e desc ibed by empe a u e p o iles.
Fu he mo e, h ee bu ne s wi h di e en designs we e used and compa ed ega ding he
Ene gies 2022,15, 262 3 o 18
emissions, lame cha ac e is ics, lame empe a u es, and hea lux. This s udy aimed o e eal
he possibili ies o uel in e changeabili y in con en ional p ocess bu ne s and o ind limi s
while inc easing he amoun o ine compounds in he uel.
2. Ma e ials and Me hods
2.1. Tes Equipmen
Combus ion es s we e ca ied ou a he bu ne es ing acili y (Figu e 1). The acili y
is designed o pe o m combus ion es s o bu ne s wi h he mal powe up o 1.8 MW and
collec expe imen al da a o he u he assessmen and e i ica ion o nume ical simula ion
esul s. Du ing he combus ion es wi h low-calo i ic uels, he bu ne s’ he mal ou pu
was se o 500 kW (and kep a his alue du ing each es ), and pa ame e s such as NO
x
and CO emissions, lue gas empe a u e, hea lux o he wall o he combus ion chambe
as a measu e o he mal e iciency, dis ibu ion o in- lame empe a u es in he ho izon al
symme y plane o he combus ion chambe , and he s abili y, shape, and dimensions o
he lame we e in es iga ed. In addi ion, he mal powe was eleased by he combus ion o
ansi na u al gas ( o a e age composi ion, see Table 1), u he dilu ed wi h ine gases
such as ni ogen and ca bon dioxide, o in es iga e hei e ec on he lame cha ac e is ics.
Ene gies 2022, 15, x FOR PEER REVIEW 3 o 18
compa ed ega ding he emissions, lame cha ac e is ics, lame empe a u es, and hea
lux. This s udy aimed o e eal he possibili ies o uel in e changeabili y in con en ional
p ocess bu ne s and o ind limi s while inc easing he amoun o ine compounds in he
uel.
2. Ma e ials and Me hods
2.1. Tes Equipmen
Combus ion es s we e ca ied ou a he bu ne es ing acili y (Figu e 1). The acili y
is designed o pe o m combus ion es s o bu ne s wi h he mal powe up o 1.8 MW and
collec expe imen al da a o he u he assessmen and e i ica ion o nume ical simula-
ion esul s. Du ing he combus ion es wi h low-calo i ic uels, he bu ne s’ he mal ou -
pu was se o 500 kW (and kep a his alue du ing each es ), and pa ame e s such as
NOx and CO emissions, lue gas empe a u e, hea lux o he wall o he combus ion
chambe as a measu e o he mal e iciency, dis ibu ion o in- lame empe a u es in he
ho izon al symme y plane o he combus ion chambe , and he s abili y, shape, and di-
mensions o he lame we e in es iga ed. In addi ion, he mal powe was eleased by he
combus ion o ansi na u al gas ( o a e age composi ion, see Table 1), u he dilu ed
wi h ine gases such as ni ogen and ca bon dioxide, o in es iga e hei e ec on he
lame cha ac e is ics.
Figu e 1. Semi-indus ial bu ne es ing acili y.
Table 1. A e age na u al gas composi ion du ing he combus ion es s.
CH4 C2H6 C3H8 i-C4H8 n-C4H8 i-C5H10 n-C5H10 C6+ CO2 N2
[% mol.] 96.486 2.593 0.127 0.043 0.021 0.005 0.003 0.024 0.333 0.365
The cen al appa a us o he acili y is a wo-shell ho izon al wa e -cooled combus-
ion chambe wi h an inne diame e o 1 m and an ou e leng h o 4 m. The on and he
ea side o he chambe a e insula ed wi h a high- empe a u e ib ous lining wi h a hick-
ness o 100 mm. The cooling shell o he combus ion chambe is di ided in o se en indi-
idual sec ions wi h an independen supply o cooling wa e . Each sec ion is equipped
wi h senso s o measu e he low a e, inle , and ou le empe a u e o cooling wa e . Wa-
e low a e is measu ed by u bine low me e s, and inle and ou le empe a u es a e
measu ed by esis ance he mome e s placed in he s eel shea h.
Be o e he lue gas is eleased in o he a mosphe e, i lows om he combus ion
chambe h ough he lue gas s ack. The e a e h ee measu emen and sampling spo s o
measu ing he p essu e in he combus ion chambe , lue gas empe a u e, and lue gas
composi ion. The lue gas analysis and lue gas empe a u e measu emen s a e p o ided
by he lue gas analyze TESTO 350-XL. The analysis box is equipped wi h elec ochemical
senso s o he eal- ime measu emen o O2, CO, CO2, NO, and NO2 concen a ions in he
d y lue gas. The lue gas empe a u e is measu ed using an R- ype he mocouple.
Figu e 1. Semi-indus ial bu ne es ing acili y.
Table 1. A e age na u al gas composi ion du ing he combus ion es s.
CH4C2H6C3H8i-C4H8n-C4H8i-C5H10 n-C5H10 C6+ CO2N2
[% mol.] 96.486 2.593 0.127 0.043 0.021 0.005 0.003 0.024 0.333 0.365
The cen al appa a us o he acili y is a wo-shell ho izon al wa e -cooled combus ion
chambe wi h an inne diame e o 1 m and an ou e leng h o 4 m. The on and he ea
side o he chambe a e insula ed wi h a high- empe a u e ib ous lining wi h a hickness
o 100 mm. The cooling shell o he combus ion chambe is di ided in o se en indi idual
sec ions wi h an independen supply o cooling wa e . Each sec ion is equipped wi h
senso s o measu e he low a e, inle , and ou le empe a u e o cooling wa e . Wa e low
a e is measu ed by u bine low me e s, and inle and ou le empe a u es a e measu ed
by esis ance he mome e s placed in he s eel shea h.
