Waste Management 47 (2016) 133–140

Contents lists available at ScienceDirect

Waste Management
journal homepage: www.elsevier.com/locate/wasman

Analysis of biomass and waste gasification lean syngases combustion

for power generation using spark ignition engines
Cosmin Marculescu ⇑, Victor Cenusßă, Florin Alexe
Faculty of Power Engineering, University Politehnica of Bucharest, 313 Splaiul Independentei, 060042 Bucharest, Romania

a r t i c l e i n f o a b s t r a c t

Article history: The paper presents a study for food processing industry waste to energy conversion using gasification and
Received 8 February 2015 internal combustion engine for power generation. The biomass we used consisted in bones and meat resi-
Revised 11 June 2015 dues sampled directly from the industrial line, characterised by high water content, about 42% in mass,
Accepted 30 June 2015
and potential health risks. Using the feedstock properties, experimentally determined, two
Available online 9 July 2015
air-gasification process configurations were assessed and numerically modelled to quantify the effects
on produced syngas properties. The study also focused on drying stage integration within the conversion
Keywords:
chain: either external or integrated into the gasifier. To comply with environmental regulations on feed-
Biomass
Waste
stock to syngas conversion both solutions were developed in a closed system using a modified
Gasification down-draft gasifier that integrates the pyrolysis, gasification and partial oxidation stages. Good quality
Spark engine syngas with up to 19.1% – CO; 17% – H2; and 1.6% – CH4 can be produced. The syngas lower heating value
Power generation may vary from 4.0 MJ/N m3 to 6.7 MJ/N m3 depending on process configuration.
The influence of syngas fuel properties on spark ignition engines performances was studied in compar-
ison to the natural gas (methane) and digestion biogas. In order to keep H2 molar quota below the det-
onation value of 64% for the engines using syngas, characterised by higher hydrogen fraction, the air
excess ratio in the combustion process must be increased to [2.2–2.8].
The results in this paper represent valuable data required by the design of waste to energy conversion
chains with intermediate gas fuel production. The data is suitable for Otto engines characterised by
power output below 1 MW, designed for natural gas consumption and fuelled with low calorific value
gas fuels.
Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction meal (MBM) and the incineration. Both solutions require additional
energy supply and the waste generators must pay for the disposal
The continuous evolution of society includes the development of these residues (Cascarosa et al., 2012). The landfill cannot be
of industry sectors that represent waste generation sources with used due to the presence of potential pathogens (Russ and
respect to the environment. The food industry has also increased Meyer-Pittroff, 2004). The trend line in waste management policy,
its production capacity as well as final residues generation. A par- according to current regulations and directives, limits the waste
ticular branch of this activity sector is represented by the meat quantities disposed of by landfilling (European Commission).
processing industry that generates considerable quantities of solid The authors’ previous research revealed a high energy potential
waste that cannot be disposed of without neutralization due to the of the meat processing industry waste such as: chicken feathers,
health risk potential. The residue mass fraction delivered by meat skins, fats, bones and offal. (Marculescu, 2012). These residues rep-
processing varies from 28% for chicken to 48% for sheep and goat of resent an important energy source even if their physical structure
the (Attorney General). About 1/3 of the meat processing industry and the presence of water rise a series of problems mainly related
feed in flow will result in residues to be disposed each year world- to the mechanical behaviour and the global energy efficiency of the
wide. For the moment two main solutions are applied for waste waste to energy conversion processes. As a result, the challenge
neutralization and disposal: the processing into a meat and bone consists in establishing the optimum process and operating param-
eters for waste valorisation (Rada, 2014). However, the conversion
solution must contain at least one stage of high temperature treat-
⇑ Corresponding author.
ment for completely neutralization of any health risk source.
E-mail addresses: cosminmarcul@yahoo.co.uk (C. Marculescu), cenusa_victor@
Usually this type of waste comes from distributed small capacity
yahoo.com (V. Cenusßă), florin.alexe@energ.pub.ro (F. Alexe).

http://dx.doi.org/10.1016/j.wasman.2015.06.043
0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
134 C. Marculescu et al. / Waste Management 47 (2016) 133–140

Table 1
6.8
Proximate analysis of residues. 6.6
6.4
Feedstock Humidity (%) Volatile (%) Fixed carbon (%) Inert (%) 6.2
6.0

