Energy assessment of wood pyrolysis coproducts for drying and power

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Energy assessment of wood pyrolysis coproducts for drying and power generation Emanuele Pereira, Marcio Martins, Luis Felipe dos Santos, and Angélica de Cassia Carneiro Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02998 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Energy assessment of wood pyrolysis coproducts for drying and power generation Emanuele Graciosa Pereiraa,*, Marcio Arêdes Martinsa, Luis Felipe S. dos Santosa, Angélica de Cássia O. Carneirob a

Federal University of Viçosa - Department of Agricultural Engineering, Av. P.H. Rolfs s/n Campus Universitário - 36570-000, Vicosa, MG, Brazil

b

Federal University of Viçosa - Department of Forest Engineering, Av. P.H. Rolfs s/n Campus Universitário - 36570-000, Vicosa, MG, Brazil

*Corresponding author. Email: [email protected]; Tel: +55 (31) 88533039; Fax: +55 (31) 38992735

Abstract: Carbonization is the process in which the wood is heated in a closed environment with controlled amount of air, producing charcoal as a solid product and secreting water vapor, organic liquids and non-condensable gases. The burning of these gases generates energy that can be used for the drying wood to be carbonized or for generation of electric power in the carbonization plant itself. The present study had the purpose to conduct an energetic survey of the fractions from charcoal production process, aiming to subsidize and to encourage the technological development of equipment for energetic use of carbonization gases. The results showed that the energy contained in non-condensed gases and in pyroligneous liquid corresponds to a total of 31% of the wood energy used. Two scenarios of synchronism of four kilns were studied in pyrolysis and a maximum and constant potential of 9.7 MW was obtained in scenario 2. The gas burner was efficient in the emission control, because it reduced CO and CH4 in the range of 97-99% and 68-91% respectively. Thus, the gas burner is a promising technology for reduction of greenhouse gases and generation of thermal energy from carbonization co-products.

Keywords: energy, charcoal, residue, pyrolysis gas.

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1. INTRODUCTION

Charcoal is an important product in the Brazilian economy and is almost exclusively used in the iron and steel sector, in which it acts as an iron ore reductor for the production of pig iron, steel, and ferroalloy.1 Charcoal is one of the main energy sources in most African countries with about 90% of urban households relying on charcoal for cooking and heating houses.2 In addition, charcoal has the potential to support millions of rural and urban livelihoods through income generation, providing urban-rural financial flows and contributing to the national economy.3 Charcoal production has, thus, become a means of alleviating rural and urban poverty in charcoal producing countries.4,5,6 The scenario of charcoal production in Brazil is still precarious. The technology of charcoal production is still based on use of small rudimentary kilns, since most of the Brazilian charcoal production is carried out by small and medium producers. The low degree of technology for conversion of wood to charcoal presents low gravimetric yields (around 25%) and high atmospheric emissions, having economic, social, and environmental impacts. Charcoal is produced by the decomposition of the wood in a controlled manner under the effect of temperature and oxygen, having the main objective of obtaining the charcoal in the form of fixed carbon (solid fraction). In this process, other fractions are generated, including a liquid fraction of pyroligneous acid and insoluble tar and a noncondensable gases fraction.7 The relative proportion of these fractions varies according to the temperature, the carbonization process, the wood species, and the type of equipment used. The charcoal production process also generates residues resulting from events, such as: (i) processing of logs, called forest residues; (ii) handling of charcoal, called charcoal fines; and (iii) the incomplete carbonization of wood, called “atiços”. The co-products generated in the production of charcoal, including the waste, have the potential to both be harnessed for the generation of energy through the use of gas burners coupled to kilns and to reduce emissions harmful to the environment and man. The energy provided by the burners can be used for the drying of the wood to be carbonized and for the generation of electric energy to be used either in the same charcoal producing unit or distributed in the electric grid. However, it is recognized that these carbonization technologies have not been able to integrate in the production chain consistently and comprehensively. This due to a great lack of knowledge on the part of society and the productive sector on the 2 ACS Paragon Plus Environment

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techniques of development and deployment of these technologies.

This urgently

requires that research validate and improve these systems or, even, develop others, as well train the producers of charcoal in these technologies.

1.1.Non-condensable gases

Non-condensable gases (NCG) resulting from thermal decomposition of wood, also known as slow pyrolysis, are a mixture of inert gases (CO2 and N2) with combustible gases (CO, CH4, and CnHn).

These are usually released into the

atmosphere in quantities that vary according to the conditions of the carbonization process. In general, the percentage of each of these gases, based on a dry basis, can be defined as shown in Table 1. Combustible gases have accumulated energy in their chemical structures that is released when combustion occurs. In this sense, the use of this energy constitutes an important alternative for the sustainability of charcoal production.

