$Hg < 1 \text{ mg}\cdot\text{kg}^{-1}$; $Zn < 400 \text{ mg}\cdot\text{kg}^{-1}$; $Cr < \text{mg}\cdot\text{kg}^{-1}$ on dry basis. As all biochars passed the quality test with respect to heavy metals and dioxins/PCB/dioxins and furans we chose to not discuss these in more detail.

**4. Conclusions and Prospects for Biochar Research**

In a lab-scale pyrolyzer and two different size lab-scale gasifiers ECN has co-produced biochar and energy from agro- and forest residues with the aim to study the quality of the biochars and try to find relationships between the production parameters and the chemical and physical properties of the biochar most important to agronomy. In our work all biochars produced demonstrate molar ratios H/C <0.6, and O/C <0.4, required by the IBI definition of biochar. The 16 PAH measurement showed all biochars to be of sufficient quality or even premium quality with respect to poly-hydrocarbon presence and heavy trace metals concentration were found to be well below high quality standards. The overall sufficient quality was a precondition for further work and this condition was met. The following conclusions can therefore zoom in to more complex relationships between production and biochar quality that need to be understood in order to be able to design biochars for certain applications or soil conditions.

In Section 1.2, a number of production parameters were identified from the literature review as being important for the quality and variation in biochar characteristics.

*The type of feedstock* (woody vs. herbaceous) proved to be an important parameter on the chemical and physical characteristics of biochar. Even at the same process production conditions, the wide range of input or starting parameters leads to the formation of biochar products that vary considerably in their elemental and ash composition, density, porosity, pore size distribution, surface area, pH and uniformity of biochars' physical structure. The wood biochars exhibit pH values lower than the agro waste biochars, at the same time coupled with higher H/C ratios, while the manures feedstock are among the very highest pH-chars. The data suggest that a greater extent of carbon enrichment occurred during gasification of wood relative to other tested feedstock, however, the effect proved to be at least partly due to the ash content, with the lowest ash (wood) feedstock having lower pH than the feedstock with high initial alkali plus Ca content. When looking at the type of feedstock, a significant effect is visible on the water holding capacity (WHC) of biochar. In this study, the lowest WHC is for the beech feedstock, which could, however, be increased significantly by using a blend of a mass fraction 80% beech and 20% agro-residue. The relation of pore volume with WHC was only present within the dataset of one feedstock while varying the feedstock covered a much wider range of WHC's than described by pore volume alone. This shows the major impact of the initial feedstock but also that the WHC cannot be solely related to the pore volume of the biochar when changing from one feedstock to another. The influence of the initial feedstock is considered a first-order parameter in the quality and characteristics of the biochar produced. More detailed attention on the starting material in future research would be very beneficial for the interpretation of the resulting char characteristics, in our opinion, based on this study.

*The temperature of production* has been shown in literature to be the most important parameter in the ratio of labile to stabile carbon and by being the key parameter for devolatilization processes. In this study it also largely determines the H/C and O/C ratios in the biochar as was expected. The most convincing argument can be found in the van Krevelen diagram (Figure 3) where the trend is from the feedstock ratios towards the origin of the graph with ever increasing temperatures. The yield of the amount of char *versus* product gas decreased with increasing temperature as can be expected. It was also
found that the higher temperature biochars have a larger WHC due to higher pore volumes hence the relation: Higher temperature> larger pore volume> larger water holding capacity within the dataset of one feedstock. The temperature, within the range investigated, is also considered a first-order parameter in the quality and characteristics of the biochar produced.

*Variation in process (especially gaseous environment)*. The largest differences are between pyrolysis and gasification for the few parameters investigated, such as the ratio (yield) of biochar and energy production. It is nearly impossible to compare pyrolysis *versus* gasification with the gaseous environment as the only variable. Especially temperatures of operations but most certainly residence times are very different for the two processes. In this study more detailed work was then performed within the gasifiers in varying the gaseous environment. Indicatively it was illustrated that for one feedstock at 600 °C the effect of the gaseous environment is very small. The effect at 750 °C is, however, not negligible and both pore volume and WHC will increase going from N₂ to steam and then to air. Although the temperature seems to be a first order parameter in this case, the gaseous environment can become important for fine-tuning the char quality with respect to porosity and WHC. To prove this beyond this first indication, more experiments are needed where the change in gaseous environment is the only variable. Within the range investigated the gaseous environment is considered only of secondary importance as a hypothesis for further work but a possibility for fine-tuning the char quality.

