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VOL. 42, 2014

A publication of

The Italian Association

of Chemical Engineering

www.aidic.it/cet
Guest Editors: Petar Sabev Varbanov, Neven Duić

ISBN 978-88-95608-33-4; ISSN 2283-9216 DOI:10.3303/CET1442012

Please cite this article as: Žandeckis A., Romagnoli F., Beloborodko A., Kirsanovs V., Blumberga D., Menind A., Hovi

M., 2014, Briquettes from mixtures of herbaceous biomass and wood: biofuel investigation and combustion tests,

Chemical Engineering Transactions, 42, 67-72 DOI:10.3303/CET1442012

Briquettes from Mixtures of Herbaceous Biomass and

Wood: Biofuel Investigation and Combustion Tests

Aivars Žandeckis*
a
, Francesco Romagnoli

a
, Anna Beloborodko

a
,

Vladimirs Kirsanovs
a
, Dagnija Blumberga

a
, Andres Menind

b
, Mart Hovi

a
Institute of Energy Systems and Environment, Riga Technical University, Kronvalda blvd. 1, Riga, Latvia

b
Institute of Technology, Estonian University of Life Sciences, Kreutzwaldi 56, Tartu, Estonia

aivars.zandeckis@rtu.lv

The background idea of the current research lies directly in the evaluation of combustion performance and

its association to the analysis of the quality of different types of herbaceous-based briquettes mixed with

wood. In total, 25 briquette samples were prepared using different proportions of herbaceous and woody

biomass. Draff, buckwheat hulls, straw, rapeseed by-product from biofuel production, oats, and common

reed were used. Briquettes fully produced from wood were used as a benchmark.

A laboratory analysis was completed to determine the following parameters of the samples: moisture and

ash content, calorific value, ash melting temperature, content of carbon, hydrogen, nitrogen, and sulphur.

The combustion tests were executed using a heating stove for all samples. The results show that some

types of non-wood biomass have a positive effect on briquette production and the combustion processes.

1. Introduction

In fact, Latvia has an important agricultural sector connected to different final end uses of the agricultural

biomass produced. Biomass residues from industrial processes are not frequently used within the

perspective of energy production for different reasons. Some of these weaknesses could represent an

opportunity for the usage of biomass residues for briquettes production, in order to provide a higher energy

value per unit of mass and to overpass the drawbacks connected to transportation, handling, treatment,

and storage (Zhang J. et al., 2014). This evidence is also reflected within the increase of the trend of the

production of briquettes from agricultural and forestry leftover during recent years (Heinimo et al., 2009).

The production costs of different briquette types can strongly vary (Stolarskia et al., 2013).

Nowadays, different methods to produce briquettes exist with an important role within its final physical and

mechanical properties (Chou et al., 2009). It is also noticeable that the quality of the briquettes produced

from agricultural biomass can be improved by adding another biomass product (Yaman et al., 2001).

Chemical and physical parameters of briquettes have an important influence on the behaviour of the

combustion process (Roy and Corscadden, 2012). The ash content in different agricultural fuels can

strongly vary, and have a strong effect on the combustion process. Driving attention to the economic

aspects, one of the non-wood briquette advantages is the price of raw materials and production costs.

Some agricultural briquettes can be successfully used for private home heating with results similar to

briquettes produced from wood (Sellin et al., 2013). The content of nitrogen in the fuel has a strong

correlation with NOx emissions in flue gas (Koyuncu et al., 2007). If the briquettes is too dry, the

combustion process can happen very quickly, reducing the retention time for the chemical reactions and

heat exchange (Yuntenwi et al., 2008). The fuel combustion time decreases when an increase of draught

and air flow occurs (Yang et al., 2004). To get more complete combustion, and a lower amount of CO

emissions, a pre-treatment of the biomass can be completed (Barmina et al., 2012).

Hence, the goal of this study is to investigate the effects of a partial or full substitution of woody biomass

with herbaceous biomass for briquette preparation. In connection to the proposed goal, the scientific

novelty lies in the significant and important extension within the understanding of risks and potential gains

in use of non-woody biomass in the multidisciplinary local Latvian context.

2. Materials and methods

A total number of 6 herbaceous types of biomass were used for the experiments. Pure woody biomass

was used to make mixtures with other types of biomass, and as a benchmarking threshold in the analysis

of the results. Briquette samples were prepared using varying proportions of herbaceous and woody

biomass (100:0, 75:25, 50:50 and 25:75). The original sources of the biomass and designation used to

describe the biomass mixtures are given in Table 1. In this article, the numbers after the designations (e.g.