Be o e he lue gas is eleased in o he a mosphe e, i lows om he combus ion
chambe h ough he lue gas s ack. The e a e h ee measu emen and sampling spo s o
measu ing he p essu e in he combus ion chambe , lue gas empe a u e, and lue gas
composi ion. The lue gas analysis and lue gas empe a u e measu emen s a e p o ided
by he lue gas analyze TESTO 350-XL. The analysis box is equipped wi h elec ochemical
senso s o he eal- ime measu emen o O
2
, CO, CO
2
, NO, and NO
2
concen a ions in he
d y lue gas. The lue gas empe a u e is measu ed using an R- ype he mocouple.
Combus ion ai is supplied o he on o he combus ion chambe using a high-
p essu e an equipped wi h a equency con e e . The maximum an ou pu is app ox-
ima ely 4500 m
N3
/h wi h a maximum o e p essu e o 11.2 kPa. The ai duc is also
Ene gies 2022,15, 262 4 o 18
equipped wi h a p ehea ing uni o combus ion ai p e- ea men . The e o e, combus ion
ai can be p ehea ed up o 500 ◦C.
The es ing acili y is equipped wi h a sophis ica ed da a collec ion and sa e y sys em.
The sys em o da a collec ion enables au oma ic da a collec ion e e y second o
wo minu es
and da a collec ion upon he ope a o ’s eques . Collec ed da a include low a es, p essu es,
and empe a u es o combus ion ai , uel, ine gases, and cooling wa e . The sa e y sys em
ensu es he sa e and eliable ope a ion o he es ing acili y using he in o ma ion om he
senso s, e.g., i p e en s p ehea ing o cooling wa e and lame blow-o .
2.2. Me hodology
The expe imen was i s ca ied ou wi h he e e ence uel—in his case, na u al gas
(composi ion Table 1). All he esul s ob ained wi h low-calo i ic uels we e compa ed o
his e e ence uel. Low-calo i ic uels a e speci ic o hei con en o non-combus ible
compounds, which lowe he LHV, bu he e is a leas one lammable compound. Thus,
he ollowing combus ion es s o low-calo i ic uels equi ed a combus ible componen —in
his case, na u al gas. E en hough me hane is usually only combus ible in syngas o
biogas, me hane was subs i u ed wi h na u al gas du ing he expe imen due o i s simila
Wobbe’s numbe (me hane—50.7 MJ/m3, a ailable na u al gas—50.5 MJ/m3).
The mixing s a ion ensu ed p ope mixing o ine gases and combus ible componen s,
as shown in Figu e 2. The s a ion can mix ou s eams (me hane—up o 100 m
N3
/h,
hyd ogen—up o 500 m
N3
/h, ca bon dioxide—up o
300 mN3/h
, and ni ogen—up o
300 mN3/h
). Each s eam is egula ed and measu ed by B onkho s and M + W ins umen s
pilo al es; he low o gases can be p ecisely egula ed, measu ed, and eco ded. In
addi ion, uel pa ame e s can be egula ed by changing he uel composi ion. The e o e,
LHV can a y in he ange om 5 o 35 MJ/m3.
Ene gies 2022, 15, x FOR PEER REVIEW 4 o 18
Combus ion ai is supplied o he on o he combus ion chambe using a high-
p essu e an equipped wi h a equency con e e . The maximum an ou pu is app oxi-
ma ely 4500 mN3/h wi h a maximum o e p essu e o 11.2 kPa. The ai duc is also
equipped wi h a p ehea ing uni o combus ion ai p e- ea men . The e o e, combus ion
ai can be p ehea ed up o 500 °C.
The es ing acili y is equipped wi h a sophis ica ed da a collec ion and sa e y sys em.
The sys em o da a collec ion enables au oma ic da a collec ion e e y second o wo
minu es and da a collec ion upon he ope a o ’s eques . Collec ed da a include low a es,
p essu es, and empe a u es o combus ion ai , uel, ine gases, and cooling wa e . The
sa e y sys em ensu es he sa e and eliable ope a ion o he es ing acili y using he in o -
ma ion om he senso s, e.g., i p e en s p ehea ing o cooling wa e and lame blow-o .
2.2. Me hodology
The expe imen was i s ca ied ou wi h he e e ence uel—in his case, na u al gas
(composi ion Table 1). All he esul s ob ained wi h low-calo i ic uels we e compa ed o
his e e ence uel. Low-calo i ic uels a e speci ic o hei con en o non-combus ible
compounds, which lowe he LHV, bu he e is a leas one lammable compound. Thus,
he ollowing combus ion es s o low-calo i ic uels equi ed a combus ible componen —
in his case, na u al gas. E en hough me hane is usually only combus ible in syngas o
biogas, me hane was subs i u ed wi h na u al gas du ing he expe imen due o i s simila
Wobbe’s numbe (me hane—50.7 MJ/m3, a ailable na u al gas—50.5 MJ/m3).
The mixing s a ion ensu ed p ope mixing o ine gases and combus ible compo-
nen s, as shown in Figu e 2. The s a ion can mix ou s eams (me hane—up o 100 mN3/h,
hyd ogen—up o 500 mN3/h, ca bon dioxide—up o 300 mN3/h, and ni ogen—up o 300
mN3/h). Each s eam is egula ed and measu ed by B onkho s and M + W ins umen s
pilo al es; he low o gases can be p ecisely egula ed, measu ed, and eco ded. In ad-
di ion, uel pa ame e s can be egula ed by changing he uel composi ion. The e o e,
LHV can a y in he ange om 5 o 35 MJ/m3.
Figu e 2. Model o a mixing s a ion.
Hyd ogen and ine gases we e supplied in bundles p essu ized up o 200 ba (300
ba in he case o ni ogen). Thei p essu e was educed o 8 ba ( he calib a ion p essu e
o he lowme e s) be o e lowing in o he mixing s a ion. Then, hese gases we e mixed,
c ea ing a combus ible mix u e. The limi a ion o he whole uni was he p essu e o he
na u al gas, which was a ailable only a he o e p essu e a 0.1 ba . The e o e, he p es-
su e o he hyd ogen and ine gas mix u e was u he dec eased be o e mixing wi h na -
u al gas and be o e en e ing he bu ne .