Syngas LHV [MJ/Nm3]

Dried 0 66.30 6.40 27.30 5.8
5.6
Raw 41.70 38.65 3.73 15.91 5.4
5.2
5.0
4.8
4.6
4.4
32 4.2
4.0 Dried syngas, solution 2
3.8
31 3.6 Dried syngas, solution 1
3.4 Humid syngas, solution 2
3.2
30
Equivalent ratio [%]

3.0 Humid syngas, solution 1

2.8
2.6
29 0 4 8 12 16 20 24 28 32 36 40 44 48 52

28 Waste humidity [%]

Solution 1 Fig. 4. Variation of syngas LHV.

27

26 Solution 2

25
0 4 8 12 16 20 24 28 32 36 40 44 48 52 power units is affected by the scale factor which strongly influ-
Waste humidity [%] ences the Rankine–Hirn thermodynamic cycles efficiency. The dis-
continuous availability of waste feed-in flow also limits the steam
Fig. 1. Gasification equivalent ratio vs. feedstock humidity. turbines applications that cover the base of the electrical load. Due
to certain characteristics of the waste related to its fast biodegrad-
ability and its potential health risk, the possibility for waste
storage is limited (in the case of food processing industry waste
32
– meat/bones residues). With respect to these restrictions the
CO in dried syngas, solution 1
30 interest for alternative fuels has emerged (Ionescu et al., 2013).
CO in dried syngas, solution 2
28 H2 in dried syngas, solution 1 The production of superior alternative fuels that can be used both
26 in the energy and transport sectors can represent a viable valorisa-
Molar fractions [%]

H2 in dried syngas, solution 2

.

24 tion of this type of waste which is currently disposed of at high

22 costs through incineration.
20 To face such problems one viable option can be the waste gasi-
18 fication and the production of gas fuel to be used in internal com-
16 bustion engines mainly because of their tolerance to fuel quality
14 standards, low scale applications and waste source time availabil-
12 ity (Cascarosa and Gasco, 2012). Due to biomass/waste variety as
well as to the gaseous bio-fuel production technologies there is a
10
0 4 8 12 16 20 24 28 32 36 40 44 48 52 continuous interest in studying the influence of these parameters
Waste humidity [%] on internal combustion engines performances. The main perfor-
mance indicator of an internal combustion reciprocating engine
Fig. 2. Main combustible gases concentration in dried syngas. is the global efficiency. The assignment of the term depends on
the delivered energy type: electricity only, or cogeneration. In
10 our case, by changing the fuel type, from natural gas to a lean
CO2 in dried syngas, solution 1 gas, the term ‘‘performance’’ refers not only to electrical efficiency,
9
CO2 in dried syngas, solution 2 but also to the decrease of power output compared to the reference
8 CH4 in dried syngas, solution 1 value (given by producers, in the technical specifications for the
Molar fractions [%]

7 CH4 in dried syngas, solution 2 engines designed for burning natural gas). The paper presents the
results of a current research on Otto engines (also suitable for
6
cogeneration applications), characterised by power output under
5 1 MW, designed for natural gas consumption and fuelled with
4 low calorific value gaseous bio-fuels. The large range of these vari-
3
ables leads to a wide variety of low quality gaseous bio-fuels with
different physical–chemical properties (Deublein and Steinhauser,
2 2008). Our study focused on analyzing these gaseous bio-fuels
1 composition using data both from engineering literature and our
0 4 8 12 16 20 24 28 32 36 40 44 48 52 calculations. Using this piece of information, a series of specific
Waste humidity [%] physical–chemical data was calculated to highlight the main differ-
ences between these gases and methane. These differences affect
Fig. 3. Variation of CO2 and CH4 fractions in dried syngas for the two proposed
solutions. the design and performances of Otto engines when shifting the
use from natural gas to gaseous bio-fuels.
This paper presents the results of a research study carried on
sources, therefore there is high interest in valorising it in small and bones and meat residues for syngas production together with the
medium scale power plants, but not exclusively. Additionally, the influence of various parameters on gas composition and the global
use of incineration and steam turbines cycles for small capacity energy efficiency of the conversion chain.
 C. Marculescu et al. / Waste Management 47 (2016) 133–140 135

90 56
89
54 CO2
88
87 52 CH4
Hot gas efficiency [%]

