Biochar is a charcoal-like substance produced from agriculture and forest wastes which contains 70% carbon. It is used as soil enhancer to increase fertility, prevent soil degradation and to sequester carbon in the soil. Biochar can store carbon in the soil for as many as hundreds to thousands of years. Biochar can be produced through pyrolysis, gasification and hydrothermal carbonization, which leaves bio-oil and syngas as by-products. Small scale production can be through pyrolysis using modified stoves and kilns which are low cost and relatively simple technologies. For large scale production, larger pyrolysis plants and adequate feedstocks are required which is more capital cost intensive. The main quality of biochar is its carbon-rich fine-grained,
highly porous structure and increased surface area that makes it an ideal soil amendment for carbon sequestration (Lehmann, 2007).
Biocharis a charcoal-like substance produced from agriculture and forest wastes. It has high active carbon surface area that is produced through anaerobic heating of biomass. Composition-wise, it contains 70% carbon and the remaining elements are hydrogen, oxygen and nitrogen. Biochar is used as soil enhancer to increase fertility, prevent soil degradation and to sequester carbon in the soil. It improves soil fertility by retaining water and nutrients in soil, encouraging beneficial soil organisms and thereby reducing the need for additional use of fertilizers. Biochar can store carbon in the soil for as many as hundreds to thousands of years (IBI, 2008). Biochar technology is different from the conventional charcoal production because it is highly efficient in the conversion of carbon and harmful pollutants are not released upon combustion. Hence, it is a cleaner and more efficient technology. If this technology is used sustainably, the by-products in the form of oil and gas can substitute for a cleaner and renewable fuel option.
One of the simplest ways of making biochar is through the thermal decomposition of the biomass (waste from agriculture and forest). It can be done in three different ways, namely, Pyrolysis, Gasification and Hydrothermal carbonization. In all these processes, biomass is heated at a high temperature in the absence of air. This releases the volatile gases leaving behind carbon rich biochar. During pyrolysis, a high proportion of carbon remains within the biochar giving it a very high recalcitrant nature. This increases the soil water and nutrient holding capacity (Forbes et al., 2006; Chan and Zhihong, 2009).
Biochar can be produced at small and large scales. Small scale production can be through pyrolysis using modified stoves and kilns which are low cost and relatively simple technologies. For large scale production, larger pyrolysis plants and adequate feedstocks are required which is more capital cost intensive (Pratt & Moran, 2010).
The intensity of pyrolysis determines the product and by-product obtained from the process. For example, more bio-oil and syngas are obtained when fast pyrolysis is done at high temperature, while slow pyrolysis yields more biochar than by-products. Figure 1 demonstrates the biochar production through the pyrolysis process. This not only produces bio-char but also produces clean energy like syngas and bio-oil which can be used for producing heat, power or combined heat and power.
Biochar producing cook-stoves are more popular in developing countries. The pyrolysis temperature of 450-500ºC might be difficult to attain in gasification stoves to make biochar. However, most of the stoves can produce 25-30% of biochar (by weight) from the initial feedstock. This is the maximum weight of biochar that can be obtained from the slow pyrolysis process. (Samuchit, 2010 and Brownsort, 2009).
The most sustainable way of gathering feedstock for biochar would be to use the agricultural and forestry wastes. Biochar can be feasible in a small scale industry like forest communities where woody biomass waste is readily available. Large scale biochar production can be done through the cultivation of crops, but adequate land is required for its cultivation. The greatest economic potential of biochar for carbon sequestration can be realized if crop residues or waste biomass are used rather than purpose grown crops (Roberts et al. 2010). Biochar applications has been introduced in Vietnam, Mongolia and India and cost effective approaches are being identified for widespread introduction of biochar in these countries (IBI, 2008).