Figu e 2. Model o a mixing s a ion.
Hyd ogen and ine gases we e supplied in bundles p essu ized up o 200 ba (
300 ba
in he case o ni ogen). Thei p essu e was educed o 8 ba ( he calib a ion p essu e o he
lowme e s) be o e lowing in o he mixing s a ion. Then, hese gases we e mixed, c ea ing
a combus ible mix u e. The limi a ion o he whole uni was he p essu e o he na u al
gas, which was a ailable only a he o e p essu e a 0.1 ba . The e o e, he p essu e o he
hyd ogen and ine gas mix u e was u he dec eased be o e mixing wi h na u al gas and
be o e en e ing he bu ne .
A i s , o e i y he possibili y o uel in e changeabili y, es s we e ca ied ou on wo
di e en ypes o con en ional expe imen al bu ne s. This means ha se e al pa ame e s
can be easily changed on each bu ne o achie e he equi ed combus ion pa ame e s ( lame
shape, emissions, e c.). Each bu ne was adjus ed o he combus ion o he na u al gas o
Ene gies 2022,15, 262 5 o 18
achie e op imal esul s ega ding he NO
x
emission and lame s abili y. Each expe imen
was ca ied ou while combus ing a uel equi alen o 500 kW.
Fo he i s se ies o expe imen s, a low-NO
x
gas-s aged bu ne wi h al e able geom-
e y (deno ed as Bu ne A) was used. This bu ne can be ope a ed ei he in he p ima y
o seconda y egime wi h mo e uel po s. Fuel en e s he combus ion chambe h ough
12 po s in he p ima y egime, dis ibu ed on wo ci cula a cs. In he seconda y egime,
ou seconda y nozzles a e used o uel injec ion. These nozzles can be posi ioned in
axial, adial, and angen ial di ec ions. Bu ne A can be seen in Figu e 3a and is capable o
u ndown 1:7.
Ene gies 2022, 15, x FOR PEER REVIEW 5 o 18
A i s , o e i y he possibili y o uel in e changeabili y, es s we e ca ied ou on
wo di e en ypes o con en ional expe imen al bu ne s. This means ha se e al pa am-
e e s can be easily changed on each bu ne o achie e he equi ed combus ion pa ame e s
( lame shape, emissions, e c.). Each bu ne was adjus ed o he combus ion o he na u al
gas o achie e op imal esul s ega ding he NO
x
emission and lame s abili y. Each ex-
pe imen was ca ied ou while combus ing a uel equi alen o 500 kW.
Fo he i s se ies o expe imen s, a low-NO
x
gas-s aged bu ne wi h al e able geom-
e y (deno ed as Bu ne A) was used. This bu ne can be ope a ed ei he in he p ima y
o seconda y egime wi h mo e uel po s. Fuel en e s he combus ion chambe h ough
12 po s in he p ima y egime, dis ibu ed on wo ci cula a cs. In he seconda y egime,
ou seconda y nozzles a e used o uel injec ion. These nozzles can be posi ioned in axial,
adial, and angen ial di ec ions. Bu ne A can be seen in Figu e 3a and is capable o u n-
down 1:7.
The second bu ne (Bu ne B) used o es ing is shown in Figu e 3b. This bu ne is
designed as an ai -s aged bu ne . The a io be ween he p ima y and seconda y ai can be
changed due o he in e changeable inne placings. Modula ion o he ai low posi i ely
a ec s he lame dilu ion, and i can elimina e empe a u e peaks whe e he mal NO
x
is
o med. The p ima y uel head has 16 po s dis ibu ed in wo ci cula ields, whe e one
o he ields ( he closes o he lame holde ) s abilizes he lame, while he o he is de-
signed o deli e he maximal he mal ou pu . The bu ne is capable o u ndown 1:10.
Bo h bu ne s we e igni ed and s abilized wi h he injec o bu ne wi h a he mal ou -
pu o 18 kW.
(a) (b)
Figu e 3. Models o commonly used con en ional bu ne s: (a) bu ne wi h s aged uel dis ibu ion
(Bu ne A); (b) bu ne wi h s aged uel dis ibu ion (Bu ne B).
A e he expe imen s wi h he wo con en ional bu ne s and p elimina y da a e al-
ua ion, i was e iden ha in o de o combus low-calo i ic uels and achie e desi ed uel
low a es, a di e en bu ne had o be used. This s ep was necessa y mainly due o ope -
a ional p oblems (especially he abili y o each he desi ed he mal ou pu while com-
bus ing low-calo i ic uel) de ec ed using he abo e bu ne s. The used bu ne (Bu ne C,
Figu e 4) e lec s all he exis ing indings. The calcula ed dimensions and he inal geom-
e y we e designed o combus low-calo i ic uel wi h he minimal LHV o 6.9 MJ/m
N3
.
This alue is he minimum ( o he mal powe o 1500 kW) achie able using he p esen ed
mixing s a ion. Low-calo i ic uel wi h his LHV can deli e maximal he mal powe , ei-
he 750 o 1500 kW, depending on he geome y o he bu ne head and pa ame e s o
he uel. Bu ne C is designed o use he same windbox as Bu ne A. The e o e, i is pos-
sible o easily e o i a con en ional bu ne wi h he low-calo i ic bu ne by changing he
inne placing. This bu ne is designed as an expe imen al de ice; many pa s a e eplace-
able and modula . Fo example, di e en ypes o lame holde s and bu ne heads wi h
Figu e 3.
Models o commonly used con en ional bu ne s: (
a
) bu ne wi h s aged uel dis ibu ion
(Bu ne A); (b) bu ne wi h s aged uel dis ibu ion (Bu ne B).
