Characterization of raw lignocellulose biomass
Understanding the chemical compositions of lignocellulose biomass is expedient to maximize the beneficial advantages of such biomass for energy production. Therefore, selected raw biomass, namely, cow dung, mango pulp, and Chromolaena odorata leaves, were characterized using the ASTM standard methods, and the results alongside comparisons with those from previous studies are discussed in this section. The proximate analysis included the volatile matter (VM), moisture content (MC), ash content (AC), and fixed carbon (FC); individual heating values were determined in the form of calorific values; ultimate analysis was used to establish the carbon, C, hydrogen, H, nitrogen, N, Sulphur, S and oxygen, O contents; and compositional analysis was employed to investigate the hemicellulose, lignin, and cellulose contents of the selected biomass. The results of bacterial load and counts are also discussed extensively. Table 1 presents the results of proximate, calorific, ultimate, and compositional details of the selected biomass.
Proximate and calorific analyses of cow dung, mango pulp, and Chromolaena odorata leaves
Cow dung had the lowest percentage of volatile matter (5.02%), followed by mango pulp, 6.87%, while Chromolaena odorata leaves had the highest percentage of 7.79% (as shown in Table 1). Comparatively, percentage volatile matter for cow dung, 66.11% obtained by Tejas9 on a dry basis was higher than those recorded in this study for cow dung, 60.75% and mango pulp, 62.78% but lower than that of Chromolaena odorata leaves, 71.49%. From Table 1, cow dung had the least percentage of volatile matter both on wet and dry basis, however, it is suggested from the literature that although cow dung has minute percentage of volatile matter, it can produce biogas earlier than any other biomass41. This is due to the microorganisms responsible for anaerobic digestion that are readily available in it since cows also feed on green leaves among others42. The significant proportion of volatile matter in the selected biomass fuel can be a positive influence for an improved ignition of the dust-air cloud and flame stability43. Of all 3 biomasses, cow dung had the highest moisture content, with over four-fifths (85.82%) of its total mass mainly composed of water, which is not unconnected to the fact that the biomass was on a wet basis; thus, this result was expected.
In comparison with other biomasses, a relatively higher percentage moisture content, 85.77%, was recorded for mango pulp, while Chromolaena odorata leaves had the lowest moisture content of 83.11% (Table 1). Although bio gasification is accompanied by an anaerobic digestion process, a rated level of moisture content is required to aid microbial activities on the biomass44; thus, slurry preparation is essential before incubation. However, the slight differences observed in the values of moisture contents could be attributed to their sources of collections, feeds, nutrients and species. Overall, all three biomasses had adequate and comparable moisture contents, with cow dung having a slightly higher value. Relatively, the VM/FC ratios of the biomass, which range from 0.69 to 1.10, were found to be suitable and imply that a higher reactivity can be achieved since ignition is easier at low temperatures with the aid of volatile matter45.
Comparatively, Table 1 shows that cow dung exhibited the highest ash content, 1.91% (on a dry basis), and the lowest volatile matter content, 5.02%. It has been reported that the ash content reduces the fuel quality because ash has an affiliation with fouling34; however, high volatile matter ascertains easy ignition46; therefore, it would be easier to ignite gas obtained from the digestion of Chromolaena odorata leaves with 7.79% volatile matter than either cow dung, 5.02%, or mango pulp, 6.87%. Blending the three biomasses promises to create a means for striking balance in terms of the required volatile matter for effective biogas combustion. On a dry basis, the average percentage ash content of 10.74% obtained for the biomasses investigated in this study is near to those obtained for rice husk (12.75%) by García et al.47 and water hyacinth leaves (13.93%) by Jimoh et al.48 but slightly higher than those obtained for reed canary grass (8.2%) and sorghum (7.2%), as remarked in the study of Lalak et al.43. Going further, in comparison with the study of Kim et al.49, where very low percentage ash contents were recorded for wood 2.18% and kenaf 5.45%, these call for critical examination. Although these biomasses are lignocellulosic, the differences observed in the results could be attributed to the type, nature, and sources of the biomass; therefore, adequate attention should be given to the nature and sources of biomass collection to be implemented for anaerobic digestion to ensure methane-rich biogas. Cow dung and mango pulp have been found in the literature and established in this study to be suitable biomasses for biogas production. When the duo is digested with Chromolaena odorata leaves, it becomes viable for yielding more methane than being digested individually. It could also be deduced from Table 1 that there exists a directly proportional relationship between ash content and fixed carbon across the selected biomass, i.e., an increase in ash content brings about an increase in fixed carbon and vice versa. In contrast, an inversely proportional relationship exists between volatile matter and moisture content, MC of the biomass, such that the higher the volatile matter, VM, the lower the moisture content and vice versa. The results further revealed a negligible difference between the fixed carbon contents of cow dung (7.25%) and Chromolaena odorata leaves (7.22%). This suggests that since cows also feed on Chromolaena odorata leaves (either completely or as a supplement) or any other green leaves for diet, the dung had similar characteristics as that of the raw Chromolaena odorata leaves50,51. Generally, the efficacy of the anaerobic digestion process has a significant relationship