Content uploaded by A. Arshanitsa
Author content
All content in this area was uploaded by A. Arshanitsa on Apr 02, 2015
Content may be subject to copyright.
151
International Scientific Colloquium
Modelling for Material Processing
Riga, September 16-17, 2010
Processing and Complex Research of the Main Characteristics of
Pelletized Lignocellulosic Materials for Clean and Effective
Energy Production
A. Arshanitsa, I. Barmina, A. Andersone, G. Telysheva, M. Zake
Abstract
The paper presents results of complex experimental study of the composition and
heating values as well as the combustion and emission characteristics of a pelletized biomass
fuel produced from softwood and wheat straw non-hydrolyzed lignocellulosic residues
(LHRs) of bioethanol production. Those biofuels can be estimated as prospective alternative
renewable energy sources for clean heat and energy production by direct combustion or
gasification. A small-scale pilot combustion system with swirl-stabilized flame of volatiles
and heat power output up to 3 kWh was used in experiments. The results of local time-
dependent measurements of the flame temperature, heat production rates and emission
characteristics of LHR granules are compared with those of commercial softwood granules.
The gasification step of LHRs granules results in an enhanced release of CO and free
hydrogen emissions in comparison with the gasification step of softwood granules. This
predetermines the possibility of LHRs granules use for syngas production. It is found that the
high content of nitrogen in the LHR granules results in a relative high mass fraction of NOx
emissions in the products. Less impact of the LHR combustion on the environment with a
higher heat output can be achieved by co-firing the LHR with softwood granules.
Introduction
The limited availability of fossil fuels and the necessity of environment protection
from greenhouses gas emissions promote an intensive research and development of alternative
energy sources for the clean and effective energy production [1, 2]. Biomass is the third
largest primary energy resource in the world, and it is estimated to be more than 10% of the
total word energy consumption [3]. Up to now, the low energy density, dissimilar structure
and high moisture content of different types of biomass restrict its utilization for the clean and
effective energy production. Granulation is a recognized way to enhance the efficiency of
plant biofuel utilization for energy production by increasing its energetic density with
simplification of biomass transportation, storage and automation of fuel supply systems.
Softwood pellets are the most widespread type of granulated biofuel. Because of the intensive
utilization of wood biomass for energy production during the last few years, the good quality
saw dust and wood chips become less available. Non-hydrolyzed lignocellulosic residues
(LHRs) of bioethanol production can be considered as an alternative material for the
granulated biofuel production. About 40-60% of LHRs must be incinerated in plant boilers.
The remaining LHRs can be utilized as a granulated biofuel that allows to obtain additionally
~0.6 MW and ~2.0 MW of heat per 1 MW of ethanol energy in case of softwood and wheat
straw hydrolysis, correspondingly [4]. The application of granulated LHRs for heat energy
152
production will make a valuable contribution to the process economy, decreasing the total cost
of the bioethanol production. Moreover, gasification of lignocellulosic residues with a high
carbon and hydrogen content can be used to produce a syngas for heat and electricity
generation. Large-scale biomass gasification plants in Austria, Great Britain, Sweden, etc.
allow a more effective and low cost utilization of different types of biomass [5-9].
The previous study of the combustion and emission characteristics of LHRs granules
produced from spruce wood by dilute acid hydrolysis and enzymatic hydrolysis has shown
that the combustion of LHRs pellets produced by a screw extrusion technology is
characterized by a higher rate of heat production and a faster ignition of the volatiles with an
enhanced release of CO2, NOx and SO2 in comparison with the softwood granules [10]. The
main disadvantages of the LHRs granules produced by the screw extrusion technology are the
lower bulk density values (450-550 kg/m3) if compared to commercial softwood granules
(650-700 kg/m3) that decreases the energy density of the biofuel. The aim of the present study
is the investigation of the combustion process including the gasification step of softwood
LHRs and LHR wheat straw granules with a high bulk density of 720-730 kg/m3. The
increasing density of the LHRs granules up to the values of commercial softwood granules
was achieved using a pellet mill technology for granulation instead of screw extrusion. To
reduce the NOx emission as well as the bottom ash content, co-firing of renewable LHR
granules with softwood granules was investigated.