DR75) describe the mass percentage of non-woody biomass in the mixture. The ultimate and proximate

analysis of the biomass samples is performed in the Environmental Monitoring Laboratory of Riga

Technical University.

Table 1: Description of the biomass used (moisture content and price are related to the source site)

Biomass Designation Source Moisture, % Price, EUR t
-1

Draff DR Residue after fermentation, beer production 86.7 n/k

Common reed CR Pure material, wetlands 15.2 32 - 38

Middlings OWM By-product from sieving of oats and wheat 12.1 n/k

Straw ST By-product from grain cultivation 16.3 28 - 35

Buckwheat hulls BH By-product from production of buckwheat 14.4 n/k

Rapeseeds RB By-product from biofuel production 8.46 200 - 270

Wood WO Saw dust from sawmilling process 43.3 20 - 40

Draff is typically given or sold to farm enterprises, is sometimes land filled. High moisture content in

unprocessed draff. Middlings are typically given or sold to farm enterprises, sometimes used in biogas

production or directly combusted. High quality buckwheat hulls are expensive and used as filling material

for pillows, mattresses, and toys. Only low quality hulls are useful for energy production. Rapeseed cake

has a high nutritional value and is commonly used for animal feed. Because of its high price, it could be

used in low amounts as a compressing additive. The briquetting operations, and the energy consumption

measurements for processing, were performed in the Biofuel Laboratory at the Estonian University of Life

Sciences using the briquetting press Weima C-150. The applied compressive force to the sample in a 50

mm press chamber was approximately 20 t. Combustion tests were performed using the stove Skladova

Tehnika Torino K according to the standard methodology (LVS EN 13240, 2002). For every test, 2 kg of

biomass fuel was used. All biomass samples were burned using the same strategy for air supply. Thermal

performance of the stove was determined according to the standard methodology (LVS EN 13240, 2002).

100

110

OWM DR BH CR RB ST

25 % 50 % 75 % 100 % 100 % Wood

a) Energy consumption, kWhe t
-1

600

700

800

900

1000

OWM DR BH CR RB ST

25 % 50 % 75 % 100 % 100 % Wood

b) Particle density, kg m-3

Figure 1. Indicators in the briquetting process: a – energy consumption, b – particle density

3. Results

The measured energy consumption of the briquetting process for the studied cases varied in the range of

51.1 - 101.1 kWh t-1 (see Figure 1.a.). Similar results are reported in a study by (Menind et al., 2012 a).

The particle density of the produced briquettes is described in Figure 1.b. All the samples of pure grainy

materials do not suit the production of high quality briquettes. For mixtures with a sawdust ratio of 50 % or

higher, it was possible to press all the materials into solid and well-shaped briquettes. In order to

guarantee the quality and mechanical durability of briquettes, it is recommended to use additional binding

materials (LVS EN 14961-1, 2010). In this specific case, straw, hay or some other material rich in lignin

may be used instead of sawdust (Menind et al., 2012 b).

3.1 Quality parameters of the briquettes

The results of proximate and ultimate biofuel analysis are presented in Table 2. Numbers represent the

average value from a repetitive analysis of samples taken before the combustion tests.

Table 2: The characteristics of the tested samples of briquettes

Ad, Mar*, Qgr.V.d, Qnet.P.ar, C, H, N, S, Ta,
w-%,d w-% MJ/kg MJ/kg w-%,d w-%,d w-%,d w-%,d °C