86 50

Molar quotas [%]

85
84 48
83 46
82
44
81 Solution 2
80 42
79 Solution 1 40
78
77 38
76 36
0 4 8 12 16 20 24 28 32 36 40 44 48 52
34
Waste humidity [%]
0.8 1 1.25 1.6
Fig. 5. Estimated hot gas efficiency for the two proposed solutions. CH4/CO2 ratios [kmol/kmol]

2. Material and methods Fig. 6. The molar quotas of CH4 and CO2, in biogas from digester, vs. CH4/CO2 molar
ratio.
2.1. Feedstock

The feedstock used in the study consists in pork bones and meat 31
residues sampled directly from the meat processing line. The entire
sampling procedure and laboratory experiments were conducted
30
in order to ensure reliability for the results. After drying the resi-

Molar mass [kg/kmol]

dues were mechanically grinded and submitted to primary and
ultimate analysis. 29
Previous analysis conducted by the research team on this type
of feedstock, revealed the properties related to the primary analy-
sis and ultimate analysis (Marculescu et al., 2013). The waste aver- 28
age humidity content is about 42%. The primary analysis was
performed using a Nabertherm calcination oven. The results are
27
presented in Table 1 (Marculescu et al., 2013).
The elemental composition of residues was determined using
elemental analyzer EA 3000 (Marculescu et al., 2013). The results 26
revealed an important combustible fraction represented by C – 0.8 1 1.25 1.6
41.5% and H – 6%. The sample also had a significant content of N CH4/CO2 ratios [kmol/kmol]
– 4.3%. No sulphur or chlorine elements were detected. The oxygen
concentration (O ffi 20.9%) is similar to the one in other biomass Fig. 7. The molar mass of biogas from digester, vs. CH4/CO2 molar ratio.
type materials.

Table 2 23 20
Gas molar compositions, HHVs and LHVs. 22 19
LHV & HHVvol [MJ/Nm3]

LHV & HHVmass [MJ/kg]

Gas (%) Digester biogas Air-gasification syngas 21 18
A B C D E F G H 20 17

Input data 19 16
H2 0.14 20.78 19.99 13.96 11.14 18 15
CH4 55.92 50.83 45.60 40.37 2.96 2.25 2.30 1.58
CO 0.14 33.52 27.97 24.60 17.85 17 14
H2S 0.1 – 16 13
HHV, vol
H2O 4.6 4.6
15 LHV, vol 12
N2&Ar 3.3 33.34 37.55 51.44 57.88
HHV, mass
O2 0.52 – 14 11
LHV, mass
CO2 35.28 40.37 45.60 50.83 4.36 7.24 3.09 6.95
13 10
SO2 – 0.44 0.40 –
0.8 1 1.25 1.6
Computed data
CH4/CO2 ratios [kmol/kmol]
Molar 26.50 27.92 29.38 30.85 22.70 23.44 24.21 25.66
mass
(kg/ Fig. 8. LHV and HHV of digester biogases, vs. CH4/CO2 molar ratio.
kmol)
HHV (MJ/ 11.7 13.9 16.3 18.9 8.0 6.7 5.4 3.8
kg)
HHV (MJ/ 16.1 18.2 20.3 22.3 8.1 7.0 5.8 4.3
By measuring the higher heating value (HHV) with a calorime-
N m3) ter (IKA C200) and correcting the value with feedstock hydrogen
LHV (MJ/ 10.5 12.5 14.6 16.9 7.4 6.2 5.0 3.5 and water content, the feedstock lower heating value (LHV) was
kg) determined (Marculescu et al., 2013).
LHV (MJ/ 14.5 16.3 18.2 20.0 7.5 6.5 5.4 4.0
For the dried feedstock the LHV is 19.2 MJ/kg while for the raw
N m3)
one it does not exceed 12.5 MJ/kg (Marculescu et al., 2013).
136 C. Marculescu et al. / Waste Management 47 (2016) 133–140