1. Experimental
1.1. Materials and Method of Investigation
Non–hydrolyzed residues of softwood 3/4 were manufactured at the Ornskoldsvik pilot
plant (Sweden) by enzymatic hydrolysis of softwood with simultaneous saccharification and
fermentation. Non–hydrolyzed residues of wheat straw 1142 were manufactured at the
SAFISIS pilot plant (France) by enzymatic hydrolysis of wheat straw with separated
saccharification and fermentation stages.
The LHRs were pelletized using a laboratory pellet mill KAHL 14-175. The water and
ash content in the granulated LHRs were determined according to CEN/TS 14774-1 and
CEN/TS 14775, correspondingly. Klason lignin (KL) was determined according to [10]. C, N
H, S in total were determined using the Analysis System Vario Macro CHNS. Higher heating
values (HHV) and combustion sulphur content (Scomb) were found according to ISO1928 and
CEN/TS 15289. Bulk densities of the granules were determined according to CEN/TS 15103.
The mechanical durability (DU) values of the granules were determined in accordance with
CEN/TS 15210-1 (Tab. 1-2).
Commercial softwood granules were used for references.
Tab. 1: Composition and heating values of origin softwood and LHRs on dry mass
Biomass
granules
HHV,
kWh/kg
Klason
lignin
content ,
%
Elemental content, %
C
H
N
Stot
Scomb
Ash
Softwood
5.4
28.8
50.2
6.3
0.24
0.13
0.02
0.60
LHR
softwood 3/4
6.2
51.5
55.2
4.8
0.43
0.32
0.15
0.40
LHR wheat
straw 1142
5.4
44.4
49.7
5.3
1.1
0.35
0.11
6.4
153
Fig. 1. The digital image of
the small-scale
experimental device
Tab. 2: The main characteristics of the LHRs and commercial softwood granules
1.2. Combustion System
The combustion process of different types of biomass granules was studied experimentally
using a compact experimental setup with the total heat output up to 3.0 kWh (Fig.1). The
setup consists of a wood biomass gasifier (1) charged with
biomass (230-260 g), a premixed swirling propane/air burner,
which provides an additional heat supply into the gasifier (2),
and a sectioned water-cooled channel (5). The primary (3) and
secondary (4) airflows were used to initiate the biomass
gasification and provide a complete burnout of volatiles. The
primary airflow was injected into the bottom part of the
gasifier at the rate 46 l/min. The secondary swirling airflow
was supplied into the upper part of the gasifier at the rate
71 l/min.
The diagnostic sections with orifices (6) were used to
insert the diagnostic tools (thermocouples, gas sampling
probes) into the flame of volatiles. The local flame
temperature was measured by Pt/Pt-Rh (10%) thermocouples
with a PC-20TR computer system for data collecting and
recording. From the calorimetric measurements of the cooling
water flow the average heat production rate was estimated for
different stages of the burnout of volatiles. The temperature
and composition of the products (T, NOx, CO2, CO, H2, O2)
were on-line registered at the channel outlet using a Testo
350XL gas analyzer.
2. Results and Discussions
The self-sustaining combustion of biomass includes the following main steps: water
evaporation, biomass gasification resulting in the formation of carbohydrates and thermal
degradation of lignin producing volatiles, ignition and combustion of the volatiles and char
combustion. The most intensive water evaporation from the biomass occurs at T ≈ 370 K.
