Opportunities to Reduce Black Carbon Emissions
John-Michael Cross, Climate Institute
Black carbon is a major contributor to climate change. Fortunately, technologies to reduce black carbon are, for the most part, available and cost-effective. The primary obstacles to black carbon mitigation are the distribution and adoption of these technologies. Black carbon emissions per capita are spread fairly evenly by region, (see Figure 2) meaning mitigation must be a global effort. Countries should adopt appropriate technologies according to each country’s emissions profile and economy.
Eight megatons of black carbon are emitted annually. Of that, 4.7 megatons, or 59%, are emitted through closed combustion sources, including cooking fires and diesel engines. The remaining 41% is emitted through the open burning of biomass (i). (See Figure 4) For the purposes of this article, open burning will be set aside, as it is too intertwined with the issues of deforestation and land degradation to go into detail here. Offered instead are means to reduce emissions in the three key areas of closed combustion: residential, transportation, and industrial sources.
Residential emissions constitute 26% of global black carbon. Emissions result from inefficient combustion of fuel for home heating and cooking, largely in the developing world. Wood and coal use account for two-thirds of residential black carbon, with the rest from burning agricultural residue, animal waste, and diesel fuel (ii).
Portable cookstove technology is the primary short-term solution to address residential black carbon. Longer-term options are tied in closely with development, such as the ability to cook with electricity or gas. The two main categories of portable cookstoves are improved-combustion stoves and solar-powered stoves.
Solar stoves eliminate the need for fuel and thus emit no black carbon. This alone makes solar stoves a promising option, but the technology presents several issues. For one, as is the case with any solar technology, lack of adequate sunshine makes the stove inoperable, which limits its usefulness. There are also durability issues, as well as cultural barriers to switching to non-flame-based heat, as this will have an effect on the taste of food. Finally, the cost of some models, US $100 for a single-family stove and $400 for a community stove, will likely prove prohibitive when compared with improved-combustion stoves (iii).
Improved-combustion stoves use traditional fuels, but with an optimized airflow that reduces emissions through cleaner burning and improved fuel efficiency. Black carbon savings vary by stove design, with the high end roughly near 50%. Stoves have mostly been designed to reduce particulate matter in general, but stoves engineered specifically for black carbon could potentially see reductions closer to 80% (iv). Envirofit International, a non-profit clean energy product developer, has designed a cookstove that reduces total particulate emissions by 80% and fuel consumption by 60%. The cookstove, which retails for US $25 and comes with a 5-year warranty, has sold 60,000 units in India in the past year and will soon be introduced to new markets (v).
The transportation sector emits nearly 19% of global black carbon. On-road engines account for 62% of these emissions, with the remainder due to off-road sources such as ships, trains, construction vehicles, and farming equipment. Both on- and off-road emissions are predominantly attributable to diesel fuel use. Black carbon from transportation is heavily concentrated in urban areas; reducing these emissions will lead to improved urban air quality (vi).
Reducing black carbon from the transport sector first requires an upgrade to higher quality, low-sulfur diesel fuel. In many countries, the sulfur content in diesel is often 500 parts per million or higher. Black carbon reductions are best achieved with the use of ultra-low sulfur diesel (ULSD), defined as 15 parts per million sulfur or less (vii). In the United States, ULSD costs a premium of US $0.08 per gallon at retail (viii).
On its own, ULSD only marginally reduces black carbon. It must be used in tandem with improved engine technologies to achieve the maximum benefit. Thus, there is a need to accelerate the turnover of diesel fleets to new, more efficient models. 2007-model diesel engine buses and trucks emit 90% less pollution than 2004 models, including a 99% reduction in black carbon and other fine particulates (ix). Ensuring that these standards are present in all of the 350,000 buses sold globally each year will lead to large gains as fleets turn over (x). Accelerating the rate of turnover will yield greater short-term benefits. Natural gas-fueled vehicles are a low-black carbon alternative, but are more expensive and require a separate fueling infrastructure (xi).
Realistically, many developing countries will continue to rely on older, heavy-polluting diesel vehicles, and a healthy market for used buses and trucks will continue to exist. Engine retrofits can be used to reduce black carbon from older vehicles. One such retrofit is the diesel particulate filter (DPF). The filter is inserted as part of the vehicle's exhaust stream and can be used in both on- and off-road vehicles. DPFs typically cost between US $5,000-7,500 and reduce particulate matter (PM) emissions, black carbon included, by 85 to 97%. However, these results have only been achieved on models built since 1994. Like new engines, DPFs only work well when in combination with ULSD (xii).
One final method of reducing black carbon from transportation is to phase out the use of two-stroke engines. Two-strokes are highly popular in the developing world both for scooters and motorized rickshaws. Four-stroke engines, while more expensive and larger, offer similar performance and greatly reduce black carbon and other pollutants. One two-stroke scooter emits fifty times the particulate matter that a car does. Retrofit technology transforms two-stroke engines into fuel-injection engines, reducing emissions by 70% at a cost of $350 (xiii). However, the logistics of retrofitting a substantial number of existing two-strokes are difficult. Black carbon reductions may be best achieved through regulation and economic incentives to steer new scooter and rickshaw purchases toward four-stroke and fuel injection technologies.
Emissions from the industrial sector account for 8% of global black carbon. While coal-fired power plants may come to mind as the culprit, in truth they emit just 0.1% of the annual total, mostly from older plants that are being phased out. Coal indeed is the source of industrial black carbon, but the vast majority is due to the coal used in iron and steel production. Coal is used in these industries to fuel coke ovens and blast furnaces. Modern techniques and existing emissions trapping technologies can significantly lower black carbon emissions (xiv). Proper regulations and self-monitoring in heavy steel and iron-producing countries, combined with technology transfer, can bring about swift drops in industrial black carbon.
Substantial black carbon reductions are possible in the transportation, residential, and industrial sectors. The technologies to do so are largely already available at competitive prices. Wider distribution, combined with economic assistance and incentives, will likely lead to large scale adoption of these technologies. Reduced black carbon emissions results in greater economic efficiency, improved human health, and cleaner air. These reasons alone warrant aggressive action on black carbon. It is, however, the opportunity to mitigate the severity of near-term climate change that gives the international community the greatest incentive to implement these technologies immediately.
(i) Bond, T. C., D. G. Streets, K. F. Yarber, S. M. Nelson, J.-H. Woo, and Z. Klimont (2004). “A technology-based global inventory of black and organic carbon emissions from combustion.” J. Geophys. Res. 109, D14203, p. 31. PDF
(iii) Ramanathan, V. and K. Balakrishnan. “Project Surya: Reduction of Air Pollution and Global Warming by Cooking with Renewable Sources.” March 2007. PDF
(v) “Our Impact.” Envirofit International. 2009.
(vii) “Ultra-Low Sulfur Diesel.” Washington State University Energy Program. 2004. PDF
(viii) “Weekly Retail On-Highway Diesel Prices.” Energy Information Administration.
(ix) Cone, Marla. “New diesel trucks and buses cut soot and smog more than 90%.” Environmental Health News. 18 June 2009.
(x) “World bus demand to exceed 350,000 vehicles in 2010.” All Business. 11 March 2007.
(xi) “New Engine Technologies.” Washington State University Energy Program. 2004. PDF
(xii) “Diesel Particulate Filters.” Washington State University Energy Program. 2004. PDF
(xiii) “Winner of the Transport and Mobility Category 2007.” World Clean Energy Awards.
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