*Residence time* is a parameter that is usually very different for slow-pyrolysis and the fluidized bed gasification processes used in this study. However, only a few experiments were performed at 600 °C which does not allow any comparison beyond the general quantities of biochar and energy produced. In general, residence time will have a larger effect at lower temperatures. It is however, worth considering this parameter for further research.

*Practical Procedures*. In this study it was shown that the PAH concentrations is not solely related to the production process but also to contact-time of gas and char and the local temperature condition (still to reach the peak temperature or already in the cooling range). Taking the char out from the bed-section of the gasifier, after some residence time at elevated temperature, prevented the condensation of tars from the gas on the chars. Tars that were obviously present in the reactor, proven by the cyclone chars of the same experiment and hence the same char-production process and the same process conditions. In general PAH content was low in all gasification obtained biochars, by separating the char and the produced gas at an early stage. The bed-derived gasification chars result in 16 PAH values 1 mg·kg⁻¹ to 2 mg·kg⁻¹ or even less compared to values of 8 mg·kg⁻¹ to 23 mg·kg⁻¹ PAH for the pyrolysis samples. Reactor design for char production should take into account best practice in practical procedures, such as early separation of char and gas, as much can be gained in terms of quality of the biochar without necessarily leading to extra costs. The production of biochar by gasification is still relatively novel, but as shown in this work, the data on the resulting biochar qualifications are highly encouraging.

Currently the relationship between biochar properties (physical properties and chemical functionalities on surface) and its applicability as a soil amendment does not yet allow a set of definite process conditions to produce biochar with desired characteristics, but every study providing solid experimental work will contribute towards this end. The need for collaboration among researchers working in different disciplines is very strong. The coupling between the char production/characteristics and the studies on biochar amendment and its effects on soil and on crop yields is essential. Only with the feed-back from soil-scientist and
found that the higher temperature biochars have a larger WHC due to higher pore volumes hence the relation: Higher temperature> larger pore volume> larger water holding capacity within the dataset of one feedstock. The temperature, within the range investigated, is also considered a first-order parameter in the quality and characteristics of the biochar produced.

*Variation in process (especially gaseous environment)*. The largest differences are between pyrolysis and gasification for the few parameters investigated, such as the ratio (yield) of biochar and energy production. It is nearly impossible to compare pyrolysis *versus* gasification with the gaseous environment as the only variable. Especially temperatures of operations but most certainly residence times are very different for the two processes. In this study more detailed work was then performed within the gasifiers in varying the gaseous environment. Indicatively it was illustrated that for one feedstock at 600 °C the effect of the gaseous environment is very small. The effect at 750 °C is, however, not negligible and both pore volume and WHC will increase going from N₂ to steam and then to air. Although the temperature seems to be a first order parameter in this case, the gaseous environment can become important for fine-tuning the char quality with respect to porosity and WHC. To prove this beyond this first indication, more experiments are needed where the change in gaseous environment is the only variable. Within the range investigated the gaseous environment is considered only of secondary importance as a hypothesis for further work but a possibility for fine-tuning the char quality.

*Residence time* is a parameter that is usually very different for slow-pyrolysis and the fluidized bed gasification processes used in this study. However, only a few experiments were performed at 600 °C which does not allow any comparison beyond the general quantities of biochar and energy produced. In general, residence time will have a larger effect at lower temperatures. It is however, worth considering this parameter for further research.

