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Influence of Thermochemical Conversion Technologies on Biochar Characteristics from Extensive Grassland for Safe Soil Application

Department of Postharvest Technologies, Leibniz Institute for Agricultural Engineering and Bioeconomy, Max-Eyth-Allee 100, 14469 Potsdam, Germany
Section of Grassland Science and Renewable Plant Resources, Faculty of Organic Agricultural Sciences, University of Kassel, Steinstr. 19, 37213 Witzenhausen, Germany
Author to whom correspondence should be addressed.
Energies 2023, 16(4), 1896;
Received: 19 December 2022 / Revised: 6 February 2023 / Accepted: 10 February 2023 / Published: 14 February 2023


Grass and other herbaceous biomass are abundant, but often under- or not utilized as a renewable resource. Here, the production of biochar from extensive late-harvest grass via multiple thermochemical conversion technologies was investigated at lab and farm scale for use in soil applications. While biochar is a product with highly diverse potential applications, it has a multitude of benefits for agricultural usage as a soil amendment, if the quality adheres to certain limit values of potentially toxic constituents. The results show that the biochar can adhere to all limit values of the European Biochar Certificate (EBC) for utilization in agriculture. Generally, the contents of heavy metals were well below the proposed EBC limits and very low PAH concentrations in the biochar were achieved. The high ash content in the grass of 7.71 wt%db resulted in high nutrient concentrations in the biochar, of benefit in soil applications, but the ash also contains chlorine, nitrogen and sulphur, which presents a challenge for the operation of the thermochemical processes themselves due to corrosion and emission limits. In the farm-scale processes, ash retention ranged from 53.7 wt%db for an autothermal batch process, reaching up to 93.7 wt%db for a batch allothermal process. The release of Cl, N and S was found to differ substantially between processes. Retention ranged from 41.7%, 22.9% and 27.6%, respectively, in a continuous allothermal farm-scale pyrolysis process, to 71.7%, 49.7% and 73.9%, with controlled lab-scale pyrolysis at 450 °C, demonstrating that process optimization may be possible.

1. Introduction

Grasslands cover a large fraction of the EU’s landscape, with currently 17.5% of the surface area classified as grasslands, providing invaluable ecosystem services [1]. However, land-use pressures and changes in agricultural practices, such as land abandonment linked to a lack of grazing, have led to disappearing grasslands and threatened ecosystems [2]. Nonetheless, a large potential for use of this resource remains. For example, in Germany, 4.495 million ha of permanent grasslands were cultivated in 2015, comprising 28% of the total agricultural area used [3]. Novel technologies are necessary to enable farmers to tap into this large biomass potential and reduce the current trend of abandonment. The combined production of heat and biochar could fill this gap.
Past research on grass utilization for biochar production has shown promising results, especially for various species from monocultures, such as Poa pratensis (Kentucky Bluegrass) [4], Miscanthus giganteus (elephant grass) [5], Pennisetum purpureum [6] or Achnatherum splendens lineus [7]. All studies have shown that biochar with a suitable composition for soil-amendment purposes can be produced from these grass species. However, there is a lack of investigations that utilize grass from less-managed sources, such as traditional pastures, where the highly diverse communities of plant species may be found, and thus the feedstock is more heterogeneous.
Restrictions on the use of plant-based char as a soil amendment are mainly based on heavy metals and polycyclic aromatic hydrocarbon (PAH) concentrations [8]. While the PAH concentration in the biochar is mainly dependent on the thermochemical conversion process design used for production [9], the heavy-metal concentration is dependent on their content within the soil and subsequent uptake in the feedstock [10]. A biochar that conforms with the proposed limits by the European Biochar Certificate (EBC) [8] for agricultural applications can provide substantial benefits when applied to fields, including increased water-holding capacity, microbial activity and nutrient retention, as well as decreasing the release of greenhouse gas emissions and nitrogen leaching from the soil [11]. Furthermore, biochar can play an important role in mitigating climate change due to its ability to store CO2 over the long term [12]. In-depth analysis of the produced biochar with a focus on potentially harmful constituents is therefore necessary to ensure that the biochar is suitable for utilization on agricultural land, especially when coming from locations such as periodically flooded polder areas, where the risk of heavy-metal contamination is increased, as is the case with the utilized feedstock here in this study.
Many different technological pathways exist for plant-based char production when aimed at use as the soil amendment termed biochar [13]. The main technology consists of pyrolysis, but also hydrochar, char produced from hydrothermal carbonization (HTC), which could potentially be considered as a soil amendment and thus biochar. Moreover, even within the thermochemical conversion route of pyrolysis, many potential technological process designs exist [14]. The process may run in an inert environment, heated via an external heat source (allothermal process), with the addition of limited amounts of oxidizer, mostly in form of air (autothermal process), and either in a batch or continuous mode of operation [13]. Additionally, the scale at which the process is performed also plays a vital role, as these substantially influence fundamental properties such as thermo- and fluid dynamics in the process. An investigation of multiple different processes at various scales could provide a deeper understanding of the process parameters and their influence on the thermochemical conversion process, as well as on the composition of the produced biochar.
The difficulties associated with the thermochemical conversion of agricultural residues, especially due to the high ash content of such feedstock when compared with woody biomass, and the subsequent combustion of released volatile products have long been identified as challenges [15,16]. However, these aspects are still not often considered in the literature on biochar production via the process of pyrolysis, especially not the combustion process of the released volatiles [12]. The pyrolysis process is thus often misleadingly presented as a stand-alone process, whereas it should always be considered as one in a series of coupled processes. When aiming at biochar production, pyrolysis should always be coupled with subsequent processes that utilize the volatile pyrolysis products. Subsequent processes are mostly direct combustion, for combined heat and biochar (in certain cases also electricity) production [17], but could also consist of other uses of the produced volatiles, including condensable and gaseous compounds [18]. In particular, small- to medium-scale plants, those that are relevant for farm-scale production of biochar, do not focus on the downstream process from the thermochemical conversion and it may be assumed that this leads to the release of substantial amounts of products of incomplete combustion, which are harmful to human health and the environment [19] as well as potentially diminishing the climate positive effects of utilization of biochar [20]. Experience from the combustion of straw may provide a guideline [21], but the differences in the feedstock composition will play a vital role. Ash constituents that are released during the pyrolysis process may also lead to corrosion within the pyrolysis reactor [22], as well as influence combustion equipment for the combustion of the released volatile pyrolysis products [23], which is in the authors’ perception the main technological hindrance for its widespread application. An analysis of the potential influence of the released feedstock constituents during the thermochemical conversion process of grass can provide information for efficient process design, for the subsequent combustion of volatile pyrolysis products for combined heat and biochar production from grass and other agricultural residues.
The focus of this paper is to generate further insights into the dependence of the biochar composition on the thermochemical conversion process design and to identify pathways for adaptation to enable the utilization of high ash-content feedstock for biochar production. To this end, the conversion of grass to biochar was studied at both the lab and farm scale. Thermochemical conversion experiments were performed in three lab-scale batch reactors, using HTC, allothermal (electrically heated) pyrolysis and autothermal pyrolysis, at various process conditions. At the farm scale, an autothermal batch reactor, as well as an allothermal batch reactor and a continuous allothermal reactor were utilized (both heated via combustion of pyrolysis volatiles). The composition of all produced chars are extensively analysed and their suitability for utilization as biochar for soil-amendment purposes is evaluated. The differences between biochars from the various production processes as well as the implications of the biochar production on the subsequent combustion of volatile products from the conversion process is investigated.