1.2.Condensable gases

The condensable fraction of carbonization gases is known as pyroligneous liquor or liquid. This liquid fraction, in turn, can be separated into two phases, forming the pyroligneous acid and the insoluble tar. Pyroligneous acid is composed mainly of acetic acid, soluble tar, methanol, and water (Table 1). The soluble tar corresponds to a fraction of the tar with low molar mass that is solubilized by acetic acid.8 The insoluble tar is a black oily product with a strong and penetrating smell of smoke. It has an extremely variable composition and may contain up to 50% phenolic derivatives, simply phenols, cresols, guaiacols and syringes.9 This vegetable tar was used in the early 1980s as fuel instead of petroleum fuel oil. However, the inevitable generation of pyroligneous liquor associated with tar production has become a critical point, given the lack of a clear, wide, and consistent direction for its use. The condensable gases are also a valuable source of energy for the carbonization plant, since they represent 35.5% of the dry mass of carbonized wood (Table 1). Some fractions of the condensable gases have a high calorific value, such as tar (25 MJ kg-1).9 3 ACS Paragon Plus Environment

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Table 1. Percentage of carbonization products on a dry weight basis Carbonization Products

% Dry Weight

Charcoal (80% Fixed Carbon)

33.0

Pirolenous acid

35.5

(Acetic Acid)

5.0

(Methanol)

2.0

(Soluble Tar)

5.0

(Water and other)

23.5

Insoluble Tar

6.5

Non Condensable Gases

25,0

(Hydrogen – 0.63 %)

0.16

(CO - 34 %)

8.50

(CO2 - 62 %)

15.5

(Methanol – 2.43 %)

0.61

(Ethanol – 0.13 %)

0.03

(Others – 0.81 %)

0.20

Total

100.0 Modified from Ferreira.

10

1.3.Residues

In general, forest activity is characterized by large production of residues in the production process,11,12 which can be differentiated into three main groups: forest harvesting material (or harvest residue), residues generated in wood processing, and wood from energy forests.13 The forest harvesting residue is formed by branches, leaves, bark, and part of the wood that is not used, such as the tips and the remaining material of the cut.14,15 The forest residue presents great potential for the generation of thermal and electric energy or both (cogeneration), through its direct combustion or incineration. In addition, forest residue can be used for the production of different fuels for further combustion, such as briquettes, pellets, and, especially, chips.12 In the production of charcoal, the solid residues generated during the processing of wood are the stools and the fines. The fines correspond to the fraction of charcoal, whose average diameter is in the range of 0-2 mm and 2-9 mm.


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In consideration of these facts, this work aimed to quantify the energy potential of co-products of charcoal production, also considering forest residues and charcoal fines for use of energy for wood drying and electricity production.

2. MATERIALS AND METHODS 2.1.Location and procedures

The experimental data were collected at the carbonization plant of the ArcelorMittal production unit, located in the municipality of São José in Goiabal, Minas Gerais. The plant consists of sixteen rectangular kilns, 5.25 m in height and 33.5 m long, interconnected by means of two 60 cm diameter metal ducts (Figure 1). The first duct located in front of the kilns is responsible for conducting the carbonization gases to the burner. The second duct draws the combustion gases back into the kilns for pre-drying the wood. The plant also has an experimental masonry gas burner, having 20 cm thick walls and the following dimensions: 4 m wide, 12.6 m long, and 6.27 m in height.

Burner

Kiln

1

2

3

4

5

6

7

8

9

Combustable Gases ustos

10

11 12

13 14

15 16

Collection point

Carbonization Gases Figure 1. ArcelorMittal kiln-burner system floor plan.

The experiment was divided in two parts. In the first part, the carbonization in kiln 11 (Figure 1) was monitored for energy analysis of the pyrolysis gas emitted from this kiln. Subsequently, the combined carbonization of the plant was investigated, that is, with several kilns sending smoke gases to the burner.


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2.2.Kiln 11 analysis

A probe of 0.01 m internal diameter was inserted into the chimney 4 m from base. The aspirated gases were sent through a cleaning system before going to a gas analyzer (Gasboard 3100, Wuhan CUBIC Optoelectronics Co. LTDA, China) for quantification of CO, CO2, CH4, H2, and CnHm concentrations. The data were collected every second, starting after 17.5 hours of ignition of the kiln and ending with 89 hours of carbonization. The gas was not analyzed during the first 17.5 hours because within this period, it was not directed to the chimney, but rather to the skylights. The carbonization cycle of the kiln corresponds to 4 days of pyrolysis and 9 days of cooling approximately. The carbonization temperature was controled by means of 12 type K thermocouples, four of them located in the upper part of the kiln and the other eight arranged in the lateral walls. The thermocouples were connected to a supervisory system composed of a programmable logic controller and a computer with the data being stored every hour. The supervisory system had graphical temperature features in function of real time, making it possible to keep the kiln top temperatures close to those recommended. The energy analysis of the non-condensable gases from kiln 11 carbonization was carried out by mass and energy balances. These balances require the determination of gas properties such as calorific value, specific heat, and density.