The second bu ne (Bu ne B) used o es ing is shown in Figu e 3b. This bu ne is
designed as an ai -s aged bu ne . The a io be ween he p ima y and seconda y ai can be
changed due o he in e changeable inne placings. Modula ion o he ai low posi i ely
a ec s he lame dilu ion, and i can elimina e empe a u e peaks whe e he mal NO
x
is
o med. The p ima y uel head has 16 po s dis ibu ed in wo ci cula ields, whe e one o
he ields ( he closes o he lame holde ) s abilizes he lame, while he o he is designed
o deli e he maximal he mal ou pu . The bu ne is capable o u ndown 1:10.
Bo h bu ne s we e igni ed and s abilized wi h he injec o bu ne wi h a he mal
ou pu o 18 kW.
A e he expe imen s wi h he wo con en ional bu ne s and p elimina y da a e alua-
ion, i was e iden ha in o de o combus low-calo i ic uels and achie e desi ed uel low
a es, a di e en bu ne had o be used. This s ep was necessa y mainly due o ope a ional
p oblems (especially he abili y o each he desi ed he mal ou pu while combus ing low-
calo i ic uel) de ec ed using he abo e bu ne s. The used bu ne (Bu ne C, Figu e 4) e lec s
all he exis ing indings. The calcula ed dimensions and he inal geome y we e designed o
combus low-calo i ic uel wi h he minimal LHV o 6.9 MJ/m
N3
. This alue is he minimum
( o he mal powe o 1500 kW) achie able using he p esen ed mixing s a ion. Low-calo i ic
uel wi h his LHV can deli e maximal he mal powe , ei he 750 o 1500 kW, depending on
he geome y o he bu ne head and pa ame e s o he uel. Bu ne C is designed o use he
same windbox as Bu ne A. The e o e, i is possible o easily e o i a con en ional bu ne
wi h he low-calo i ic bu ne by changing he inne placing. This bu ne is designed as an
expe imen al de ice; many pa s a e eplaceable and modula . Fo example, di e en ypes o
lame holde s and bu ne heads wi h di e en geome ies and uel dis ibu ion can be used.
The main ea u e o he bu ne is ha uel and ai a e no s aged, bu bo h a e deli e ed as a
single s eam. Once mixed in he combus ion chambe , hey appea as homogenous lames.
Bo h bu ne s’ head and swi l gene a o we e designed o gene a e a u bulen low o ensu e
p ope low-calo i ic uel and ai mixing, i.e., pe ec combus ion.

Ene gies 2022,15, 262 6 o 18
Ene gies 2022, 15, x FOR PEER REVIEW 6 o 18
di e en geome ies and uel dis ibu ion can be used. The main ea u e o he bu ne is
ha uel and ai a e no s aged, bu bo h a e deli e ed as a single s eam. Once mixed in
he combus ion chambe , hey appea as homogenous lames. Bo h bu ne s’ head and
swi l gene a o we e designed o gene a e a u bulen low o ensu e p ope low-calo i ic
uel and ai mixing, i.e., pe ec combus ion.
Figu e 4. Model o low-calo i ic bu ne designed a e he i s expe imen s (deno ed as Bu ne C).
Di e en ope a ional pa ame e s we e obse ed in he expe imen . In he i s es
(TEST A), he in luence o CO2 and N2 addi ion in o he noble uel on NOx emissions, lue
gas empe a u e, and lame s abili y was in es iga ed. Due o he di e en bu ne geom-
e ies, which limi ed he o e all uel amoun ha could be bu ned unde ce ain condi-
ions, an expe imen al ma ix o each es was c ea ed o show he di e ences be ween
he expe imen s. A e each change in he combus ion pa ame e s, he combus ion cham-
be ope a ed o 30 min be o e he measu emen s we e eco ded in o de o elimina e he
luc ua ions in he da a and ensu e op imal esul s. Fu he mo e, all he s a ed emission
esul s we e measu ed a 3% O2 in he lue gas a he sampling spo in he lue gas duc
be o e connec ing o he chimney. The echnological scheme o he combus ion chambe
is shown in Figu e 5.
Figu e 4. Model o low-calo i ic bu ne designed a e he i s expe imen s (deno ed as Bu ne C).
Di e en ope a ional pa ame e s we e obse ed in he expe imen . In he i s es
(TEST A), he in luence o CO
2
and N
2
addi ion in o he noble uel on NO
x
emissions,
lue gas empe a u e, and lame s abili y was in es iga ed. Due o he di e en bu ne
geome ies, which limi ed he o e all uel amoun ha could be bu ned unde ce ain
condi ions, an expe imen al ma ix o each es was c ea ed o show he di e ences
be ween he expe imen s. A e each change in he combus ion pa ame e s, he combus ion
chambe ope a ed o 30 min be o e he measu emen s we e eco ded in o de o elimina e
he luc ua ions in he da a and ensu e op imal esul s. Fu he mo e, all he s a ed emission
esul s we e measu ed a 3% O
2
in he lue gas a he sampling spo in he lue gas duc
be o e connec ing o he chimney. The echnological scheme o he combus ion chambe is
shown in Figu e 5.
Ene gies 2022, 15, x FOR PEER REVIEW 7 o 18
Figu e 5. Model o low-calo i ic bu ne designed a e he i s expe imen s (deno ed as Bu ne C).
TEST B was ocused on he lame empe a u e. Fo his es , R- ype he mocouples
we e used o measu e he empe a u e in he ho izon al plane o he lame. In o al, eigh
R- ype he mocouples we e used o co e he empe a u es along he ho izon al plane o
symme y. Measu ing poin s we e loca ed 50 cm om each o he along he whole com-
bus ion chambe . Each he mocouple was connec ed o he G aph ec midi LOGGER
GL220, which eco ded he measu ed alues. Tempe a u es we e measu ed a six di e -
en posi ions: nea he wall (5 cm), 10, 20, 30, 40, and 50 cm om he wall.