*Practical Procedures*. In this study it was shown that the PAH concentrations is not solely related to the production process but also to contact-time of gas and char and the local temperature condition (still to reach the peak temperature or already in the cooling range). Taking the char out from the bed-section of the gasifier, after some residence time at elevated temperature, prevented the condensation of tars from the gas on the chars. Tars that were obviously present in the reactor, proven by the cyclone chars of the same experiment and hence the same char-production process and the same process conditions. In general PAH content was low in all gasification obtained biochars, by separating the char and the produced gas at an early stage. The bed-derived gasification chars result in 16 PAH values 1 mg·kg⁻¹ to 2 mg·kg⁻¹ or even less compared to values of 8 mg·kg⁻¹ to 23 mg·kg⁻¹ PAH for the pyrolysis samples. Reactor design for char production should take into account best practice in practical procedures, such as early separation of char and gas, as much can be gained in terms of quality of the biochar without necessarily leading to extra costs. The production of biochar by gasification is still relatively novel, but as shown in this work, the data on the resulting biochar qualifications are highly encouraging.

Currently the relationship between biochar properties (physical properties and chemical functionalities on surface) and its applicability as a soil amendment does not yet allow a set of definite process conditions to produce biochar with desired characteristics, but every study providing solid experimental work will contribute towards this end. The need for collaboration among researchers working in different disciplines is very strong. The coupling between the char production/characteristics and the studies on biochar amendment and its effects on soil and on crop yields is essential. Only with the feed-back from soil-scientist and
agricultural scientists will it be possible to establish the appropriate process conditions to produce a biochar with desired characteristics and increase the success rate of biochar applications.

## Acknowledgments

The work presented in this paper was carried out with financial support from the EU FP7 project FERTIPLUS (Grant Agreement N° 289853), the Dutch national TO2 scheme project Optimalisatie Glastuinbouw (Optimizing greenhouse horticulture) and the Dutch Ministry of Economic affairs. We would like to thank and acknowledge a number of people whose help was very important in the realization of this work. Herman Bodenstaff, Dennis Slort, Peter Heere and Edwin Brouwer for preparing and carrying out the tests at the lab-scale installations at ECN, Marco Geusebroek and Arnold Toonen for preparing and carrying out the sampling (gas, solid, liquid). Finally we kindly thank the reviewers and editor of this journal for their constructive remarks and suggestions.

## Author Contributions

The authors of this manuscript have equally contributed to its realization.

## Conflicts of Interest

The authors declare no conflict of interest.

## References

1. Laird, D.A. The charcoal vision: A win-win-win scenario for simultaneously producing bioenergy, permanently sequestering carbon, while improving soil and water quality. *Agronomy* **2008**, *100*, 178–181.

2. Montanarella, L.; Lugato, E. The Application of Biochar in the EU: Challenges and Opportunities, Review. *Agronomy* **2013**, *3*, 462–473.

3. Field, J.L.; Keske, C.M.H.; Birch, G.L.; Defoort, M.W.; Cotrufo, M.F. Distributed biochar and bioenergy coproduction: a regionally specific case study of environmental benefits and economic impacts. *GCB Bioenergy* **2013**, *5*, 177–191.

4. Lehmann, J.; Joseph, S. *Biochar for Environmental Management: Science and Technology*; Earthscan: Sterling, VA, USA, 2012; pp. 127–146.

5. Sohi, S.; Lopez-Capel, E.; Krull, E.; Bol, R. Biochar's roles in soil and climate change: A review of research needs. *CSIRO Land Water Sci. Rep.* **2009**, *5*, 1–57.

6. Kimetu, J.M.; Lehmann, J.; Ngoze, S.O.; Mugendi, D.N.; Kinyangi, J.M.; Riha, S.; Verchot, L.; Recha, J.W.; Pell, A.N. Reversibility of Soil Productivity Decline with Organic Matter of Differing Quality Along a Degradation Gradient. *Ecosystem* **2008**, *11*, 726–739.

7. Gaskin, J.W.; Speir, R.A.; Harris, K.; Das, K.C.; Lee, R.D.; Morris, L.A.; Fisher, D.S. Effect of peanut hull and pine chip biochar on soil nutrients, corn nutrient status, and yield. *Agrono. J.* **2010**, *102*, 623–633.

8. Major, J.; Rondon, M.; Molina, D. Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol. *Plant and Soil* **2010**, *333*, 117–128.
agricultural scientists will it be possible to establish the appropriate process conditions to produce a biochar
with desired characteristics and increase the success rate of biochar applications.