When deploying biochar technology a potential barrier could be that poor and isolated communities have to be convinced to accept the new technology instead of their traditional practices. An imminent risk of this technology as identified by Biofuel Watch and others is that when promoted at a large scale with dual goals of achieving agricultural as well as environmental benefits, environmental goals may be overridden by the agricultural goals. This is due to the fact that the investors might give more value to its agronomic benefits than its carbon sequestration potential (Pratt & Moran, 2010). Development of this technology for carbon sequestration could also result in the destruction of virgin forest as they might be cleared for large scale plantation to fulfill biomass feedstock requirements.
Status of the technology
Biochar carbon sequestration is fundamentally different from other forms of bio-sequestration. The issues of permanence, land tenure, leakage, and additionally are less significant for biochar projects than for projects sequestering carbon in biomass or soil through management of plant productivity. Biochar carbon sequestration might avoid difficulties such as accurate monitoring of soil carbon which are the main barriers to inclusion of agricultural soil management in emissions trading. Using turnover rate and quantity of carbon has been suggested as a method to be used in assessment of the carbon sequestration potential (Gaunt and Cowie, 2009) and that could be done independently from biochar’s use as soil amendment or other non-fuel purposes.
Biochar technology adoption in terms of its use and scale varies widely depending upon the local and regional contexts like the type of feedstock, production technology, economic setting and the expected use of the biochar. Though there are larger scale units producing biochar from agricultural waste, recently, this technology has been applied at micro scale in the form of biochar cook stoves to small scale in the developing countries.
Figure 2 presents the status of biochar producing stoves globally.
The optimistic scenario in Figure 3 shows that the use of biochar can sequester 2.2 gigatons of carbon annually by 2050 (IBI, 2008). In agricultural soils, biochar has been experimentally shown to double grain yields, improve soil fertility and increase water retention (Luizao. et al., 2009). Although modern biochar technology is still under research, some researchers claim that it has significant potential for mitigating climate change together with generating social, economic and environmental benefits (see also below).
The technical potential of biochar is determined by a number of factors, such as, scale of production, auxiliary benefits upon its application to the soil and its sustainability. Markets for the sale of feedstock for pyrolysis process are still underdeveloped. However, the economic feasibility is largely dependent on the value of the bio-products and the differences in the cost of production. Currently, the biochar process is being implemented at a small scale in developing countries (IBI, 2008).
- Substantial amounts of carbon can be sequestered in a very stable form.
- Addition of biochar to soil has been associated with enhanced nutrient use efficiency, water holding capacity, and microbial activity.
- In the process of manufacturing biochar, both heat and gases can be captured to produce energy carriers such as electricity, hydrogen, or bio-oil. Further, other valuable co-products including wood flavoring and adhesives can also be obtained as a byproduct of biochar (Czernik and Bridgwater, 2004).
- Biochar applications sometimes disturb the physical and chemical balances of nutrients in the rhizosphere.
- Biochar generally helps the growth of undesirable weeds.
- Biochar manufacturing is relatively expensive.
- Land and wildlife habitat conservation as biochar can be used for forestry management and hence wildlife habitat conservation.
- There are health benefits as biochar stoves are more efficient and produce less air pollutants.
- Promoting biochar does not jeopardize the food security by displacing the cropland with biochar feedstock.
- Farmers can have an additional source of income through collection and sale of agri-residues.
- The grain yields in agricultural soils are shown to increase by the use of biochar.
- Use of locally available feedstock reduces dependence on fossil fuel.
- Employment opportunities can be created in the course of development of biochar technology.
- Revenue can be generated through carbon trading.
- Since biochar can be used as a fertilizer, alternative fertilizers no longer need to be purchased (imported) which helps developing countries to reduce trade and fiscal deficits.
Galinato et al. (2011) have examined the potential economic returns to farmers if they utilise biochar as a substitute for agricultural lime under three price scenarios: (1) $350.74/MT, (2) $114.05/MT, and (3) $87/MT (figure 4).
