Greenhouse-gas emissions from biofuel combustion in Asia1

Energy 24 (1999) 841–855 www.elsevier.com/locate/energy

Greenhouse-gas emissions from biofuel combustion in Asia夽 David G. Streets*, Stephanie T. Waldhoff Decision and Information Sciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Received 26 January 1999

Abstract An inventory of biofuel combustion is used to develop estimates of the emissions of carbon-containing greenhouse gases (CO2, CO, CH4, and NMHC) in Asian countries. It is estimated that biofuels contributed 573 Tg-C (teragrams of carbon; 1 Tg ⫽ 1012 g) in 1990, about 28% of the total carbon emissions from energy use in Asia. China (259 Tg-C) and India (187 Tg-C) were the largest emitting countries. The majority of the emissions, 504 Tg-C, were in the form of CO2; however, emissions of non-CO2 greenhouse gases were significant: 57 Tg-C as CO, 6.4 Tg-C as CH4, and 5.9 Tg-C as NMHC. Because of the high rates of incomplete combustion in typical biofuel stoves and cookers and the high global warming potentials (GWP) of the products of incomplete combustion (PICs), biofuels comprise an even larger share of energyrelated emissions when measured in terms of total GWP (in CO2 equivalents): 38% over a 20-year time horizon and 31% over a 100-year time horizon. Even when the biofuel is assumed to be harvested on a completely sustainable basis (all CO2 emissions reabsorbed in the following growing season), PIC emissions from biofuel combustion account for 4.5% of the total carbon emissions and 23% of CO2 equivalents on a short-term (20-year) GWP basis.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Biomass is a primary fuel for much of the world’s population. Fuelwood, crop residue, and dried animal waste are burned in large quantities in traditional domestic stoves and cookers for the purposes of space heating and food preparation. This is particularly true in rural areas of the

夽 This work was carried out at Argonne National Laboratory, managed by the University of Chicago for the U.S. Department of Energy under Contract No. W-31-109-ENG-38. * Corresponding author. Fax: ⫹ 630-252-5217; e-mail: [email protected]

0360-5442/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 9 9 ) 0 0 0 3 0 - 4

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densely populated, but economically underdeveloped, countries of Asia. In an earlier paper [1], we presented an inventory of biofuel use in Asia. We estimated that biofuel use amounted to about 22 EJ (1 EJ ⫽ 1018 J) in Asia in 1990, almost 24% of total energy use. In some of the less-developed countries, biomass contributed more than 80% of total primary energy, e.g. Bhutan 92%, Nepal 91%, Laos 85%, and Myanmar 81%. Though per capita consumption of biofuels is declining as traditional fuels are supplanted by commercial fuels (kerosene, oil, natural gas, coal, and electricity), the rural population is still increasing, so that biofuel use is presently stable or only slowly declining. Thus, the contribution of biofuels to the energy mix in Asia will remain significant for many years to come. Biofuel consumption is often assumed to be neutral with respect to emissions of carbonaceous greenhouse gases—that is to say, all the CO2 emitted in the burning process is taken up in the following growing season by replacement crops and trees. However, only when the biofuel is harvested on a sustainable basis—a new tree planted for each tree cut down for fuelwood—and only when all the carbon in the fuel is converted to CO2 can the process be considered truly carbon-neutral. While some countries are working toward sustainable harvesting, it is not the common practice in most of the developing world, and many upland areas of Asia have been severely degraded by unsustainable harvesting of fuelwood. In addition, most biomass stoves and cookers in the developing world have low combustion efficiencies. Combustion efficiency is inversely related to the formation of products of incomplete combustion (PIC) [2]. In other words, lower combustion efficiencies lead to higher PIC emissions. Three of the most important components of PIC are carbon monoxide (CO), methane (CH4), and nonmethane hydrocarbons (NMHC). On a carbon-weight basis, each of these species contributes more to global warming than does CO2. In the case of NMHC, this refers to the average over all constituent hydrocarbon compounds. The purpose of this paper is to examine the impact of biofuel combustion on total releases of carbon-containing greenhouse gases in Asia and to estimate the global warming potential of biofuel combustion products in 1990. Emissions are presented for three biofuel types, four chemical species, and each country of Asia, so that mitigation strategies can be directed toward the most significant sources and geographical regions. 2. Methodology The development of a biofuels inventory for Asia was an outgrowth of the RAINS-ASIA project, sponsored by the World Bank and the Asian Development Bank [3]. This project recognized that the countries of Asia are using ever larger quantities of fossil fuels and biofuels to drive their rapidly growing economies. Damage to the environment was thought to be likely without the use of emission controls and other preventative measures. The RAINS-ASIA computer model [4–7] was developed to simulate acid deposition and ambient concentrations of pollutants in 94 Asian regions within 23 countries and international sea lanes. Initial focus was on acidifying emissions, particularly sulfur dioxide (SO2) [6] and nitrogen oxides (NOx) [7]. A detailed accounting of regional fossil-fuel consumption is an important foundation of the RAINS-ASIA model. Data on biofuel consumption, however, were largely absent from initial versions of the model. With the goal of extending the model to incorporate biofuels, primary sources were used to