2.3.Combined kilns analysis

The operation of the carbonization plant occurs without synchronism. That is to say that there is no control related to the quantity of kilns in pyrolysis, resulting in a great variation in the composition and potential of the gas in the burner. In practice, synchronization does not happen because it requires a high degree of organization of the workers to lay the timber and to uncoil the charcoal in a synchronized way, in addition to studies that indicate how best to synchronize the kilns. In this work, the non - condensable gases resulting from the joint operation of the kilns (without synchronism) were analyzed to determine the thermal potential of the burner. From these data, two scenarios of kilns synchronization were simulated in which there are always four kilns in carbonization daily and twelve kilns in cooling. In scenario 1, the ignition is performed every day in a kiln, and in scenario 2, it is 6 ACS Paragon Plus Environment

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performed every day in two kilns. For the evaluation of these scenarios, a 16 days average cycle was considered, with 4 days of carbonization followed by 12 days of cooling. For the thermal analysis of the joint operation of the kilns and determination of the potential as a function of the number of kilns in pyrolysis, the following procedures of data collection were adopted: a probe with 0.01 m internal diameter was installed in the burner inlet duct, as indicated at the collection point of Figure 1. Another gas collection point was installed in the burner chimney at 4 m from base in order to investigate the gas after burning. Temperatures were monitored at these two points by means of K-type thermocouples. During the nine-day period, the gas entering and exiting the burner was evaluated as a function of the carbonization kilns. Before and after the firing, the gas was analyzed for temperature and concentration of H2, CO, CH4, CO2, and O2, using the same gas analyzer and treatment system described in the previous item. In addition, atmospheric air inlet flow data was collected in the burner by means of a rotating blade anemometer.

2.4.Distillation of the pyrolignous liquid

Fractional distillation of the condensable gases was carried out in order to characterize this co-product of the carbonization for its potential as an energy input. For this experiment, 951.5 g of pyroligneous liquid resulted from a sample of Eucalyptus sp. with dry mass equal to 2369.6 g, yielding 40.15% for production of pyroligneous liquid (composed of pyroligneous acid and insoluble tar). This value is in agreement with the contents presented in Table 1 (42%). The pyroligneous liquid was distilled for the separation of three phases in the following temperature ranges:

•

64 - 94 oC

•

94 - 98 oC

•

> 98 oC Each phase of the distillate was quantified and analyzed in an adiabatic bomb

calorimeter according to ABNT NBR 863316 for the determination of the higher calorific value.

2.5.Calorific value of non-condensable gases 7 ACS Paragon Plus Environment

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The Lower Calorific Value (LCV) was determined from the volumetric composition of the gas on a dry basis, according to the equation:17

LCV = 0.126 CO + 0.358(CH4 + Cn Hm ) + 0.108 H2 (1)

In which: LCV

- Lower calorific value (J Nm-3), where Nm-3 denotes "normal m3", i.e. defined under normal conditions (temperature 0°C and 1 atm pressure);

CO

- Volumetric concentration of carbon monoxide (%);

CH4

- Volumetric concentration of methane (%);

CnHm - Volumetric concentration of short chain hydrocarbonas (%); H2

- Volumetric concentration of hydrogen gas (%).

2.6.Heat potential

The energy potential of the non-condensable gases was calculated by means of Equation 2: H = LCV Q

(2)

In which: H

- Heat Potential (W);

LCV - Lower Calorific Value (J Nm-3); Q

- Normal gas flow (Nm³ s-1).

2.7.Enthalpy

The amount of heat or enthalpy of non-condensable gases and atmospheric air was determined for the energy balance study. The enthalpy was calculated according to Equation 3: 8 ACS Paragon Plus Environment

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Q = m Cv ∆T

(3)

In which: Q

- Heat (J);

m

- Mass (kg);

Cv

- Specific heat at constant volume (kJ (kg.K)-1);

∆T

- Variation of temperature (K).

2.8.Specific heat

The specific heat at constant volume (Cv) of the non-condensable gases was determined for the enthalpy calculations using Equation 4:

Cv = ∑ xi Cv i

(4)

In which: xi - Mass concentration of each "i" component of the generated gas (-);

Cvi

- Specific heat of each component "i" at constant volume (J (kg.K)-1).

2.9.Gas specific mass

The density of the non-condensable gases generated under normal conditions was determined as follows (Equation 5):

ρg = ∑C x ρx (5) In which:

Cx ρg

- Corresponds to the volumetric concentration of each component of the generated gas (-); - Density of the gas mixture under normal conditions (kg m-3);

ρx

- Density of each component under normal conditions (kg m-3).