DR25 1.66 7.09 20.8 17.7 48.30 7.13 1.08 0.02 1,510
DR50 2.65 7.50 20.7 17.5 48.52 7.37 2.40 0.03 >1,520
DR75 4.01 7.49 20.8 17.6 46.69 7.39 3.12 0.04 >1,530
DR100 4.91 7.82 21.0 17.7 46.29 7.26 4.05 0.05 >1,530
CR25 1.61 6.70 20.2 17.4 47.59 6.10 0.18 0.01 1,370
CR50 2.93 6.84 19.6 16.9 46.47 6.21 0.29 0.03 >1,530
CR75 3.93 6.61 19.1 16.4 44.92 5.95 0.37 0.03 >1,520
CR100 4.89 6.65 18.8 16.2 44.53 5.84 0.38 0.04 >1,530
OWM25 1.30 7.50 20.2 16.9 47.38 7.90 0.61 0.06 1,200
OWM50 1.97 7.95 19.7 16.4 45.97 7.91 1.31 0.08 1,300
OWM75 2.87 8.42 19.4 16.0 45.21 7.96 1.68 0.09 1,360
OWM100 3.61 8.66 19.0 15.6 43.58 7.52 1.84 0.09 1,400
ST25 1.51 6.95 20.2 17.2 46.60 7.03 0.21 0.05 1,090
ST50 2.54 7.06 19.8 16.8 46.85 6.76 0.39 0.08 1,130
ST75 3.69 7.33 19.2 16.2 45.33 6.92 0.42 0.09 1,180
ST7100 4.72 6.97 18.9 16.2 45.08 5.77 0.42 0.10 1,200
BH25 1.23 7.10 20.3 17.3 48.09 7.16 0.28 0.03 1,200
BH50 1.50 7.75 20.0 16.9 46.84 6.81 0.42 0.04 1,490
BH75 1.92 8.01 20.0 16.9 47.74 6.33 0.61 0.03 1,480
BH100 2.19 12.37 19.8 15.9 47.20 6.15 0.65 0.08 1,480
RB25 3.95 6.93 21.0 17.9 48.29 7.35 1.06 0.03 1,220
RB50 6.57 7.10 21.0 17.9 48.24 7.28 1.73 0.04 1,260
RB75 8.71 7.30 21.1 17.9 48.64 7.44 2.46 0.05 1,220
RB100 11.3 7.52 21.3 17.9 48.02 8.00 3.05 0.06 1,230
WO100 0.82 6.39 20.7 17.8 50.19 6.98 0.14 0.01 1,310

In the case of a non-standard deviation, additional analyses were applied. Variations between the results

of the final analysis are seen due to the low mass of the sample (10 mg), an inhomogeneity of the sample

(owing to the mixture of materials with different physical properties). Such variations are not typical for

non-mixture samples. For samples where one material has significantly higher ash melting temperature,

the overall melting temperature strongly depends on the proportions of each biomass. A rapid change of

form is seen after reaching the critical temperature for both of the materials. The results are rounded to the

nearest 10 °C according to the standard methodology (LVS CEN/TS 15370-1, 2007).

The moisture content for all the samples varied from 6.39 w-% to 12.4 w-%. The non-woody biomass used

for the experiments has a much higher ash content in comparison to the wood sawdust. The gross calorific

value is fully dependent on the amount of combustible elements in the solid fuel. Typically, wood presents

a higher gross calorific value than herbaceous biomass. Briquettes from draff, rapeseeds and oat, and

wheat middling show a higher nitrogen content than other samples. Some of the analysed biomass has a

high ash melting temperature. In fact, a low melting temperature of ash can cause significant problems in

terms of both the reduction of efficiency, and damages to the technical aspects of the combustion unit.

3.2 Combustion experiments
The main results related to the thermal performances of the studied combustion tests are presented in

Figure 2. The combustion of wood briquettes has a very fast ignition, and an intense combustion in the first

stage of the test. However, adding herbaceous biomass allows for the stabilisation and extension of the

combustion process. The concentration of free oxygen in flue gas shows a strong correlation with the

duration of the combustion tests. An intense combustion results in a lack of free oxygen within the

combustion chamber and a higher flue gas temperature

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Thermal power [kW]

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Thermal efficiency [%]

0,4

0,6

0,8

1,0

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Combustion time [h]

330

380

430

480

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Flue gas temperature [ C]

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Chemical heat losses in the flue gas [%]

25 50 75 100

Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Termal heat losses in the flue gas [%]

Figure 2: Thermal performance of the stove. Pure wood briquettes as a reference test

An increase in the share of herbaceous biomass in general caused lower emissions of CO (see Figure 3).

The reason for that is that the less intense combustion process with a higher retention time for chemical

reactions and a higher concentration of free oxygen in the combustion chamber. A lower trend for CO

emissions can be observed in the case of full non-woody biomass (except rapeseed briquettes). The

concentration of free oxygen in the flue gas shows a strong correlation with the duration of the combustion

tests. Roy and Corscadden investigated a similar trend – grassy briquettes showed lower emissions of

CO, and higher excess oxygen in comparison to wood briquettes.