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compile an inventory of the consumption of biofuels (fuelwood, crop residue, and animal waste) in each of the 94 RAINS-ASIA regions. This inventory was subsequently used to derive emissions of acid-deposition precursors (SO2 and NOx) [1]. The present paper examines the impact of biofuel combustion on carbon emissions (in the form of CO2, CO, CH4, and NMHC) and its potential contribution to global warming in terms of CO2 equivalents. Finally, emissions from biofuel combustion are compared with emissions from fossil-fuel combustion, which are calculated using the RAINS-ASIA energy consumption data base. The inventory of biofuel consumption is combined in this work with greenhouse-gas emission factors for biofuel combustion in Asian domestic stoves and cookers. Emission factors are known to vary significantly when burning conditions are altered [2,8–10]. Specifically, it has been observed that PIC formation increases as the flaming stage is reduced and the smoldering stage is increased. This is because the smoldering stage allows the least amount of mixing between oxygen and carbon, so a greater share of the carbon in the fuel is not completely combusted. Variations in burning conditions can result from differences in fuel type, fuel moisture, stove design, wind speed, and many other factors. These variations make it difficult to determine average durations of the flaming and smoldering stages and therefore make it problematic to determine average emission factors. Table 1 summarizes the available literature [2,8,9,11–14] on emission factors for the combustion of biofuels. All emission factors have been converted to a comparable basis of gigagrams of Table 1 Emission factors (Gg-C/PJ) for biofuel combustion in domestic stoves Source

Fuel

CO2

CO

CH4

NMHC

Delmas [9] IPCC [11] Zhang [12] Zhang [12] Smith [13] Smith [13] Smith [2] Piccot [14]

Fuelwood Fuelwood Chinese fuelwood Indian fuelwood Fuelwood (heating) Fuelwood (cooking) Fuelwood Fuelwood

25.69

1.93 2.14 1.97 2.14 3.64 2.19 2.74

0.10 0.23 0.18 0.47

0.16 0.6a 0.17 0.50

0.44 0.39–1.55

0.77a

Zhang [12] Zhang [12] Smith [13]

Chinese crop residue Indian crop residue Coconut husks

23.73 23.40

3.04 2.53 3.47

0.28 0.60

0.19 0.17

Zhang [12] Smith [13] Piccot [14]

Indian animal waste Animal waste Animal waste

20.97

2.10 2.83

1.05

1.20

Lobert [8] IPCC [11]

Biomass Other biomass

23.97

a

Units are Gg-pollutant/PJ. Emission factor is for ‘hydrocarbons’.

