Biochar is the black carbonaceous residue formed from the thermochemical conversion of biomass in an inert atmosphere [1]; these materials need to have similar or superior performance than commercially used activated carbons for their implementation across different applications. This is achieved by a combination of cheap availability of feedstock, accompanied by diverse physical and chemical properties, giving carbon-rich biochars the potential to be used in a range of applications [2], including soil amendment, as enrichment fertilisers [3, 4], catalysts [5], adsorbents [3], and in energy storage [6].
Pyrolysis is frequently used to produce biochars from biomass and wood substrates [2]. The process can be divided into two classes: fast pyrolysis is governed by a high heating rate (10–200°C/s) with a residence time of a few seconds, primarily to produce bio-oils, while slow pyrolysis comprises a lower heating rate (1-100°C/min) with contact times ranging from a few minutes to several days and is the preferred route in biochar production [7, 8]. Surface area and porosity are the commonest determining factors of biochar performance, as the availability of active sites and accessible internal volume govern interaction potentials important in adsorption and cation exchange, while also influencing water holding capacities [9].
Over the years, a plethora of materials have been used to produce carbon- based materials. The abundance of wastes produced from households and agricultural residues, and the feasibility to transform these into carbon materials, has attracted researchers to work towards the concept of a circular economy within biochars production. Biochars have well-developed pore networks, ranging from micro- to macropores, and high surface areas that make them suitable for adsorption [10]. The pore network extends throughout the material and provides active binding sites for heavy metals that readily sorb on the surface and within the pore network. Biochars made from renewable sources gave comparable adsorption capacities to commercial activated carbons, even though the surface areas were significantly smaller [11]. As a consequence of these characteristics, many biochars have been applied in water treatment processes, extending the circularity of their manufacture. Table 1 summarises different feedstock used to produce biochars and their application in water treatment systems.
Table 1
Different feedstock for biochar production and target compounds in water treatment
Feedstock for biochar production | Pollutant species | Reference |
Grape bagasse | Copper | [12] |
Modified waste potato peels, commercial coffee waste | Cobalt ions, heavy metals | [13, 14] |
Walnut wood | Lead and methylene blue | [15] |
Rosid angiosperm | Metaldehyde | [16] |
Peanut shell | Metal ion | [17] |
Anaerobically digested biomass | Heavy metal | [18] |
Hazelnut shell | Chromium (VI) | [19] |
Apple wastes | Heavy metal | [20] |
In addition to water treatment, there are several other areas where biochars have been tested. For example, biochar based acid catalysts derived from rice husk [21], coconut shells [22] and pyrolysed hard wood [23] have found potential application in biodiesel production; biochars as soil enhancement materials can maintain nutrients within soil and control cation exchange, which reduces nutrient leaching from soils [11], while potassium hydroxide activated biochar could be used within supercapacitors [24]. There are several parameters involved in the production of biochars: operating temperature, gas flowrate, residence time, furnace ramp rate, and pressure that can influence the yield and quality of the final product. The influence of these key parameters are outlined below, allowing for variable selection to obtain optimised biochars materials.
Pyrolysis temperature is considered one of the key factors influencing the properties of biochars; the breakdown of heavy hydrocarbons decreases the quantity of the final product, as more volatiles are removed from the system [25]. Many researchers have reported a reduction in biochar yield on increasing the pyrolysis temperature, which is expected [16, 26, 27] as, at high temperatures, secondary reactions occur that further breakdown the char formed at initial temperatures into liquid and gaseous phases, i.e. releasing more volatile components [28]. While higher temperatures enables the development of micropores and an enhanced pore structure [8], a disadvantage of extreme temperatures is that the formation of ash hinders the growth of the pore network and surface area [29], and a fine balance exists in determining eh optimal temperature for biochars formation. By contrast, too low a temperature can result in insignificant changes in pore volume and surface area, as the system is unable to completely devolatilise volatile constituents, and the final product may be subject to pore blockage and an underdeveloped pore network [30]. Previous studies indicate that a temperature range between 400 and 800°C is most appropriate for biochar production.
During pyrolysis, vapours are formed, and these can participate in reactions with the char, modifying its characteristics if not purged from the system [28]. Carrier gases are used to ensure an inert atmosphere for pyrolysis, and nitrogen is the most common carrier gas used being cheaper and more readily available than other inert gases. Increased gas flowrate has been shown to marginally decrease the biochar yield, due to the removal of vapours from the system, preventing repolymerisation [28]; previous work has shown a reduction in yield from 28.4% to ~ 27% on increasing the nitrogen flowrate from 50 to 400 mL/min [31], with similar observations for other systems [12, 32, 33], suggesting that low to moderate flowrates will produce little effect on yield. By contrast, gas flowrate has been shown to markedly affect surface area and total pore volume, with an increase in nitrogen flowrate (50 to 150 mL/min) reported to cause an increase of > 300 m2/g in surface area and a ten-fold increase in total pore volume for Algerian date pits derived activated carbon [34]. Notably, very high gas flowrates decrease biochar yield and pore volume [34, 35], hence, moderate gas flowrates between 150 and 300 mL/min are suggested for optimum characteristics.
A low heating rate mitigates the possibility of thermal cracking of biomass and rules out secondary pyrolysis reactions to enhance the biochar yield [28]. A very high heating rate would melt the biochar particles and increase the gaseous and liquid components, thereby decreasing the quantity of the final product [2]. An excessive heating rate also results in accumulation within particles, resulting in blocked pore entrances, due to shortage of time for the volatile matter to diffuse [36], while depolymerisation of biomass and prevalence of secondary pyrolysis result in a reduced biochar yield [37, 38], and can decrease surface area [39]. To avoid micropore coalescence or collapse of the carbon matrix altogether, a high rate of volatile matter generation must be avoided [2], which rules out the use of high heating rates, hence, an optimum range of 10 and 30°C/min is preferred. Residence time is influenced by temperature, gas flowrate and heating rate; to promote repolymerisation, and improve biochar yield, sufficient residence time is necessary for reaction [40], however, several researchers have reported that the yield is not proportional to residence time [41, 42]. Residence times between 30 and 60 min have been reported to yield maximum pore volume for chemically activated biochars from corn cob [43], while an increase in the surface area was reported by for residence time increasing from 10 to 60 min [44]; however further increase reduced surface area. Complications, arising from interaction between other process conditions and residence time, make it a challenging parameter to analyse; hence, it is a key component to investigate during biochar production with residence times between 20 and 60 min being of interest. By contrast the influence of pressure on biochar production is relatively straightforward. Extreme, high pressures prevent the release of volatile matter from the system and result in the formation of spherical cavities [45], with continuous decrease in surface areas reported upon increasing the pressure from 1 to > 20 bar [45, 46]. Pressures slightly higher than atmospheric pressure can increase the residence time of reaction constituents, which assists char formation [47], and carbon content in the final product was suggested to be pressure dependent [28]. Feedstock with different physical and chemical compositions, react differently to operational parameters and produce biochars with variability in characteristics [48]. Previous studies have discussed the relationship between biochar performance and process parameters [49, 50]. However, there is a limitation and lack of understanding of simultaneous influence of these parameters on produced biochars. In order to understand the optimal conditions required to produce target characteristics in biochars obtained from waste sources, the effects of these process parameters on biochars produced from Scottish woods were investigated in this study. The raw materials are available in abundance and procured locally, therefore, there is a considerable reduction in carbon footprint associated with supply and transport, offering the potential for circularity in the formation of biochars materials for local applications.