Assessment of energy parameters of biomass and biochars, leachability of heavy metals and phytotoxicity of their ashes

The analysis of the tested materials reveled an increase in the contents of carbon and nitrogen after pyrolysis. Furthermore, the contents of hydrogen, sulfur, and oxygen were reduced, as shown in Table 1. Such a change in the elemental composition can be attributed to the fact that, during pyrolysis, light organic fractions are being decomposed. These are mainly substances that break down to release compounds with significant amounts of hydrogen (light hydrocarbons and simple structure polymers). The carbon content in materials ranged from 34 to 68%. However, higher values (exceeding 60% of the C content) related to biochars were produced from miscanthus straw, sawdust, and wheat straw. According to Tripathi et al. [29], the proportions in which carbon, hydrogen, and oxygen occur in biomass have a decisive effect on its calorific value. The authors stated that the high content of N and S in biomass can result in the formation of high-energy C–N, H–N, C–S, and H–S bonds. On the other hand, the low content of N and S in biomass is more environmentally friendly when the biomass is combusted, because it does not contribute to the formation of acid rains and greenhouse gas emissions. What is more, the ash content seems to be an important factor having impact on the dynamics of changes in the elemental composition. Its high value, usually combined with a low volatile matter content, significantly limited variations in the elemental composition [30]. The content of sulfur in the tested materials ranged from 0.01% (S) to 0.65% (PL). Smaller variations were found for the hydrogen content. The highest contents of hydrogen were determined in sawdust and miscanthus straw and were over 6%. These are the most typical values for these types of biomass. In these cases, the pyrolysis resulted in the creation of a material which is more similar to lignite or low-quality coal and it means that it can be used as a fuel.

The pyrolysis process leads to a series of chemical reactions as a result of which the increase of the condensation and aromatization degree takes place, which translates into the change in mol relations O/C (carbonization degree) and H/C (aromatization degree) [31]. It is generally recognized that the O/C ratio in biochar should not exceed 0.4, and the H/C ratio below 0.6, which is also confirmed by our results. The results of Do et al. [32] showed that the pyrolysis temperature increase from 300 to 800 °C increases the losses of O and H, which result from the removal of water, CO, and CO₂ from biomass. Finally, biochars become highly carbonaceous at higher temperatures, and the H/C and O/C ratios of biochars decrease gradually with increasing temperatures of pyrolysis. Our study revealed the H/C ratio for all biochars of below 0.6. The O/C ratio of below 0.4 was obtained in WSB, MSB, and PLB biochars. This ratio was slightly higher (0.41) in BB and SB biochars produced from woody biomass. This indicates that the aromatic C structures were more stable. The theory is confirmed by the study of Kim et al. [33], who argued that there is a relationship between the H/C and O/C ratios and the pyrolysis temperature and the biomass type. The authors stated that dehydration and decarboxylation reactions significantly depend on the type of biomass (e.g., carbon content, humidity level) and the temperature. Tripathi et al. [29] argued that the calorific value of biochar increases with increasing temperature and with the decreasing H/C and O/C molar ratios. This thesis was not fully confirmed by our results, as we did not discover such a dependency. This was particularly evident for PL and PLB treatments.

Humidity is an important problem related to the combustion of biomass, which, in the case of plant feedstocks, can reach even 50% [32, 34]. The high moisture level makes the biomass combustion less profitable, not only because of the process itself, but also due to high transportation costs (low energy density). As in the case of all combustion fuels, moisture significantly affects the lower calorific value and plays an important role in the combustion process. Combustion of fuels having a high moisture content increases the volume of exhaust gases and, consequently lowers the combustion temperature [35]. Additionally, increased losses related to partial and incomplete combustions can be observed, as the temperature in the combustion chamber will not be sufficient to combust all flammable substances in flue gases [36].

Moisture content in feedstocks was similar and ranged from 5.3 to 7.6% (Fig. 1). It is worth mentioning that, before pyrolysis, feedstocks were pre-dried at 65 °C. The main objective of this operation was to limit the influence of feedstock moisture on the process. This approach allowed to evaluate the process effect on the specific material. Therefore, the humidity may be considered analytical (air-dry state). Given that pyrolysis was carried out at above 280 °C, in all cases, we expected to observe reduction in the moisture content. The highest moisture reduction compared to feedstocks was achieved for bark biochar (BB) and sawdust biochar (SB). Among all the tested biochars, the lowest humidity was determined for bark biochar (1.3%), and the largest in miscanthus straw (4.8%) (Fig. 1).