b

27.87 14.40

26.50

3.4ab 0.48ab

0.37 1.67 2.14

0.12 0.23

0.34 0.6a

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carbon released per petajoule of energy input (Gg-C/PJ; 1 Gg ⫽ 109 g; 1 PJ ⫽ 1015 J). This presents a problem for nonmethane hydrocarbons (NMHC), which encompass a range of organic compounds. However, most researchers convert the weight of individual compounds into the weight of total NMHC, expressed as carbon, that is appropriate for their test conditions; we did not perform any such conversions ourselves. Also, emission factors are sometimes expressed in terms of nonmethane volatile organic compounds (NMVOC), a larger set of compounds than NMHC. In particular, the emission factors used in this paper for fossil-fuel combustion are strictly for NMVOC [11], and thus our estimate of the relative contribution of biomass emissions to total emissions may err on the low side. Because many definitions of this class of compounds exist, and because some definitions are used interchangeably, we took care to ensure that only comparable emission factors were used in this work. Some of the emission factors in Table 1 were obtained using real stoves and cookers tested in the field in Asia with typical Asian fuels. Others were from laboratory studies using a variety of stoves and fuels. In addition, the range of combinations tested was variable, leading to apparent discrepancies in ‘average’ emission factors. After a close examination of the literature and discussions with several researchers in this field, a set of emission factors was selected that, in our opinion, is most applicable to the Asian setting. These values are presented in Table 2 for combinations of the four emitted species and the three biofuel types. Table 2 shows, for example, that when fuelwood is burned in a typical Asian stove, 84% of the carbon stored in the fuel is released as CO2, 8% as CO, 1% as CH4, and 1% as NMHC. An additional 1% of the carbon is released as particulate matter, and approximately 5% of the carbon remains as ash. When dried animal waste is burned, the proportions of CH4 and NMHC are considerably higher. By contrast, in a typical, efficient fossil-fuel-fired boiler, almost all the carbon (98% for coal, increasing to 99.5% for natural gas [11]) is converted to CO2, i.e. the combustion is almost complete. Consumption data for each biofuel type (fuelwood, crop residue, and animal waste) were extracted from the inventory [1] and multiplied by the fuel-specific emission factors from Table 2 to yield emissions of each of the four carbon-containing greenhouse gases by fuel type for each Asian country (Table 3). Total biofuel emissions by country are presented in Table 4. Note that China, Hong Kong, and Taiwan are presented separately in this work. Carbon emissions from biofuel combustion were then compared with carbon emissions from fossil-fuel combustion. Fossil-fuel consumption data for 1990 were obtained from the RAINSASIA model, and emission factors for each species were obtained from IPCC guidelines [11]. Consumption data and emission factors were then combined to yield emissions of the same four

Table 2 Emission factors (Gg-C/PJ) used in this study Fuel

CO2

CO

CH4

NMHC

Fuelwood Crop residue Animal waste

22.66 23.73 20.97

2.19 3.04 2.83

0.18 0.28 1.05

0.17 0.19 1.22

Bangladesh Bhutan Brunei Cambodia China Hong Kong India Indonesia Japan North Korea South Korea Laos Malaysia Mongolia Myanmar Nepal Pakistan Philippines Singapore Sri Lanka Taiwan Thailand Vietnam Asia

Country

2508.5 242.4 312.1 30.2 15.9 1.5 1425.9 137.8 87733.5 8479.1 22.6 2.2 99359.8 9602.7 20828.6 2013.0 1995.7 192.9 2451.8 237.0 636.2 61.5 642.9 62.1 1296.4 125.3 230.4 22.3 4858.0 469.5 3677.9 355.5 10293.8 994.9 4228.4 408.7 0.0 0.0 2067.4 199.8 4.6 0.4 5082.6 491.2 9763.2 943.6 259436.3 25073.5

19.9 2.5 0.1 11.3 696.9 0.2 789.3 165.5 15.9 19.5 5.1 5.1 10.3 1.8 38.6 29.2 81.8 33.6 0.0 16.4 0.0 40.4 77.6 2060.8