2. Materials and Methods

2.1. Feedstock

The feedstock consists of grass from a periodically flooded polder area in the Lower Oder Valley National Park (53°02′32.6″ N 14°17′50.0″ E). The grass from the national park consists predominantly of meadow foxtail and reed grasses [24], but no specific determination of the utilized grass was performed. The grass was cut, dried, turned, rowed and baled on the field. Square bales with dimensions of 1.35 × 1.2 × 0.9 m with an approximate weight of 650 kg were produced from the grass. Due to the conservation management practices in the national park, the utilized grass was from areas that were harvested once, with a harvest date after August 15, as shown in Figure 1. The grass was harvested in the years 2020 and 2021. For experiments, grass from the harvest in 2021 was used, if not otherwise stated. The investigated conversion technologies required different feedstock particle size and densification properties and thus, grass was either used as stalks, or further processed by milling, briquetting or pelleting. In the initial step for briquetting and pelleting, the grass bale was taken apart and milled using a Tomahawk 505XL with an 8 mm sieve (Teagle Machinery Ltd., Truro, UK). Then briquettes were pressed in a 60 mm piston press. For pelleting, the grass pieces were additionally milled in a hammer mill with a 6 mm sieve and pressed to 8 mm diameter.

2.2. Biochar Production

Biochars were produced on a lab scale using an hydrothermal carbonization (HTC) [25] reactor (Parr, US), a top-lit up-draft (TLUD) reactor and a nitrogen-flushed pyrolysis oven. In farm-scale pyrolysis, biochars were produced using a batch-fed autothermal Carbontwister (Prodana, DE), a batch-fed allothermal VarioL (SPSC, DE) and a continuous allothermal C63-F (Biomacon, DE). Table 1 presents an overview of the performed experiments, and experimental details for each thermochemical conversion unit are provided in Section 2.3. Depending on the thermochemical conversion reactor, biochars were produced in (1) lab- (L) and farm-scale (F) units; (2) via HTC (H), autothermal pyrolysis (A), or allothermal pyrolysis in an inert environment (I); (3) as batch (B) or continuous (C) process; and (4) from stalks (S), milled (M), briquetted (B) or pelleted (P) grass, as indicated in the experimental code. Between one and four replicates were performed using the different thermochemical conversion routes. For the farm-scale units with only one replication, the long operation of the continuous unit (FI-CP) over multiple days and a large amount of input material may provide a representative sample size, though the single experiment of unit FI-BB is a limitation of this study. The highest heating temperature (HHT) and the retention time can be controlled in the lab-scale experiments, HTC (LH_BP) and pyrolysis oven (LI-BP450, -550, and -650); in the lab-scale TLUD as well as all farm-scale units both parameters depend on a multitude of factors, cannot be controlled and may only be reported here.

2.3. Thermochemical Conversion Units

The HTC experiment (LH-BP) was performed on grass pellets at a solid content of 15 wt% in an 18.75 L reactor system (Model 4555, T 316 Stainless Steel, Parr Instrument Co., Moline, IL, USA) with a 6000 W heating system and temperature controller (Model 4848BM) using SpecView data acquisition software. The reactor was heated at a rate of 2 °C∙h−1, to reach a final temperature of 220 °C, with a stirring rate of 125 rotations per minute and a holding time of 5 h. These conditions have been chosen on the basis of previous experience with similar feedstock as well as pre-tests to achieve complete conversion of the feedstock. A detailed experimental procedure was employed as described in more detail in [26]. Further HTC experiments are reported in the Supplementary Materials.
The 3.9 L batch fixed-bed TLUD (LA-BM) reactor consisted of a steel pipe with 100 mm inner diameter and a depth of 500 mm that was filled with milled grass from the harvest in 2020. Approximately 10 mL of Ethanol (EC No. 200-578-6) was initially used for starting the conversion process at the top surface. Air was continuously supplied from below the fuel bed at a rate of 0.05 kg∙m−2∙s−1. This led to a mean maximum conversion temperature [27] of 720 °C within the migrating pyrolytic front. Process conditions including the highest heating temperature, retention time and heating rate are mainly dependent on the air supply and cannot be directly controlled.
Batch pyrolysis experiments (LI-BP) were performed in a pre-heated nitrogen flush oven with a holding time of 90 min, which was chosen on the basis of pre-tests to achieve complete conversion of feedstock. Three pyrolysis temperatures of 450 °C, 550 °C and 650 °C were tested in duplicate. The range of conversion temperatures was chosen on the basis of previous investigations using grass as feedstock [4,5,6,7].
Autothermal batch-fed farm-scale experiments (FA-BS) were performed with a Carbontwister (Prodana, DE) on a farm in the state of Brandenburg, Germany. The system has a total volume of 0.6 m3 and a total of 650 kg of grass stalks from the harvest in 2022 was converted over four batches. The conversion of each batch took approximately 2 h and the target conversion temperature within the fuel bed was around 600 °C, while at certain locations within the bed, temperatures up to 900 °C were measured. Further details of the experiments can be found in the Supplementary Materials.
The batch allothermal farm-scale experiment (FI-BB) was performed using a Vario L+ (SP-SC, DE) on a farm in the state of Baden-Württemberg, Germany. The batch unit with an inner volume of 1.9 m3 was partially filled with grass briquettes. A total of 201 kg of grass was converted over 4 h. No temperature sensors were present within the device. Further details of the experiments can be found in the Supplementary Materials.
The continuous allothermal farm-scale experiment (LI-BP) was performed using a C63-F (Biomacon, DE) on a farm in the state of Lower Saxony, Germany. The unit uses a screw pyrolysis reactor that is externally heated by the combustion of the released pyrolysis vapours (allothermal). A temperature of around 800 °C was reported by the unit’s control system within the combustion region, but it is unclear to what conversion temperature this may lead to in the unit in the pyrolysis region. A total of 2705 kg grass pellets were converted over a continuous operation of 86 h and thus with an average feeding rate of 31.5 kg∙h−1. The biochar production process ran without complications, while the excess heat was utilized to satisfy the requirements of the farm.

