DOI: 10.1515/eces-2018-0003 ECOL CHEM ENG S. 2018;25(1):51-59
Milan GERŠL
1*
, Tjaša KANDUČ
2
, Dalibo MATÝSEK
3
, Ma in ŠOTNAR
1
and Jan MAREČEK
1
THE ROLE OF MINERAL PHASES
IN THE BIOGAS PRODUCTION TECHNOLOGY
ROLA FAZ MINERALNYCH W TECHNOLOGII PRODUKCJI BIOGAZU
Abs ac : In he ield o elec ic powe indus y, enewable ene gy sou ces, e ilise s, eclama ion, and was e
managemen , biomass is widely s udied and used. Mine als a e p esen in e e y s ep o biogas ans o ma ion, bu
hei o ms, occu ence, and composi ion ha e no been s udied ye . Howe e , he e is no comp ehensi e s udy
esea ch ha would add ess he p esence o mine al phases in he p ocess o biogas p oduc ion. This aim o he
s udy is de e mina ion o he amoun and composi ion o he mine al phases p esen in e men a ion esidues
esul ing om di e en p oduc ion echnologies. Diges a e mine al composi ion was analysed using 46 samples
om ag icul u al biogas plan s and uni e si y es ing biogas eac o . The majo i y o samples con ained he
amo phous phase. Mino i y phases consis ed o qua z, albi e, o hoclase, musco i e, and amphibole. Opal-CT was
ound in ele en samples (1.26 o 12.1% w .). The elemen s p esen in gas-liquid luids o in liquids, gases and
ae osols wi hin he biogas echnology sys em may c ea e mine al phases, namely he amo phous phase o he
c ys alline phase unde ce ain condi ions. Opal-CT may en e he e men e as pa o plan issues e e ed o as
phy oli hs, o as an unwan ed admix u e o di e en o igin. I may also o igina e om he p esen amo phous
SiO
2
.
Keywo ds: biogas p oduc ion, diges a e, X- ay di ac ion analysis, Rie ield me hod, opal-CT
In oduc ion
Biogas is a ype o uni e sal enewable uel which can be used o elec ic powe and
hea gene a ion o as uel in ehicle engines. I has been sugges ed ha majo pa o he
EU 27’s 2020 enewable ene gy a ge would come om bioene gy and a leas 25% o
bioene gy could o igina e biogas p oduced om we o ganic ma e ials including manu e,
whole c op silage, we ood and eed was es [1]. Se e al s udies we e pe o med o educe
G eenhouse Gasses (GHG) emissions om an h opogenic ac i i ies [2] and de ine
en i onmen al e ec s o biogas deploymen on ca bon balance [3-5].
1
Depa men o Ag icul u e, Food and En i onmen al Enginee ing, Facul y o Ag onomy, Mendel Uni e si y
in B no, Zemědělská 1, 613 00 B no, Czech Republic
2
Depa men o En i onmen al Sciences, Jože S e an Ins i u e, Jamo a ces a 39, 1000 Ljubljana, Slo enia
3
Ins i u e o Clean Technologies o Ex ac ion and U iliza ion o Ene gy Resou ces, Facul y o Mining and
Geology, VŠB - Technical Uni e si y o Os a a, 17. lis opadu 15/2172, 708 33 Os a a, Czech Republic
*
Co esponding au ho : milan.ge [email protected]
Milan Ge šl, Tjaša Kanduč, Dalibo Ma ýsek, Ma in Šo na and Jan Ma eček
52
The p ocess o anae obic decomposi ion o biodeg adable ma e ials p ima ily p oduces
biogas. Raw biogas gene ally consis s o me hane (40-75%) and ca bon dioxide (15-60%);
he wa e con en is abou 5-10%. I includes ace amoun s o such componen s as
hyd ogen sulphide (0.005-2%), ammonia (< 1%), halogena ed hyd oca bons (< 0.6%),
siloxanes (0-0.02%), ni ogen (max. 2%), and oxygen (max. 1%); ca bon monoxide
(< 0.6%) can be p esen and migh be incon enien when no emo ed [6, 7].