Then follows thermal decomposition of hemicelluloses at T ≈ 470-570 K, of cellulose at
T ≈ 520-630 K and of lignin at T ≈ 450-770 K. The main products of the biomass gasification
are: CO, H2, CH4, CO2, H2O, N2, gaseous olefins, aromatics, primary and secondary
Sample
Water
content,%
LHV,
kWh/kg
Bulk
density,
kg/m3
Energy density,
MW*h/m3
DU,%
LHR softwood 3/4
7.6
5.4
735
4.0
97.7
LHR wheat straw
1142
9.2
4.6
720
3.3
98.8
softwood
7.2
4.7
699
3.3
98.6
1
2
4
3
5
6
154
condensed oils and charcoal [8, 9]. The simplified reaction of the biomass gasification at
about 10% water content (hydrothermal gasification) can be expressed as:
Carbohydrate matter (C6H10O5) +6O2+mH2O+Heat→6CO+5H2+nH2O (2.1)
The results of the experimental study have shown that for the given main
characteristics of the LHRs and commercial softwood (Tab. 2) at constant rates of primary and
secondary air supply in the combustor, the process of granulated LHR gasification (t < 500 s)
results in an intensive formation of a mixture of the combustible volatiles CO and H2 (Fig. 2)
with approximately constant ratio of H2/CO≈0,66 during the gasification of the different
biomass types. Higher contents of CO and H2 in the products are observed during the
gasification of the lignocellulosic residues (LHR softwood 3/4 and LHR wheat straw 1142)
with a higher Klason lignin content in comparison with the softwood (Fig.2, Tab. 1) that is
important for the syngas production by the LHRs granules gasification.
Fig. 2. Time dependent variations of the mass fraction of CO and H2 at the gasification stage
of different types of granulated biomass fuel (t < 500 s)
At the next stage of the biomass combustion (t > 500 s), the ignition and burnout of the
volatiles results in a fast increase of the temperature up to 1600-1700 K with a correlating
increase of the heat production rates and CO2 volume fraction in the products, while the mass
fraction of CO in the products decreases to a minimum value (50-150 ppm) (Figs. 2, 3). The
simplified reaction of the biomass combustion can be written as:
Carbohydrate matter (C6H10O5)+6O2 +(m-n)H2O→6CO2+(5+m-n)H2O +Heat (2.2)
As one can see from Tab. 1-2 and Fig.3, the higher content of carbon and the higher
heating value of the LHR softwood granules result in a higher amount of the total heat
production (7.7 MJ/kg) if compared to that of softwood (7.1 MJ/kg) and LHR wheat straw
(5.4 MJ/kg). The lower heat output is observed for the LHRs wheat straw, which can be
explained by the prolonged char combustion at the end stage of the wheat straw burnout in the
bottom part of the gasifier that is not equipped with a water-cooled jacket determining the
reduced amount of the produced heat energy. These features of the LHRs wheat straw must be
accounted for when develop a large-scale boiler for the combustion of granulated LHRs.
0
3000
6000
9000
0250 500
time,s
Mass fraction,ppm
CO,wood CO,LHR 3/4
CO,LHR 1142
0
2400
4800
7200
0250 500
time,s
Mass fraction,ppm
H2,wood H2,LHR 3/4
H2,LHR 1142
155
Fig. 3. Time dependent variations of the flame
temperature, heat production rate and volume
fraction of CO2 in the products at different
stages of the biomass combustion.
The higher mass fraction of NOx in the
products (up to 500 ppm) is observed for the
burnout of the LHR wheat straw with a higher
content of nitrogen in granulated biomass (Tab.
1, Fig. 4). For the softwood granules, the mass
fraction of NOx in the products decreases to
100 ppm. The more effective utilization of
non–hydrolyzed residues with a higher heat energy production (up to 8 MJ/kg for the LHR
softwood and 6.4 MJ/kg for the LHR wheat straw) and the reduced total amount of NOx
emission by 27% for the LHR softwood and by 25% for the LHR wheat straw were observed
at co-firing the LHR with softwood granules at the mass ratio of LHR and softwood equal
to1.0 (Fig. 4).
Fig. 4. The effect of co-firing the LHRs with softwood granules on the formation of NOx at
different stages of the biomass combustion
0
180
360
0700 1400 2100
time,s
Mass fraction,ppm
NOx,w ood NOx,LHR 3/4
NOx,LHR 3/4+wood
0
200
400
600
0700 1400 2100
time,s
Mass fraction,ppm
NOx,w ood NOx,LHR 1142
NOx,LHR 1142+wood
300
800
1300
1800
0700 1400 2100
time,s
Temperature,K
T2,wood T2,LHR 3/4
T2,LHR 1142
0
710
1420
2130
0700 1400 2100
time,s
Heat production rate,J/s
Qsum,wood Qsum,LHR 3/4
Qsum,LHR 1142
0
6
12
18
0700 1400 2100
time,s
Volume fraction,%
CO2,wood CO2,LHR 3/4
CO2,LHR 1142
156
Besides, the co-firing of the LHRs wheat straw with softwood granules has resulted in
a significant decrease of the total ash content from 6.4 to 3.5 % for dry biomass.