2.10. Stoichiometric air


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Stoichiometric air is defined as the amount of air necessary and sufficient to supply oxygen in the stoichiometric amount to react with all the fuel. The complete combustion of 1 kg of eucalyptus wood (CH1,4O0,6) requires 5.73 kg of air (N2-78%, O221%, CO2-0,03%).18 The equivalence ratio (φ) is a parameter that characterizes the type of thermochemical conversion and is defined as a fraction of the stoichiometric amount of air applied. Ratios below 0.25 are indicative of predominance of pyrolysis, having richer gas production but also more tar. Higher ratios are indicative of combustion with excess air, with the production of a gas with low calorific value.19 In this research, an equivalence ratio was assumed for the carbonization process equal to 0.15. Thus, actual air was calculated according to Equation 6:

 oxidant     fuel  actual φ=  oxidant     fuel  stoichiomé tric

(6)

2.11. Carbonization gas flow

Measurements of carbonization gas flow in the kilns and burners are very difficult to perform in practice. This is due to the fact that the gas flows are very weak, nearing the bottom of the scale for the most sensitive flow measurement instruments. Thus, the non-condensable gas flow was calculated by means of the mass balance of N2 in the control volume of kiln 11. The N2 mass flow entering the kiln from the atmospheric air is equal to the mass flow that leaves with the carbonization gas, since there is no formation, adsorption, or reaction with N2 gas in the carbonization process:

Σm& N 2 ent = Σm& N 2 out (7)

%N2 (m / m) ent . ρair . Qair = %N2 (m / m) out . ρgas . Qgas (8)

In which:


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m& N 2 in

- Mass flow of N2 entering kiln (kg s-1);

& N2 out m

- Mass flow of N2 exiting kiln (kg s-1);

N2 (m/m) in

- Mass concentration of N2 entering kiln (%);

ρair

- Specific mass of atmospheric air (kg m-3);

Qair

- Flow of atmospheric air (m3 s-1);

N2 (m/m) out - Mass concentration of N2 exiting kiln (%); ρgas

- Specific mass of carbonization gas (kg m-3);

Qgas

- Flow of carbonization gas (m3 s-1).

2.12. Mass balance

The mass balance was used to quantify the mass entering and exiting, using kiln 11 as the control volume and, thus, allowing for quantification of mass of the carbonization co-products. For steady-state control volumes, where there is no mass accumulation, the mass balance can be represented by Equation 9.

∑m = ∑m in

(9)

out

In which:

∑m ∑m

in

= mwood + mair

(10)

out

= mchar + mCG + mNCG + losses

(11)

In which: m

wood

- Dry wood mass (kg);

m

air

- Atmospheric air mass (kg);

m

char

- Charcoal mass (kg);

m

CG

- Condensable gas mass (kg);

m

NCG

- Non-condensable gas mass (kg).

2.13. Energy balance


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The energy balance was used to quantify the inputs and outputs of energy, considering kiln 11 as a control volume. This analysis allows for quantification of the energy coming from the wood, as divided between the charcoal and the various coproducts, such as condensable and non-condensable gases. The First Law of Thermodynamics allows us to establish the energy balance, usually expressed in the form of energy rates. However, since the purpose of the analysis is to account for the energy contained in inputs and outputs, it was decided to use the balance in the form of energy rather than rate. As there is no accumulation of mass inside the control volumes and the analyzed system does not perform work, the energy balance can be found from the difference between the sum of the energy of the products in the entrance and exit in terms of heat.

∑E

in

− ∑ Eout = Q

(12)

In which:

∑E

in

∑E

out

= E wood + E air

(13)

= E char + ECG + E NCG + losses

(14)

In which: E

wood

- Energy contained in the wood (J);

E

char

- Energy contained in the charcoal (J);

E

CG

- Energy contained in the condensable gases (J).

E

NCG

- Energy contained in the non-condensable gases (J);

Since Ewood, Echar, ECG and ENCG were determined experimentally, the losses were estimated using Equation 14.

2.14. Amount of residue

The amount of residues needed to compensate for the variation of the thermal potential of the burner was calculated based on the calorific value of the forest residues (19 MJ kg-1) and the charcoal fines (33 MJ kg-1).20 The energy required to stabilize the energy generation in the burner is a known variable obtained through the temporal 12 ACS Paragon Plus Environment

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analysis of the thermal potential. Thus, by establishing a ratio between forest and charcoal fine residues (Equation 15), an equation can be estimated to determine the mass quantities of these variables (Equation 16):

m forest = X m fines (15)

In which: X

- Proportion of the mass of forest and charcoal fine residues (-).

E = 19 m forest + 33,79 m fines (16)

In which: E

- Amount of energy necessary (MJ);

m forest

- Amounts of forest residue used (kg);

m fines

- Amount of fine residue used (kg).

