Influence of Temperature on Composition of Wood Pyrolysis Products

Experiments on birch wood chips of a size < 20 mm have been conducted in a laboratory scale batch pyrolysis reactor to study the effect of temperature on the pyrolysis product yields and the balance of elements in the products. The investigated variable was the pyrolysis temperature, which was equal to 300, 400 and 500°C in consecutive pyrolysis runs. Products were separated into fi ve fractions: gases, tar, water, char, and a low-boiling liquid fraction. The paper presents the mass share of each fraction as a function of temperature. The content of carbon, hydrogen and nitrogen was analysed in each fraction. The paper summarizes the weights of different fractions and the distribution of elements in each fraction. The gas fraction was analysed with the use of a gas chromatograph with thermal conductivity detector, while the liquid fraction was analysed in a gas chromatograph with mass detector. To analyse the content of C, H and N in tar and charcoal, an elemental analyser was used. The preformed study has revealed that the increase of the pyrolysis temperature leads to the increase of the amount of char and to the decrease of the amount of tars (heavy tars and low boiling tars). The amount of water in the pyrolysis products does not change as signifi cantly as the amount of tars, which is the result of constant amount of water obtained in the pyrolysis process. The amount of pyrogenic water increases with the increasing temperature of the process.


INTRODUCTION 1. UVOD
Pyrolysis is an endothermic thermochemical process used to receive a variety of chemicals or fuels.As a result of the pyrolysis of wood, liquid, gaseous and solid products are obtained (Kardaś, 2014).The relative amount of each pyrolysis product (gas, char, liquid) can be modifi ed by adjusting parameters of the operating process.The major operating parameters are: temperature, sweep gas fl ow rate, heating rate, and pressure (Akhtar, 2012).Numerous studies exhibit dramatic infl uence of the pyrolysis temperature on product distribution.During the pyrolysis in a solar reactor (Zeng, 2015), the gas yield increased by more than three times when the temperature increased from 600 °C to 2000 °C.The maximum gas yields of 50.9 % (mainly CO and a little of CH4) were achieved for beech wood pyrolysis at 2000 °C and 50 °C/s heating rate with 6 Ndm 3 / min argon fl ow rate.The highest temperature gives the highest gas yields.The most important gas yield increases (15.3 % to 37.1 %) and liquid yield decreases (70.7 % to 51.6 %) were found between 600 and 1000 °C (Zeng, 2015).The char yield is approximately constant above 600 °C and depends on the heating rate.This is roughly between 10-30 % for slow-pyrolysis and 5-20 % for fast pyrolysis (Blasi, 2009).The temperature also affects the liquid yield.Numerous works on biomass pyrolysis showed yield increase with temperature in the range 400-550 °C, with further yield decrease (Bridgwater, 1999).The liquid product was composed of aqueous and bio-oil phases.The former phase, including all the watersoluble organic compounds, consists of water coming from both the moisture in the wood and the dehydration reactions taking place during the pyrolysis process (Antonakou, 2006).The main constituent of the aqueous phase was water (∼90 wt%).The watersoluble organic compounds were mainly acetic and formic acid, and they contained low molecularweight oxygenated organic compounds such as aldehydes, ketones, alcohols, phenols and others (Wei, 2006;Mohan, 2006).All the water-soluble organic compounds could be recovered as useful chemicals (Zhang, 2005).Yorgun (2015) presents the results of pyrolysis of paulownia wood and shows how the pyrolytic aqueous chase fraction increases with temperature.At the temperature of 623 K, the aqueous fraction amounted to 19.9 % of the product mass, while at 873 K, it increased to 21.1 %.There is a large number of works addressing various process conditions and infl uence of these parameters on the obtained products (Wang, 2010;Septien, 2012;Yorgun, 2015).In the present paper, due to a very large number of variables infl uencing the pyrolysis, only one parameter was chosen for examination -temperature.Different amounts of individual pyrolysis products at different temperatures are due to the degree of thermal conversion of biomass and the reactions occurring at a given temperature.