18.8 2.3 0.1 10.7 658.2 0.2 745.4 156.3 15.0 18.4 4.8 4.8 9.7 1.7 36.4 27.6 77.2 31.7 0.0 15.5 0.0 38.1 73.2 1946.3

NMHC 7512.9 962.5 57.0 7.3 5.6 0.7 321.3 41.2 136309.5 17462.3 28.5 3.7 35430.0 4538.9 6155.0 788.5 0.0 0.0 3784.8 484.9 166.2 21.3 98.1 12.6 1124.6 144.1 60.5 7.8 1019.6 130.6 699.7 89.6 2747.1 351.9 2909.3 372.7 0.0 0.0 792.8 101.6 125.9 16.1 4088.7 523.8 1874.3 240.1 205311.4 26302.0

CO

CO2

CH4

CO2

CO

Crop residue

Fuelwood

88.6 0.7 0.1 3.8 1608.4 0.3 418.1 72.6 0.0 44.7 2.0 1.2 13.3 0.7 12.0 8.3 32.4 34.3 0.0 9.4 1.5 48.2 22.1 2422.6

CH4 60.2 0.5 0.0 2.6 1091.4 0.2 283.7 49.3 0.0 30.3 1.3 0.8 9.0 0.5 8.2 5.6 22.0 23.3 0.0 6.3 1.0 32.7 15.0 1643.9

NMHC

Table 3 Greenhouse-gas emissions from biofuel combustion in 1990, by fuel type and species (Gg-C)

1503.5 24.0 0.0 79.3 3862.7 0.0 28860.1 243.4 0.0 0.0 0.0 38.1 0.0 193.0 300.4 276.0 3238.9 0.0 0.0 0.0 0.0 0.0 560.5 39179.8

CO2 202.9 3.2 0.0 10.7 521.3 0.0 3894.8 32.8 0.0 0.0 0.0 5.1 0.0 26.0 40.5 37.2 437.1 0.0 0.0 0.0 0.0 0.0 75.6 5287.5

CO

Animal waste

75.3 1.2 0.0 4.0 193.4 0.0 1445.1 12.2 0.0 0.0 0.0 1.9 0.0 9.7 15.0 13.8 162.2 0.0 0.0 0.0 0.0 0.0 28.1 1961.8

CH4

87.5 1.4 0.0 4.6 224.7 0.0 1679.0 14.2 0.0 0.0 0.0 2.2 0.0 11.2 17.5 16.1 188.4 0.0 0.0 0.0 0.0 0.0 32.6 2279.4

NMHC

D.G. Streets, S.T. Waldhoff / Energy 24 (1999) 841–855 845

Bangladesh Bhutan Brunei Cambodia China Hong Kong India Indonesia Japan North Korea South Korea Laos Malaysia Mongolia Myanmar Nepal Pakistan Philippines Singapore Sri Lanka Taiwan Thailand Vietnam Asia

Country

CO

11524.9 1407.8 393.1 40.7 21.5 2.2 1826.5 189.7 227905.7 26462.7 51.1 5.9 163649.9 18036.4 27227.0 2834.3 1995.7 192.9 6236.6 721.9 802.4 82.8 779.1 79.8 2421.0 269.4 483.9 56.1 6178.0 640.6 4653.6 482.3 16279.8 1783.9 7137.7 781.4 0.0 0.0 2860.2 301.4 130.5 16.5 9171.3 1015.0 12198.0 1259.3 503927.5 56663.0

CO2

Biofuels

183.8 4.4 0.2 19.1 2498.7 0.5 2652.5 250.3 15.9 64.2 7.1 8.2 23.6 12.2 65.6 51.3 276.4 67.9 0.0 25.8 1.5 88.6 127.8 6445.6

CH4 166.5 4.2 0.1 17.9 1974.3 0.4 2708.1 219.8 15.0 48.7 6.1 7.8 18.7 13.4 62.1 49.3 287.6 55.0 0.0 21.8 1.0 70.8 120.8 5869.4