2.4. Physico-Chemical Analysis

Multiple analyses have been performed on the grass feedstock as well as on the produced chars. In general, a minimum of three individual samples were taken for each experiment or conversion device. Results are reported as mean values of these individual samples, while for measurements below the limit of detection, the quantification limit was used. For the grass feedstock measurements were performed on stalks, briquetted and pelleted grass. While here, mean values from all these measurements are reported and the individual measurements for each fraction can be found in the Supplementary Materials. CHNS was measured using a Vario EL III (Elementar, Germany) with three replicates of each sample. The oxygen content was calculated via Equation (1), according to DIN 51733.
O = 100 C H N S A s h   w t % d b
A TOC 5050A was utilized to measure the organic carbon content (Corg) in accordance with EN13137. An ICP-OES emission spectrometer, series iCAP 6000 (Thermo Fisher Scientific, US) was utilized for the determination of nutrients and heavy metals, with preparation via microwave disintegration with nitric acid. Dry matter was measured at 105 °C according to DIN 51718. The ash content was determined using a muffle furnace following DIN 38414 EN 12879 at 550 °C peak temperature. Higher heating values (HHV) were measured using a C200 bomb calorimeter (IKA, PRC). The pH and electric conductivity (EC) were determined using a ph-con720 (WTW, Germany) following DIN 38 404-C5 and DIN 38 404-C8, respectively. PAH analyses were performed by Eurofins Umwelt Ost GmbH following DIN EN 16181:2019-08.

2.5. Assessments

Retention is used to describe the degree of total ash, as well as individual constituents, retained in the biochar. It was determined as the relationship between the measured constituent content in the biochar to that in the feedstock as presented in Equation (2) [21].
r e t e n t i o n = ( wt % d b   in   biochar )   ( wt % d b   i n   b i o m a s s ) b i o c h a r   y i e l d   w t % d b
where the biochar yield is the ratio of the mass of biochar to the mass of feedstock as dry matter and wt%db is the weight percentage of the constituent in the biochar and feedstock on a dry matter basis. Carbon retention is also often called carbon yield.

3. Results

The intended utilization pathway for the produced biochar in this study was soil application, and thus the biochar has been evaluated concerning its potential safety risks as a soil amendment in the following section. As a basis for the analysis, the guidelines of the European Biochar Certificate (EBC) [8] are considered. Additionally, the implications of the main constituents of biochar as well as those of the minor constituents for the application as a soil amendment and their dependence as well as the influence on the production technology design are investigated.

3.1. Biochar Quality and Main Constituents

The elemental composition of the grass feedstock as well as the yield and elemental composition of the produced biochars are presented in Table 2. Looking first at the ultimate analysis of the grass feedstock, it can be seen that its composition stands out among other traditional woody feedstock for biochar production with its high ash and hydrogen contents. The ash content at 7.79 wt%db is much higher than the typical values found in wood (0.1–1 wt%db), but falls within the ranges found for grasses and straws (2–10 wt%db) [28], which are a challenge for thermal applications. The carbon content (46.2 wt%db) is comparable to other herbaceous biomass [22], but at the low end compared to wood (47–54 wt%db) [28], while the content of hydrogen is quite high here at 7.0 wt%db, in comparison to woody, straw, grain crops and herbaceous biomass, which range between 5.1 wt%db and 7.2 wt%db [22]. Since during pyrolysis and devolatilization, hydrogen-containing compounds contribute substantially to the heating value of the volatile product fraction, this high H content may be beneficial. For the minor elements of the ultimate analysis, the content of N (1.1 wt%db) is comparable to other herbaceous biomass, while the content of S with 0.286 wt%db is high compared with a range between 0.015 wt%db and 0.270 wt%db for a variety of biomass [22]. This high S content may pose a challenge for the conversion system’s heat exchange and exhaust system as sulfuric acid may be formed, which can lead to increased corrosion [22]. These challenges could be overcome by additional pretreatment to reduce minerals in grassy biomass to improve feedstock properties for thermal conversion processes [29].
For the biochars, an important result is the degree of carbonization achieved in the conversion process, and it is expressed as the molar H/Corg ratio and regulated in the EBC to a limit value of <0.7. Comparing the biochars in Table 2 shows that all produced biochars are below this limit with the exception of LH-BP, the hydrochar (char produced from HTC). This was to be expected as the H/Corg ratio of hydrochars is generally higher than the limit proposed in the EBC. In hydrothermal carbonization, no devolatilization occurs and the majority of hydrogen from the feedstock remains in the product, causing high H/Corg ratios. For all pyrolytic conversion processes, the H/Corg ratio is well below the proposed threshold with a general trend of lower values for allothermal (LI-BP, FI-BB and FI-CP) compared with autothermal (LA-BM and FA-BS) conversion. This trend is also reflected and influenced by the overall yield of biochar from the process as well as the carbon yield from the feedstock in the product. Through the addition of air and thus an oxidizer in the autothermal processes (LA-BM and FA-BS), the overall product yields as well as the yields of carbon are lower in comparison with the allothermal (LI-BP, FI-BB and FI-CP) pyrolytic processes.
The largest biochar yield as well as the carbon yield can be achieved via HTC (LH-BP) with 61.0 wt%db and 80.0 wt%db, respectively, which are nearly five-fold higher than in the lowest-yielding process of autothermal farm-scale pyrolytic conversion with 12.9 wt%db and 16.8 wt%db, respectively. The high overall and carbon yields from HTC are a substantial advantage over pyrolytic processes, while the high organic content in the liquid product fraction can be disadvantageous, requiring further treatment, e.g., in a biogas plant [30,31]. From the high H/Corg ratio, it can be deduced that a large fraction of the carbon has a lower stability compared to pyrolyzed char, leading to more rapid degradation of this fraction when added to the soil [32,33]. In the long term, however, it is still unclear which process, HTC, allothermal or autothermal pyrolysis, will lead to the highest retention of carbon in the soil [34,35,36].