- Reduced GHG emission:Reduced use of fertilizer results in reduced emissions from production and use of other fertilizer products. Retention of nutrients like nitrogen in the soil limit consequent emission of nitrous oxide into the atmosphere. As agricultural wastes are turned into biochar, emission of methane resulting from natural decomposition of biomass is reduced. By 2100, use of biochar can sequester 5.5–9.5 GtC/yr (Lehmann et al, 2006). Similarly, biochar increases the microbial life in the soil and increases carbon storage in the soil.
- Enhanced soil fertility and food security:Biochar increases the number of soil microbes, retains nutrients in the soil and hence increases the soil fertility and subsequently there is increased food security. In Laos, Asai et al. (2009) reported that application of biochar improved saturated hydraulic conductivity of top soil. However, biochar may not be suitable for all situations. Derived biochar may enhance the loss of forest humus (Wardle et al., 2008). Therefore identification of specific niches for biochar application is crucial to exploiting its benefits.
- Reduced water pollution:Groundwater and surface water pollution through leaching, erosion, etc., is reduced through lower use of chemical fertilizer and reduced degradation of soils. As the nutrients and agrochemicals are retained in the soil due to use of biochar, pollutants produced through agriculture in water is reduced. Mizuta (2004) notes that biochar can remove nitrate and phosphate from water. Biochar also has an affinity for organic compounds which can help retaining toxic organic compounds from water (Kookana et al., 2006).
- Waste management:Biochar technology offers a simple and sustainable solution to waste management because agricultural wastes are used as feedstocks.During the pyrolysis process no waste is produced and by-products include syngas and bio-oils can be recycled and used further.
- Reduced deforestation and increased cropland diversity:Since biochar technology emphasizes the use of agricultural wastes as feedstock, deforestation is prevented and biodiversity inside soil can be significantly enhanced. Hence, by converting agricultural waste into a powerful soil enhancer with sustainable biochar, cropland diversity can be preserved and deforestation discouraged.
The rice-wheat cropping system in the Indo-Gangetic plains of India produces substantial quantities of crop residues, and if these residues can be pyrolysed, 50% of the carbon in biomass is returned to the soil as biochar This would increase soil fertility and crop yields, while sequestering carbon. In addition, pyrolysis of plant materials with applications of biochar to soil can actually result in a net carbon reduction from the atmosphere of 20%, making it a carbon sequestering process (Lehmann, 2007). It has been projected that about 309 million tonnes of biochar could be produced annually, the application of which might offset about 50% of carbon emissions (292 teragram C yr-1) from fossil fuel (Lal, 2005).
Since biochar technology is relatively new, costs and impacts associated with it are just beginning to be explored. A potential financial barrier to the development and transfer of the technology could be: the high costs of large scale pyrolysis plants, the required investments in biomass feedstocks, infrastructure and access to the upfront capital.
According to Granatstein et al. (2009), the total cost of biochar production ranges from US $194- US$424 per ton of feedstock. The study was based on biomass from sustainable forest, non-farm and ranch-based biochar production. The cost of biochar stoves can range from $6-$40 depending on the type and application of the stoves (Ravenhill, 2012).
The carbon price is critical for the cost effectiveness of biochar projects. The cost effectiveness of biochar stove and kiln projects in developing countries (Asia) up to 2030 is -43.7 US$/tCO2e with carbon price of 6 US$/tCO2e and -157.41 US$/tCO2e for high carbon price of 30 US$/tCO2e in the developing countries. The negative cost-effectiveness value indicates that if the biochar stove and kiln project is implemented in Asia it would offer a net savings to the society (Pratt & Moran, 2010).
A carbon market for sequestered carbon from biochar does not exist yet. In particular there has been no proper way of evaluation of the cost and benefit associated with the application of biochar and mitigation of GHG emissions. Furthermore, there is need to establish the carbon market for biochar. However, the existence of a current methodology for stabilization of organic matter in avoidance of methane emission represents an important precedent (Sohi et al., 2009).
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