NMHC

Table 4 Greenhouse-gas emissions in 1990, by species (Gg-C)

13283.0 442.4 24.0 2053.2 258841.4 57.9 187046.9 30531.4 2219.5 7071.4 898.4 874.9 2732.7 565.6 6946.3 5236.5 18627.7 8042.0 0.0 3209.2 149.5 10345.7 13705.9 572905.5

Total C

CO

5010.5 24.7 26.8 0.2 983.1 30.7 1064.0 25.9 710435.7 5340.5 10012.0 77.4 158179.3 1214.4 46015.0 1005.8 301045.8 5983.1 37482.5 215.6 62829.5 1245.8 45.2 2.1 18155.6 569.6 3215.7 18.8 910.5 32.3 264.0 6.7 17887.3 253.2 9600.6 280.0 6868.8 83.6 1065.8 45.7 36400.4 698.7 22834.6 525.9 5301.7 69.0 1455634.4 17749.7

CO2

Fossil fuels

1.0 0.0 0.2 0.3 1008.2 0.9 51.9 13.6 67.6 8.5 98.2 0.0 4.7 0.6 0.2 0.3 3.9 2.5 1.1 0.3 7.9 4.9 3.2 1280.0

CH4 3.3 0.0 4.1 3.4 472.6 10.7 155.1 134.6 798.7 26.6 146.4 0.3 76.0 2.4 4.3 0.9 34.0 37.5 11.5 6.2 92.9 70.9 8.7 2101.1

5039.5 27.0 1018.1 1093.6 717257.0 10101.0 159600.7 47169.0 307895.2 37733.2 64319.9 47.6 18805.9 3237.5 947.3 271.9 18178.4 9920.6 6965.0 1118.0 37199.9 23436.3 5382.6 1476765.2

NMVOC Total C

72.5 94.2 2.3 65.2 26.5 0.6 54.0 39.3 0.7 15.8 1.4 94.8 12.7 14.9 88.0 95.1 50.6 44.8 0.0 74.2 0.4 30.6 71.8 28.0

% Biofuel of total C

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Fig. 1.

847

Dependence of global warming potentials on time horizon.

carbon-containing greenhouse gases from fossil-fuel combustion in Asia in 1990 (Table 4). The biofuel contribution in each country was calculated as a percentage of total carbon released. Estimates of the global warming potentials (GWP) of each species were gathered [2,11,15,16] and combined with the emission estimates to approximate their impact on global warming. Because the non-CO2 species have large heat-trapping capabilities, but relatively short atmospheric lifetimes, their impacts on global warming are highly dependent on the time horizon chosen; this is illustrated in Fig. 1 for the species examined here. The GWP values used in this study, as shown in Fig. 1 and Table 5, are derived from the work of Smith et al. [2]. The role of the trace gases is assessed using three measures: absolute carbon emissions, short-term GWP (20 years) and long-term GWP (100 years) (Table 6). 3. Results Table 4 presents the emissions of greenhouse gases by species and country. Emissions are presented in gigagrams (1 Gg ⫽ 109 g ⫽ 1000 t) of carbon (Gg-C) or teragrams (1 Tg ⫽ 1012 Table 5 Global warming potentialsa, 20-year and 100-year time frames

20-year 100-year a

CO2

CO

CH4

NMHC

1.0 1.0

4.5 1.9

22.0 7.5

12.0 4.1

Calculated on a carbon weight basis. Source: Ref. [2].