The highest concentration of NOx was observed during the combustion tests of the draff, rapeseeds and

oat and wheat middlings briquettes. A higher share of herbaceous biomass in general caused a higher

concentration of NOx in the flue gas. This behaviour is strongly linked to the fuel-NOx formation

mechanism. During biomass combustion, nitrogen content in fuel (Zhang et al., 2008) and the amount of

free oxygen available for chemical reactions are the main factors affecting the formation of NOx (Dias et

al., 2004). In this case, both these factors are present and negatively affect the amount of NOx emissions.

Beside the technical aspects and the design of the combustion appliance, the ash content in the fuel, the

temperature, and the flow velocity in the combustion chamber and flues are the main parameters affecting

the amount of dust emissions. Herbaceous biomass presented a much higher ash content, and higher

emissions of dust in comparison with wood. However, there is no obvious trend observed between these

two parameters. Similar results are obtained by the Roy and Corscadden (Roy and Corscadden, 2012).

Only in the case of rapeseed briquettes is there a higher ash content in fuel caused by the continuous

increase of dust emissions. The intensity of the combustion process, represented as combustion time and

thermal power (see Figure 3), can be stated as a parameter with a possible high influence on the formation

of dust emissions. This could explain the non- significant increase of dust emissions with a higher share of

herbaceous biomass in briquettes and a higher amount of ash as a result.

10000

20000

30000

40000

50000

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

CO [mg/Nm3 at 10% O2]

200

350

500

650

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

NOx [mg/Nm3 at 10% O2]

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

O2 in the flue gas [%]

500

1000

1500

2000

25 50 75 100
Share of non-wood biomass [%]

DR CR OWM ST

BH RB WO

Dust [mg/Nm3 at 10% O2]

Figure 3: The environmental performance of the stove. Pure wood briquettes as a reference test

4. Discussions and conclusions

The results of the combustion tests prove that several types of herbaceous biomass can stabilize and

prolong the combustion process by reducing combustion intensity. This can result in a much longer and

safer combustion process with a lower emission of CO. For every appliance, fuel and/or environmental

conditions, the optimal amount of combustion air and supply strategy will be different.

A high ash content should not create any significant mechanical problems in the case a herbaceous

biomass is used in the appliances designed for wood logs and briquettes. In automated combustion

appliances equipped with a fuel supply and an ash removal system, this aspect can cause significant

problems in terms of both the reduction of efficiency, and disorders to the technical parts of the combustion

unit. However, some of the analysed non-woody biomass has a high ash melting temperature and can be

used to prevent the problems of ash sintering.

Due to the diverse properties of herbaceous biomass, it can be effectively used in mixtures to improve the

specific parameters of the final product. However, for every type of non-woody biomass, actual drawbacks

related to the production process and final use have to be identified and understood. Based on the results

of this study, high energy consumption of the production process in some cases can be stated as an

example of this.

The foreseeable increase of the use of biomass fuel in medium and large power plants, and combined

heat and power plants, will reduce the availability of biomass in the local markets. The use of both

herbaceous biomass and leftover biomass from agricultural and industrial processes represent a potential

solution to offset this lack. In relation to this aspect, as well as the overall sustainability connected to the

use of local biomass energy sources, an important beneficial contribution will be received. However, it is

recommended that a more extended and holistic analysis of the overall energy performances (not only

taking the production phase into account) must be taken in order to provide the real beneficial effect.

In general, the use of agricultural left-overs, residues and by-products represent a potential to gain several

economic and environmental benefits such as industrial symbiosis, use of waste, the partial replacement

of wood as a main source of solid biofuels, a more efficient use of energy sources, a lower amount of

harmful emissions etc. However, any decision has to be based on real-life testing, and a deep

understanding of the advantages and drawbacks of using a particular biomass.

Acknowledgements

This work has been supported by the European Regional Development Fund from the European

Commission within the framework of “Central Baltic INTERREG IV Programme 2007-2013” for the

implementation of project “ECOHOUSING” and by the European Community’s Seventh Framework

Programme’s project “BioWALK4Biofuels”.

Nomenclature

Ad ash content of the test fuel (on dry basis), w-%,d;

Mar water content of the fuel (as fired basis), w-%;

C, H, N, S respectively, carbon, hydrogen, nitrogen and sulfur content of test fuel (on dry basis), w-%,d;

Ta ash melting temperature, °C;

Qgr. V.d gross calorific value of the fuel (on dry basis), MJ kg
-1

;

Qnet.P.ar lower calorific value of the fuel (as fired basis), MJ kg
-1

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