Conclusions
1. The non-hydrolyzed lignocellulosic residues of bioethanol production from wood
and wheat straw are new efficient biofuels with higher heating values and higher carbon
contents that can be used for the heat energy production by direct combustion.
2. The increased lignin content in the LHRs granules provides the biomass
gasification with a higher CO and H2 mass fractions in comparison with that for softwood
granules. This fact allows predicting some potential for the LHRs granules’ use as a biofuel in
gas-generator facilities for heat and energy production.
3. The LHRs granules (especially from wheat straw) contain a relatively high
concentration of nitrogen, sulphur [10] and ash in comparison with softwood granules. For a
more efficient utilization of the LHRs granules with a higher heat energy output, a reduced
harmful emission and ash content joint LHR and softwood granule combustion can be
applied.
References
[1] Asif, M., Muneer, T.: Energy supply, its demand and security issues for developed and emerging economies.
Renewable and sustainable Energy Reviews, Vol. 11, 2007, pp. 1388-1413.
[2] Renewable Energy. Annual Energy Review, Report No. DOE/EIA-0384, U.S. Energy Information
Administration, 2008, pp. 1-24. http://www.eia.doe.gov/emeu/aer/renew.html
[3] ECN Biomass, Coal & Environmental Research. Energy Research Centre of the Netherland, 2007, pp. 2.
http://www.ecn.nl/fileadmin/ecn/units/bio/Leaflets/b-08-017_ECN_BKM.pdf
[4] Wingren, A., Galbe, M., Zachi, G.: Energy consideration for SSF-based softwood ethanol plant. Bioresource
Technology 99, 2008, pp. 2121-2131.
[5] Simader, R. G.: 2 MWel biomass gasification plant in Güssing (Austria). The Austrian Energy Agency, 2004,
pp. 1-6. http://www.opet-chp.net/download/wp3/g%FCssingaustria.pdf
[6] Kurkela, E., Kurkela, M. (ed.): Advanced Biomass Gasification for High-Efficiency Power. Publishable Final
Activity Report, Project No 019761, 2009, pp. 1-56. http://www.vtt.fi/inf/pdf/tiedotteet/2009/T2511.pdf
[7] Ellis, R.: Biomass-Fueled Power Plants, an overview. Utility Engineering Corporation, pp. 1-5
http://www.ue-corp.com/news/wp_biomass.pdf
[8] Run Cang Sun: Cereal straw as resource for sustainable biomaterials and biofuels. Elsevier, Oxford, UK,
2010, pp. 284.
[9] Wang, L., Weller C. L., Jones, D. D., Milford A. Hanna: Contemporary issues in thermal gasification of
biomass and its application to electricity band fuel production. Biomass and Bioenergy, 32, 2008, pp. 573-
581.
[10] A. Arshanitsa, I. Barmina, T. Dizhbite, G. Telisheva, M. Zake: Combustion of Granulated Plant Biofuel.
The 5th UEAA General Assembly and the Associated Workshop on “Renewable Energy Resources,
Production and Technologies”, Zinātne, 2008, Rīga, pp. 37-46.
Authors
M.chem. Arshanitsa, Alexandrs
Dr. habil. chem. Telysheva, Galina
M.chem. Andersone, Anna M.chem. Andersone, Anna
Latvian State Institute of Wood Chemistry
27 Dzērbenes str.
LV-1006, Riga, Latvia
E-mail: arsanica.a@inbox.lv
ligno@edi.lv
Dr. phys. Zaķe, Maija
Dr.sc.ing. Barmina, Inesa
Institute of Physics, University of Latvia
32 Miera str
LV-2169, Salaspils, Latvia
E-mail: mzfi@sal.lv
barmina@sal.lv