3.2. PAH Content

The concentration of PAHs in biochar is generally dependent on the production technology [17]. Therefore, these compounds have only been analysed in the biochars produced in the farm-scale processes, as these results will be more representative than lab-produced biochars. In Table 3, the PAH concentrations from the biochars produced at the candidate material stage for soil amendment are presented, together with the proposed limit values for safe utilization as a soil amendment on agricultural land as found in the EBC regulations. In this analysis, a sum of 16 PAHs (EPA16) defined by the United States Environmental Protection Agency (EPA) as particularly environmentally relevant and representative of their general contents in a product is presented. Of these, eight compounds (EPA8) are particularly cancerogenic and are separately listed. Further details and the concentrations of individual PAHs can be found in the Supplementary Materials. Two of the three tested processes achieved measurements below or within the proposed limits by the EBC. It appears that the concentrations are higher for both batch pyrolysis processes. In the batch processes, the producer gas from the thermochemical conversion may pass through layers where the conversion has already occurred and the char may absorb PAHs or if the thermal profile in the reactor is not optimal, these may condense and remain in the biochars [37]. For FI-BB biochar, PAH concentrations were low, but above the EBC thresholds. This might be a result of the grass briquettes, which may not be the perfect size and structure for this reactor. Especially since the technology provider reported PAH concentrations below the limit for various other biomass feedstock. There is still substantial discussion if the low limits for PAHs as proposed by the EBC are necessary for safe application as a soil amendment, especially since it may be the case that only about 1% of the total amount of PAHs may be bioavailable [38]. Thus, although the batch reactors struggle with meeting EBC regulations, in all cases very low concentrations are achieved. These results and especially those from the continuous experiment FI-CP demonstrate that with good process control a biochar with minimal PAH concentration can be produced from grass as feedstock.

3.3. Heavy Metals

The content of heavy metals in the char is mainly dependent on their content in the feedstock. The feedstock content in turn depends on the concentration and bio-availability of heavy metals in the soil where the feedstock was grown. The grass was grown in a periodically flooded polder area of the Oder River. Periodical flooding provides a substantial risk factor for heavy-metal contamination of soils through polluted river water. However, the content of heavy metals in the feedstock as well as in the produced biochar is below the limit values for safe utilization as a soil amendment on agricultural land, as proposed by the EBC (refer to Table 4).
Since the limit values of the heavy metals vary widely from 1 mg kg−1 to 400 mg kg−1 (refer to Table 4), the measured values are shown as a percentage of the proposed EBC limit values in Figure 2 to allow easier comparison of the processes. All relevant values are within the proposed limits of the EBC with the exception of Cd in the biochar produced from the lab-scale autothermal (LA-BM) process. This shows that the heavy-metal concentration within the biochars is non-toxic and safe for soil application for agricultural purposes. The Cd measurement of 3.02 mg∙kg−1 is more than double the proposed limit of 1.5 mg∙kg−1. This high Cd content is due to a high Cd content in the feedstock harvested in 2020 (refer to the Supplementary Materials). This supports a need for regular testing of biochars, especially from risk areas such as polders, to ensure their safe use as a soil amendment. For most compounds, the concentrations generally are well below 50% of the limit value, with only Zn being around this fraction, demonstrating that even high ash-content fuels from risk areas can be well suited for soil-amendment purposes in terms of their heavy-metal contents.
The exceptionally high concentrations of Cr and Ni in the biochar from the experiment FI-BB are most likely caused by the introduction from new stainless-steel parts of the reactor as a result of abrasion during the experiment (refer also to Section 3.5). It may be assumed that such an introduction of steel alloy elements will not occur after the first few uses.
The content of Hg has been measured in the feedstock and biochars produced in most lab-scale experiments but has not been measured in biochars from farm-scale experiments. The low content of this element in the feedstock, below the detection limit of <0.5 µg∙kg−1, as well as in all lab-scale experiments, ensures that there will also be concentrations lower than the limit value of 1 mg∙kg−1 in all the produced biochar.

3.4. Minor Elements and CO2 Sequestration

The high ash content in the grass is a defining feature as feedstock and leads to higher values in the produced char, which may be beneficial for soil applications since a large portion of the ash is made up of plant-essential nutrients. These main elements remain partly in the biochar, but the share is highly affected by the conversion technology used and the underlying process conditions (refer to Table 2 and Figure 3).
When comparing the different processes, the hydrochar shows a lower concentration of non-gaseous elements, such as P, K, Mg, Ca and Fe than the biochars from pyrolysis (refer to Figure 3). This trend is in line with the results for heavy metals and can be explained by the hydrothermal process, in which these elements are partially released into the liquid phase of the conversion process (refer to Section 3.3). In addition, the higher solid yield means there is a lower content of inorganics in the solids. Within the different pyrolysis processes, no clear trend can be observed. Overall, the contents of the plant-essential nutrients, nitrogen (0.9–1.6 wt%), phosphorous (0.5–0.8 wt%) and potassium (0.5–2.9 wt%), differ in some cases very substantially between the different biochars (refer to Table 2 and Figure 3).
However, the key properties of biochar in soil are its ability to store carbon long term and improve soil quality. Since the biochars produced in these experiments show a high degree of carbonization, indicated by the low H/Corg ratio (refer to Table 2) below 0.3 for pyrochar and 1.1 for hydrochar, as well as a molar O/C ratio less than 0.2, the predicted half-life stability of biochar is larger than 1000 years [39]. For the biochars from the farm-scale technologies, a carbon sink of 1.68, 1.93 and 1.50 t of CO2eq per t of the biochar for units FA-BS, FI-BB and FI-CP, respectively, can be generated, as assessed elsewhere in more detail [40]. This, in turn, means that the investigated grass-type biochars have a high potential for CO2 sequestration and thus climate-change mitigation.
For soil application, the concentration of plant-essential nutrients (refer to Table 2 and Figure 3) in biochar plays a critical role in estimating the potential beneficial effects of biochar on crop yield and soil fertility and must be reported by law [8]. In general, it can be estimated that the plant availability of P, N and K in biochar in the first year after application is around 15%, 1% and 50%, respectively [41]. Plant availability of biochar constituents and the effects of biochar on soil fertility and thus crop yields are highly dependent on feedstock conversion processes, soil conditions and crops, among other factors [42,43,44]. Depending on the soil application rate, pyrochar can increase soil pH and thus affect overall nutrient availability [42]. Also, yield depressions have been reported by applying raw biochar to soil [43]. Hence, further analyses, which are outside the scope of this article, would need to be performed to determine the exact plant availability of nutrients and the effect of biochar on soil fertility [41], especially since biochar has not only the ability to enrich the soil with nutrients, but also to enable more efficient use of nutrients [45,46].

