Les systèmes de biochar utilisant la biomasse comme source d’énergie pour les pays en développement

What is “biochar”?

Biochar is the solid product remaining after biomass is heated to temperatures typically between 300°C and 700°C under oxygen-deprived conditions, a process known as “pyrolysis. In contrast to the original biomass feedstock that mainly contains cellulose, hemicellulose, and lignin, biochar falls into the spectrum of materials called “charcoal” or “black carbon” but excludes black carbon derived from fossil fuels or non-biomass wastes.

Biochar can be produced from almost as many types of feedstock as there are types of biomass including: agricultural wastes, rice husks, bagasse, paper products, animal manures, and even urban green waste. However focus is placed on use of “true wastes” in order to minimize disruption to local carbon and nutrient recycling.

Biochar retains between 10 percent and 70 percent (on average about 50 percent) of the carbon present in the original biomass and slows down the rate of carbon decomposition by one or two orders of magnitude, that is, in the scale of centuries or millennia.

Production systems range in scale from small household cookstoves, to large industrial pyrolysis plants. Because it can be created over a wide range of temperatures and could be applied to a diversity of soil types, it is important to understand how different production conditions can result in different types of biochars, and how these chars interact with different types of soils.

The practice of amending soils with charcoal for fertility management goes back millennia. New instances of traditional practice using biochar are still being discovered around the world. Some of the most remarkable and currently best-understood properties of biochars include their effects on soil nutrient dynamics and the high stability of the carbon of which they are composed.

Nowadays, biochar is best thought of as a system-defined term referring to black carbon that is produced intentionally to manage carbon for climate change mitigation purposes combined with a downstream application to improve soils quality and for its agricultural effects.

Why this recent interest and development of biochar projects?

The surge of interest in climate-smart agriculture has sparked curiosity in using biochar as a tool to fight climate change while also improving soil fertility. Biochar systems are particularly relevant in developing country contexts and could be leveraged to address global challenges associated with food production and climate change.

The potential for biochar to improve soil fertility could result in increased crop yields from previously degraded soils for smallholder farmers. Improved cookstoves that produce biochar as well as heat for cooking could reduce indoor air pollution and time spent on fuel gathering.

Both of these results could be beneficial to forests. Enhanced food production capacity could potentially decrease the need to clear more forested land for agriculture, and more efficient cookstoves could decrease wood gathering from forests already in decline.

A typology of biochar systems categorizing projects based on production technology, energy recovery type, feedstock choice, and scale indicated that cooking energy dominated at the household scale, and projects generating electricity occurring at larger scales. The farm scale was most likely to produce biochar without energy capture, indicating that the system may be driven primarily by the agronomic benefits of biochar.

What are the benefits and risks related to the development of biochar systems?

Biochar systems are inherently complex and further research and life cycle analysis are needed to better understand their associated opportunities and risks in developing countries. Four main factors need to be taken into account:

1. the impacts on soil health and agricultural productivity; this includes soil pH, nutrient availability, soil moisture, soil organic matter and the amount of biochar applied;
2. the impacts on climate change; Carbon storage and stabilization is probably the most direct and important quality for climate change mitigation efforts based on biochar. Indirect factors contributing to emission reductions include: a) the generation of renewable energy through combustion of gas (called” syngas”) as by-product of biochar production; b) waste diversion, thus avoiding methane and nitrous oxide emissions from decomposing wastes; c) reduced fertilizer manufacturing. The risks of negative climate impact related to biochar lie primarily in the negative feedbacks that may occur directly or indirectly during biochar production and application. Such risks include emissions of methane and nitrous oxide during inefficient pyrolysis and degradation of soil organic matter after biochar application on unsuitable soils. Also, biomass cookstoves are significant sources of black carbon, which could have a global impact on the climate and on human health. The great advantage of biochar is that it is one of the few GHG reduction strategies that can actually withdraw carbon dioxide from the atmosphere but the overall net climate impact of biochar could only be assessed through a full life-cycle assessment ;
3. the social impacts; it is important to take into account that biochar systems can affect energy, health, economics, and food security. Biochar use can reduce pressure on wooded ecosystems and decrease the burden (mainly on women) of fuel gathering. Improved cookstoves can also reduce indoor air pollution throughout the developing world especially for women and children. In addition, by improving crop yields or crop resilience, biochar systems could help buffer practitioners against crop shortages and hunger. Energy produced from biochar could potentially be used for such critical functions as refrigeration of vaccines, water pumping, and lighting after sunset. Risks of biochar use can include potential emission of toxins and inhalation of dust and small particulate matter.
4. competing uses of biomass. The use of dedicated energy crops for biochar production could, among others, divert food crops for fuel production, divert arable land from food crops, affect direct and indirect land use change. For example, the costs and benefits must therefore be weighed, of leaving biomass in situ, versus using it to produce biochar that is then added to the soil.