De ailed composi ion o biogas and e men a ion esidues is signi ican ly in luenced by
he ype o inpu ma e ial, especially om he pe spec i e o mino componen s ha can be
conside ed ha m ul. The o al o 12 o 53% o ganic ma e ial is decomposed du ing
anae obic e men a ion [8, 9].
The high con en o hea y me als in diges a e inhibi ed he use as a e ilise . Hea y
me als en e diges a e as pa o he eed ma e ial and as suppo ing subs ances o inc ease
mic obial ac i i y o he p ocesses unde way in he biogas plan [10]. The elemen s p esen
in gas-liquid luids o in liquids, gases and ae osols wi hin he biogas echnology sys em
may c ea e mine al phases, namely he amo phous phase o he c ys alline phase unde
ce ain condi ions.
The main sou ces o o ganosilicon compounds in subs a es include many ypes o
cosme ic p oduc s ha in he aqueous phase o he was ewa e ea men p ocess cause
siloxanes o bind o he solid componen s o sewage sludge. Fo biogas, which o igina ed
om he decomposed sewage sludge, he measu ed siloxane concen a ions inc ease in he
long e m; his is caused by he e e -inc easing consump ion o p oduc s con aining silicon
compounds. The biogas plan s used o p ocessing ma e ials coming om ag icul u e ha e
usually low concen a ion o halogena ed and o ganic silicon compounds. Howe e , low
amoun s o o ganic silicon compounds ha e been de ec ed in biogas om plan s ha
p ocess only manu e and ene gy c ops e en hough he o igin o hese was no clea [11].
P oblems occu i he amoun o siloxanes in he biogas su passes a c i ical alue ha
jeopa dizes he al es and pis ons o he engine o he powe gene a o h ough he
o ma ion o SiO
2
. Siloxane compounds decompose in engines unde high empe a u es.
The composi ion o he eleased uel gas needs o be analysed o a oid se e e damage o
he engine ha is caused e en by small amoun s o siloxanes p esen when deposi ed o e
a longe pe iod [12-14]. EU has no s anda ds on maximum pe missible siloxane con en in
biogas. Aus ia and he Ne he lands whe e he siloxane speci ica ions a e 10 mg Si/m
3
and
5 ppm, espec i ely, a e he only excep ions [15].
Exceeding siloxane concen a ions leads o a conside able educ ion in echnology
se ice and main enance pe iod and disp opo iona e inc ease he cos s o main enance [16,
17]. A e men a ion esidue is o med as a biogas p oduc ion co-p oduc . Re e ed o as
“diges a e” [18], he ma e ial is used as a e ilise , aw ma e ial o compos ing o solid
uel. The combus ion o he ma e does no inc ease he ca bon con en in i s na u al cycle
[2]. Diges a e con ains a high amoun o phospho us, po assium, and o al ni ogen, he
la e ound in he o m o ammoniacal ni ogen; all o he subs ances a e bene icial o
ag icul u al soils [19, 20]. pH o he e men a ion esidue is usually 7.5 o 8.3 [21]. The
chemical and mine al composi ion o e men a ion esidues is in luenced by he ype o he
aw ma e ial being p ocessed, he manne o p e- ea men , he ope a ing empe a u e in he
diges e , he esidence ime, he con en o solids e c., plus he s o ing me hod and pe iod
a e ac o s. The usual con en o o al solids is 4 o 15% o liquid e men a ion esidues and
30 o 45% o non-liquid e men a ion esidues.
The ole o mine al phases in he biogas p oduc ion echnology
53
Howe e , he e is no comp ehensi e s udy ha would add ess he p esence o mine al
phases in he p ocess o biogas p oduc ion. The aim o his s udy is o de e mine he amoun
and composi ion o he a ious mine al phases p esen in e men a ion esidues esul ing
om di e en biogas p oduc ion echnologies.