TEST C was ocused on measu emen s o hea lux in o he wall o he combus ion
chambe . This was allowed due o he unique cons uc ion o he combus ion chambe ,
di ided in o se en di e en sec ions and equipped wi h he mocouples and lowme e s
o measu e cooling wa e p ope ies o calcula e he o e all hea lux.
Du ing TEST A, limi s (bo h in design and echnological) o each bu ne we e e-
ealed. Thus, o TEST B and TEST C, only a ew di e en uel composi ions we e selec ed,
which we e adjus ed acco ding o he esul s ob ained du ing TEST A and o ob ain da a
desc ibing he end wi hou unnecessa y was e o he uel. Be o e each es , condi ions in
he combus ion we e s abilized; i was i al o con ol he low a e o uel and combus ion
ai o achie e s able condi ions. Al hough measu emen s we e conduc ed in exac in e -
als, maximum e o was used o ensu e he epea abili y and accu acy o each es .
3. Resul s
3.1. TEST A—Emissions, Flame S abili y, Flue Gas Tempe a u e
The expe imen s wi h Bu ne A e ealed ha he cons uc ion o his bu ne is no
su icien o a dilu ion highe han 10 m
N3
/h (LHV 28.67 MJ/m
3
) o CO
2
and 20 m
N3
/h (LHV
24.58 MJ/m
3
) o N
2
, as is e iden om he expe imen al ma ix o TEST A (Table 2). The
limi a ion was ha he bu ne head, o iginally designed o noble uels, could no dis ib-
u e mo e uel. I was e iden ha i would be possible o achie e a highe low a e wi h
highe o e p essu e o uel, bu his change would a ec he compa ison o he esul s.
E en hough he amoun o he added ine gases was low and he educ ion o NO
x
in
he lue gas was lowe , i was possible o obse e signs o dependence. The di e ence
be ween he e ec o N
2
and CO
2
is also e iden . CO
2
as a la ge molecule can abso b much
Figu e 5. The echnological scheme o he combus ion chambe .
TEST B was ocused on he lame empe a u e. Fo his es , R- ype he mocouples
we e used o measu e he empe a u e in he ho izon al plane o he lame. In o al, eigh
R- ype he mocouples we e used o co e he empe a u es along he ho izon al plane
o symme y. Measu ing poin s we e loca ed 50 cm om each o he along he whole
Ene gies 2022,15, 262 7 o 18
combus ion chambe . Each he mocouple was connec ed o he G aph ec midi LOGGER
GL220, which eco ded he measu ed alues. Tempe a u es we e measu ed a six di e en
posi ions: nea he wall (5 cm), 10, 20, 30, 40, and 50 cm om he wall.
TEST C was ocused on measu emen s o hea lux in o he wall o he combus ion
chambe . This was allowed due o he unique cons uc ion o he combus ion chambe ,
di ided in o se en di e en sec ions and equipped wi h he mocouples and lowme e s o
measu e cooling wa e p ope ies o calcula e he o e all hea lux.
Du ing TEST A, limi s (bo h in design and echnological) o each bu ne we e e-
ealed. Thus, o TEST B and TEST C, only a ew di e en uel composi ions we e selec ed,
which we e adjus ed acco ding o he esul s ob ained du ing TEST A and o ob ain da a
desc ibing he end wi hou unnecessa y was e o he uel. Be o e each es , condi ions in
he combus ion we e s abilized; i was i al o con ol he low a e o uel and combus ion
ai o achie e s able condi ions. Al hough measu emen s we e conduc ed in exac in e als,
maximum e o was used o ensu e he epea abili y and accu acy o each es .
3. Resul s
3.1. TEST A—Emissions, Flame S abili y, Flue Gas Tempe a u e
The expe imen s wi h Bu ne A e ealed ha he cons uc ion o his bu ne is no
su icien o a dilu ion highe han 10 m
N3
/h (LHV 28.67 MJ/m
3
) o CO
2
and 20 m
N3
/h
(LHV 24.58 MJ/m
3
) o N
2
, as is e iden om he expe imen al ma ix o TEST A (Table 2).
The limi a ion was ha he bu ne head, o iginally designed o noble uels, could no
dis ibu e mo e uel. I was e iden ha i would be possible o achie e a highe low a e
wi h highe o e p essu e o uel, bu his change would a ec he compa ison o he esul s.
E en hough he amoun o he added ine gases was low and he educ ion o NO
x
in he
lue gas was lowe , i was possible o obse e signs o dependence. The di e ence be ween
he e ec o N
2
and CO
2
is also e iden . CO
2
as a la ge molecule can abso b much mo e
hea , i.e., educe he lame empe a u e, educing NO molecules’ nuclea ion. The e o e,
bo h ine gases had a di ec impac on lame cha ac e is ics. A highe amoun o ine gas
in he uel inally esul ed in he ins abili y o he lame. A low a es o 20 m
N3
/h o CO
2
and 30 m
N3
/h o N
2
, he lame co e became much colde , and a high amoun o CO was
measu ed in he lue gas. I could also be no iced ha he lame wi h mo e ine gas in
he uel changed i s colo om yellow o blue and became much mo e s able ( he lame
was isually sha pe ) un il i eached he u ning poin , and he lame had he endency o
quench. Resul s o TEST A o Bu ne A a e shown in Table 3.
Table 2. Expe imen al ma ix—TEST A—Bu ne A (•indica es ha he es was ca ied ou ).
Amoun o N2[mN3/h] 0 10 20 30 40 50 60 0 0 0 0 0 0
Amoun o CO2[mN3/h] 0 0 0 0 0 0 0 10 20 30 40 50 60
Fuel/Ine gas a io [% ol.]
100/0 83/17 71/29 62/38 56/44 50/50 45/55 83/17 71/29 62/38 56/44 50/50 45/55
Bu ne A P ima y egime • • -----•-----
Bu ne A Seconda y egime • • • - - - - •-----
Table 3. Expe imen al ma ix—TEST A—Bu ne A.