Table 3 8.4
Elemental analysis for dry biomass, % mass. Syngas F
8.0

Hydrogen/(air+fuel) [kmol/kmol]
7.6 Syngas E
Biomass C H S N O Ash

.
7.2 Syngas G
#1 (wood) 43.21 5.94 1.24 0.65 45.50 3.46
6.8 Syngas H
#2 (food industry residue) 41.48 5.99 – 4.30 20.93 27.30
6.4
6.0
5.6
5.2
Nevertheless the raw feedstock has a lower heating value compa- 4.8
rable to the low quality coal (Romanian lignite – 6.5 to 7.5 4.4
MJ/kg; German Silesian lignite – 6.9 to 8 MJ/kg) and certain types 4.0
of biomass (wood – 8 to 19 MJ/kg, agriculture residues 12 to 3.6
3.2
19 MJ/kg), depending of humidity (Marculescu, 2012). 1 1.3 1.6 1.9 2.2 2.5 2.8
Air excess coefficient [-]
2.2. Gasification process Fig. 10. Hydrogen quota in intake mixtures (air + syngases), vs. the excess air ratio.

Two main steps were performed to asses the gasification con-

version chain (process stages and parameters) suitable for the
feedstock. The first step consisted in a preliminary experimental 3. Results and discussions
air gasification campaign at atmospheric pressure conducted on
feedstock samples (300 g) using a tubular batch reactor. The gasi- 3.1. Syngas production
fication experiments were performed only to estimate the gross
process parameters (temperature and the Equivalent Ratio – ER) Two gasification solutions were proposed and analyzed. Both
used in the mathematical model and to check if the feedstock is solutions use a modified down-draft gasifier that offers the advan-
suitable for air gasification. tage of reduced tar content in the syngas (the feedstock is charac-
The experimental configuration is similar to externally heated terised by fat content due to meat residues attached to the bones
atmospheric pressure air gasification under transitory regime leading to a high tar fraction in the syngas). This process integrates
(Kobayashi et al., 2009). The processing temperature ranged from the pyrolysis, partial oxidation and gasification stages in this order.
600 °C to 850 °C, a typical temperature for the air gasification pro- Consequently, the volatile matter from the pyrolysis stage is oxi-
cesses. Syngas LHV was calculated using the gas components. Its dized in the combustion zone, minimizing the heavy hydrocarbon
value varies from 5.5 MJ/kg to 6.8 MJ/kg at 850 °C. As the syngas presence in the syngas. The high humidity (about 42% in mass) and
was obtained by externally heating the reactor these values are lar- the physical–mechanical properties of the feedstock lead to the
gely superior to the ones that could be obtained under real indus- necessity of pre-drying as separate first stage treatment. To solve
trial conditions. In that case a quota of the feedstock is partially this problem we used two different configurations. Both have
oxidized to ensure the gasification endothermic conditions energy recovery within the conversion chain itself. Solution 1 con-
(Antonopoulos et al., 2012). This oxidized quantity depends on sists in achieving the drying stage within the gasifying reactor and
feed-stock elemental composition, humidity, gasification tempera- also in air pre-heating using syngas sensitive heat. Solution 2 uses
ture and it is determined based on the energy balance of the gasi- air pre-heating combined with feedstock external drying thus
fication process. offering a higher degree of energy recovery. The processes related
Based on gasification experimental results and feedstock prop- to these solutions were numerically modelled using specific gasifi-
erties the second step consisted in establishing a gasification con- cation process equations at thermodynamic equilibrium as well as
version line at theoretical level. Within this second step a series of energy and mass balance equations.
estimations were made with respect to syngas composition, gasifi- For the air-gasification process, the equivalent ratio variation
cation efficiency and global energy efficiency. Consequently, a depending on waste humidity is presented in Fig. 1.
combined experimental and computational approach was applied The molar fractions of the main combustible gas species in the
aiming for the optimum waste–syngas–energy conversion chain. syngas, CO and H2, are presented in Fig. 2.

22 Biogas D 54 Syngas H
Biogas C
fuel/(air+fuel) [kmol/kmol]

fuel/(air+fuel) [kmol/kmol]

20 50 Syngas G
Biogas B
18 46 Syngas F
Biogas A
16 42 Syngas E
Methane
14 38
12 34
10 30
8 26
6 22
4 18
1 1.2 1.4 1.6 1.8 2 2.2 1 1.3 1.6 1.9 2.2 2.5 2.8
Air excess coefficient [-] Air excess coefficient [-]
(a) Digester biogases (b) Air gasification syngases
Fig. 9. Gaseous fuel quotas in intake mixture, vs. the excess air ratio (kair ex). (a) Digester biogases. (b) Air gasification syngases.
 C. Marculescu et al. / Waste Management 47 (2016) 133–140 137