3.5. Ash Retention

The amount of ash that remains in the solid biochar product (retention) has substantial implications for the overall process. On the one hand, retained ash constituents in the biochar are added to the soil and may have a positive long-term effect on the soil quality. On the other hand, all ash constituents that are released during the conversion process may have a detrimental impact, on the conversion reactor itself via corrosion, on the combustion process of the released volatile products from the conversion process and subsequently may cause corrosion in heat exchangers as well as exhaust equipment. The high ash content of the feedstock grass, 7.79 wt%db, is one of the main differences between grass and other biomass feedstock such as woody biomass [28]. Therefore, an analysis of the retention of ash and inorganic constituents in biochar may provide insights into how the yield of especially soil-beneficial and combustion-negative compounds in biochar can be maximized.
The total retention of ash from the feedstock in the produced biochar ranges from 53.7 wt%db, in the autothermal farm-scale experiment (FA-BS) to 93.7 wt%db, in the allothermal farm-scale experiment (FI-BB). This wide range demonstrates the dependence of the release of ash from the feedstock in the conversion on the process conditions. Surprisingly, the retention of ash in the hydrochar is, at 75.4 wt%db, comparable to that of biochar produced via pyrolysis. This demonstrates that the devolatilization and entrainment of ash constituents during pyrolysis may release similar amounts of inorganic constituents from the feedstock as hydrothermal processing, where ash may be solved and transferred into the liquid-product fraction [47].
Detailed studies on the fate of inorganic constituents during biomass pyrolysis and combustion can be found in the literature [48,49,50,51,52,53]. It is well understood that inorganics can substantially influence combustion reactions [23], lead to undesired emissions and cause corrosion of downstream equipment [22]. Although much is known about the release of inorganic feedstock constituents during thermochemical conversion, no clear and general assumptions can be made, as the release of constituents is still dependent on the feedstock and process conditions.
The Cr and Ni retention in the biochar from the batch farm-scale processes (FA-BS and FI-BB) appear exceptionally high, with a rate of over 100%, in comparison with the other biochars. This is probably due to the fact that they were produced in relatively new units made of high temperature-resistant steel alloys. Thus, Cr and Ni can be introduced into the biochar as a result of the production process, as discussed previously in Section 3.3. This demonstrates that especially the biochar from new or modified units may have excessive concentrations of certain heavy metals.
A similar influence has been identified for the As and Pb contents. Although generally the As content in the grass feedstock was below the detection limit, the As content in the grass pellets was detected at an average concentration of 0.18 mg·kg−1 (refer to Table 4). Subsequently, even higher concentrations appeared in the biochar produced from these pellets, with >100% of retention in all conversion technologies where pellets were utilized, leading to a biochar concentration of up to 1.02 mg·kg−1 (refer to Table 4). For Pb, the concentration in the pellets is at 0.62 mg·kg−1 (refer to Table 4), while in the biochar only detectable from lab-scale pyrolysis and amounts there to more than the content in the pellets, with a retention of up 162.7% (refer to Table 5). It is generally unclear what led to retention rates >100% of various compounds, although most likely this is due to the heterogeneity of the feedstock and the produced biochar, where a higher concentration may be present in certain parts of the sample. This demonstrates the challenges of performing mass balances in heterogeneous systems. Although in all cases both the As and Pb concentrations remained well below the EBC limits (refer to Table 4), these measurements demonstrate the challenges when performing an ash balance.
Cd, Zn and Hg are volatile and may be released during the thermochemical conversion, contributing to fly ash, and may accumulate when cooling in heat exchangers [22]. For grate combustion, compounds such as As, Cd, Pb, Zn and Hg are considered to be more dominant in fine fly ash [22]. In Table 5, it can be seen that, especially in low-temperature pyrolytic processes, the retention of these inorganic elements with >90% at 450 °C is generally high. This will reduce this fraction of the fly ash from the subsequent combustion process.
For the pyrolysis experiments (LI-BP450, -N550 and –N650) a general trend of greater retention of compounds such as Cd, Cr, Cu, P, K, Mg, Ca, Fe, N and S with lower temperatures can be seen in Table 5, although surprisingly the retention at 550 °C (LI-BP550) is in some cases lower than at 650 °C (LI-BP650). Higher temperatures generally lead to an increasing release of volatile compounds, which are the main cause for lower retention in the biochar product. Additionally, higher temperatures also lead to higher heating rates within a particle, greater carbon conversion and an increased gas velocity of released volatiles from the feedstock particles, which may all contribute to an increase in the release of inorganic feedstock constituents. As many nutrients are included here, a lower pyrolysis temperature in the conversion process appears beneficial in that regard.
For P, the lab-scale experiments showed minimal release, which is similar to the results found for the thermal decomposition of bran under combustion conditions up to 900 °C [52]. In contrast, in two of the farm-scale processes, FA-BS and FI-CP, only 42.7–71.0% of P could be accounted for in the biochar. The high retention rates from the well-controlled lab experiments could be an indication that similar retention of P should be possible with an optimized process design.
For a large fraction of the inorganic constituents, there is no clear trend visible, when comparing the present results with previous investigations. Although the release of K and Mg has been reported at well below <10% for bran and wheat straw under grate-fired combustion conditions up to 700 °C [52,54], for grass the release is generally >10% for both compounds at temperatures >450 °C. When looking at the autothermal farm-scale experiment (FA-BS), both retentions of K and Mg of 32% and 39%, respectively, appear comparable to grate combustion at temperatures ≥1100 °C [52,54]. Such high temperatures were not reached in FA-BS, suggesting that higher retention of K and Mg may be possible with optimized process design. Conversely, the retention of Cl for wheat straw combustion has been reported at around 40% up to 700 °C, with near-complete release at higher temperatures [54]. This appears very similar to the results presented here, with >50% for the lab-scale pyrolysis experiments and 41.7% for the farm-scale experiment FI-CP, where temperatures between 700 °C and 800 °C may have been reached.
Nitrogen and sulphur are particularly combustion-relevant feedstock constituents due to their oxidation to NOx and SO2, respectively, during combustion [55]. The release of both constituents from the fuel is in part dependent on the feedstock, but mostly on the conversion conditions. While higher temperatures and residence time reduce N retention [56], S may already be completely released under combustion conditions at 700 °C [52]. Under lab-scale pyrolysis conditions, the behaviour of grass has been found here similar to the reported values of wheat straw where release rates of N of 30–50 wt%daf have been reported [57]. As stated previously, the grass has an exceptionally high S content when compared to other biomass (refer to Section 3.1). The pyrolysis experiments show that the release of S is temperature dependent here, with a much higher retention of 73.9 wt%db at 450 °C conversion temperature compared to only 55.8 wt%db at 650 °C. The yield in farm-scale equipment is generally much lower than from lab-scale experiments, especially in the autothermal process, where an oxidation process directly in the conversion process leads to a particularly low retention of only 14.1 wt%db. Adapting process conditions could therefore provide a substantial benefit with regard to SO2 emissions.