Expe imen and sample collec ion
The mine al composi ion o he e men a ion esidues was e alua ed du ing 2013-2014
using 30 samples om nine ag icul u al biogas plan s, wi h a leas h ee samples pe biogas
plan aken in each o he wo seasons (summe and win e ) (Table 1). A he same ime,
12 samples we e in es iga ed om six uni e si y labo a o y e men e s. The e men a ion
esidue om he all sampling poin s was sou ced om liquid manu e, maize silage and
so ghum silage, hay silage and was e om gas onomy. P ocesses in he biogas plan s we e
no al e ed by chemicals.
A biogas plan wi h an in eed d y ma e (Table 1) con en o less han 15% ( ypically
abou 4%) is e e ed o as a liquid e men a ion biogas plan . Liquid e men a ion biogas
plan s ope a e on a con inuous basis; samples we e aken om he e minal diges a e
s o age. A biogas plan wi h d y ma e con en highe han 15% ( ypically 30-45%) is
e med a non-liquid e men a ion biogas plan . Mendel Uni e si y’s labo a o y e men e s
had he capaci y o 130 dm
3
wi h a d y ma e con en o abou 4%. They ope a e on
a discon inuous basis and diges a e samples we e aken a e 28 days o eac ion.
Samples o mine al coa ings (Table 1) we e aken om cogene a ion uni ’s engine
cylinde s and pis ons. The uni is equipped wi h a pe ol engine and ope a ed as pa o
a liquid e men a ion biogas plan . Fou silage samples we e aken om silage pi s. This
in ol ed s anda d silage made o maize hyb ids in ended o biogas p oduc ion.
Table 1
Subs a es and ma e ials speci ica ions o e alua ed samples
Type o biogas plan Fe men a ion ype Raw subs a e Numbe o samples
Ag icul u al A Liquid Mix u e: maize silage,
so ghum silage, manu e 12
Ag icul u al B Liquid Mix u e: maize silage, hay
silage, manu e 6
Ag icul u al C Liquid Mix u e: maize silage, hay
silage, animal aeces, slu y 6
Ag icul u al Non-liquid Mix u e: maize silage,
manu e, gas onomy was e 6
Labo a o y 130 l
e men e s Liquid Mix u e: maize silage,
so ghum silage, slu y, o
gas onomy was e 12
Raw maize silage Silage Maize issue (lea s) 4
Samples o mine al
coa ings cogene a ion uni ’s Cylinde s and pis ons 12
Milan Ge šl, Tjaša Kanduč, Dalibo Ma ýsek, Ma in Šo na and Jan Ma eček
54
Analy ical me hods
Bulk pa ame e s
The collec ed samples we e d ied a 105°C, subsequen ly milled by a cu ing mill o
make a ac ion below 0.100 mm, and homogenised. Solids, o al o ganic ca bon (TOC),
o al ino ganic ca bon (TIC), combus ibles a 550°C and he hea o combus ion we e
de e mined in all he samples.
X- ay di ac ion
Quan i a i e phase analyses we e ca ied ou by he X- ay powde di ac ion me hod
(XRD) using he B uke D8 Ad ance di ac ome e , adia ion CoKα/Fe, 40 kV/40 mA, he
angula in e al 2-80° 2θ, s ep 0.014° 2θ, 0.75 sec., wi h he Lynxeye posi ion sensi i e
de ec o . The homogenised powde composi ions o samples wi h he addi ion o in e nal
s anda d (ZnO) we e used o he analysis, wi h 6% w . added o he sample e.g. [22]. The
quan i a i e phase analysis was ca ied ou by he Rie eld me hod [23-26] using he Topas
p og am, e sion 4.2.
Elec on mic oscopy
Mic oscopic images we e aken using he elec on mic oscope, FEI Quan a 650 FEG
(VŠB-TUO ICT Os a a), i ed wi h wa eleng h-dispe si e and ene gy dispe si e
de ec o s, as well as wi h EBSD and CL de ec o s. The mic oscopic images we e aken by
a backsca e ed elec on de ec o (BSED). Nei he silage no diges a e samples we e
adjus ed using classic mine alogy me hods. The specimens we e scanned a an accele a ion
ol age o 15 kV, using he elec on beam o he diame e o 6 µm, and unde p essu e in
a 50 Pa chambe .