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Table 6 Contribution of biofuels to carbon emissions and global warming in 1990

Carbon emissions (Gg-C) Biofuels Fossil fuels Total % Biofuel 20-year GWP (Gg-CE) Biofuels Fossil fuels Total % Biofuel 100-year GWP (Gg-CE) Biofuels Fossil fuels Total % Biofuel a

CO2

CO

CH4

503 928 1 455 634 1 959 562 26 503 928 1 455 635 1 959 562 26 503 928 1 455 635 1 959 562 26

56 663 17 750 74 413 76 254 984 79 874 334 857 76 107 660 33 724 141 384 76

6445 1280 7725 83 141 803 28 160 169 964 83 48 342 9600 57 942 83

NMHCa 5870 2101 7971 74 70 433 25 213 95 646 74 24 065 8614 32 679 74

Total 572 905 1 476 765 2 049 671 28 971 147 1 588 881 2 560 028 38 683 994 1 507 573 2 191 567 31

Biofuels are estimated as NMHC and fossil fuels as NMVOC, see text.

g ⫽ 106 t) of carbon (Tg-C); note that 1 Tg-CO2 contains 12/44 Tg-C, and so forth for the other species. Carbonaceous emissions from biofuel combustion in Asia in 1990 are estimated to be 573 Tg-C. Emissions from fossil-fuel combustion, calculated from RAINS-ASIA data in precisely the same manner, are estimated to be 1477 Tg-C. Thus, biofuels represented about 28% of the total carbon emissions from all energy-related combustion activities, Asia-wide, in 1990. As expected, the two countries with the largest absolute carbon emissions from biofuel combustion were China (259 Tg-C) and India (187 Tg-C). China and India together contributed more than 75% of all the carbon emissions from biofuel combustion in Asia. Indonesia (31 Tg-C), Pakistan (19 Tg-C), and Vietnam (14 Tg-C) were the next largest contributors. Though emissions from biofuel combustion in China and India were high, they represented only 27% and 54%, respectively, of total energy-related emissions, because of the extensive use of fossil fuels in those countries. In many of the smaller, less-industrialized countries, greenhouse-gas emissions from biofuel combustion represented a very high proportion of total energy-related emissions, e.g. Nepal 95%, Laos 95%, Bhutan 94%, and Myanmar 88%. The majority of carbon emissions from biofuel combustion, 504 Tg-C, were in the form of CO2. However, emissions of non-CO2 greenhouse gases were significant: 57 Tg-C were emitted as CO, 6.4 Tg-C as CH4, and 5.9 Tg-C as NMHC. Thus, for biofuel combustion, 88.0% of total carbonaceous emissions in Asia was released as CO2, 9.9% was released as CO, 1.1% was released as CH4, and 1.0% was released as NMHC. This is in sharp contrast to the distribution of species from fossil-fuel combustion, in which 98.6% was released as CO2, followed by 1.2% CO, 0.1% CH4, and 0.1% NMHC. Poor combustion conditions, like those found in typical domestic biofuel stoves, greatly enhance the production of non-CO2 greenhouse gases. Table 3 shows that the combustion of dried animal waste generated the most NMHC Asiawide (39% of total), despite there being six times as much fuelwood burned as animal waste (on a PJ basis) and almost five times as much crop residue. Emissions of CH4 are roughly equally distributed among the three fuel types, and emissions of CO are dominated by the combustion of fuelwood and crop residue (each about 45% of the total).