4. General Discussion

Grass is a widely available biomass and, in many contexts, underutilized or wasted. The combined production of heat and biochar appears, therefore, to be a promising way for grass utilization. This could contribute to increasing bioenergy production, the reduction of fossil fuel carbon emissions and increase the utilization of alternative renewable biomass for energy provision. Although it is shown here that a high-quality biochar can be produced, which could be awarded the European Biochar Certificate (EBC) for agricultural application, there is a lack of available combustion technologies for its production, since existing technologies are generally designed for woody biomass as a feedstock. In addition, knowledge about the influence of the utilization of grass on existing technologies is scarce.
Within the EBC limit values for the molar hydrogen to carbon ratio and the concentrations of heavy metals, PAHs and PCBs/PCDDs are the decisive aspects. The results here show that various pyrolytic processes can produce biochars that adhere to all regulations. Outliers have been identified though, especially for heavy-metal concentrations, which should regularly be monitored, especially if the feedstock comes from risk areas, such as periodically flooded areas, as is the case in the present investigation. PAHs are mainly process dependent and low concentrations may be achieved with good process design and control. Although not measured here, PCBs and PCDDs are measured by biochar technology providers and, as long as this is the case, the risk of producing such compounds is minimal. This shows that, in principle, grass can provide a good feedstock for the combined heat and biochar production.
The main difference between grass or other herbaceous biomass, when compared with woody biomass, is the high ash and inorganics content. As stated previously, the release of inorganic constituents from the feedstock during the conversion process may have a substantial detrimental influence on the subsequent combustion of the volatile pyrolysis products and heat-exchange processes. Therefore, maximizing the retention of inorganic constituents within the solid biochar product appears a highly beneficial and achievable goal. Generally, it appears that in pyrolysis processes, lower temperatures and inert conditions reduce the release of inorganic constituents. This may be explained by the three potential pathways of release, consisting of (1) the devolatilization of elements or compounds, (2) the entrainment of particles into the gas stream and (3) the oxidation of constituents from the feedstock. Since (1) devolatilization is mainly temperature dependent at constant pressure, this may explain the lower retention of inorganic constituents at higher conversion temperatures [51,58]. Regarding (2), the entrainment of particles, especially small particles that disconnect from the feedstock surface, is dependent on the velocity of the surrounding gas stream [50]. Regarding (3), when supplying air to the conversion process, as in experiments LA-BM and FA-BS, the gas velocity is high compared to processes with no addition of gas, and oxidation reactions on the particle surface may cause bonds between the carbon matrix and inorganic constituent to disintegrate. Both attributes of the addition of air may lead to the particularly low ash yields of 69.5 wt%db and 53.7 wt%db for LA-BM and FA-BS, respectively. For the grate combustion of herbaceous biomass, a distribution of 80–90 wt%db of ash remaining as bottom ash has been reported [22]. Thus with a range of 53.7–93.7 wt%db of ash retained in the produced biochar here appears similar or even lower. This comparison shows that, although in grate combustion where almost all organic material is oxidized, higher conversion temperatures and gas velocities are present when compared with pyrolysis, the release of ash constituents is comparable or lower to the experiments performed here. Additionally, it has previously been shown that secondary capture of SO2, as well as HCl, is possible if released volatiles come in contact with char [59]. In the autothermal batch process FI-BB, secondary capture of inorganic compounds could play a role due to the fixed-bed process design where initially unconverted biomass and subsequently produced biochar will come into contact with the volatile pyrolysis products and contribute to the highest ash retention of 93.7%. This demonstrates that with improved process design and favourable conditions, such as low gas velocities, relatively low conversion temperatures and re-capture of inorganics in the produced char, increased inorganics yields in the biochar may be possible in the future.

5. Conclusions

The analysis of biochars produced from multiple thermochemical conversion technologies has shown that a high-quality and safe soil amendment may be produced from grass as a feedstock. While the concentrations of heavy metals are generally below the limit values proposed by the EBC, low PAH concentrations may be achieved with proper process design. To ensure continuous safety an EBC certification of the produced biochars should always be performed. The comparatively high nutrient content of the produced grass biochar may provide beneficial attributes when utilized as a soil amendment.
The high ash content is a defining attribute when comparing grass with other biomass and especially with woody biomass, which is currently the feedstock that biochar production almost exclusively uses. It could be shown here that the release of inorganic constituents may be limited by adjusting the process conditions to utilize lower temperatures and a non-oxidizing atmosphere.
The utilization of grass as feedstock for biochar production on the farm level shows great potential for application to agricultural soils, benefitting soil properties with the co-benefits of an independent fossil-free and carbon-positive energy production. Due to the substantial release of inorganic biomass constituents during the thermochemical conversion process, it is currently unclear if existing technology may utilize this high ash-content feedstock without detrimental long-term effects on the efficiency and emissions profile of the combustion units. Therefore, further development will be necessary, with a special focus on technologies that focus on utilizing high ash-content agricultural biomass. The development of the combined production of the solid biochar, which may retain inorganic feedstock constituents, with the combustion of the organic devolatilization products may enable the efficient and clean thermochemical utilization of a multitude of high ash-content agricultural residues.