Regime CO2Addi ion
[mN3/h]
N2Addi ion
[mN3/h] NOx[ppm] NOx[mg/mN3]Flue Gas
Tempe a u e [◦C]
P ima y 0 0 72 147 629
P ima y 0 10 63 129 647
P ima y 10 0 53 109 654
Seconda y 0 0 46 94 683
Seconda y 0 10 39 79 676
Seconda y 0 20 27 56 692
Seconda y 10 0 29 60 682
Ene gies 2022,15, 262 8 o 18
The geome y o Bu ne B was designed o a highe he mal ou pu han ha o Bu ne
A. The e o e, i is possible o add mo e ine gases in o he uel. This bu ne combus s all he
uel in he p ima y egime, and, o NO
x
educ ion, he seconda y ai dis ibu ion is used. In
bo h p ima y and seconda y egimes, up o 60 m
N3
/h (LHV 15.64 MJ/m
3
) o CO
2
and N
2
was
added. All he uel was in a single s eam, c ea ing be e bu ning condi ions o low-calo i ic
uel compa ed o Bu ne A. Hence, i was possible o achie e highe low a es wi h good
lame s abili y. The educ ion o NO
x
was apid, and i s end was appa en . Assump ions
om he measu emen wi h Bu ne A we e con i med. CO
2
a ec s he educ ion mo e han
N
2
, as is e iden om he esul s in Table 4. In bo h cases, a highe con en o non-combus ible
compound esul ed in highe ou le nozzle eloci ies. This phenomenon imp o ed he lame
s abili y and in ensi ied he mixing o uel and combus ion ai . Al hough he lame was s able
du ing he whole p ocess/ es , he colo o he lame u ned om yellow o blue as he lame
empe a u e d opped. The expe imen al ma ix o Bu ne B is shown in Table 4, while he
measu ed alues a e summa ized in Table 5.
Table 4. Expe imen al ma ix—TEST A—Bu ne B (•indica es ha he es was ca ied ou ).
Amoun o N2[mN3/h] 0 10 20 30 40 50 60 0 0 0 0 0 0
Amoun o CO2[mN3/h] 0 0 0 0 0 0 0 10 20 30 40 50 60
Fuel/Ine gas a io [% ol.]
100/0 83/17 71/29 62/38 56/44 50/50 45/55 83/17 71/29 62/38 56/44 50/50 45/55
Bu ne B P ima y egime •••••••••••••
Bu ne B Seconda y egime •••••••••••••
Table 5. Expe imen al ma ix—TEST A—Bu ne B.
Regime CO2Addi ion
[mN3/h]
N2Addi ion
[mN3/h] NOx[ppm] NOx[mg/mN3]Flue Gas
Tempe a u e [◦C]
P ima y 0 0 92 189 678
P ima y 0 10 89 182 639
P ima y 0 20 78 159 619
P ima y 0 30 66 135 597
P ima y 0 40 54 110 558
P ima y 0 50 40 82 554
P ima y 0 60 34 70 547
P ima y 10 0 55 113 622
P ima y 20 0 40 83 615
P ima y 30 0 28 58 568
P ima y 40 0 20 42 552
P ima y 50 0 16 33 552
P ima y 60 0 15 31 551
Seconda y 0 0 58 118 634
Seconda y 0 10 53 108 607
Seconda y 0 20 49 100 577
Seconda y 0 30 45 92 574
Seconda y 0 40 42 86 567
Seconda y 0 50 37 77 566
Seconda y 0 60 32 66 566
Seconda y 10 0 47 97 611
Seconda y 20 0 38 77 580
Seconda y 30 0 28 57 570
Seconda y 40 0 22 45 569
Seconda y 50 0 18 38 564
Seconda y 60 0 15 31 550
Bu ne C was designed o high low a es a he bu ne head; he e o e, he lame was
uns able du ing he combus ion o na u al gas wi hou any ni ogen o ca bon dioxide. As
a esul , uel mixing wi h combus ion ai was insu icien , and i was possible o obse e
a long lame (3 m). The addi ion o ni ogen and ca bon dioxide changed he lame
Ene gies 2022,15, 262 9 o 18
cha ac e is ics, he lame became sho e , and almos all he yellow colo in he lame was
subs i u ed wi h he blue colo . Wi h he addi ion o 120 m
N3
/h o N
2
in he uel, he lame
was only 1.5 m long, o 1 m o CO
2
. Due o he high eloci y a he ip o he bu ne head,
p ope mixing o uel and combus ion ai was ensu ed because o he high u bulen low.
Ne e heless, he bu ne was designed e en o highe low a es. I was impossible
o inc ease he a io o ine compounds in he uel because o he insu icien low om he
bundles. The egime wi h CO
2
addi ion was e alua ed as highly p oblema ic; a high low
a es, such as 120 m
N3
/h o CO
2,
he emaining CO
2
in he cylinde s comple ely oze o ,
and a u he inc ease in low a e was no possible. The expe imen al ma ix o Bu ne C
is shown in Table 6, and he measu ed alues a e displayed in Table 7.
Table 6. Expe imen al ma ix—TEST A—Bu ne C (•indica es ha he es was ca ied ou ).
Amoun o N2[mN3/h] 0 30 60 90 120 0 0 0 0
Amoun o CO2[mN3/h] 0 0 0 0 0 30 60 90 120
Fuel/Ine gas a io [% ol.] 100/0 62/38 45/55 36/64 29/71 62/38 45/55 36/64 29/71
Bu ne C •••••••••
Table 7. Expe imen al ma ix—TEST A—Bu ne C.