Acknowledgments

The work presented in this paper was carried out with financial support from the EU FP7 project FERTIPLUS (Grant Agreement N° 289853), the Dutch national TO2 scheme project Optimalisatie Glastuinbouw (Optimizing greenhouse horticulture) and the Dutch Ministry of Economic affairs. We would like to thank and acknowledge a number of people whose help was very important in the realization of this work. Herman Bodenstaff, Dennis Slort, Peter Heere and Edwin Brouwer for preparing and carrying out the tests at the lab-scale installations at ECN, Marco Geusebroek and Arnold Toonen for preparing and carrying out the sampling (gas, solid, liquid). Finally we kindly thank the reviewers and editor of this journal for their constructive remarks and suggestions.

Author Contributions

The authors of this manuscript have equally contributed to its realization.

Conflicts of Interest

The authors declare no conflict of interest.

References

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27 Jeffery, S.; Verheijen, F.G.A. A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis. *Agric. Ecosyst. Environ.* **2011**, *144*, 175–187.

28 Ojeda, G.; Mattana, S.; Àvila, A.; Alcañiz, J.M.; Volkmann, M.; Bachmann, J. Are soil-water functions affected by biochar application? *Geoderma* **2015**, *249*, 1–11.

29 Yao, Y.; Gao, B.; Zhang, M.; Inyang, M.; Zimmerman, R.A. Short Communication, Effect of Biochar Amendment on Sorption and Leaching of Nitrate, Ammonium, And Phosphate in a Sandy Soil. *Chemosphere* **2012**, *89*, 1467–1471.

30 Mimmo, T.; Panzacchi, P.M.; Baratieri, C.A.; Davies, G. Effect of pyrolysis temperature on *Miscanthus* (*Miscanthus × giganteus*) biochar physical, chemical and functional properties. *Biomass Bioenergy* **2014**, *62*, 149–157.

31 Manya, J.J. Pyrolysis for biochar purposes: A review to establish current knowledge gaps and research needs. *Environ. Sci. Technol.* **2012**, *46*, 7939–7954.

32 Mukherjee, A.; Zimmerman, A.R.; Harris, W. Surface chemistry variations among a series of laboratory-produced biochars. *Geoderma* **2011**, *163*, 247–255.

33 Li X.; Shen, Q.; Zhang, D.; Mei, X.; Ran, W. Functional Groups Determine Biochar Properties (pH and EC) as Studied by Two-Dimensional ¹³C NMR Correlation Spectroscopy. *PLoS ONE* **2013**, doi:10.1371/journal.pone.0065949.

34 Sun, H.; Hockaday, W.C.; Masiello, C.A.; Zygourakis, K. Multiple Controls on the Chemical and Physical Structure of Biochars. *Ind. Eng. Chem. Res.* **2012**, *51*, 3587–3597.

35 Crombie, K.; Mašek, O. Pyrolysis biochar systems, balance between bioenergy and carbon sequestration. *GCB Bioenergy* **2015**, *7*, 349–361.

36 Mašek, O.; Brownsort, P.; Cross, A.; Sohi, S. Influence of production conditions on the yield and environmental stability of biochars. *Fuel.* **2013**, *103*, 151–155.

37 Budai, A.; Wang, L.; Gronli, M.; Strand, L.T.; Antal, M.J.; Abiven, S.; Dieguez-Alonso, A.; Anca-Couce, A.; Rasse, D.P. Surface Properties and Chemical Composition of Corncob and *Miscanthus* Biochars: Effects of Production Temperature and Method. *J. Agric. Food Chem.* **2014**, *62*, 3791–3799.

38 Sohi, S.P.; Krull, E.; Lopez-Capel, E.; Bol, R. A Review of Biochar and Its Use and Function in Soil. *Adv. Agron.* **2010**, *105*, 47–82.

39 Singh, B.; Singh, B.P.; Cowie, A.L. Characterisation and evaluation of biochars for their application as a soil amendment. *Soil Res.* **2010**, *48*, 516–525.

40 Novak, J.M.; Lima, I.; Xing, B.; Gaskin, J.W.; Steiner, C.; Das, K.; Ahmedna, M.; Rehrah, D.; Watts, D.W.; Busscher, W.J. Characterization of designer biochar produced at different temperatures and their effects on a loamy sand. *Annals. Environ. Sci.* **2009**, *3*, 195–206.