Supplementary Materials

The following supporting information can be downloaded at: Table S1: Yield, elemental composition and the ash content of the grass feedstock and the produced biochar. The mean value in wt-% on a dry basis and the standard deviation of the mean. Table S2: PAH concentrations of the produced biochars from farm-scale equipment in mg∙kg−1. Values for individual compounds as well as total EPA16 and 8. Table S3: Heavy-metal concentrations of the grass feedstock and produced biochars. The mean concentration in mg∙kg−1 on a dry basis (except Hg in µg∙kg−1) and the standard deviation of the mean. Table S4: Elemental nutrient content of grass feedstock and produced biochars. The mean concentration in mg∙kg−1 on a dry basis and the standard deviation of the mean. Table S5: The ash content (wt-%) and yield (%) of ash in the produced biochar, as well as the retention of inorganic elements from the feedstock in the product biochar (%).

Author Contributions

Conceptualization, T.H. (Thomas Heinrich), K.K., J.A.L. and T.H. (Thomas Hoffmann); methodology, T.H. (Thomas Heinrich); validation, T.H. (Thomas Heinrich), K.K., J.A.L. and T.H. (Thomas Hoffmann); formal analysis, T.H. (Thomas Heinrich) and K.K.; resources, T.H. (Thomas Heinrich), K.K., J.A.L. and T.H. (Thomas Hoffmann); data curation, T.H. (Thomas Heinrich); writing—original draft preparation, T.H. (Thomas Heinrich) and K.K.; writing—review and editing, T.H. (Thomas Heinrich), K.K., J.A.L. and T.H. (Thomas Hoffmann); visualization, T.H. (Thomas Heinrich); project administration, T.H. (Thomas Hoffmann); funding acquisition, T.H. (Thomas Hoffmann). All authors have read and agreed to the published version of the manuscript.


This research has been performed within the GO-GRASS project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 862674.

Data Availability Statement

Data available on request.