Volatile matter content is an important parameter that significantly affects the combustion process [28]. Literature data indicate that biomass contains up to 2.5 times more volatile matter than coal, which has a major effect on the conditions of its ignition and combustion [37]. During combustion of high volatile fuels, large amounts of flammable substances (mainly CO, low hydrocarbons, and monocyclic aromatic hydrocarbons) are released, which reveals itself by higher flames. This requires the introduction of additional air to ensure complete burnout of these products [34, 37]. Higher volatile matter content than in the case of hard coal requires specially designed solutions for biomass combustion. Our results revealed that the thermal conversion of organic materials by pyrolysis reduced the volatile matter content (Fig. 1), which is fully consistent with information given in the literature. In this case, volatile matter content ranged from 37.0% (WSB) to 84.6% (S) (Fig. 1). The thermal process was found to reduce the volatile matter content in all materials, as shown in Fig. 1. According to this data, the intensity of raw material devolatilization during pyrolysis was relative to changes in the volatile matter content. The most intense release of volatiles was observed for straws of wheat and miscanthus, as almost 50% of them were released from these materials. As stated by Mitchual et al. [38], when biomass is heated to 400–500 °C, volatile gases are released, giving the so-called volatile matter. Volatile matter (flammable gas and smoke) is created when biomass is heated up under sufficient air, time, temperature, and turbulence conditions. According to Patel and Gami [39], the higher the volatile matter content in the fuel, the larger the amount of high-pressure secondary air is required for effective combustion. On the other hand, incomplete combustion of volatile matter leads to dark smoke, heat loss, pollution hazard, and soot deposition on boiler surfaces [39].

Fixed carbon content (C_fd) represents the amount of matter which will form coke during combustion. A high value of this parameter indicates that fuel will be burned in two steps: pyrolysis and coke combustion; however, the latter will last longer. These fuels tend to combust in the form of embers. Sadiku et al. [28] noted that the fixed carbon of any material gives a rough estimate of the heating value of a fuel and acts as the main heat generator during the combustion. For the analyzed materials, fixed carbon content ranged from 7.2% (PL) to 48.7% (WSB) (Fig. 2). In all cases, pyrolysis increased the carbon content. The largest increase in this parameter was found for biochar produced from poultry litter (280%), and the lowest in bark biochar (6.7%). Such properties of biochar favor its use as fuel similar to coal. Similar changes in fixed carbon content were also obtained by other researchers [10, 30, 40].

As in the case of hard and brown coal, combustion of biomass generates solid waste, mostly in the form of the bottom ash. The amount of that waste depends mainly on the type of biomass used [37]. Usually, the ash content after combustion of biofuels is significantly lower than for fossil fuels like hard coal or lignite. This has some operating advantages, i.e., longer periods between ash removal. The analysis of feedstocks showed that the ash content ranged from 0.80 to 41.5%. It was found that bark biochar (BB) and poultry litter biochar (PLB) had the highest ash contents among the tested biochars (Fig. 2). For these materials, the ash content was 48.8% and 42.9%, respectively. Its high value determined for BB sample was mainly related to the high amount of mineral impurities (mainly sand) sourced from the wood harvesting process. This was also the case for thermally unconverted bark (B). On the other hand, the ash content of less than 1% was marked for sawdust (S) and biochar produced from it (SB). Such a low ash content is normal for pure organic materials, devoid of impurities. Nevertheless, as we expected, pyrolysis caused the ash content decrease in all materials. Higher ash content for biochars could have either positive or negative results, depending upon utilization methods. He et al. [36] stated that feedstocks such as cotton stalk biochar and rape straw with low ash content should be used in fuel production, whereas materials such as rice straw biochar, due to their high ash content, should be used as fertilizers. With this in mind, in our study, S, WS, MS and biochars obtained from them would be the best fuels. On the other hand, B, BB, PL, and PLB should be used as fertilizers.