41 Karhu, K.; Mattila, T.; Bergström, I.; Regina, K. Biochar addition to agricultural soil increased CH₄ uptake and water holding capacity—Results from a short-term pilot field study. *Agr. Ecosyst. Environ.* **2011**, *140*, 309–313.

42 Yu, O.Y.; Raichle, B.; Sink, S. Impact of biochar on the water holding capacity of loamy sand soil. *Int. J. Energy Environ. Eng.* **2013**, *4*, 1–9.
43 Zhang, J.; You, C. Water Holding Capacity and Absorption Properties of Wood Chars. *Energy Fuels* **2013**, *27*, 2643–2648.

44 Akhtar, S.S.; Li, G.; Andersen, M.N.; Liu, F. Biochar enhances yield and quality of tomato under reduced irrigation. *Agric. Water Manag.* **2014**, *138*, 37–44.

45 Kinney, T.J.; Masiello, C.A.; Dugan, B.; Hockaday, W.C.; Dean, M.R.; Zygourakis, K.; Barnes, R.T. Hydrologic properties of biochars produced at different temperatures. *Biomass Bioenergy* **2012**, *41*, 34–43.

46 Gray, M.; Johnson, M.; Dragila, M.I.; Kleber, M. Water uptake in biochars: The roles of porosity and Hydrophobicity. *Biomass Bioenergy* **2014**, *61*, 196–205.

47 Jeffery, S.; Bezemer, T.M.; Cornelis, G.; Kuyper, T.W.; Lehmann, J.; Mommer, L.; Saran, P.S.; van de Voorde, T.F.J.; Wardlevand, D.A.; Groenigen, J.W. The way forward in biochar research: targeting trade-offs, between the potential wins. *GCB Bioenergy* **2015**, *7*, 1–13.

48 Shen, W.; Li, Z.; Liu, Y. Surface Chemical Functional Groups Modification of Porous Carbon. *Recent Pat. Chem. Eng.* **2008**, *1*, 27–40.

49 Morales, V.L.; Pérez-Reche, F.J.; Hapca, S.M.; Hanley, K.L.; Lehmann, J.; Zhang, W. Reverse Engineering of Biochar. *Bioresour. Technol.* **2015**, *183*, 163–174.

50 IBI. *Standardized Product Definition and Product Testing Guidelines for Biochar that Isused in Soil*; The International Biochar Initiative (IBI): Philadelphia, PA, USA, 2012.

51 EBC. *European Biochar Certificate: Guidelines for a Sustainable Production of Biochar*; European Biochar Foundation (EBC): Arbaz, Switzerland, 2015.

52 Peterson, S.C.; Jackson, M.A. Simplifying pyrolysis: Using gasification to produce corn stover and wheat straw biochar for sorptive and horticultural media. *Ind. Crop. Prod.* **2014**, *53*, 228–235.

53 Lugato, E.; Vaccari, F.; Genesio, L.; Baronti, S.; Pozzi, A.; Rack, M.; Woods, J.; Simonetti, G.; Montanarella, L.; Miglietta, F. An energy-biochar chain involving biomass gasification and rice cultivation in Northern Italy. *GCB Bioenergy* **2012**, doi:10.1111/gcbb.12028.

54 *The Biomass Environmental Assessment Tool*, 2nd ed.; Provided by Defra, the Biomass Energy Centre and the Environment Agency as an aid to assessing the likely environmental impact of a biomass energy project; Defra: London, UK, 1990.

55 Tsai, W.T.; Liu, S.C.; Chen, H.R.; Chang, Y.M.; Tsai, Y.L. Textural and chemical properties of swine-manure-derived biochar pertinent to its potential use as a soil amendment. *Chemosphere* **2012**, *89*, 198–203.

56 Gaunt J.L.; Lehmann, J. Energy Balance and Emissions Associated with Biochar Sequestration and Pyrolysis Bioenergy Production. *J. Environ. Sci. Technol.* **2008**, *42*, 4152–4158.