3. RESULTS AND DISCUSSION 3.1.Destillation

Table 2 presents the results of three tests for fractional distillation of the pyrolignous liquid resulting from wood carbonization at laboratory scale.

Table 2. Percentage and higher calorific value of distilled fractions. % of the pyrolignous liquor o

64 - 94 C 94 - 98 oC > 98 oC Losses

Dist 1 6.38 66.12 24.16 3.35

Dist 2 8.83 54.84 27.30 9.03

Dist 3 9.93 54.29 33.61 2.17

Average 8.38 58.42 28.36

Standard Deviation 1.82 6.68 4.82

HCV (MJ kg-1) 10.78 25.96

Among the constituents of pyroligneous liquor (Table 1), methanol has a boiling point in the range of 64 to 94 oC, equal to 64.7 oC.21 Thus, the first fraction of the distillation (64 to 94 oC) corresponds predominantly to methanol, totalizing a mean 13 ACS Paragon Plus Environment

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fraction of 8.38 ± 1.82% of the pyroligneous liquid. The mean calorific value of this fraction was 10.78 MJ kg-1, indicating that it can be used for complementary energy production. The second and largest fraction separated in the temperature range of 94 to 98 oC is composed mostly of water (58.42 ± 6.68%). This fact is confirmed both by the boiling point of water, which is 97.9 oC,22 for these experiment conditions, and also by the fact that it was not possible to obtain HCV readings in the tests with these fractions. The constitutive water is one that can be observed only when the wood undergoes some type of degradation. For example, thermal degradation occurs and resultsin formation of water molecules. 3.46 MJ kg-1 is necessary to remove the constitutive water, according to Skaar.23 The residue of the fractional distillation of the pyroligneous liquid is the soluble and insoluble tar, representing 28.36 + 4.82% of the liquid. Sena24 also found a similar tar content of 30% by fractional distillation of pyroligneous liquid from Eucalyptus sp.. The high tar content suggests that this residue is, in fact, an additional energy stream. The chemical components present in the tar have great binding energy, thus releasing more energy (25.96 MJ kg -1) when such bonds are broken. Table 3 presents the results of the mass balance of this experiment in comparison with the data obtained by Ferreira.10

Table 3. Percentage on a dry weight basis of co-products of charcoal. Products of Carbonization Charcoal Methanol Soluble and insoluble tar Water and others

Current work (% Dry weight) 33.27 3.36 11.37 23.46

Literature (% Dry weight) 30 3 11.5 23.5

It can be seen from Table 3 that the carbonization fractions obtained in this work approximated the values quoted by Ferreira.10 The small differences are explained by the fact that the products of carbonization are dependent on the conditions of the carbonization process, which are difficult to be precisely reproduced.

3.2.Kiln 11 energy analysis 3.2.1. Calorific value of gases and thermal potential


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Figure 2 shows concentration and Lower Calorific Value (LCV) of the noncondensable gases emitted by kiln 11 throughout the carbonization process.

14

7 CO

10

6

CH4 H2

5

LCV

8

4

6

3

4

2

2

1

0

LCV (MJ/Nm3)

12

Concentration v/v (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 18 21 25 28 31 35 38 41 45 48 51 55 58 61 65 68 71 75 78 81 85 88

Time (hours)

Figure 2. LCV of non-condensable gases.

The composition variation of the non-condensable gases throughout the carbonization process results in the temporal variation of the calorific values of the carbonization gas (Figure 2). It is observed that the lower (0.4 - 2 MJ Nm-3) and higher (4 - 6 MJ Nm-3) LCV values are in the range of 0 - 29.7 and 35 - 39.9 hours of carbonization, respectively. Interestingly, the smaller (0.32 - 2.81) and larger (5.94 8.58) CH4 volumetric concentration values were observed at these same intervals. Among the fuel gases present, methane is the one with the highest LCV (35.8 MJ Nm3 25

) , which justifies its greater contribution in the variation of LCV of the carbonization

gas. The non-condensable gas can be considered of low calorific value at the beginning and end of the process when compared to other fuels (Table 4). The carbonization process inside the kilns begins with drying of the wood, having greater water emission and lesser gas formation. As the temperature increases, the wood degradation process begins and releases flammable gases, thus increasing the calorific value of the carbonization gas. At the end of the process, the wood decomposition rate decreases, resulting in a gas of low calorific value.

Table 4. Lower calorific value of gaseous fuels. Fuel

LCV (MJ Nm-3)


Reference

Energy & Fuels

Biogas

21.6

Silva et al.26

Liquefied Petroleum Gas

118.1

Morais27

Natural Gas

39.29

Sobrinho et al.28

Hydrogen

10.98

Rottava28

As discussed in section 3.1, condensable gases are an important part of the energy contribution to the carbonization plant. Figure 3 illustrates this fact, providing a comparative analysis of the potential of the non-condensable gases with and without the potential of the condensable gases.