Resul s
Fe men a ion esidues and aw maize silage
The o al o ganic ca bon con en (TOC) and he highe hea ing alue p o ided
pa ame e s o e alua ion o he me hods ha se e o de e mining he a ios o o ganic
and ino ganic componen s wi hin he es ed e men a ion esidues. TOC in he e alua ed
samples anged 32.0-52.0% w . (median = 41.2% w ., a iance = 38.6% w .). The diges a e
highe hea ing alue was 12.67-18.07 MJ·kg
–1
. All he s udied samples con ained
an amo phous phase o 77.5 o 98.7% w . (median = 91.8% w ., a iance = 43.5% w .)
based on he x- ay di ac ion me hod (Table 2).
All he samples con ain calci e (1.0 o 5.6% w .); i possibly comes om he
alloch honous en i onmen o om au och honous eac ions. Thei p esence s abilises
e men e pH as well as pH o he e men a ion esidue.
Mine al phases o he au och honous o igin a e ep esen ed by syl i e, i s de ec ion
in e al anging om o 4.1% w ., wi h cell pa ame e s a
0
= 6.29459(28). This is a new ype
o mine al ha ypically o ms om elemen s p esen in biomass, which is possible only
a e he comple ion o he echnological p ocess in e men e s. The g ow h po en ial o he
mine al is de e mined by physico-chemical condi ions occu ing a he empo a y s o age
acili y o e men a ion esidues.
The ole o mine al phases in he biogas p oduc ion echnology
55
Table 2
Iden i ied mine al phases in he diges ion esidues and aw maize silage
Mine al phase
Liquid
e men a ion
A
Liquid
e men a ion
B
Liquid
e men a ion
C
Non-liquid
e men a ion
Labo a o y
e men e s
(Liquid .)
Raw maize
silage
n = 12 n = 6 n = 6 n = 6 n = 12 n = 4
Maize silage,
so ghum
silage, manu e
Maize silage,
hay silage,
manu e
Maize silage,
hay silage,
animal aeces,
slu y
Maize silage,
manu e, ood
was e
Maize silage,
so ghum
silage, slu y,
ood was e
Maize silage
subs a e
Amo phous con en
[% w .] 87.6-98.7 88.2-96.1 90.3-92.4 77.5-90.4 82.5-92.3 99.2-99.9
Qua z [% w .] 0.3-1.9 1.6-5.6 bellow LOD 7.3-11.8 LOD-0.6 LOD-0.8
Illi -musco i e
[% w .] LOD-3.6
Albi e [% w .] LOD-2.5 LOD-2.5 2.1-2.9
O hoclase
[% w .] LOD-0.9 LOD-0.9 LOD-2.3
Calci e [% w .] LOD-2.5 1.85-2.3 LOD-5.6 LOD-1.8
Ho nblende
[% w .] LOD-0.1
Opal-CT [% w .] LOD-12.1 LOD-15.2
Syl i e [% w .] LOD-2.56 LOD-4.1 LOD-0.07
To al o ganic
ca bon (TOC)
[% w .] 44.7-52.0 41.5-48.6 42.4-44.6 32.0-42.0 34.7-37.4
The highe
hea ing alue
[MJ · kg
–1
] 17.06-18.07 16.82-17.57 15.66-16.56 12.67-15.49 11.48-17.74
n - numbe o samples; LOD - limi o de ec ion
Elec on mic oscopy and maize lea mic o analysis
The aw maize silage was p epa ed om lea es o maize in ended o p epa ing silage
o be used a a biogas plan was ound o con ain phy oli hs shaped as wo connec ed o als
and co esponding o SiO
2
in e ms o composi ion. An elec on mic op obe es ed he
phy oli h poin composi ion o elemen s (Table 3). In addi ion o he elemen s shown below
Al, Ca, Cl, Fe, K, Mg, Na, P, and S we e also de ec ed in he lea es.