412 439 Syn-gaz G

408 438 Syn-gaz H
437 Syn-gaz E
404

temperature [°C]

temperature [°C]
Syn-gaz F
400 436

.
435
396
434
392
Methane 433
388 Biogas A 432
384 Biogas B 431
380 Biogas C
430
Biogas D
376 429
1 1.2 1.4 1.6 1.8 2 2.2 1 1.3 1.6 1.9 2.2 2.5 2.8
Air excess coefficient [-] Air excess coefficient [-]
(a) Digester biogases (b) Air gasification syngases
Fig. 11. The instantaneous temperatures after compression, vs. the excess air ratio. (a) Digester biogases. (b) Air gasification syngases.

2,300 2,300
2,200 Methane 2,200 Syn-gaz E
2,100 Biogas A 2,100 Syn-gaz F
Biogas B 2,000 Syn-gaz G
temperature [°C] .

2,000

temperature [°C]
Biogas C 1,900 Syn-gaz H
1,900
Biogas D 1,800
1,800
1,700
1,700
1,600
1,600 1,500
1,500 1,400
1,400 1,300
1,300 1,200
1 1.2 1.4 1.6 1.8 2 2.2 1 1.3 1.6 1.9 2.2 2.5 2.8
Air excess coefficient [-] Air excess coefficient [-]
(a) Digester bio-gas (b) Air gasification syn-gases
Fig. 12. The instantaneous maximal temperature after the burning, vs. the excess air ratio. (a) Digester bio-gases. (b) Air gasification syn-gases.

24 Biogas D 21 Syn-gaz F
22 Biogas C 20
Syn-gaz E
19
CO2 molar quota [%]

CO2 molar quota [%]

20 Biogas B
18 Syn-gaz H
.

18 Biogas A
17
Methane 16 Syn-gaz G
16
15
14
14
12 13
10 12
8 11
10
6 9
4 8
1 1.2 1.4 1.6 1.8 2 2.2 1 1.3 1.6 1.9 2.2 2.5 2.8
Air excess coefficient [-] Air excess coefficient [-]
(a) Digester bio-gases (b) Air gasification syn-gases
Fig. 13. Molar rate of CO2 in dry flue gases, vs. the excess air coefficient. (a) Digester bio-gases. (b) Air gasification syn-gases.

The CO concentration is higher than hydrogen concentration for therefore the expected syngas LHV will be higher if separate waste
both solutions. This can be explained by the fact that CO is drying is applied.
obtained both from the C and H2O reaction as well as from the par- The concentration of inert gases (CO2) in syngas as well as the
tial oxidation of C. The same mol number of CO and H2 results from combustible gas fraction (low CH4 concentration) depend on the
the C and H2O reaction. Hydrogen results only from the C and H2O gasification process configuration and feedstock humidity as pre-
reaction (Sharma, 2011). Both main combustible gases concentra- sented in Fig. 3 (Surjosatyo et al., 2014; Loha et al., 2011).
tions (CO and H2) decrease with feedstock humidity because the Analyzing the diagram from Fig. 3, we notice that CO2 concentra-
fraction of C that combines to H2O decreases while the C fraction tion varies rapidly with feedstock water content. For the same
that is oxidized in CO and CO2 increases to cover the heat demand feedstock the CO2 fraction in syngas is higher in solution 1 than
of the vaporising process. Consequently the lower heating value in solution 2. At higher humidity, which is our case, the differences
(LHV) of syngas decreases when waste water content increases. between the two solutions are less significant. The methane con-
CO and H2 are higher for solution 2 compared to solution 1, centration in syngas is comparable in both cases and does not
138 C. Marculescu et al. / Waste Management 47 (2016) 133–140