with the types and characteristics of biomass. Furthermore, characteristics of the biomass were discovered to have a great impact on degradation and retention times52. The calorific value is the quantity of energy that is generated per unit of mass per unit volume of the biogas fuel when it is completely burnt in the presence of oxygen 53. Additionally, there were technical agreements between the contents of volatile matter and calorific values obtained for cow dung, 14.37 MJ/kg, mango pulp 13.77 MJ/kg, and Chromolaena odorata leaves, 16.16 MJ/kg, respectively. Evidently, from Table 1, it could be said that volatile matter and calorific values are proportionate. All 3 biomasses had attractive and significant calorific values, but Chromolaena odorata leaves had the highest percentage of volatile matter, which subsequently suggested why they had the highest calorific value. Although Boie’s model is the best and well suited for predicting the calorific values of typical biomass, this study used the ultimate analysis as input variables in three model equations (i.e., Yin35; Sheng and Azevedo36; Boie37) to predict the calorific values of the investigated biomass. In comparison with the experimental results of this study, all the predicted calorific values had significant increments, such that percentage deviations of 26.00–33.59%, 24.42–33.32%, and 17.71–33.42% were recorded using the models of Yin35, Sheng and Azevedo36, and Boie37, respectively. This suggests that further studies are still required for the prediction of biomass calorific values54; however, the use of calorimetry for the determination of calorific values remains important.
A ternary plot is a triangular coordinate system having the edges of the triangle as the axes where each edge corresponds to a composition of the system. Figure 2 presents a ternary plot that was used to visualize and understand the overall proximate compositional variations of the investigated biomass. The mean proximate compositions of the sampled biomass were plotted using the principles of Singh et al.53 and Vassilev et al.55. Specifically, three key data were plotted: the volatile matter, VM, ash content, and fixed carbon, FC, all in percentages55. Critical examination of the plot showed proximity among the sampled biomass, which is an indication of the similarities in the chemical components of the biomass53. Thus, these results suggest their suitability for energy production, especially when blended.
Ternary plot of volatile matter (%), ash content (%), and fixed carbon (%) on a wet basis.
Ultimate analysis of biomass
One of the fast-growing alternative renewable energy forms that could serve as a replacement for fossil fuels is biomass energy. The suitability and quality of biogas to be produced by any selected biomass could be informed by ultimate analysis; hence, these factors necessitated the elemental, CHNSO, and investigative contents shown in Table 1. Carbon (39.98–43.08%), hydrogen (6.74–9.86%), nitrogen (1.34–1.53%), Sulfur (0.12–0.46%), and oxygen (46.69–51.82%) of cow dung, mango pulp, and Chromolaena odorata leaves are similar to those obtained for lignocellulose biomass investigated in past studies (Okolie et al.5; Kobra et al.13; Wannapokin et al.14; Dahunsi et al.34; Adegun and Yaru56: Fang et al.57). It is clear that all the biomass has low carbon contents and high oxygen contents, which are consistent with those reported for grasses and manure by Harpreet58 but inconsistent compared to coal by the same study. The results also indicated that the higher the oxygen concentration of typical biomass was, the lower the carbon content, which is observable in Table 1. Nitrogen and sulfur contents are reported not to be important in biofuel production because they tend to release harmful and toxic gases48. Therefore, negligible sulfur contents obtained for the selected biomass suggest their suitability for biogas production with a minute possibility of releasing voluminous toxic gases, which could negatively affect humans and the environment. Percentage contents of sulfur (0.46, 0.12, and 0.25) % and nitrogen (1.53, 1.34, and 1.51) % for cow dung, mango pulp, and Chromolaena odorata leaves, respectively, are considered acceptable because they imply low concentrations of oxides of sulfur and nitrogen present in biogas obtained as a result of their digestion53. Therefore, during biogas combustion, the possibility of releasing toxic gases that would otherwise cause undesirable environmental impacts is infinitesimal. These results match those obtained for similar lignocellulose biomass considered in the research conducted by Singh et al.53. Furthermore, the range of calorific values (13.77–16.16 MJ/kg) is indeed interesting and is found to be in accordance with the proximate and ultimate analyses that depict favourable levels of fixed carbon contents, i.e., above 13% and ash contents of less than 2%, respectively; with these results, the carbon contents promise to positively contribute to increasing the calorific values45. From Fig. 3, the calorific values of the selected biomass were all consistent with those obtained in previously studied lignocellulose biomass Jimoh et al.48; Rambo et al.59; Magdalena et al.60; Stelaski, et al.61. These results suggest the characteristic similarities of lignocellulosic biomass reported in the literature; however, slight variations observed in the calorific values presented in Fig. 3 could be ascribed to the locality of the biomass, climatic and environmental states, discrepancies in determination processes, nutrition, and chemical structure of the types of biomass investigated48,49,62. The predicted calorific values (using Eqs. 10–12, developed by Yin35, Sheng and Azevedo36, and Boie37, respectively) from those experimentally obtained in this study. This could be a result of inadequate data used for modeling on the part of the authors who developed the models because the greater the data point, the greater the coefficient of correlation (R-square value) tends towards unity and consequently enhancing the accuracy of the model.