The authors would like to acknowledge the European Union for financing the Three C project (NWE 1010) through the INTERREG European regional development fund. We would also like to thank Philipp Grundmann for the scientific coordinating the GO-GRASS project.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Dynamic grassland management in a section of the Lower Oder Valley National Park near the city of Schwedt (upper-left corner), displaying the release date for agricultural practices, including harvest, in the year 2021. The feedstock utilized in this study was sourced from areas harvested after August 15 (Source: Lower Oder Valley National Park administration).
Figure 1. Dynamic grassland management in a section of the Lower Oder Valley National Park near the city of Schwedt (upper-left corner), displaying the release date for agricultural practices, including harvest, in the year 2021. The feedstock utilized in this study was sourced from areas harvested after August 15 (Source: Lower Oder Valley National Park administration).
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Figure 2. The concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn are presented as percentage of the limit values for agricultural application proposed by the EBC. The EBC limits are indicated on the x-axis labels (mg∙kg−1) and set the maximum of 100% on the y-axis.
Figure 2. The concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn are presented as percentage of the limit values for agricultural application proposed by the EBC. The EBC limits are indicated on the x-axis labels (mg∙kg−1) and set the maximum of 100% on the y-axis.
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Figure 3. Concentration of various nutrients contained in the grass feedstock and biochar product. Presented are the mean values of multiple samples and the error bars mark the standard deviation of the measurements.
Figure 3. Concentration of various nutrients contained in the grass feedstock and biochar product. Presented are the mean values of multiple samples and the error bars mark the standard deviation of the measurements.
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Table 1. Specifications of the presented experiments. Including the process type, the given code, the replications of performed experiments, the total amount of input material on a dry basis (db), the estimated highest heating temperature (HHT), as well as the estimated retention time. The code is composed of (1) lab- (L) and farm-scale (F); (2) via HTC (H), autothermal pyrolysis (A), or allothermal pyrolysis in an inert environment (I); (3) as batch (B) or continuous (C) process; and (4) from stalks (S), milled (M), briquetted (B) or pelleted (P) grass.
Table 1. Specifications of the presented experiments. Including the process type, the given code, the replications of performed experiments, the total amount of input material on a dry basis (db), the estimated highest heating temperature (HHT), as well as the estimated retention time. The code is composed of (1) lab- (L) and farm-scale (F); (2) via HTC (H), autothermal pyrolysis (A), or allothermal pyrolysis in an inert environment (I); (3) as batch (B) or continuous (C) process; and (4) from stalks (S), milled (M), briquetted (B) or pelleted (P) grass.
ReactorProcess Type
CodeReplicationTotal Input (kgdb)HHT (°C)Retention Time (h)
Lab scale
Top-lit up-draft (TLUD)Auto-LA-BM10.37200.5
Pyrolysis ovenAllo-LI-BP45025.54501.5
Pyrolysis ovenAllo-LI-BP55025.55501.5
Pyrolysis ovenAllo-LI-BP65025.56501.5
Farm scale
VarioLAllo-FI-BB1172.1NM 14.0
C63-FAllo-FI-CP12458.6800NM 1
1 NM = not measured.
Table 2. Biochar and carbon yield of the process and phyisco-chemical characteristics of feedstock and biochar (ultimate analysis, dry matter content (DM), organic carbon (COrg), H/COrg ratio, ash content pH and electrical conductivity (EC)). For all parameters the mean values and standard deviation are presented (n = 3), if determined.
Table 2. Biochar and carbon yield of the process and phyisco-chemical characteristics of feedstock and biochar (ultimate analysis, dry matter content (DM), organic carbon (COrg), H/COrg ratio, ash content pH and electrical conductivity (EC)). For all parameters the mean values and standard deviation are presented (n = 3), if determined.
Ash (wt%db)pH
GrassNA 1NA88.2 ± 0.146.2 ± 1.938.1 ± 2.57.0 ± 0.21.1 ± 0.190.3 ± 0.030.38 ± 0.032.1 ± 0.147.79 ± 0.35.8 ± 0.234.8 ± 12.4
LH-BP61.0 ± 1.980.022.5 ± 0.860.5 ± 1.757.0 ± 2.15.6 ± 0.31.2 ± 0.140.1 ± 0.150.22 ± 0.011.1 ± 0.039.3 ± 2.24.2 ± 0.211.9 ± 0.3
LA-BM23.8 ± 0.032.1101.1 ± 0.062.1 ± 0.061.2 ± 0.01.5 ± 0.01.3 ± 0.000.6 ± 0.000.11 ± 0.000.3 ± 0.0023.6 9.3 ± 0.0NM 2
LI-BP45032.5 ± 1.748.2100.2 ± 0.368.4 ± 1.070.5 ± 2.62.0 ± 0.11.6 ± 0.060.7 ± 0.050.07 ± 0.010.3 ± 0.0120.6 ± 1.810.6 ± 0.073.7 ± 6.8
LI-BP55027.4 ± 2.142.5100.3 ± 0.271.4 ± 1.872.7 ± 3.01.5 ± 0.21.4 ± 0.080.6 ± 0.040.03 ± 0.010.2 ± 0.0422.1 ± 1.010.4 ± 0.180.4 ± 1.2
LI-BP65026.6 ± 0.840.898.7 ± 1.370.7 ± 2.574.3 ± 1.11.2 ± 0.01.3 ± 0.030.6 ± 0.050.05 ± 0.010.2 ± 0.0124.2 ± 1.810.5 ± 0.082.9 ± 5.9
FA-BS12.9 ± 0.116.898.6 ± 0.560.4 ± 2.962.8 ± 2.91.8 ± 0.10.9 ± 0.140.3 ± 0.050.03 ± 0.040.3 ± 0.0333.7 ± 4.09.9 ± 0.036.9 ± 8.2
FI-BB36.3 ± 0.655.263.8 ± 1.170.1 ± 1.771.2 ± 2.51.3 ± 0.11.3 ± 0.070.4 ± 0.040.07 ± 0.010.2 ± 0.0319.6 ± 1.09.6 ± 0.132.1 ± 1.3
FI-CP20.5 ± ± 0.172.4 ± 0.371.6 ± 2.11.0 ± 0.01.3 ± 0.050.4 ± 0.01−0.02 ± 0.020.1 ± 0.0127.1 ± 2.610.8 ± 0.052.7 ± 1.5
1 NA = not applicable. 2 NM = not measured.
Table 3. PAH concentrations of the produced biochars from farm-scale equipment together with the EBC-Agro limit values.
Table 3. PAH concentrations of the produced biochars from farm-scale equipment together with the EBC-Agro limit values.
EPA8 (mg∙kg−1)EPA16 (mg∙kg−1)
EBC-Agro1.06.0 ± 2.2
1 BDL = below detection limit.
Table 4. Heavy-metal concentrations of the grass feedstock and produced biochars on a dry basis. For all parameters, the mean value and the standard deviation of the mean are presented (n = 3). Additionally, the limit values for the contents in biochar proposed by the EBC for agricultural application are given. Detection limit 0.01 mg∙kg−1 for all elements, except Hg with a detection limit of 0.05 µg∙kg−1, of fresh matter (FM) biochar.
Table 4. Heavy-metal concentrations of the grass feedstock and produced biochars on a dry basis. For all parameters, the mean value and the standard deviation of the mean are presented (n = 3). Additionally, the limit values for the contents in biochar proposed by the EBC for agricultural application are given. Detection limit 0.01 mg∙kg−1 for all elements, except Hg with a detection limit of 0.05 µg∙kg−1, of fresh matter (FM) biochar.
Grass0.18 ± 0.060.12 ± 0.031.08 ± 0.666.57 ± 1.39<0.5 10.43 ± 0.140.68 ± 0.3753.69 ± 14.12
LH-BP0.19 ± 0.100.37 ± 0.252.67 ± 1.1710.82 ± 0.89<0.5 13.27 ± 2.490.91 ± 0.5475.48 ± 7.29
LA-BMNM 23.02 ± 0.000.89 ± 0.0017.99 ± 0.00NM 23.34 ± 0.00<0.01 1260.43 ± 0.00
LI-BP4501.02 ± 0.300.33 ± 0.112.08 ± 0.5518.40 ± 3.34<0.5 11.92 ± 0.212.02 ± 1.51235.90 ± 149.77
LI-BP5501.00 ± 0.340.20 ± 0.142.21 ± 0.7020.59 ± 2.56<0.5 12.06 ± 0.103.14 ± 0.87195.37 ± 84.98
LI-BP6501.01 ± 0.270.17 ± 0.052.14 ± 0.5433.88 ± 11.72<0.5 12.23 ± 0.112.65 ± 0.58188.94 ± 31.34
FA-BSNM 21.09 ± 0.2915.99 ± 7.7225.06 ± 1.49NM 214.95 ± 8.643.89 ± 1.59241.68 ± 34.87
FI-BB0.90 ± 0.070.17 ± 0.0344.51 ± 15.3319.09 ± 0.88NM 237.90 ± 3.54<0.01 1160.65 ± 10.98
FI-CP0.99 ± 0.050.12 ± 0.013.63 ± 1.3919.09 ± 0.45NM 22.69 ± 0.40<0.01 1147.63 ± 1.09
1 Below detection limit. 2 NM = not measured.
Table 5. The retention of overall ash in the produced biochar as well as that of the individual elements as a percentage (%) of the content in the grass feedstock.
Table 5. The retention of overall ash in the produced biochar as well as that of the individual elements as a percentage (%) of the content in the grass feedstock.
Percentage of Element (%)
LA-BM69.5NA 170.621.968.869.0NA 182.171.678.866.872.429.841.636.747.7
FA-BS53.7NA 113.7213.751.7166.7NA 141.142.734.039.444.7333.2NA 113.414.1
FI-BB93.7NA 147.52653.7103.13489.2NA 184.6107.187.8103.291.981.534.345.461.7
1 NA = not applicable.
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Heinrich, T.; Kaetzl, K.; Libra, J.A.; Hoffmann, T. Influence of Thermochemical Conversion Technologies on Biochar Characteristics from Extensive Grassland for Safe Soil Application. Energies 2023, 16, 1896.

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Heinrich T, Kaetzl K, Libra JA, Hoffmann T. Influence of Thermochemical Conversion Technologies on Biochar Characteristics from Extensive Grassland for Safe Soil Application. Energies. 2023; 16(4):1896.

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Heinrich, Thomas, Korbinian Kaetzl, Judy A. Libra, and Thomas Hoffmann. 2023. "Influence of Thermochemical Conversion Technologies on Biochar Characteristics from Extensive Grassland for Safe Soil Application" Energies 16, no. 4: 1896.

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