exceed 1.8% in volume and its concentration decreases when the reports) of the ratios between molar fractions of CH4 and CO2
waste humidity increases. (mCH4/mCO2) varies from simple to double within range [0.8–1.6].
Based on estimated syngas composition the lower heating value For these ratios, in the lower part of columns A–D of Table 2, the
was computed. The variation of syngas LHV with feedstock initial calculated values of equivalent molar masses, and calorific values,
humidity is presented in Fig. 4. HHV and LHV are shown.
In the hypothetical case of waste with minimum water content, The variations of CH4 and CO2 molar fractions within gas fuels
the obtained LHV is about 6.6–6.7 MJ/N m3. This parameter contin- composition, their equivalent molar masses and HHV/LHV as func-
ually decreases with feedstock humidity. At 41% humidity (charac- tion of mCH4/mCO2 ratio, are shown in the Figs. 6–8.
teristic of the raw waste) the expected LHV of the syngas is about We notice that molar masses are about double as compared to
3.6–3.7 MJ/N m3. These values are considerably different from the pure CH4 while HHV and LHV are about half the pure CH4. For the
experimental ones due to the reasons presented in the previous analysis performed on gaseous fuels produced by air-gasification
section of this paper. We also notice that the differences between the input data was the elemental composition of the biomass
syngas LHV delivered in the two cases become smaller with (Table 3).
humidity increase. Beyond the estimated values presented in the Based on this data and using an original computation method,
diagrams as function of feedstock initial humidity, interesting for developed by the research team, the air gasification of the raw
our analysis are the values that correspond to our raw feedstock feedstock was modelled. The feedstock humidity was different
(about 41–42%). In this case the composition of dried syngas is: from 0, according to real conditions. The conversion process
CO – 14.4%; H2 – 12.8%; CO2 – 6.9%; CH4 – 1.6%; N2 – 64.3 includes the syngas heat recovery for biomass drying. The drying
(solution 1), respectively CO – 19.1%; H2 – 17%; CO2 – 6.7%; CH4 – water vapours are exhausted alongside with the syngas flow.
1.6% N2 – 55.6% (solution 2). The upper part of columns E–H from Table 2 presents the syn-
The gasification process efficiencies were computed using the gas composition, arranged decreasingly with respect to HHV and
hot gas efficiency formula because within the waste to energy con- LHV according to the following criteria:
version chain the internal energy recovery was applied at different
thermal levels. For example the gasification air is preheated using column E, wood biomass, column F, wood biomass,
the syngas sensitive heat. Fig. 5 presents the estimated hot gas effi- humidity = 30% humidity = 45%
ciency. Due to the different configurations used in the analysis, the column G, meat waste, column H, meat waste,
differences between the calculated efficiencies are also important. humidity = 30% humidity = 45%
The gasification efficiency is about 80.5% for solution 1 and
85.5% for solution 2 when applied to the raw feedstock (waste
directly from the meat processing line). As the all the necessary
The syngas composition reported in Table 2 is free of ash and tar, in
energy required in the gasification and drying processes is
engine intake, after dedusting and tar removal processing. The
self-provided, the gasification process efficiency is less important
Nitrogen in the feedstock could also lead to NH3 formation in the
than syngas quality, because the feed-in feedstock is not a fuel
syngas. Because of its low quota, the ammonia was neglected in
(which must by paid for) but a residue. Therefore the most impor-