Compared to the A_d feedstock, the highest change in the ash content was observed for straw materials (WS and MS). In the case of materials with either low (S) or high (B, PL) ash content, the change was not so intensive and was below 40% for all these three materials. It is crucial to select the right type of biomass for combustion, as it can affect not only the amount of the ash formed, but also the chemical composition and leaching of important compounds (e.g., heavy metals, sulfate ions, chlorides) [34]. Ashes from the combustion of biomass have higher contents of CaO, SO₃, Cl and lower contents of SiO₂, Al₂O₃ and Fe₂O₃ than residues after burning of coal [7, 37, 41, 42]. That is why, many literature sources report that bottom ash remaining after biomass combustion can be used as an additive in the production of fertilizers [43‐46]. In that case, it is important to assess the concentration of heavy metals, as they cannot exceed the permissible values.

Net calorific value (q_p,net,d) is one of the most important thermo-physical parameters describing energy potential of materials. This parameter constitutes a basis for assessing the quality of fuel as an energy source. Net calorific value is associated with moisture and strongly depends on the fuel chemical composition [30]. In this study, the highest gross calorific value (q_v,gr,d) was determined for miscanthus straw biochar and sawdust biochar and reached 26.59 and 23.42 MJ kg⁻¹, respectively (Fig. 3). The calorific value of WSB and MSB was 29% and 28%, respectively, higher than the calorific value of feedstocks. Also, Ul Saqib et al. [47] indicated a 21% increase in the calorific value of leaf biochar compared to the raw biomass. The lowest q_p,net,d was determined for untreated bark (11.88 MJ kg⁻¹) and poultry litter (11.98 MJ kg⁻¹). Lower heating values of sawdust, wheat straw, and miscanthus straw were similar, which finds confirmation in the literature [31, 43, 44]. Gross calorific value of biochars was higher on average by 36% than calorific value of feedstocks used in the pyrolysis process, which suggests that they are a better solid fuel. This is due to the fact that pyrolysis results in the release of volatile matter and increase of fixed carbon and thus a higher carbonization degree. Additionally, according to He et al. [36], the higher the C content, the lower the O content and, consequently, the gross calorific value undergoes a constant increase. However, these authors demonstrated that when the pyrolysis was carried out at 300 °C, 75–84% of primary energy of plant biomass was retained in biochars. On the other hand, at 600 °C, 47–51% of energy was stored in biochars and the remaining 20% was retained in gases. In turn, Tsai et al. [33] stated that in the case of biochars, the gross calorific value increases at up to 500 °C and then remains relatively constant. Similar results were obtained by Poudel et al. [48] for empty fruit bunches and palm kernel shell and by Wróbel et al. [49] for woody biomass.

Analysis of the FVI (Fig. 2) showed that materials’ applicability for energy production was different. Only for three samples, the value of this parameter exceeded 100 which seems to be a threshold value for application as fuel. What is also important, not in all cases values for biochar were more favorable. In the case of wheat and miscanthus straws, there was a decrease in the FVI value, which was probably caused by lower bulk density of these materials. The highest value was obtained both for raw sawdust and biochar obtained from it. More importantly, the value for biochar was two times higher than for untreated material, 1037 and 2334 GJ/m³, respectively. The highest increase was observed for biochar from bark, which was five times higher than for untreated feedstock (25 and 127 GJ/m³, respectively). Our results showed that one of the biochars produced can be considered a potentially valuable material for energy production. The FVI of biochar produced from bark showed that, due to its high ash content, this material can create threats, as it can cause fouling and slugging problems during combustion. It needs to be mentioned that FVI gives only general information about the application ability of the fuel, that is why further tests should be made to determine fouling or slagging factors. The study of Sadiku et al. [28] proved that the FVI is positively correlated with the fixed carbon content and negatively correlated with volatile matter content (Table 4). Our results also confirmed that the FVI increases with the increasing fixed carbon content and decreases with the increasing volatile matter content. These correlations are mainly related to the ash content change. When ash content rises, the volatile and fixed carbons lower.

The measurement results bulk and absolute densities were summarized in Table 2. A slight decrease in bulk density was determined for all materials. However, this is normal, because the overall shape of the material is preserved during pyrolysis conducted at relatively low temperatures. On the other hand, the absolute density increases, because new pores are formed in the process and this affects the material volume. The highest absolute density value was determined in poultry litter biochar (1.91 g/cm³), and the lowest in miscanthus straw (1.43 g/cm³). It is expected that high density will affect the fuel value index of biomass for energy generation, as these parameters show a positive correlation (Table 4). High-density materials show a higher mass in relation to volume and thereby they have a higher combustion yield. In our study, the highest bulk density was determined in bark and sawdust, and the lowest in maize straw. According to Mitchual et al. [38], higher material density at room temperature and low pressure allows for the production of higher density briquettes.

