The application of biomass gasification technology is very important in the sense that it helps to relieve the dwindling supply of natural gas from fossil fuels, and the desired product of its gasification process is syngas. This syngas is a mixture of CO and H2; however, by-products such as char, tar, soot, ash, and condensates are also produced. This study, therefore, investigated selected by-products recovered from the gasification process of pinewood chips with specific reference to their potential application in other areas when used as blends. Three samples of the gasification by-products were obtained from a downdraft biomass gasifier system and were characterized in terms of chemical and physical properties. FTIR analysis confirmed similar spectra in all char-resin blends. For fine carbon particles- (soot-) resin blends, almost the same functional groups as observed in char-resin blends appeared. In bomb calorimeter measurements, 70% resin/30% char blends gave highest calorific value, followed by 50% resin/50% soot blends with values of 35.23 MJ/kg and 34.75 MJ/kg consecutively. Provided these by-products meet certain criteria, they could be used in other areas such as varnishes, water purification, and wind turbine blades.
The need for renewable energy technology has drastically increased; this is made possible by its number of advantages over its disadvantages. Renewable energy technology is the use of energy sources that are continually replenished by nature. This energy can be produced from sun, wind, water, municipal waste, and plants. The renewable energy technologies convert these fuels into usable forms of energy which include the production of power, heat, and chemicals [
The techniques used in the conversion of the fuels to usable forms of energy are different and one of them is the biomass gasification technology. Out of these biomass gasification technologies, the fixed bed downdraft gasifier is more preferable due to low entrainment of particulates and lower tar content in the syngas as compared to other gasifier types [
The process of biomass gasification is characterized by four different process steps that take place in the gasifier. The first step is the dehydration of the biomass feedstock, which occurs at a temperature range of 70°C to 200°C. The feedstock material normally contains some amount of moisture that is less than 20%, depending on the source and other conditions [
Studies conducted by Klinghoffer in 2013 suggested that char from a biomass gasification plant can be used in catalytic applications for tar reforming [
Tar generated from the air gasification of woody biomass in a two-stage gasifier was studied by Mun et al. in 2011 [
Formation of fine carbon particles (soot) is a very complex phenomenon involving both homogeneous and heterogeneous processes; however, it may act as a condensation nucleus of polyaromatic hydrocarbons (PHAs) and other organic substances and can also be used as a possible NO
When these by-products of gasification are blended, a possible synergistic effect may arise during their further application in other process steps. The blending of resin with char or soot could enhance the chemical and physical properties of both samples. The blended materials can also be further used as gasification feedstocks for syngas production. However, very limited data is available in the literature detailing the use of gasification by-products as blends in other process applications. This study, therefore, sought to investigate the possibility of blending selected by-products of gasification with a view to characterize them so as to establish their potential applications in other areas when used as blends.
The pinewood material dumped by a nearby sawmill industry was used to feed the downdraft gasifier situated at Melani village, Eastern Cape Province of South Africa. Before feeding into the gasifier, the raw pinewood was cut into blocks with a size range of ~8 cm × 8 cm × 4 cm. Pinewood was used for the experiment because of its availability as compared to other biomass feedstocks and because a homogeneous starting material needs to be obtained as this allows for better comparison of different reaction conditions. However, in comparison to other biomass feedstocks, pinewood has a relatively higher calorific value (19 MJ/kg) and a low sulphur content (0.02% of dry wt.) which allows for less emission of SO2 that is a source of acid rain [
The blocks were loaded into the reactor at a rate of ~107 kg/hr. The temperature of the combustion zone of the downdraft gasifier system was in the range of 1200°C and 1500°C, which allows for the disintegration of some quantities of tar produced during the gasification process. The downdraft gasifier is designed to tolerate feedstock with moisture content of less than 20%, although it can also accommodate feedstock moisture content of up to 25%.
The composition of the by-products produced in a gasification process is dependent on the type of gasification system employed and the type of feedstock used [
The composition of pinewood in terms of proximate and ultimate analyses.
Proximate analysis | Ultimate analysis | ||
---|---|---|---|
Properties | Composition | Element | Composition |
(wt%) | (wt%) | ||
Moisture content | 15.2 | C | 50.6 |
Volatile matter content | 70.2 | O | 22.2 |
Ash content | 5.92 | H | 6.3 |
Fixed carbon | 8.41 | N | 0.3 |
During the gasification process, some quantities of char, resin, soot, ash, and condensates were produced as by-products. Of these, three samples which included the gasifier char, resin, and soot were collected in sample bottles for analysis. The scrubber soot is a black collection of fine carbon particles found on top of the scrubber water in the cooling pond. The char is incompletely combusted material found in the char compartment part of the gasifier. The gasifier resin is a sticky material produced during devolatilisation stage of the gasification process; this was collected from the condensate tank. The samples were collected and prepared randomly in blends of 30% resin/70% char, 50% resin/50% char, and 70 resin/30% char. These blend ratios were randomly chosen without recourse to standard ratios from the literature. The reason for blending was to establish their combined effect when used in other applications.
To determine the usefulness of a material requires an understanding of its properties and composition, and the steps taken to establish their potential use in other areas began with the characterization of such material using different analytical techniques. The samples used for this study were characterized using Fourier Transform Infrared (FTIR) and oxygen calorimeter. These are the most important analytical instruments when attempting to predict or describe the application of gasification by-products based on their composition and properties.