our computation. The energy analysis focused on the power gener-
tant criteria in our analysis are the syngas composition/LHV and
ation using spark ignition engines taking into account a small toler-
the possibility of using this fuel for an internal combustion engine.
ance in syngas pollutants.
As well as for the bio-chemical process (anaerobic digestion):
3.2. Gas fuel type influence on engine combustion thermodynamics
(1) humid biogas/syngas was considered for simulation – H2O
about 4.6%; (2) the equivalent syngases molar masses together
The syngas properties influence on internal combustion engine
with their HHV and LHV are presented in the lower section of E–
behaviour was studied compared to digester biogas and methane
H columns. It can be noticed that:
as reference. The digester biogas was used in the analysis because
spark ignition engines are widely used for power generation being
 the main combustible gases in the syngas are CO (33.5–17.9%)
modified to burn lean gas fuel instead of methane (Sadeghinezhad
and H2 (20.8–11.1%), and the non-combustibles are N2&Ar from
et al., 2014; Munasinghe and Khanal, 2010). Consequently our
air (33.3–57.9%) together with CO2 (4.4–7%);
interest was in observing if significant modifications occur at ther-
 due to the high organic oxygen presence in biomass#1, the air
modynamic level when syngas is used.
required by the gasification process is lower, reducing the
By analyzing the properties of biogas from anaerobic digestion
N2&Ar from air and increasing the combustible fractions quota
we noticed that common bibliographic sources (Deublein and
for a higher HHV/LHV as compared to the syngas produced from
Steinhauser, 2008; Arroyo et al., 2014) do not classify these fuels
biomass#2;
upon their molar structure as a consequence of raw organic mate-
 molar masses (lmix) are lower, because the syngas molar
rial elemental composition. In our analysis we neglected the distri-
masses are lower as compared to biogas;
bution of low content gas species such as: C2H4, C2H6, C3H6, C3H8,
 due to CO lower heating values, and to the presence of increased
C4H10, C5H12 and SO2. We assumed H2, CO, H2S, N2&Ar and O2 to be
non combustible gas fractions, the syngas HHV and LHV are
constant as they are low but not negligible. The cited sources con-
from 2 to 4 times lower as compared to biogas.
sider the biogas dried, a purely theoretical assumption. Based on
real conditions our analysis considered the biogases with water The study focused on the influence of feed-in gas type on the
vapour content at a quota that can be technologically achieved combustion process and the resulting flue gas characteristics.
through partial condensing of H2O vapours. In that case the water Consequently the combustion at stoichiometric ratio, and the one
vapours will be around 4.6% in volume. with excess air (kair ex – defined as the ratio between the real air
The results of our analysis are presented in Table 2. As this data consumption and the stoichiometric air demand) was modelled
represent the input for our research on engine combustion a com- using the considered gas fuels under the following conditions:
mon basis was considered, as presented below. The first columns (A) for digestion biogas, similar values as for CH4 Otto engines,
A–D contain the compositions considered: (1) the sum of main kair ex 2 [1–2.2]; (B) for syngas kair ex 2 [1–2.8].
gas components, combustible (CH4), and non-combustible (CO2) For stoichiometric conditions (kair ex = 1) the fuel gas molar frac-
is constant (91.2%); (2) the usual value (according to bibliographic tion in the engine intake gaseous mixture is:
 C. Marculescu et al. / Waste Management 47 (2016) 133–140 139