57 Galinato, S.P.; Yoder, J.K.; Granatstein, D. The economic value of biochar in crop production and carbon sequestration. *Energy Policy* **2011**, *39*, 6344–6350.

58 Ahrenfeldt, J.; Thomsen, T.P.; Henriksen, U.; Clausen, R. Biomass gasification cogeneration: A review of state of the art technology and near future perspectives. *Appl. Therm. Eng.* **2013**, *50*, 1407–1417.

59 Hansen, V.; Müller-Stöver, D.; Ahrenfeldt, J.; Holm, J.K.; Henriksen, U.B.; Hauggaard-Nielsen, H. Gasification biochar as a valuable by-product for carbon sequestration and soil amendment. *Biomass Bioenergy* **2015**, *72*, 300–308.
43 Zhang, J.; You, C. Water Holding Capacity and Absorption Properties of Wood Chars. *Energy Fuels* **2013**, *27*, 2643–2648.

44 Akhtar, S.S.; Li, G.; Andersen, M.N.; Liu, F. Biochar enhances yield and quality of tomato under reduced irrigation. *Agric. Water Manag.* **2014**, *138*, 37–44.

45 Kinney, T.J.; Masiello, C.A.; Dugan, B.; Hockaday, W.C.; Dean, M.R.; Zygourakis, K.; Barnes, R.T. Hydrologic properties of biochars produced at different temperatures. *Biomass Bioenergy* **2012**, *41*, 34–43.

46 Gray, M.; Johnson, M.; Dragila, M.I.; Kleber, M. Water uptake in biochars: The roles of porosity and Hydrophobicity. *Biomass Bioenergy* **2014**, *61*, 196–205.

47 Jeffery, S.; Bezemer, T.M.; Cornelis, G.; Kuyper, T.W.; Lehmann, J.; Mommer, L.; Saran, P.S.; van de Voorde, T.F.J.; Wardlevand, D.A.; Groenigen, J.W. The way forward in biochar research: targeting trade-offs, between the potential wins. *GCB Bioenergy* **2015**, *7*, 1–13.

48 Shen, W.; Li, Z.; Liu, Y. Surface Chemical Functional Groups Modification of Porous Carbon. *Recent Pat. Chem. Eng.* **2008**, *1*, 27–40.

49 Morales, V.L.; Pérez-Reche, F.J.; Hapca, S.M.; Hanley, K.L.; Lehmann, J.; Zhang, W. Reverse Engineering of Biochar. *Bioresour. Technol.* **2015**, *183*, 163–174.

50 IBI. *Standardized Product Definition and Product Testing Guidelines for Biochar that Isused in Soil*; The International Biochar Initiative (IBI): Philadelphia, PA, USA, 2012.

51 EBC. *European Biochar Certificate: Guidelines for a Sustainable Production of Biochar*; European Biochar Foundation (EBC): Arbaz, Switzerland, 2015.

52 Peterson, S.C.; Jackson, M.A. Simplifying pyrolysis: Using gasification to produce corn stover and wheat straw biochar for sorptive and horticultural media. *Ind. Crop. Prod.* **2014**, *53*, 228–235.

53 Lugato, E.; Vaccari, F.; Genesio, L.; Baronti, S.; Pozzi, A.; Rack, M.; Woods, J.; Simonetti, G.; Montanarella, L.; Miglietta, F. An energy-biochar chain involving biomass gasification and rice cultivation in Northern Italy. *GCB Bioenergy* **2012**, doi:10.1111/gcbb.12028.

54 *The Biomass Environmental Assessment Tool*, 2nd ed.; Provided by Defra, the Biomass Energy Centre and the Environment Agency as an aid to assessing the likely environmental impact of a biomass energy project; Defra: London, UK, 1990.

55 Tsai, W.T.; Liu, S.C.; Chen, H.R.; Chang, Y.M.; Tsai, Y.L. Textural and chemical properties of swine-manure-derived biochar pertinent to its potential use as a soil amendment. *Chemosphere* **2012**, *89*, 198–203.

56 Gaunt J.L.; Lehmann, J. Energy Balance and Emissions Associated with Biochar Sequestration and Pyrolysis Bioenergy Production. *J. Environ. Sci. Technol.* **2008**, *42*, 4152–4158.