3.00

Non-condensable gases

2.50

Non-condensable gases and pyroligneous liquid

Potencial (MW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

2.00 1.50 1.00 0.50 0.00 15.00

35.00

55.00 Time (hours)

75.00

95.00

Figure 3. Thermal potential of non-condensable gases with and without condensable gases. The thermal potential variation of the gases represents a problem for cogeneration, due to the instability of the generated enthalpy. The energy density of the carbonization gases changes within the same pyrolysis cycle, starting with a low energy phase because of the water content and, later, forming a medium heat gas. Some measures can be taken to circumvent this limitation. First, it is possible to reduce the moisture of the wood to be carbonized by using of a drying system or do not route these emissions to the burner during drying. A second measure is the use of several kilns operating in synchrony that can attenuate the temporal variation of the thermal potential. The synchronization of several kilns coupled to a single burner results in a gas mixture with higher calorific value, allowing the maintenance of high temperatures in the chamber and carrying out the combustion of the gases during all stages of the carbonization.


Page 17 of 27

3.2.2. Mass balance

The mass balance in kiln 11 allowed for determining the generated amounts of each gas throughout the carbonization process (Figure 4). These results are shown in a comparative way with data found in the literature.10

20,000

18,418.0 17,353.5

Literature This work

Quantity (Kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

15,000

10,100.2 10,000

5,640.7 5,000

760.5

187.2 123.0

1,345.5

0

H2

CO

CO2

CH4

Figure 4. Quantity produced of each gas in this work in comparison with the values of the literature. The mass quantity of gases produced depends on several factors, such as temperature, raw material, type of kiln, and residence time, among others. Thus, since these conditions are variable even in the same plant, the amount of gases varies from one process to another. The production of large quantities of combustible gases (CO, H2, and CH4) favor the energy utilization, since more energy can be potentially generated by the burning of these gases. However, improvements are sought in the gravimetric yield of the carbonization process, increasing the production of charcoal and consequently generating low amounts of gases.

3.2.3. Energy balance


Energy & Fuels

The energy balance was determined from the experimental values of calorific value of wood, charcoal, condensable, and non-condensable gases. With the results of the energy balance, it was possible to analyze the energy partition in each co-product of the carbonization and compare with the reference values. The reference values were determined by means of the energy balance from literature data (Figure 5). The literature data used were those presented in Table 1 and also the values found by Lana,30 presenting charcoal bulk density of 175.04 kg m-3 and a higher calorific value of 32.53 MJ kg-1 . 80 70

69

Reference values

71

This work

60 Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

50 40 30

21

20

20 10

10

9

0 Charcoal

Non-condensable gases Soluble and insoluble tar

Figure 5. Distribution of coproduct energy.

It is observed in Figure 5 that most of the total energy of the co-products corresponds to the energy of the charcoal, being 71% the value found in this work and 69% referring to that calculated according to the data found in the literature. The energy percentage for the non-condensable gases (10%) corresponds to half of the energy contained in the soluble and insoluble tar (20%). This experimental evidence allows us to conclude that tar is an important co-product for the generation of thermal and electric energy in the plant. The values presented in Figure 5 are close to the values obtained by Vilela et al.,

31

who developed a semi-continuous pyrolysis process technology characterized by

the use of metal kiln. In this work, 70% of the co-product energy corresponded to charcoal, 28% to the non-condensable gases, and 10% to the tar. The total energy balance of the kiln 11 is represented by a flowchart shown in Figure 6.


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6. Energy balance of the carbonization process. 3.3.Combined kiln analysis 3.3.1. Concentration of non-condensable gases

Table 5 shows the average composition of the gas samples at the entrance of the firing cell and at the chimney outlet, as a function of the number of kilns in the pyrolysis stage.

Table 5. Comparison between gas concentrations before and after burning. Gas composition – mean values (% v/v, d.b.) Sampling point Number of kilns % CO % CO2 % CH4 % O2 %H2

Entrance to the firing cell 3 6.14 11.82 1.41 9.55 1.72

4 5.67 10.24 2.13 11.03 1.61

5 4.64 6.34 1.23 14.60 0.97

Chimney outlet 3 0.07 11.97 0.16 8.20 0.01

4 0.15 12.29 0.68 8.07 0.03

5 0.01 10.85 0.10 7.67 0.00

It can be observed in Table 5 that there is no direct relationship between the level of CO, CO2, and CH4 emissions and the number of kilns in operation. This shows the need to synchronize the phases of the carbonization in order to obtain a richer and more homogeneous gas in the burner in terms of concentration of the components, especially the fuels. The variation in gas concentration between the entrance of the firing cell and the chimney output can be visualized in Table 6.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Table 6. Variation in gas concentration. Number of kilns 3 4 5