An oxygen bomb calorimeter (Eco Cal2K) was used to measure the heating value of the pure and blended samples of the by-products. The calorimeter was first calibrated with a 0.5 g of benzoic acid (C7H6O2) before measurements were taken. About 1 g of each sample was weighed using a watch glass. The weighed samples were then transferred into a crucible in the outer electrode connected to the lid of the vessel. The vessel was then pressurized up to 3000 kpa using oxygen gas. The vessel was then taken into the calorimeter for firing to take place. For each mass of the sample input in the calorimeter, the heating value was measured in units of MJ/kg.
Fourier Transform Infrared (FTIR) spectroscopy is a technique used to obtain an infrared spectrum of absorption, emission, and photoconductivity [
The heating values, also known as calorific value or energy value of the pure and blended materials, were determined using a CAL 2K oxygen calorimeter. This was undertaken in order to determine the energy content of the gasification by-products and to compare them with that of their blends so as to be able to predict their potential application in other process steps.
The calorific values of pure samples of the gasification by-products are presented in Figure
Calorific value of pure samples of gasification by-products.
In Figure
Figure
The calorific values of blended samples of resin and char.
The calorific values of resin-char blends varied as the blends were mixed at different ratios. The sample with the highest calorific value (35.86 MJ/kg) is the 30% resin/70% char blend, compared to 33.82 MJ/kg and 35.23 MJ/kg for the 50% resin/50% char and 70% resin/30% char blends, respectively. This difference is attributed to the higher ratio of char in the blend as compared with other blends, as well as differences in chemical composition between char and resin. Char is a solid residue that is composed primarily of carbon while resin is a combination of single and multiring aromatics such as toluene, benzene, and styrene including methylated naphthalene as well as oxygenated compounds such as dibenzofuran and phenol and also contains a lot of water which lowers its calorific value [
The calorific values of mixtures of resin-soot, blended at 30%/70%, 50%/50%, and 70%/30% ratios, are shown in Figure
The calorific values of blended samples of resin and soot.
There is also a variation in the calorific values of the samples of resin-soot blends which is also attributed to the ratio of mixing. However, the blend of resin and soot with the highest calorific value is the 50% resin/50% soot blend with the value of 34.75 MJ/Kg. This is contrary to the resin-char blend which gave the highest calorific value with the 30% resin/70% char blend, resulting in a calorific value of 35.86 MJ/kg. 30% of resin blended with 70% of soot resulted in a calorific value of 32.48 MJ/kg. When the quantity of resin in the blend was increased from 30% to 50%, a calorific value of 34.75 MJ/kg was obtained. This calorific value dropped again to 33.7 MJ/kg when the quantity of resin in the blend was increased from 50% to 70%. Soot is a solid material rich in carbon, and calorific value increases with an increase in the concentration of carbon [
The functional groups present in the pure by-products and their blends were determined to establish if blending could have an impact in the dominant chemical bonds and to also help determine their applications in other areas based on these functional groups. R refers to the gasifier resin while S and C refer to soot and char, respectively.
Figure
FTIR spectrum of pinewood.
Figures
FTIR spectra of pure samples of gasification by-products.
It can be noted from Figure
The functional groups present in resin are an indication that it can be used as a raw material in the manufacture of plastics and fibres which are sources of aromatic polymers, through a single ring aromatic separation; however, this requires a high level of processing to separate the resin into its components since gasifier resins can contain hundreds of different components [
Figure
FTIR spectra of blended samples of gasification by-products (resin and char blends).
The same kind of functional groups found in the samples of the pure materials could also be noticed with all the blends of resin and char. The presence of these functional groups is an indication that the blends of the by-products may be useful as a catalyst for the production of chemical compounds and the decomposition of tar as previously stated.
Figure
FTIR spectra of blended samples of resin and soot.
The FTIR spectra of the blended by-products shown in Figures
The by-products of the gasification of pinewood blocks retained the same functional groups as those of their parent material used for the gasification process, but only with a slight shift in the absorption regions of the spectra. The FTIR was found useful in identification of the functional groups of the studied by-products in order to determine the possible applications of the blends. The higher calorific values of the blends also make them good candidates as feedstocks for energy production. However, results obtained indicated that the blends of resin and char are much better to be used in other process applications than those of the resin and soot due to higher carbon concentration although the results did not differ significantly. The blending of the materials provided a medium for compositional exchange which ultimately resulted in strengthened blends. Since the production of syngas from the gasification process requires high or moderate feed calorific value and relatively high carbon concentration, then the higher contents of carbon in resin-char blends imply that the blends could be used as gasification feedstocks. Regasification of these by-products will reduce levels of pollution and toxic sulphur emissions to the environment. These materials could be used in other areas provided they meet certain requirements, that is, in terms of quality, safety, and so forth.
Since the characteristics of the three blending ratios (30%/70%, 50%/50%, and 70%/30%) have been determined, then a need to investigate the characteristics of the blending ratios that were omitted in this paper is recommended.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge Mr. Anthony Anukam for his immense and valuable contribution towards this paper. The research was also supported by the National Research Foundation (NRF) and the Fort Hare Institute of Technology (FHIT) in form of funding provided and their support is gratefully acknowledged.