57 Galinato, S.P.; Yoder, J.K.; Granatstein, D. The economic value of biochar in crop production and carbon sequestration. *Energy Policy* **2011**, *39*, 6344–6350.

58 Ahrenfeldt, J.; Thomsen, T.P.; Henriksen, U.; Clausen, R. Biomass gasification cogeneration: A review of state of the art technology and near future perspectives. *Appl. Therm. Eng.* **2013**, *50*, 1407–1417.

59 Hansen, V.; Müller-Stöver, D.; Ahrenfeldt, J.; Holm, J.K.; Henriksen, U.B.; Hauggaard-Nielsen, H. Gasification biochar as a valuable by-product for carbon sequestration and soil amendment. *Biomass Bioenergy* **2015**, *72*, 300–308.
60 Shackley, S.; Carter, S.; Knowles, T.; Middelink, E.; Haefele, S.; Sohi, S.; Cross, A.; Haszeldine, S. Sustainable gasification–biochar systems? A case-study of rice-husk gasification in Cambodia, Part I: Context, chemical properties, environmental and health and safety issues. *Energy Policy* **2012**, *42*, 49–58.

61 Shackley, S.; Carter, S.; Knowles, T.; Middelink, E.; Haefele, S.; Haszeldine, S. Sustainable gasification–biochar systems? A case-study of rice-husk gasification in Cambodia, Part II: Field trial results, carbon abatement, economic assessment and conclusions. *Energy Policy* **2012**, *41*, 618–623.

62 Project FERTIPLUS ‘Reducing mineral fertilizers and agro-chemicals by recycling treated organic waste as compost and bio-char’. Available online: http://www.fertiplus.eu (accessed on 26 October 2015).

63 Beuth Standard Solutions. Available online: http://www.beuth.de/en/ (accessed on 26 October 2015).

64 BSI Shop. Available online: http://shop.bsigroup.com/Navigate-by/Standards (accessed on 26 October 2015).

65 Dong, C.D.; Chen, F.; Chen, C.W. Determination of Polycyclic Aromatic Hydrocarbons in Industrial Harbor Sediments by GC-MS. Int. *J. Environ. Res. Public Health* **2012**, *9*, 2175–2188.

66 Van Krevelen, D.W. Graphical-statistical method for the study of structure and reaction processes of coal. *Fuel* **1950**, *29*, 269–84.

67 Spokas, K.A. Review of the stability of biochar in soils: Predictability of O:C molar ratios. *Carbon Manag.* **2010**, *1*, 289–303.

68 Buss, W.; Mašek, O.; Graham, M.; Wüst, D. Inherent organic compounds in biochar—Their content, composition and potential toxic effects, *J. Environ. Manag.* **2015**, doi:10.1016/j.jenvman.2015.03.035.

69 Hale, S.E.; Lehmann, J.; Rutherford, D.; Zimmerman, A.R.; Bachmann, R.T.; Shitumbanuma, V.; O’Toole, A.; Sundqvist, K.L.; Arp, H.P.; Cornelissen, G. Quantifying the Total and Bioavailable Polycyclic Aromatic Hydrocarbons and Dioxins in Biochars. *Environ. Sci. Technol.* **2012**, *46*, 2830–2838.

70 Spokas, K.; Novak, J.M.; Stewart, C.E.; Cantrell, K.B.; Uchimiya, M.; Du Saire, M.G.; Ro, K.S. Qualitative analysis of volatile organic compounds on biochars. *Chemosphere* **2011**, *85*, 869–882.

71 Yargicoglu, E.N.; Sadasivam, B.Y.; Reddy, K.R.; Spokas, K. Physical and chemical characterization of waste wood derived biochars. *Waste Manag.* **2015**, *36*, 256–268.

72 Yargicoglu, E.N.; Reddy, K.R. Characterization and Surface Analysis of Commercially Available Biochars for Geoenvironmental Applications. In Proceedings of the IFCEE2015, San Antonio, TX, USA, 17–21 March 2015.