Regime CO2Addi ion
[mN3/h]
N2Addi ion
[mN3/h]
NOx
[ppm]
NOx
[mg/mN3]
Flue Gas
Tempe a u e [◦C]
P ima y 0 0 73 149 701
P ima y 0 30 61 125 673
P ima y 0 60 56 115 669
P ima y 0 90 49 101 662
P ima y 0 120 43 88 656
P ima y 30 0 45 92 653
P ima y 60 0 36 73 633
P ima y 90 0 28 58 632
P ima y 120 0 20 40 620
The measu ed alues o all egimes indica ed in he p e ious ma ixes we e displayed
in g aphs in o de o unde s and he acqui ed da a be e . In all igu es, a simila end
is isible. When ine gas was added, measu ed NO
x
in he lue gas s a ed o d op,
co esponding wi h he empe a u e dec ease. The alues o he bu ne wi h s aged uel
dis ibu ion can be seen in Figu e 6a. Since added ine gases we e ela i ely low, he
change was also mild. Figu e 6b shows a mo e signi ican dec ease, whe e measu ed da a
o he bu ne wi h s aged ai a e p esen ed. A simila pa e n can be obse ed in Figu e 6c,
displaying measu ed alues on a low-calo i ic bu ne .
Tempe a u es o he lue gas a e also p esen ed in he g aphs in Figu e 7. As men ioned
abo e, when ine gas is p esen in he uel, i immedia ely impac s he empe a u e, which,
in gene al, dec eases. The alues o he bu ne wi h s aged uel dis ibu ion can be seen in
Figu e 7a. In his case, he opposi e end is e iden . The empe a u e is inc easing wi h
added ine gas. The cause o his could be he highe ou le speed o he uel, which leads
o enhanced mixing and a mo e in ensi e eac ion. Figu e 7b shows a dec ease, whe e
measu ed da a o he bu ne wi h s aged ai a e p esen ed. A simila pa e n can be
obse ed in Figu e 7c, displaying measu ed alues on a low-calo i ic bu ne .
Ene gies 2022,15, 262 16 o 18
Ene gies 2022, 15, x FOR PEER REVIEW 16 o 18
E alua ion o he da a collec ed in he Bu ne C combus ion es e ealed simila
ends a e bo h N
2
and CO
2
addi ion. The mos signi ican di e ence was isible a e
adding 30 m
N3
/h o CO
2
. A u he inc ease in ine gas low did no cause a signi ican
di e ence. I was essen ial o s abilize he lame e en wi h a small amoun o enla gemen
o he uel o imp o e he ou le eloci ies. Al hough mo e ine gas was added o he uel,
i did no cause a signi ican luc ua ion in he hea lux abso p ion in single sec ions. The
e alua ed da a a e shown in Figu e 13.
(a) (b)
Figu e 13. In luence o ine gas addi ion on hea lux dis ibu ion—low-calo i ic bu ne —Bu ne
C: (a) addi ion o N
2
; (b) addi ion o CO
2
.
The e o analysis o hea lux was pe o med acco ding o Equa ion (1) o he s and-
a d de ia ion 𝜎󰇗

o calcula ed hea lux o he wall o he i- h chambe ’s sec ion [kW/m
2
].
Va iables in he equa ions a e hea lux, 𝜎

—s anda d de ia ion o wa e low a e
h ough he i- h chambe ’s sec ion [m
3
/h], 𝑉— olume ic low a e o cooling wa e
h ough he i- h chambe ’s sec ion [m
3
/h], 𝜎
,
— he s anda d de ia ion o he ou le
empe a u e o cooling wa e ou o he i- h chambe ’s sec ion [°C], 𝜎

— he s anda d
de ia ion o he inle empe a u e o cooling wa e in he chambe [°C], ∆𝑡— he ou le
empe a u e o cooling wa e ou o he i- h chambe ’s sec ion [°C].
𝜎󰇗

𝑞󰇗∙󰇩𝜎

𝑉𝜎
,
𝜎


∆𝑡󰇪
𝑓
𝑜𝑟 𝑖1,2,…,7 (1)
Fo he i s sec ion, he mos signi ican e o in all measu emen s was less han ±5%,
while i was usually less han ±3% in he o he sec ions.
4. Conclusions
The use o al e na i e uels in s anda dized bu ne s wi hou any modi ica ion is lim-
i ed. I is possible o injec he al e na i e low-calo i ic uel in o he nobble one, bu he
amoun has o be speci ied be o ehand so ha he ope a ing condi ions can emain s able.
The bu ne can usually handle he uel combus ion wi h a signi ican ly lowe LHV han
ini ially designed, bu i s abili y o deli e he mal powe will dec ease. This is mainly
due o he highe amoun o uel lowing h ough nozzles ini ially designed o p ocess
uel wi h a much highe LHV. Mo eo e , o p o ide s able combus ion condi ions on con-
en ional bu ne s while combus ing low-calo i ic uel, i is necessa y o suppo he lame
co e o s abilize he lame, which could blow o due o he inc eased pa ial p essu e o
incombus ible componen s.
Bo h ine gases (N
2
and CO
2
) can dec ease peak empe a u es in he lame, esul ing
in lowe NO
x
emissions. Howe e , CO
2
can abso b mo e hea han N
2,
and i can be mo e
bene icial o NO
x
emission-wise. When combus ing in he bu ne wi h a s aged gas dis-
ibu ion, e en a mino uel dilu ion wi h 16% ol. o N
2
(26% w ) o CO
2
(34% w ) can
educe NOx up o 30%. When he uel dilu ion is highe , he educ ion can be e en mo e
apid. This was p o en on he bu ne wi h seconda y ai -s aged dis ibu ion, whe e he
Figu e 13.
In luence o ine gas addi ion on hea lux dis ibu ion—low-calo i ic bu ne —Bu ne C:
(a) addi ion o N2; (b) addi ion o CO2.
The e o analysis o hea lux was pe o med acco ding o Equa ion (1) o he
s anda d de ia ion
σ.
qi
o calcula ed hea lux o he wall o he i- h chambe ’s sec ion
[kW/m
2
]. Va iables in he equa ions a e hea lux,
σVi
—s anda d de ia ion o wa e low
a e h ough he i- h chambe ’s sec ion [m
3
/h],
Vi
— olume ic low a e o cooling wa e
h ough he i- h chambe ’s sec ion [m
3
/h],
σ ou ,i
— he s anda d de ia ion o he ou le
empe a u e o cooling wa e ou o he i- h chambe ’s sec ion [
◦
C],
σ in
— he s anda d
de ia ion o he inle empe a u e o cooling wa e in he chambe [
◦
C],
∆ i
— he ou le
empe a u e o cooling wa e ou o he i- h chambe ’s sec ion [◦C].