CO -98.86 -97.35 -99.78

Variation in gas composition (%) CO2 CH4 O2 +1.27 -88.65 -14.14 +20.02 -68.08 -26.84 +71.14 -91.87 -47.47

H2 -99.41 -98.13 -100

As shown in Table 6, the burning of the gases resulted in the reduction of most of the average emissions of carbon monoxide and methane for the three scenarios studied when 3, 4, and 5 kilns were in the pyrolysis stage. Thus, it is found that the burner is an efficient device for reducing the emission of harmful gases to atmosphere. Cardoso et al.32 reported a 96% reduction in methane concentration after gas flaring, and Halouani & Farhat33 were able to eliminate the fuel gases in a plant with four metal kilns connected to a single firing unit. It should be pointed out that carbonization technology can still be economically interesting for the possibility of carbon credit generating projects, since there is an emission reduction, mainly methane, which is 21 times more harmful than carbon dioxide.34 The reduction in the oxygen levels in the chimney, verified in the three scenarios, is associated with the elevation of CO2 concentration, indicating the oxidation of the fuels gases.

3.3.2. Thermal potential of the burner

Figure 7 shows the results of the burner thermal potential for each gas concentration measurement point, each point being associated with a number of carbonization kiln. It should be noted that the energy considered refers only to that contained in the non-condensable gas


Page 21 of 27

5 Potential (MW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4 3 2 1 0

1

2

3

4

5

6

7

8

9

10 2

11

12

13

14

Measurement points 1 Number of kilns 1 2 3 4 5

2

3

4

5

6

7

8

9

10

11

12

13

14

Hours after ignition 64

12

12

35

59

106

13

12

34

32

36

39

34

36

24

13

13

36

60

107

59

34

34

70

74

77

57

59

15

38

38

58

83

11

36

58

82

86

91

94

97

97

46

46

67

91

57

10

15

18

10

12

35

59

34

81

Figure 7. Thermal potential of the burner as a function of the number of kiln and the carbonization stage. The calculated thermal potential for each measurement day specified in Figure 7 does not refer to a synchronized kilns system. It is observed that each point has a condition based on number of kilns and hours of carbonization, which consequently generates different potentials of energy. The thermal potential is not directly related to the number of kilns in pyrolysis, since it also depends on the stage of each kiln in operation. At point 3, for example, the thermal potential of the burner of the carbonization plant was 3.9 MW. However, at point 10, the thermal potential of the furnace, receiving gases from the same number of kilns, was 1.8 MW. However, the utilization of the energy from burning the gases requires a more uniform generation of enthalpy. The synchronism of the kilns allows for achieving this condition, in addition to leading to a greater potential of generation. Figure 8 shows the generation of energy in two scenarios of different synchronisms, considering only the burning of non-condensable gases.


Energy & Fuels

6.0 5.0

Potential (MW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

4.0 3.0 2.0

scenario 1 scenario 2

1.0 0.0 0

2

4

6

8

10

12

14

16

Days

Figure 8. Potential of energy generation in two scenarios of kilns synchronization.

It can be seen from Figure 8 that the highest values of the thermal potential of the burner were 5 MW in scenario 2. Thus, different combinations of phases of the kilns culminate in different energetic potentials, since the carbonization gas has a variable composition over the course of the process. The study of synchronizing kilns allows one to find out how best to combine the kilns so that the gas mixture in the burner has the highest calorific value. Figure 8 also shows that although the kilns are following a theoretical synchronism, there are still significant oscillations in the temporal generation of energy. One solution to this fact would be the use of residues from the forest harvest and the charcoal production process. Castro20 conducted experimental research on energy potential of these wastes, and the main results can be seen in Table 7. The author estimated the amount of residues based on the productivity (ton ha-1) of Eucalyptus sp. of the analyzed area.

Table 7. Energy potential of forest residues and charcoal production residues. Residue

HHV (MJ kg-1)

Residue Volume (ton ha-1) 10.69

Potential (MJ ha-1)

Bark

19.59

Branches

18.39

3.25

638x100

Fines (2 – 9 mm)

33.79

5.3

1,795x100

Total

1,971x100

4,404x100


Page 23 of 27

The residues characterized in Table 7 can be incinerated in the smoke burners in order to standardize the generation of energy. In Figure 9, the two synchronizing kilns scenarios were modified by adding the energy of the condensable gases. The amount of residue required for the burner's energy potential to be constant were calculated.