As it results from the literature data [46, 50‐52], in all cases, the porosity of biochars increased after pyrolysis. This was directly connected to the volatile matter content decrease and thereby the formation of a free space in the material structure. Our results showed a slight increase in porosity for materials with high ash content, i.e., those in which the amount of the released volatiles was small relative to their dry weight. This phenomenon finds its confirmation in literature [52, 53]. Porosity is a particularly important parameter when using biochars for anything other than energy purposes. For example, when using biochars as sorbents or fertilizers, porosity is directly related with the material’s effect on the biological, chemical, and physical properties of soil.

Our investigations proved that it is advisable to use pyrolytic biochars (produced at 300 °C) for energy purposes. The volatile matter content is an important aspect and, in the cases analyzed by us, it was similar to that typical for lignite [53, 54]. This can increase the popularity of using this type of fuel in conventional coal boilers. In some materials (except for bark biochar and poultry litter), the ash content was less than 10%. This result is very advantageous from the operational point of view and allows to reduce the necessary boiler maintenance. The calorific value as a main criterion in the assessment of energy potential of raw fuel, was also more promising in the case of biochars. However, one should take into account the energy consumption of the pyrolysis process, having a direct impact on the final costs of acquiring energy from this medium, and which was omitted in this study. It is also worth mentioning that pyrolysis of biomass can significantly reduce its moisture content, affecting its quality in terms of energy production [54]. Since the ash content in the analyzed materials can be very different, it is important to determine the future destination of the ash produced during combustion. High amounts of heavy metals in the analyzed materials (e.g., in poultry litter) can prevent these materials from being used as fertilizers, but, in some cases, they can be applied for the production of sorbents [55].

The potential utilization of ash is influenced by contaminants such as heavy metals and the extent to which the ash is sintered. Clean biomass contains minerals and important trace elements and, therefore, can be recycled to forest grounds; however, these trace metals must be clearly quantified and their impacts studied if they are to be applied to soils [56]. Results of the trace element analysis, presented in Table 3, show that heavy metal contents varied to a large extent for both feedstocks and biochars. In general, all biochars had increased contents of the tested metals compared to feedstocks used in their production, as shown in Table 2. This relation was foreseeable, because thermal conversion affects the combustible matter, and ash remains in biochar. Pels et al. [46] and Hossain et al. [47] reported that increased pyrolysis temperature increases the content of trace elements, which is a consequence of a more intense discharge of flammable substances. The temperature used in our study (300 °C) was selected considering not only the further heavy metal content, but also the possibility of using the resulting biochars as fertilizers. The highest contents of heavy metals were observed in samples related to poultry litter (PL and PLB). Very high concentrations of zinc, iron, and copper can reduce the applicability of their ashes as fertilizers. The correlation analysis confirmed that there is a strong relationship between the content of ash and the content of the tested heavy metals in materials used for the study (Table 4). According to Demirbas [55], the composition of ash is dependent on the plant species, growth conditions, and ash fraction. For example, Vamvuka [56] suggested that the high concentration of Ni and Cr present in the tested olive kernel ash was most likely due to the soil parent material. According to the author, the soil type is laden with Ni/Cr which is transferred to the plant [56].

With view of using ashes in the nature, the analysis of the heavy metal content in the obtained ashes was carried out. Due to the possible leaching into underground and surface waters, heavy metals contained in ash residue may have negative environmental impacts if irrationally managed and disposed [57]. The chemical composition and leaching behavior of biomass ashes were analyzed. As shown in Fig. 4, leachability of heavy metals of biomass ash was highly diversified and conditioned by the analyzed element and material type. Our investigations showed that ashes produced from thermally unconverted biomass had higher leachability of heavy metals. The cadmium content wad determined only in WS, WSB, and S ash extracts and ranged from 0.006 mg to 0.022 mg/kg. The highest Cu content was determined in the extracts: PL (0.75 mg/kg), PLB (0.57 mg/kg), and WS (0.37 mg/kg). The contents of other elements ranged as follows: for Cu from 0.01 mg/kg (MS) to 0.75 mg/kg (PL), for Fe from trace contents (PLB, BB) to 4.84 mg/kg (MS), for Ni from 0.03 mg/kg (S) to 1.35 mg/kg (WS), for Pb from 0 mg/kg (PLB) to 0.13 mg/kg (MS), for Zn from 0.05 mg/kg (BB) to 2.14 mg/kg (S), for Cr from 0.03 mg/kg (B) to 0.53 mg/kg (PLB), and for Mn from 0.17 mg/kg (PLB) to 87 mg/kg (S). After analyzing the literature data on the heavy metal content in extracts from ashes of various biomasses, one can conclude that the mean content of individual elements is, usually: 3.5 mg/kg for Cr, 0.052 mg/kg for Ni, < 0.0014 mg/kg for Cd, and 0.0027 mg/kg for Pb [3, 11, 58]. On the other hand, taking into account regulatory limits (2003/33/EC), the not-hazardous limits for leachates for Cr, Ni, Cd, and Pb are: 4 mg, 5 mg, 0.6 mg, and 5 mg/kg, respectively. It should be emphasized that the content of these elements in the analyzed extracts was lower than that given in the literature and in regulatory limits (2003/33/EC).