Table 3
Elemen al composi ion o phy oli hs ob ained om maize lea es
W [%] A [%]
C 12.26-28.58 19.49-40.45
O 36.81-40.93 39.12-50.27
Si 27.28-45.59 16.49-30.99
Al, Ca, Cl, Fe, K, Mg, Na, P, S bellow LOD bellow LOD
Poin s analysis numbe n = 12; LOD - limi o de ec ion
Resul s o XRD analysis o he engine laye
Chalcan hi e and chalcocyani e a e he p edominan mine als ound in he engine
cylinde and pis on coa ings; qua z con aining 7.44% is ound in place 4 (Table 4).

Milan Ge šl, Tjaša Kanduč, Dalibo Ma ýsek, Ma in Šo na and Jan Ma eček
56
Table 4
Mine al phase in he coa ings o he cogene a ion uni 's engine
Phase Mine al Con en [% w .]
Phase 1 Chalcan hi e 67.58
Phase 2 Chalcocyani e 11.65
Phase 3 Bona i e 8.89
Phase 4 Qua z 7.44
Phase 5 Teno i e 1.89
Phase 6 Ramsbecki e 1.75
Phase 7 An le i e 0.79
Discussion - The e ec o mine als ound in he diges ion esidues
Qua z ep esen s a mine al phase o alloch honous o igin in all he s udied samples.
Mine als such as illi e-musco i e, albi e, o hoclase and ho nblende come om ag icul u al
soil. They en e he echnological p ocess as pa o con amina ion o he subs a e o
silage p oduc ion. The p opo ion o he de ec ed mine al phases in he e men a ion
esidues a ies as pe he local soil si ua ion. The highes amoun o mine al admix u es
was ound in biogas plan s wi h d y e men a ion.
The amo phous phase con en as de e mined by he XRD me hod and TOC eached he
co ela ion coe icien o 0.87 in he da a se . The esul s a e suppo ed by he highe
hea ing alue analyses. This p ocedu e is used o alida ing he esul s o he mine al
phase analysis.
The quan i y o he admix u e can howe e be assumed o be e y simila in all he
echnologies s udied. Fo liquid e men a ion echnology, he mine als supplied a e g a i y-
sepa a ed in he e men o , sedimen on he bo om and a e no p esen in he exi ing
diges a e, unlike he non-liquid echnology, whe e diges a e is always quan i a i ely
emo ed om he e men e . These mine als occu na u ally in his p ocess and a e
e u ned o he soil when he e men a ion esidue is applied as a e ilise .
Opal-CT was ound in ele en cases (1.26-12.1% w ., median = 4.1% w .), he cell
pa ame e s being a
0
= 4.9996(11), and c
0
= 7.0068(31). I may en e e men e s as pa o
plan issues - phy oli hs, o as admix u e o a di e en o igin; i may also come om he
p esen amo phous SiO
2
.
Analysis o coa ings collec ed om O o engine cylinde heads was he subjec o
s udies ca ied ou by [27] who concluded ha he heo e ical O/Si mass a e in he silica e
uni (SiO
4
)
4
- is 2.278; analysing he O/Si a ios indica ed 1.67-2.46. I can be concluded
ha he silica laye on he cylinde head su ace is one o he o hosilica e o m. The
au ho s also analysed so deposi samples ex ac ed om he pis on ing g oo e zone o
SEM-EDX analysis. Because he heo e ical mass a e in he silicon dioxide, SiO
2
, is 1.14,
his so pas e deposi can be assumed o be silicon dioxide.
When bu ning biogas, siloxanes become oxidised o o m a e y ine powde
composed mainly o SiO
2
which is o med on he inne me al su aces in he combus ion
chambe s o cogene a ion uni engines, e.g. pis on walls, cylinde heads, spa k plugs and
al es. In he gas engine combus ion p ocess, o ganic silicon compounds o m a solid
insoluble coa on he inne pis on su ace ha cause engine damage [27].
Using he XRD me hod, SiO
2
was ound in he o m o qua z in he ea ed samples o
maize silage, he amoun being max. o 0.8% w . As no o he mine als we e ound which
would come om soil, he p esence canno be explained h ough emnan s o soils on plan s
The ole o mine al phases in he biogas p oduc ion echnology
57
ed in o he sys em. Since SiO
2
was ound in samples o maize lea es in he o m o
phy oli hs using he me hod o elec on mic oscopy wi h an elec on mic op obe, i can be
assumed ha SiO
2
ound in he silage comes om maize issue phy oli hs.