73 Cantrell, K.B.; Hunt, P.G.; Uchimiya, M.; Novak, J.M.; Ro, K.S. Impact of pyrolysis temperature and manure source on physicochemical characteristics of biochar. *Bioresour. Technol.* **2012**, *107*, 419–428.

74 Park, J.; Lee, Y.; Ryu, C.; Park Y.K. Slow pyrolysis of rice straw: Analysis of products properties, carbon and energy yields. *Bioresour. Technol.* **2014**, *155*, 63–70.
60 Shackley, S.; Carter, S.; Knowles, T.; Middelink, E.; Haefele, S.; Sohi, S.; Cross, A.; Haszeldine, S. Sustainable gasification–biochar systems? A case-study of rice-husk gasification in Cambodia, Part I: Context, chemical properties, environmental and health and safety issues. *Energy Policy* **2012**, *42*, 49–58.

61 Shackley, S.; Carter, S.; Knowles, T.; Middelink, E.; Haefele, S.; Haszeldine, S. Sustainable gasification–biochar systems? A case-study of rice-husk gasification in Cambodia, Part II: Field trial results, carbon abatement, economic assessment and conclusions. *Energy Policy* **2012**, *41*, 618–623.

62 Project FERTIPLUS ‘Reducing mineral fertilizers and agro-chemicals by recycling treated organic waste as compost and bio-char’. Available online: http://www.fertiplus.eu (accessed on 26 October 2015).

63 Beuth Standard Solutions. Available online: http://www.beuth.de/en/ (accessed on 26 October 2015).

64 BSI Shop. Available online: http://shop.bsigroup.com/Navigate-by/Standards (accessed on 26 October 2015).

65 Dong, C.D.; Chen, F.; Chen, C.W. Determination of Polycyclic Aromatic Hydrocarbons in Industrial Harbor Sediments by GC-MS. Int. *J. Environ. Res. Public Health* **2012**, *9*, 2175–2188.

66 Van Krevelen, D.W. Graphical-statistical method for the study of structure and reaction processes of coal. *Fuel* **1950**, *29*, 269–84.

67 Spokas, K.A. Review of the stability of biochar in soils: Predictability of O:C molar ratios. *Carbon Manag.* **2010**, *1*, 289–303.

68 Buss, W.; Mašek, O.; Graham, M.; Wüst, D. Inherent organic compounds in biochar—Their content, composition and potential toxic effects, *J. Environ. Manag.* **2015**, doi:10.1016/j.jenvman.2015.03.035.

69 Hale, S.E.; Lehmann, J.; Rutherford, D.; Zimmerman, A.R.; Bachmann, R.T.; Shitumbanuma, V.; O’Toole, A.; Sundqvist, K.L.; Arp, H.P.; Cornelissen, G. Quantifying the Total and Bioavailable Polycyclic Aromatic Hydrocarbons and Dioxins in Biochars. *Environ. Sci. Technol.* **2012**, *46*, 2830–2838.

70 Spokas, K.; Novak, J.M.; Stewart, C.E.; Cantrell, K.B.; Uchimiya, M.; Du Saire, M.G.; Ro, K.S. Qualitative analysis of volatile organic compounds on biochars. *Chemosphere* **2011**, *85*, 869–882.

71 Yargicoglu, E.N.; Sadasivam, B.Y.; Reddy, K.R.; Spokas, K. Physical and chemical characterization of waste wood derived biochars. *Waste Manag.* **2015**, *36*, 256–268.

72 Yargicoglu, E.N.; Reddy, K.R. Characterization and Surface Analysis of Commercially Available Biochars for Geoenvironmental Applications. In Proceedings of the IFCEE2015, San Antonio, TX, USA, 17–21 March 2015.

73 Cantrell, K.B.; Hunt, P.G.; Uchimiya, M.; Novak, J.M.; Ro, K.S. Impact of pyrolysis temperature and manure source on physicochemical characteristics of biochar. *Bioresour. Technol.* **2012**, *107*, 419–428.

74 Park, J.; Lee, Y.; Ryu, C.; Park Y.K. Slow pyrolysis of rice straw: Analysis of products properties, carbon and energy yields. *Bioresour. Technol.* **2014**, *155*, 63–70.