Scenario 2

Scenario1 A) Potential Charcoal fines and forest residue 8000

6 4000 4 2000

2

8

6000

6 4000 4 2000

2

0

0 0

2

4

6

8

10

12

0

14

8000

Residue (kg)

6000

Charcoal fines and forest residue

10

Potential (MW)

8

B)

Potential

Residue (Kg)

10

Potential (MW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 0

2

4

6

8

10

12

14

Days

Days

Figure 9. Thermal potential and amount of waste needed to generate constant energy in A) scenario 1 (8.6 MW) and B) scenario 2 (9.7 MW). It can be seen from Figure 9 that when considering the energy of condensable and non-condensable gases, the maximum potential reached was 8.6 and 9.7 MW for scenarios 1 and 2, respectively. The amounts of forest residue and charcoal fines required for such maximum thermal potential values to be constant are shown in Figure 9 as a function of time. The curves corresponding to the amounts of residues represent only one example when considering that 50% of forest residues and 50% of charcoal fines will be used. It should be noted that the proportions of these two types of residues can be combined in various ways according to Equation 15. Thus, 35 and 60 tons of each type of residue, forest or carbonization, would be required in scenarios 1 and 2, respectively, over 15 days of carbonization.

4. CONCLUSIONS

The total energy balance for a single furnace of an industrial charcoal production unit allowed for quantifying that 21% of the energy of the wood corresponds to the energy of the pyroligneous liquid, 67.2% to the energy contained in the charcoal, 10% to the non-condensable gases, and 1.8% is lost to the environment. These values 23 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

indicate that the energy of the co-products of the carbonization presents potential in use for drying the wood or for production of electric energy. The comparative analysis of the carbonization gas composition before and after the firing in the furnace showed that the firing is efficient in the emission control. Reductions of CO and CH4 in the range of 97 to 99% and 68 to 91%, respectively, were observed, which would contribute to the mitigation of the greenhouse effect. The two furnace synchronism scenarios studied indicated that this methodology is fundamental for the increase of the thermal potential. This is because the phases of the carbonization process are synchronized so that the gas in the furnace contains larger amounts of CH4, CO, and H2 that are responsible for combustion. It was possible to reach a constant thermal potential of 8.601 and 9.742 MW for two synchronism scenarios, respectively, which differed in the number of simultaneous ignition furnaces, having a total of 16 furnaces. In these scenarios, the energy released in the combustion of condensable gases, non-condensable gases, and contribution of coal fines and forest residues was considered.


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Energy & Fuels

5. REFERENCES 1. Pereira, E. G.; Martins, M. A.; Pecenka, R.; Carneiro, A. C. O. Pyrolysis gases burners: Sustainability for integrated production of charcoal, heat and electricity. Renewable and Sustainable Energy Reviews 2017, 75, 592-600. 2. Baumert, S.; Luz, A. C.; Fisher, J.; Vollmer, F.; Ryan, C. M.; Patenaude, G.; Miras, P. Z.; Artur, L.; Nhantumbo, I.; Macqueen, D. Charcoal supply chains from Mabalane to Maputo: Who benefits? Energy for Sustainable Development 2016, 33, 129–138. 3. Smith, H. E.; Hudson, M. D.; Schreckenberg, K. Livelihood diversification: The role of charcoal production in southern Malawi. Energy for Sustainable Development 2017, 36, 22–36. 4. Aabeyir, R.; Bredu, S. A.; Agyare, W. A.; Weir, M. J. C. Empirical evidence of the impact of commercial charcoal production on Woodland in the ForestSavannah transition zone, Ghana. Energy for Sustainable Development 2016, 33, 84–95. 5. Jones, D.; Ryan, C. M.; Fisher, J. Charcoal as a diversification strategy: The flexible role of charcoal production in the livelihoods of smallholders in central Mozambique. Energy for Sustainable Development 2016, 32, 14–21. 6. Mulenga, B. P.; Hadunka, P.; Richardson, R. B. Rural households’ participation in charcoal production in Zambia: Does agricultural productivity play a role? Journal of Forest Economics 2017, 26, 56–62. 7. Mckendry, P. Energy production from biomass: part 2, conversion technologies. Bioresource Technology 2002, 83, 47-54. 8. Wenzl, H. F. J. The chemical technology of wood, Academic Press, New York: 1970. 9. Masuda, H. Carvão e coque aplicados à metalurgia, ABM, Belo Horizonte: 1983. 10. Ferreira, O. C. O futuro do carvão vegetal na siderurgia: emissão de gases de efeito estufa na produção e consumo do carvão vegetal. Revista Economia & Energia 2000, 21. 11. Brand, M. A.; Muñiz, G. I. B.; Silva, D. A.; Klock, U. Caracterização do rendimento e quantificação dos resíduos gerados em serraria através do balanço de materiais. Revista Floresta 2002, 32, 247-259. 12. Wiecheteck, M. Aproveitamento de resíduos e subprodutos florestais, alternativas tecnológicas e propostas de políticas ao uso de resíduos florestais para fins energéticos. Boletim técnico, Curitiba, 40 p., 2009.


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Energy assessment of wood pyrolysis coproducts for drying and power

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