C yp oc ys alline qua z (opal-CT) was also ound in a o al o 12 samples o
e men a ion esidues o liquid e men a ion echnology. Since he mine al is p esen in he
mic oc ys alline o m o opal, i is no capable o sedimen a ion and is en ained along wi h
biogas in o he combus ion chambe o he cogene a ion uni ia he pipeline, whe e i
con e s in o s able qua z a high empe a u e and unde high p essu e.
Conclusions
Mine al phase composi ion was iden i ied in samples o e men a ion esidues om
di e en echnological p ocesses o biogas p oduc ion: (1) liquid e men a ion; (2)
non-liquid e men a ion; and (3) liquid e men a ion in uni e si y's labo a o y e men e s.
The o al o ganic ca bon (TOC) con en anged om 32.0 o 52.0% w .
(median = 41.2% w .) in all samples. The majo i y o samples consis ed o he amo phous
phase (77.5 o 98.7% w ., median = 91.8% w .), while he mino i y phases consis ed o
qua z, albi e, o hoclase, musco i e and amphibole (LOD up o 1.92% w .). Va iable
p opo ions o he mine al subs ances we e de ec ed in he e men a ion esidues e lec ing
he local pedological si ua ion. The la ges amoun o hese admix u es was ound in biogas
plan s wi h d y e men a ion. These mine als occu na u ally in his p ocess and a e
e u ned o he soil when he e men a ion esidue is applied as a e ilise . I , howe e , he
e men a ion esidue is used as uel, hey can cause inc eased wea o machine pa s, educe
he pumping po en ial and cause u he complica ions. All he samples we e ound o
con ain calci e and/o magnesium calci e (1.0-5.7% w .). These mine als possibly o igina e
om he alloch honous en i onmen o au och honous eac ions. Thei p esence s abilises
pH in he e men e and ha o he e men a ion esidue.
Opal-CT was ound in ele en cases (1.26 o 12.1% w ., median = 4.1% w .). Unwan ed
Opal-CT is en e ing e men e s ei he as phy oli hs - plan issues, o as an admix u e o
unknown o igin. Opal-CT may also de elop as au och onous amo phous SiO
2
. Any link
be ween he occu ence o opal-CT and he p oduc ion echnology/ he composi ion o inpu
aw ma e ials was no iden i ied.
The p esence o SiO
2
de ec ed on he su ace o he cogene a ion uni 's cylinde s and
pis ons p o ed ha SiO
2
coa ing can de eloped du ing he ag icul u al p ocesses wi h none
siloxanes p esen . I mobilised as an ae osol, i can be ca ied along wi h biogas in o he
cogene a ion uni . The SiO
2
ae osol hen o ms he base o qua z, a s able mine al, o
gene a e. Simul aneously, o he s able mine als gene a e om p esen sulphu and coppe
eleased om sliding bea ings o c anksha s (Table 4) o o m a coa on he inne su aces
o engines.
The posi i e symp oms he e include he binding o elemen s sui able o plan nu i ion
o mine al phases, which leads o he s abilisa ion and g adual elease when he
e men a ion esidue is applied o he soil.
Acknowledgemen s
This s udy was inanced by he IGA IP_4/2016 on Mendel Uni e si y in B no. The
analy ical wo k was done a he “BAT labo a o y cen e”, Mendel Uni e si y in B no. We
also hank o he Slo enian Resea ch Agency (ARRS) and Resea ch P og amme P1-0143
Milan Ge šl, Tjaša Kanduč, Dalibo Ma ýsek, Ma in Šo na and Jan Ma eček
58
(Cycling o subs ances in he en i onmen , mass balances, modelling o en i onmen al
p ocesses and isk assessmen ). Special hanks o d . Magdalena Misz-Kennan (Uni e si y
o Silesia, Poland).
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