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The amounts of each species emitted during biofuel combustion vary by country, with areas that burn large amounts of animal waste (such as the Indian sub-continent) having proportions of CH4 and NMHC that are 30–40% higher than countries in which the primary biofuel is in the form of fuelwood or crop residue. Thus, India is estimated to have higher CH4 (2.7 Tg-C) and NMHC (2.7 Tg-C) emissions than China (2.5 Tg-C and 2.0 Tg-C, respectively). Fig. 2 shows per capita emissions of the carbonaceous greenhouse gases from biofuel combustion in each country. Total per capita emissions of carbon were highest in North Korea, Bhutan, Nepal, and Mongolia (325 kg-C, 309 kg-C, 277 kg-C, and 266 kg-C, respectively). These high values reflect a combination of the essentially rural nature of the societies, extensive use of biofuels at the household level, and a propensity toward the use of dried animal waste with its higher PIC emission rates. On the basis of kg-C per capita, however, CO2 emissions dominate the mix of greenhouse gases released. The results of this study have been compared with other attempts to estimate emissions of these gases on global and continental scales. The Emission Database for Global Atmospheric Research (EDGAR) project inventoried CO2, CO, and CH4 emissions for Asia [17], partly in support of the Global Emissions Inventory Activity (GEIA). The results from the EDGAR project for 1990 biofuel emissions of CO2, CO, and CH4 in Asia were 759 Tg-C, 45 Tg-C, and 6.3 Tg-C, respectively. The most significant difference between the EDGAR study and the present work is the result for CO2; this study found CO2 emissions from biofuel combustion in Asia to be approximately 34% lower than the EDGAR estimate. There are two key methodological reasons for this discrepancy. First, in the EDGAR calculation of CO2 emissions it is assumed that all the carbon in the biofuel is fully oxidized to CO2 during combustion. Allowing for incomplete combustion would lower the EDGAR CO2 estimate by 10–15%. Second, the carbon content assumed for biofuels in the EDGAR study (25.5 kg-C/GJ) is about 10% higher than in this work (22.7 kg-C/GJ). Any

Fig. 2. National per capita greenhouse-gas emissions from biofuels in 1990, by chemical species.

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remaining discrepancy is probably due to a higher estimate in the EDGAR inventory for the total amount of biofuel burned. There is also an apparent discrepancy between the CO estimates. This can largely be accounted for by a significant difference between the CO emission factors used in this study and in the EDGAR project. The inferred CO emission factor for solid biofuel combustion in the EDGAR inventory is 1.77 Gg-C/PJ, lower than our values in Table 2 for fuelwood (2.19 Gg-C/PJ), crop residue (3.04 Gg-C/PJ), and animal waste (2.83 Gg-C/PJ). It does not seem that the test data in the literature (Table 1) can support the low value used in the EDGAR inventory, even allowing for the wide range of uncertainty in published estimates. The CH4 estimates in the two studies appear to be in close agreement, though if the assumed values for the total amount of biofuel burned are indeed different between the two studies, then there may in reality be an emissions discrepancy of about 15%. The CH4 estimate from this inventory is supported by the work of Piccot et al. [14], which placed 1990 worldwide CH4 emissions from animal waste burning at 2.20 Tg-C. This is consistent with our estimate of 1.96 Tg-C, with the reasonable implication that about 90% of animal waste combustion for fuel occurs in Asia. The estimates for fuelwood emissions are similarly supported, with Piccot et al. suggesting worldwide CH4 emissions from fuelwood combustion between 5.87 and 9.21 Tg-C. Because fuelwood is a more common fuel worldwide, we would expect our figure for Asia to comprise a relatively smaller share, which it does at 2.06 Tg-C. Finally, the total carbon emissions from this inventory of 1.48 Pg-C (1 Pg ⫽ 1015 g) are corroborated by an estimate made by the World Energy Council of 1.47 Pg-C, under the Reference Case B energy scenario [18]. As an aside, this analysis by the World Energy Council projects that total carbon emissions in Asia will rise to 2.98 Pg-C by the year 2020. The concept of global warming potential (GWP) was developed to compare the ability of a greenhouse gas to trap heat in the atmosphere, relative to another gas. Carbon dioxide has been chosen as the reference gas. Thus, the GWP of a greenhouse gas is the ratio of the global warming (or radiative forcing, both direct and indirect) caused by unit mass of that gas, relative to unit mass of CO2 over a selected period of time. In this work, we examine 20-year and 100-year time horizons. The units used for GWP in this work are gigagrams of carbon equivalent, or Gg-CE. The EPA web site on GWP [16] provides an excellent summary of GWP definitions, calculations, and values. The GWP of methane has been well established [2,15,16], but the GWP of CO and NMHC are less certain. Both CO and NMHC are believed to have negligible direct radiative forcing effects [19]. However, they do have significant indirect effects, due to their involvement in chemical reactions in the troposphere that influence the formation of O3 and OH radicals [19]. And, of course, their ultimate fate is conversion to CO2. The net indirect effects of CO and NMHC are believed to be positive, i.e. they contribute to global warming, rather than cooling, but the values of their GWP are uncertain. The values used by Smith et al. [2] are based on 1990 IPCC estimates [20]. Subsequent IPCC reports [19,15] have not included estimates of the GWP of CO and NMHC, because IPCC scientists “...are now aware of additional complications affecting such calculations and are less sure of the results” ([19], p. 62). It is concluded that “CO and NMHC will...make positive indirect contributions, although they are believed to be less significant than the contribution from CH4 and more difficult to assess due to temporal and spatial variations in concentration” ([19], p. 61). In the absence of better information, we use the original 1990 IPCC values