Extracts derived from leaching of ashes had a wide range of pH values, i.e., from 5.50 to 11.40 (Table 5). High pH values of ash extracts confirm the possibility of using ashes for fertilizing purposes (liming). However, it is necessary to monitor the solubility of individual minerals in ashes from various types of biomass to avoid the potential risk of uncontrolled disturbances of soil and groundwater pH.

The EC measured in the resulting extracts ranged from 0.33 to 5.51 mS/cm (Table 5). This parameter value indicates that easily soluble salts were present in extracts. This is of particular importance when it comes to the plant’s response to any possible increase in the substrate salinity, which may result in a limited availability of water [59, 60]. According to Ribeiro et al. [59], amendment with biomass fly ash slightly elevated soil pH, to values within the most adequate range for plant growth. Results showed small raises in the availability of essential plant macronutrients—Ca, K, and Mg—especially in the top layer of the soils, where the amendment materials were applied. The mobilization of cations to the groundwater was always minimal, which is promising, since it means little contamination of groundwater [59].

Phytotoxicity analysis showed that, in general, each of the tested ash extracts had a beneficial effect on the growth of Lepidium sativum L. compared to the control treatment (Fig. 5). Our results suggest that the extracts contained substances that not only did not inhibit the germination of Lepidium sativum L., but also stimulated the growth of the plant at a very early stage. This is important information, especially given the fact that plants are susceptible to adverse environmental factors in the early stages of growth. The highest plant growth stimulation was determined after the application of MS (59%) and B (45%) extracts. Compared to the control, the growth inhibition of Lepidium sativum L. of 26% was noted only after using the PL ash extract. The comparison of the effect of ashes from feedstocks and biochars revealed that PL showed significantly higher phytotoxicity to Lepidium sativum L. than PLB. In the case of other treatments, no significant changes in the phytotoxicity were found between the feedstock ash extract and the biochar ash extract. In addition to the highest Cu content in extract, PLB also had the highest pH and EC values. On the other hand, the results obtained by Lu et al. [61] showed that the effect of pyrolysis temperature on chemical speciation distribution, leaching toxicity, and bio-available contents of heavy metals in biochar was inconsistent, and the potential risk of soil and groundwater contamination after the application of biochar was lower than in the case of sewage sludge. In the study of Rombola et al. [60] on relationships between chemical characteristics and phytotoxicity of biochar from poultry litter pyrolysis, it was found that higher phytotoxicity of poultry litter than of corn biochars was tentatively attributed to hydrophilic biodegradable substances derived from lipids or proteins removed by water leaching or microbial treatments. The combustion process could release active forms not only of trace elements, but also microelements, which, in turn, caused a negative response of Lepidium sativum L. The investigations conducted by Gondek et al. [62] showed that the thermal conversion of mixtures of sewage sludge and plant biomass significantly inhibited sprouting and growth of L. sativum roots. The inhibition of L. sativum root growth ranged from 13 to 59% for thermally unconverted materials and from 9 to 38% for biochars. In the study, Oleszczuk et al. [63] reported slightly different relationships—the tested biochars were more toxic to V. fischeri than to L. sativum. Other studies by Mierzwa-Hersztek et al. [64] showed that the type of feedstock from which biochars were produced has a significant effect on the toxicity of biochars. According to Ribeiro et al. [59], ash amendment and ash + sludge amendment resulted in similar Lolium perenne growth compared to the control. However, biomass grown in Cao-amended pots showed the highest root size.