Calorific values for previous and present studies.
Carbon–nitrogen (C/N) ratio
The estimated carbon/nitrogen ratios of selected biomass ranged between 27.61 and 29.84, with cow dung and Chromolaena odorata leaves having nearly the same value of approximately 28 each, although with a slight difference of 0.24, while mango pulp had the highest ratio of 29.84 (see Tables 1 and 2). Previous studies have shown that anaerobic bacteria source their foods from carbon and nitrogen such that carbon is needed for energy, while a combination of carbon and nitrogen is used for building new cell structures63. The process of anaerobic digestion is inclined to be the proportion of carbon/nitrogen present because it is an indication of the nutrient level of the biomass64. Moreover, a high C/N ratio tends to cause insufficient nitrogen for maintaining biomass(es) cells and consequently brings about an ammonia nitrogen supply in the digester64. Additionally, a high C/N ratio is an indication of rapid consumption of nitrogen by methanogens, which results in lower gas production65. In contrast, a low C/N ratio can lead to possible ammonium inhibition of microorganism activities in the digester, resulting in an optimum pH value to exceed 8.5, which is toxic to methanogenic bacteria64,65. Thus, to strike a balance between the two levels (high/low) of C/N ratios, a combination of biomass with considerably low and high ratios of C/N is proposed, such as organic waste blended with sewage or animal manure; thus, the biomass investigated in this study can form a good blend for biogas production. The advantages of this approach include not only having an optimum operational C/N ratio but also having a higher methane content yield when codigested compared to sole digestion49. Practically, several previous studies had an optimum biomass C/N ratio for anaerobic digestion, ranging between an average value of 20 and 3563,64. All the respective C/N ratios obtained in this study, that is, cow (28.16), mango pulp (29.84) Chromolaena odorata leaves (27.61), and the overall average (28.4) recorded, fell within the optimum range found in the literature. This formed the basis upon which the biomass could be considered an excellent anaerobic digestive for biogas production63,64. Although the C/N ratios recorded for this present study fall within the optimum range, they varied from those reported for rice husk (66.17) and walnut shells by Hongtao et al.66 and wood pellet (84.33) by Kim et al.49. The variations may be due to different feedstocks given to the cows, soil compositions, and climatic conditions9,60.