 9.41% molar, when using pure CH4; digestion biogases, in the excess air range kair ex 2 [2.2–2.8] all syn-
 5.69–20.51% molar, when using biogas; gases generate end combustion temperatures lower than digestion
 39.59–54.07% molar, when using syngas. gases when operated at kair ex 2 [1.6–2.2].
Fig. 13a and b show that gaseous bio-fuels with significant com-
Fig. 9a and b present the gaseous fuel molar quotas in the intake position differences generate combustion flue gases with similar
mixture vs. the excess air ratio – kair ex. It can be noticed that these composition when the process excess air is comparable.
quotas decrease by 1/x variation when air excess increases. The study revealed the importance of both primary energy
The fact that gaseous bio-fuels quota in engines intake mixtures sources and derived fuels conversion process types in the opera-
are several times higher as compared to CH4 use, leads us to the tion of natural aspiration Otto engines when shifting from natural
conclusion that even for stoichiometric conditions the engines car- gas to biomass and waste based gas fuels.
burettor requires adapting when shifting from natural gas use to
biogases or syngases (Arroyo et al., 2013).
Due to syngases high quota in engine intake mixtures and to the 3.3. Power generation
consistent presence of H2 an additional problem emerges in rela-
tion to the increased detonation potential during compression. To estimate the electrical output that can be achieved by burn-
Consequently, the question will be: is it suitable to maintain the ing the syngas into an internal combustion engine we used speci-
same kair ex as for using CH4? (Porpatham et al., 2012; Sahoo fications for series manufactured power equipment available on
et al., 2012). To solve this issue the H2 quota in intake mixtures the market. The waste feed in flow was chosen 110 kg/h. This mass
was calculated. The results are shown in Fig. 10. flow is characteristic to medium industry capacities for meat pro-
It can be noticed that in order to keep H2 molar quota below the cessing (Russ and Meyer-Pittroff, 2004). The engine-generator out-
detonation limit, mH2/mair+syngas 6 4%, the kair ex must be increased up put was calculated according to the new combustible used as well
to [2.2–2.8] (increasing with the syngas HHV/LHV ratio decrease). as to its efficiency. Compared to natural gas, when the global effi-
These values exceed the maximum usual ones for Otto engines ciency of the unit is about 24.24%, if syngas is used the efficiency
running on CH4, where kair ex 6 2.2 (Raman and Ram, 2013). may decrease to 18.2%.
We expect that kair ex increase will have as immediate effect the The electrical output that can be generated will range from
decrease of the instant after burning temperatures and lower val- 42 kW to 48 kW depending on the gasifying conditions: direct air
ues of the average superior temperature of the cycle (Tav warm), gasification or air gasification with preliminary waste external dry-
affecting the brake power at flywheel and the mechanical net effi- ing. This power represents approximately 15% of thermal power to
ciency of the Otto engine, compared to CH4 burning. be obtained by waste stoichiometric combustion.
To quantify this effect we considered the case of natural aspira-
tion engines with Vmax/Vmin = 8. Using the real gases properties
equations developed in Cenusßă and Alexe (2007), we performed 4. Conclusions
the numerical models of:
The paper presents the results of an original approach of
 non-isentropic compression, down to Vmin; waste to energy conversion by using thermo-chemical processes
 isochoric fast combustion, quasi-adiabatic. applied to an un-exploited fuel source – residues from the meat
processing industry. The combined experimental campaign and
The results are shown in the Figs. 11a and b, 12a and b theoretical computation revealed the energy potential of this
and 13a and b. waste type with respect to quality gas fuel production which
Fig. 11a and b revealed that for different intake mixtures com- can be used in internal combustion engines for power generation.
positions, and the same initial temperature, the instantaneous Laboratory scale experiments proved the possibility of direct
temperatures after compression: gasification and good quality syngas production. The estimation
of industrial scale gasification was completed by extrapolating
 are higher when using syngases, as compared to digestion bio- the results and using engineering computational tools.
gases, for the same kair ex; Good quality syngas with up to 19.1% – CO; 17% – H2; and
 for digester biogases, decrease when their HHV increases; for 1.6% – CH4 can be produced.
the same HHV, increase along with the kair ex; The influence of syngas fuels properties on combustion in spark
 for syngases having the same HHV, decrease when the kair ex ignition engines was studied and compared to biogas and methane
increases. for different initial temperatures of gases mixtures and excess air
i.e. the stoichiometric ratios (molar and mass) between gas and
The instantaneous temperatures at the end of combustion pro- air as well as the flue gas temperatures after combustion.
cess depend on: (a) the initial temperature of the cycle, (b) intake For the applications using syngases, characterised by higher
mixtures composition and (c) HHV values. Fig. 12a and b show that hydrogen fraction, in order to keep H2 molar quota below the deto-
the instantaneous temperatures after burning: nation value, mH2/mair+syngas 6 4%, the kair ex must be increased up to
[2.2–2.8]. These values exceed the maximum usual ones for Otto
 are comparable for syngases and digestion biogases, for the engines running on natural gas that are inferior to 2.2. The data is
same kair ex; suitable for Otto engines characterised by power output below
 decrease in both cases when the kair ex increases. 1 MWel, designed for natural gas consumption and fuelled with lean
syngas.
Both results are not favourable to syngas fuelled engines Using data from the power equipment series manufactured and
because: (1) syngases have lower HHV compared to biogases and available on the market, the electrical output that can be recovered
(2) in order to avoid detonation, the syngas fuelled engines must from a waste source of about 110 kg/h may reach 45 kW.
work with higher excess air ratios kair ex. The results sustain the possibility of using residues from the
Despite higher peak temperatures reached by wood syngases food industry as a renewable energy source for alternative fuel
stoichiometric combustion (E and F) compared to average quality and power generation.
140 C. Marculescu et al. / Waste Management 47 (2016) 133–140

Acknowledgement Kobayashi, N., Tanaka, M., Piao, G., et al., 2009. High temperature air-blown woody
biomass gasification model for the estimation of an entrained down-flow
gasifiers. Waste Manage. 29 (1), 245–251.
This work was supported by a grant of the Romanian National Loha, C., Chatterjee, P.K., Chattopadhyay, H., 2011. Performance of fluidized bed
Authority for Scientific Research, CNDI – UEFISCDI, project number steam gasification of biomass – modeling and experiment. Energy Convers.
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PN-II-PT-PCCA-2011-3.2-1687 (62/2012).
Marculescu, C., 2012. Comparative analysis on waste to energy conversion chains
using thermal–chemical processes. Energy Procedia 18, 604–611.
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