σ.
qi
=.
qi·
u
u
"σVi
Vi2
+
σ2
ou ,i+σ2
in
∆ 2
i# o i =1, 2, . . . , 7 (1)
Fo he i s sec ion, he mos signi ican e o in all measu emen s was less han
±
5%,
while i was usually less han ±3% in he o he sec ions.
4. Conclusions
The use o al e na i e uels in s anda dized bu ne s wi hou any modi ica ion is
limi ed. I is possible o injec he al e na i e low-calo i ic uel in o he nobble one, bu
he amoun has o be speci ied be o ehand so ha he ope a ing condi ions can emain
s able. The bu ne can usually handle he uel combus ion wi h a signi ican ly lowe
LHV han ini ially designed, bu i s abili y o deli e he mal powe will dec ease. This
is mainly due o he highe amoun o uel lowing h ough nozzles ini ially designed o
p ocess uel wi h a much highe LHV. Mo eo e , o p o ide s able combus ion condi ions
on con en ional bu ne s while combus ing low-calo i ic uel, i is necessa y o suppo he
lame co e o s abilize he lame, which could blow o due o he inc eased pa ial p essu e
o incombus ible componen s.
Bo h ine gases (N
2
and CO
2
) can dec ease peak empe a u es in he lame, esul ing
in lowe NO
x
emissions. Howe e , CO
2
can abso b mo e hea han N
2,
and i can be
mo e bene icial o NO
x
emission-wise. When combus ing in he bu ne wi h a s aged gas
dis ibu ion, e en a mino uel dilu ion wi h 16% ol. o N
2
(26% w ) o CO
2
(34% w ) can
educe NOx up o 30%. When he uel dilu ion is highe , he educ ion can be e en mo e
apid. This was p o en on he bu ne wi h seconda y ai -s aged dis ibu ion, whe e he
educ ion in NO
x
was ca. 80% (dilu ion o he uel 55% ol. o N
2
(67% w ) o CO
2
(
74% w
)
can signi ican ly a ec NO
x
educ ion up o 80%). A simila pa e n was also appa en wi h
he combus ion es s in he low-calo i ic bu ne .
A c ucial ac o ha has o be conside ed is he lame shape. When he ine gas
is added o he uel, i s olume inc eases, and he ou le nozzle eloci y inc eases; his
p ocedu e can also be used o modi y he lame shape and di ec ly in luence he he mal
s ess o he de ice.

Ene gies 2022,15, 262 17 o 18
The addi ion o ine gas in o he con en ional uel can be bene icial; i can imp o e he
e iciency o s eam mixing, which di ec ly impac s he he mal e iciency, bu i is challenging
o ind he u ning poin . A u he inc ease in he ine gas a io has a nega i e e ec . The
e alua ion o he expe imen s e ealed ha he imp o emen o hea lux could each up
o 5% compa ed o he combus ion o na u al gas in a con en ional bu ne . Howe e , he
e ec in he new low-calo i ic bu ne was negligible. An inc eased hea lux ansi ion in o he
walls can be explained by he imp o ed mixing e iciency, bu a change in he pa ial p essu e
o CO
2
o N
2
can also play a ole. Fu he mo e, a highe uel olume also esul s in highe
ou le eloci ies, which in ensi y he u bulen lue in he combus ion chambe . Thus, un il he
u ning poin is eached, i can imp o e he hea lux owa ds he walls.
When co-combus ing uels, such as na u al gas and biogas, i is mo e bene icial o equip
he bu ne wi h wo di e en se s o nozzles (i he same he mal powe is expec ed). Howe e ,
i a sligh dec ease in he mal powe is no an issue, one se o nozzles could be used.
Fu u e wo k will be ocused on he da a alida ion and op imiza ion o he low-
calo i ic bu ne .
5. Pa en s
Sk yja, P.; Hudák, I.; Bˇeloh adský, P.; S ehlík, P.; Vu B nˇe, B no. Czech Republic:
Bu ne Fo Low Calo i ic Fuels. EP3364105 (B1). (2018).
Sk yja, P.; Hudák, I.; Bˇeloh adský, P.; S ehlík, P.; Vu B nˇe, B no. Czech Republic:
Bu ne Head o Low Calo i ic Fuels. US2018231245A1. (2018).
Au ho Con ibu ions:
Concep ualiza ion, I.H. and P.S.; me hodology, I.H.; alida ion, I.H., J.B.
and Z.J.; o mal analysis, P.S.; in es iga ion, I.H. and J.B.; w i ing—o iginal d a p epa a ion, I.H.;
w i ing— e iew and edi ing, I.H., P.S. and J.B.; isualiza ion, I.H.; supe ision, M.K. and Z.J.; p ojec
adminis a ion, Z.J. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
This esea ch was unded by he Czech Minis y o Educa ion, You h, and Spo s/EU Ope a-
ional P og amme Resea ch, De elopmen and Educa ion, g an no. CZ.02.1.01/0.0/0.0/16_026/0008413
“S a egic pa ne ship o en i onmen al echnologies and ene gy p oduc ion”. Fu he mo e, he au ho s
g a e ully acknowledge he inancial suppo p o ided by he Technology Agency o he Czech Republic
(TACR) wi hin he esea ch p ojec Na ional Cen e s o Compe ence, speci ically h ough he p ojec
Na ional Cen e o Ene gy (TN1000007).
Ins i u ional Re iew Boa d S a emen : No applicable.
Da a A ailabili y S a emen :
The da a p esen ed in his s udy a e a ailable on eques om he
co esponding au ho .
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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