What are the lessons learn from existing projects of biochar?

Regarding climate impact, all systems analyzed demonstrate that the emissions from biochar production (whether cookstove or village-scale unit), transportation, and stove or kiln construction are minimal compared to the net balance of the system.

The net GHG balance of a number of case studies ranges from −0.4 to −1.8 tonnes of CO2 per tonne of dry matter, with a Kenya cookstove project having the highest amounts of GHG reductions due to the avoided emissions from traditional cooking. A Vietnam case study highlights the role of reduced agricultural inputs, specifically fertilizers in reducing GHG emissions. Ensuring the sustainability of the feedstock for biochar production is the first and most important step in achieving GHG reductions. Avoided emissions from traditional biomass management practices, such as traditional cooking, can also play an important role when the feedstock is derived from a non or slowly renewable source such as wood.

Regarding the economic dimension, the most important result is that each project has a very short payback period—within one year of when surplus crops are monetized. In the case of cookstoves, the capital cost over the lifetime of the stove is small and the operating costs are minimal. In contrast, a pyrolysis unit at village-scale in Senegal has significant costs, which are only offset by the large revenues from surplus crop sales.

The yield of the crops to which the biochar is applied plays the largest role in determining the economic balance, implying that the farmer’s choice of crops can be as important as the type of soil to which the biochar is applied. Determining the minimum biochar application rate that still achieves the desired agronomic response is important in this context for determining the economic balance as well as the price the farmer receives for the surplus crop. Overall, the economics of biochar projects analyzed in the case studies are largely determined by the price farmers receive (or lack thereof) for surplus crops due to biochar additions to the soil.

Are there socio-cultural barriers to the adoption of biochar projects?

Implementation of biochar systems requires a highly location-specific understanding of people and of their needs, values, and expectations. The barriers identified in a survey included:

• Lack of awareness of biochar and a need for education and demonstration projects to show farmers that making and using biochar would be worth their time;
• Labor barriers, for example extra labor required to operate slash-and-char systems and gather dispersed feedstocks;
• Restricted availability of biochar production technologies;
• Environmental concerns related to changing resource use patterns.

Regarding the perceived benefits of biochar systems, respondents cited soil improvement, increased crop yields, decreased fertilizer use, improved water use efficiency, clean cookstoves, income benefits, and environmental hygiene. Half of respondents to the survey said that carbon payments would be nice but that they were not counting on them. Many felt that the existence of the traditional practice of charcoal made their job of communicating the benefits of biochar much easier.

What should be done to implement these projects in practice?

World Bank and other development institutions, could engage in knowledge- and technology-oriented services, as well as in financing services for biochar projects or programs. These could include, but not be limited to, development of carbon finance-related methodologies for different biochar systems. Institutions like the World Bank, particularly through its technical advisory and convening services, could help forge effective alliances between the research community and development practitioners on the ground. The World Bank is indeed committed to supporting new and innovative ways to reduce the emissions and impacts of black carbon and methane. In addition to direct financing of biochar projects, facilitating research and providing knowledge services are other ways the World Bank and other organizations can support the development of biochar systems.

Involving the private sector will be crucial in bridging the funding gap that typically constrains the implementation of new technologies with long lead times and considerable research requirements. Innovative financing solutions will be needed. Again, there is potential for the World Bank to play a role in helping to set up the complex structures required. Given the wide-ranging potential of biochar systems, it is important to build synergies with other projects and programs.

Clearly, lessons learned from existing projects will be extremely valuable for future projects and programs.