Air Pollution as a Climate Forcing: A Workshop
Day 3 Presentations
Carbon Balances, Global Warming Commitments, and Health Implications of Avoidable Emissions from Residential Energy Use in China: Evidence from an Emissions Database
Rufus D. Edwards* and Kirk R. Smith*+
* Environmental Health Sciences, University of California, Berkeley, CA, U.S.A.
+ Environment Program, East-West Center, Honolulu, HI, U.S.A.
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Abstract
The burning of solid fuels for household energy in the developing world often occurs with efficiencies considerably below those of large-scale combustion. For example, biomass fuels in Chinese stoves typically achieve overall efficiencies of 10-25% compared to the 35-45% of small-scale coal-fired boilers and 55-65% of industrial boilers. Industrial energy conversion efficiencies are even higher in more developed nations. As importantly, unlike large-scale combustion, the overall efficiency of simple household stoves is significantly related to the combustion efficiency of the fuel (conversion of fuel carbon to CO2) as well as to the heat transfer efficiency (to the pot). Thus, a significant portion of the pollution is in the form of products of incomplete combustion (PIC), i.e., much fuel carbon is diverted into non-CO2 airborne emissions such as CO, CH4, NMHC, and particles. These PIC include major health-damaging pollutants (HDP) as well as greenhouse-related pollutants (GRP).
As a result of the poor combustion efficiency and poor overall efficiency, HDP and GRP emissions per delivered energy are therefore considerably higher from residential fuel combustion than most large-scale combustion processes or uses of internal combustion engines. The cost-effectiveness of control of PIC emissions from small-scale combustion can also be greater than other sectors as cost of improvements in stove design are relatively low compared to many industrial measures. This implies that GRP control measures can be highly cost-effective in this sector. Given that such controls often will also reduce HDP, the overall benefits can be quite high. With the advent of the clean development mechanism such measures may accrue an additional financial incentive.
The residential sector has traditionally been overlooked in evaluating least-cost options and no-regret policies options for reducing GHG emissions mainly because residential stoves are individually small and are generally not perceived as emitting large quantities of greenhouse gases. Although this is true for each stove, it does not take into account the huge numbers of these devices in China and the developing world, and the poor combustion efficiencies. Traditional biomass fuels including firewood, brushwood and crop residues accounted for 12.7% and rural coal and coal product use accounted for 11.8% of Chinese national primary energy consumption in 2000. Because of PIC emission, however, the fractions of Chinese GRP and HDP are substantially greater.
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Figure 1: Figure 1. 20-year global warming commitments as g carbon per MJ delivered for residential stove and fuel types in China. 20-year GWP from Smith et al 2000 were used on a mole basis (per carbon atom) as follows: CO2 1.0, CH4 22.6, CO 4.5, TNMHC 12 (total non-methane hydrocarbon has been computed collectively but the warming of individual species may exceed this value). 20-year GWP of 150 for NOx was taken from IPCC 1990. Global warming commitments due to each PIC appear as a separate color. GWC CO2 does not appear for wheat residues as harvesting is renewable, and for brushwood and fuel wood the "non-renewable" points correspond to the global warming commitment assuming non-renewable harvesting (inclusion of CO2 in GWPs). The degree of renewable and nonrenewable harvesting will determine the actual GWC between these two extremes. Gases included in Kyoto negotiations and emitted from residential fuel combustion include GWC CO2 and GWC CH4 As a result a considerable fraction of the current global warming emissions of developing nations is not available for reduction and accumulation of carbon credit in the clean development mechanism of the Kyoto protocol. |
The burning of renewably harvested fuel wood (and other biomass) has often been assumed to be GHG neutral as eventually all the CO2 will be recycled and taken up by vegetation in the next growing season. Unfortunately this picture is flawed as most PIC emitted from incomplete combustion have higher global warming impact per carbon atom than CO2. Thus inefficiently burned biomass fuels have a global warming contribution even if renewably harvested. In China, for example, considering only CO2 and CH4, the burning of renewable brushwood and crop residues appears only marginally worse than emissions from LPG (liquified petroleum gas), even though LPG releases fossil carbon. For wood, even if only a small fraction is harvested non-renewably, it becomes worse than LPG. Inclusion of the other PIC changes this picture dramatically in favor of LPG, because of the large PIC emission from biomass in simple stoves (See Figure 1). LPG also produces substantially fewer HDP per unit energy delivered than do the solid household fuels. The counter-intuitive implication of these results is that LPG, a non-renewable fuel, can in many circumstances be promoted over these renewable fuels on both grounds: global warming and health.
Decreasing combustion efficiency seems to lead to near linear increases in PIC emissions. This facilitates the development of models to predict emissions of GHG and HDP from relatively simple information on stove and fuel use that can be incorporated into national surveys. Increased predictive ability may be obtained by including additional parameters that can be easily monitored in the field such as CO/CO2 ratios as a proxy for nominal combustion efficiency. These models form a mechanism for computing greenhouse credits in the residential sector, and monitoring compliance with a control regime. Not only would this allow assessment of stoves that have already been disseminated, and policies for fuel switching, potential merits of improved stoves and fuel switching can easily be compared prior to implementation for those communities involved. Incentives and financial interest in greenhouse credit ould also have a secondary benefit of concurrent reduction of HDP, making them truly no regret scenarios.
Acknowledgment. The analysis in this work uses the database of emissions from household stoves developed by the East-West Center and Tsinghua University, Beijing, under a Cooperative Agreement with the USEPA.
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