Birch wood samples 2.1. Uzorci drva breze
Chips of birch of a grain size <20 mm were used as fuel in the experiment.The wood was analysed for its technical and elemental characteristics.The measured parameters included humidity, heat of combustion, elemental composition, ash and volatiles.The characteristics of the used fuel were important for interpretation of the obtained results.The results of the performed analysis are shown in Tab. 1. Pyrolysis experiments were carried out in a hightemperature pyrolysis reactor ŚMIGŁY (Fig. 1.).The reactor has the shape of a cylindrical vessel with an internal diameter of 98 mm.The active part of the reactor chamber, located in the central section, is wrapped with induction heating and thermal insulation.The upper part is a chamber with a valve spacer loading the reactor.The bottom of the active part of the reactor chamber is provided with valve spacer revision.The reactor is designed for pyrolysis at temperatures up to 950 °C.The system is equipped with bleed air coolers to cool the gases discharged from the reactor.The stream of the pyrolysis gas formed in the reactor passes through the system of fi ve scrubbers.During the experiment, the fi rst washer worked at room temperature and was fi lled with isopropanol.The next four muds were placed in a cryostat at -20 °C, two of which were fi lled with isopropanol, while the next two were blank.The pyrolysis gas was analysed by GC-ECD.The contents of CO, CO 2 , H 2 and CH 4 were considered in the gas analysis.Low temperatures, at which the scrubbers were kept, supported the condensation of the liquid fraction with low volatility components and protected against evaporation of the isopropanol scrubbers.
After the experiment, the contents of all fl uids were pooled in a fl ask, the fl ushing was washed in isopropanol, and the washings were poured into the fl ask.Then the fl ask was refi lled with fresh isopropanol to give a total volume of 0.5 dm 3 .This procedure was used in all experiments.Next, a liquid sample was analysed for water content using a Carl-Fischer titrator.Another sample of the liquid was placed in a tarred glass evaporator, reweighed and left under the hood until the isopropanol, water and low-boiling fractions evaporated.The remaining fraction, here denoted as heavy tar, was weighed and then analysed in terms of combustion heat, and the contents of H, N and C. Once the reactor had cooled, another solid fraction -char, was unloaded.This fraction was also weighed and examined in terms of heat content.The gaseous fraction was determined by chromatography, and its calorifi c value was calculated from the composition of the pyrolysis gas.

Results of thermogravimetric analysis 2.3. Rezultati termogravimetrijske analize
In order to plan the experiment in the pyrolysis reactor, preliminary experiments were performed in a thermogravimetric analyser.Figure 2 presents a graph of thermogravimetric pyrolysis of birch wood.The graph shows the weight change with temperature.The most intense degassing can be noticed for the temperature of 370 °C.The fi gure also presents the derivative weight of the temperature.In the graph, points correspond to the discharge of solid residue at the temperature at which the experiments were conducted in the pyrolysis reactor.For the temperatures of 300, 400 and 500 °C, the solid residue was 82, 29 and 23 % of the initial mass, respectively.The most intensive degassing was 0.9 % of weight/°C and applied to the temperature of 370 °C.The TGA studies are the introduction to the experimental research.They provide the information on the temperature at which the pyrolysis of wood starts and on pyrolysis residues at specifi c temperatures.However, these studies do not answer the question what products are obtained during the pyrolysis.Products of the pyrolysis of birch chips at 300 °C were divided into fi ve fractions, which were in three states of matter: solid -char, gas -pyrolysis gas, and liquid -water, heavy tars, and low-boiling liquids.The percentage of each fraction is presented in Fig. 3.The  The obtained fraction was characterized by the combustion heat slightly greater than that of the wood used for the experiments (17.37 MJ/kg) and amounting to 18.84 MJ/kg.The determination of the water content in the products showed the increase of 20 grams of water per 100 grams of timber sample.The amount of tar obtained in the experiment was low and amounted to lowboiling liquids and heavy tars, with 2 and 3 % contents, respectively.Figure 4 presents the contributions of individual elementary components (C, O, H, N) in the products of pyrolysis.

REZULTATI
The main combustible components in the pyrolysis gas are hydrogen, carbon monoxide and methane.In the experiments carried out at the temperature of 300 ° C, low content of combustible gases was recorded.The average calorifi c value of the pyrolysis gas was 1.57 MJ/Nm 3 .

Pyrolysis runs to 400 °C 3.2. Piroliza pri 400 °C
In a series of experiments at 400 °C, the share of char decreased in relation to the lower temperature and accounted for 34 % of the products.However, the content of carbon in the char increased to 78 %.The solids were up to 54 % of the biomass carbon.To a lesser extent, elemental carbon was stored in heavy tars (16.3 %) and in pyrolysis gas (10 %).At the expense of smaller amount of solid residue, the share of other fractions increased.Half of the hydrogen (49.3 %) contained in the fuel becomes water.The hydrogen in the char and heavy tars spread out evenly and amounted to 17.8 % and 17.3 %.Nitrogen, similarly to the experiment at 300 °C, was mainly concentrated in the char, in which 78.4 % of the nitrogen from the fuel was recorded.Mass shares of individual fractions of the pyrolysis products are presented in Fig. 5, while the distributions of carbon, oxygen, hydrogen and nitrogen in the products of pyrolysis are shown in Fig. 6.In the studies of pyrolysis at 400 °C, more than 30 % of fl ammable gases were recorded.The main combustible gas was carbon monoxide.The average calorifi c value of the gas mixture was 4.23 MJ/Nm 3 .

Pyrolysis runs to 500 °C 3.2. Piroliza pri 500 °C
The pyrolysis carried out at 500 °C resulted in a small amount of solid fraction (28 %) with high carbon content, and a large amount of liquid fraction.The solid fraction mainly consisted of carbon, constituting 85 % of its weight, and contained 48.95 % of the biomass carbon.Carbon substantially ended as heavy tar (35 %) and, to a lesser extent, as the pyrolysis gas and low boiling liquid fraction (about 5 %).The total yield of liquid products was 62 % of the substrate mass.The main components of the liquid