Determination of energy densities using H/C and O/C atomic ratios
Naturally, asides from the proximate and ultimate analyses of biomass, lignocellulosic biomass also differ from one another based on their compositional formulations, which is a function of fuel efficacy, i.e., ability to produce adequate energy in forms of heat and electricity33). The classification of atomic ratios, involving hydrogen, oxygen, and carbon, such as H:C and O:C atomic ratio diagrams, is a useful approach that can be used to understand the fuel calorific value54,69. A plot of the Van Krevelen diagram shown in Fig. 4 was plotted with respect to atomic ratios of hydrogen to carbon, H/C against oxygen to carbon, and O/C, and the correlations between them were used to locate the energy density (in terms of individual position relative to one another) of eleven (including those of this study) selected biomass samples in the Van Krevelen diagram presented in Fig. 3. It has been reported that there is a directly proportional relationship between atomic ratios and the energy content of biomass fuel 70, which implies that the energy density of fuel biomass has a direct correlation (inverse proportional relationship) with atomic ratios, i.e., H/C and O/C. Compared to atomic ratios of cow dung 0.18 H/C and 1.09 O/C, mango pulp 0.17 H/C and 1.30 O/C, the atomic ratios H/C and O/C of Chromolaena odorata leaves are 0.24 and 1.12, respectively. Therefore, Chromolaena odorata leaves had the highest value of H/C, 0.24, and mango pulp with the least H/C value of 0.17 but with the highest value of O/C 1.30, while cow dung recorded the least O/C value of 1.09 compared to others. These ratios were observed to be higher than those of other types of fuels, anthracite, 0.01 H/C, 0.01 O/C70, hard coal 0.06 H/C, 0.08 O/C43, and miscanthus 0.12 H/C, 0.71 O/C43 (Fig. 3). Comparatively, high hydrogen/carbon and oxygen/carbon atomic ratios were the factors that lowered the heating values of cow dung, mango pulp, and Chromolaena odorata leaves, respectively. However, it caused an increase in the heating values when compared to PRB coal, lignite, peat, teak, and melina54,70,71, where the atomic ratios for this study were observed to be lower in decreasing order of magnitude of the calorific values (Fig. 3). This implies that, compared to those of this study, higher atomic ratios (H:C and O:C) were recorded for other biomasses. Digestion feedstock with relatively low O:C ratios have more energy densities with higher calorific values; thus, comparatively, greater chemical energy is obtainable in C–C bonds than in C-O bonds53. The calorific values obtained in this study are traceable to the percentage of fixed carbon as a result of lower O:C and H:C atomic ratios. Table 2 reveals that all ratios obtained for this study fall within those compiled and adapted from the literature. Thus, the biomass investigated can be adopted for biogas production or processed into the desired renewable energy form.
Van Krevelen diagram showing atomic ratios of H:C against O:C for past and present studies.
Compositional analysis
Typical lignocellulose biomass, such as those investigated in this study, primarily comprises cellulose, hemicellulose, and lignin51. Although they have associated virtues with each other, their performances under anaerobic digestion are different. Accurate compositional analysis of lignocellulose biomass gives room for evaluating the conversion yields as well as process economics, especially in biogas conversion processes11. The percentage hemicellulose, lignin and cellulose contents of the biomass were cow dung (10.76, 6.33, 12.03) %, mango pulp (7.47, 0.22, 3.71) % and Chromolaena odorata leaves (11.37, 0.90, 5.15) %, respectively (Table 1). It has been reported that the considerable cellulose and lignin contents should be relatively small if digestion is to be aided because those contents are not easily bioconvertible in anaerobic environments as a result of their rigid structure72. This hinders the anaerobic digestion process and consequent reduction in the rate of biogas generation; therefore, the higher the lignin content becomes, the lower the corresponding biogas yield72,73. However, to obtain energy from combustion, it is stated that a considerably larger amount of lignin is preferable, as a higher calorific value of biomass has a strong positive correlation with lignin content. From Table 3, generally, the content of lignins present in nonwoody biomass was discovered to be lower but higher in woody biomass. Cow dung had the highest lignin and cellulose values compared to other biomasses. Additionally, it is characterized by a higher calorific value because a higher calorific value is associated with higher lignin and extractives59, while similar trends were observed for Chromolaena odorata leaves followed by mango pulp. The considerable amounts of lignin, hemicellulose and cellulose contents in the selected biomass are indications that when harnessed adequately, they can produce useful energy through anaerobic digestion. Similarly, the slight difference observed between lignin concentrations of mango pulp and Chromolaena odorata leaves could be attributed to the nutrients of the hosting trees. All three biomasses characterized had significant cellulose contents, with mango pulp having the lowest value of 3.71%, while that of cow dung topped the list with a value (12.03%) more than twice that of Chromolaena odorata leaves (5.15%) and more than thrice that of mango pulp (3.71%). Variations due to the types and nature of biomass were observed between the results of compositional analysis recorded for this study and those found in the literature (Table 3).
Results of microbial load
Furthermore, to ensure the suitability of any biomass for anaerobic digestion, specifically manure, this study characterized cow dung for bacterial load and count, which was determined as colony-forming units per gram (cfu/g). An important factor that can inform the quality of any manure is the total viable count because changes in microbial varieties may result in changes in dung functionality in the digester throughout the digestion process78. Thus, the results obtained for this characterization revealed that cow dung has total bacterial counts of 5.78 × 108 and 3.93 × 105 on wet and dry bases, respectively. It was also clear from the results obtained that the fresh cow-dung sample was enriched in microbial colonies, evidently from the various species found, such as Escherichia coli, Staphylococcus aureus, Bacillus cereus, Pseudomonas aureginosa, Proteus morganii, and Micrococcus spp. The microbial contents of cow dung may explain its bioefficacy, thus justifying its usage for biogas production79. Additionally, manure quality and suitability as feedstock depend on microbial presence, and the digestate of cow dung when codigested with other lignocellulose biomass could serve as a potential fertilizer.