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for the GWP of CO and NMHC, as interpreted by Smith et al. [2], and as presented in Fig. 1 and Table 5. The high GWP of the PIC enhances the global warming potency of biofuel combustion. Although only about 1% of the carbon is released as CH4, the 20-year GWP of CH4 is 22. Similarly, the 20-year GWP of CO and NMHC are 4.5 and 12, respectively. Note that these GWP values are calculated on a carbon-weight basis rather than actual weight. Thus, for example, the IPCC-reported GWP for CO is 7 kg-CO2/kg-CO on a 20-year time horizon. This value needs to be multiplied by the conversion factor of (12/44 kg-C/kg-CO2)/(12/28 kg-C/kg-CO) to yield Smith’s GWP value of 4.5. One reason for treating trace greenhouse gases in this way is to avoid having to determine a single molecular weight for NMHC, which consists of a number of different chemical compounds. As shown in Table 6, on a simple weight basis the PIC from biofuel combustion represent 69 Tg-C of emissions, or 12% of the total carbon emissions from biofuel combustion (573 Tg-C). On a 20-year GWP basis, however, the PIC emissions contribute 467 Tg-CE, or 48% of the total carbon releases from biofuel combustion (971 Tg-CE). Thus, in the short term, the PIC are almost as important as the CO2. On a 100-year GWP basis, the contribution is less, 180 Tg-CE out of a total of 684 Tg-CE (26%), because of the relatively short lifetimes of the PIC in the atmosphere. Biofuel combustion released 573 Tg-C in Asia in 1990, about 28% of the total releases from all fuel combustion (biofuels plus fossil fuels) of 2050 Tg-C. However, on a 20-year GWP basis, it contributed 971 Tg-CE, 38% of the total emissions of 2560 Tg-CE; and on a 100-year GWP basis, it contributed 684 Tg-CE, 31% of the total emissions of 2192 Tg-CE (Table 6). In an absolute sense, then, biofuel combustion is a major factor in the release of greenhouse gases. Although biofuel combustion is often regarded as greenhouse-gas neutral, i.e. all the CO2 emissions are reabsorbed by new growth in subsequent growing seasons, this is an oversimplification. Neutrality comes closest to being true when the biofuels are harvested sustainably; however, this is not the case in much of Asia, particularly in the upland regions of countries like Thailand, Nepal, and southwest China. In addition, none of the products of incomplete combustion (PIC) are absorbed by new plant growth. Note that we neglect the fact that some of this vegetative matter, if not burned, may ultimately be returned to the atmosphere as CO2 through long-term natural decomposition. If it is assumed that all the biofuel in Asia is harvested sustainably, then only the PIC portion of the emissions becomes significant, because the CO2 emissions are reabsorbed. If the CO2 emissions from biofuel combustion (504 Tg-C) are subtracted from the total carbon releases from fuel combustion (2050 Tg-C), then a net release of 1546 Tg-C is obtained. In 1990, therefore, the PIC emissions from biofuel combustion of 69 Tg-C, represented about 4.5% of the net carbon released. However, when this exercise is repeated for 20-year and 100-year GWP, this figure increases to 23% and 11%, respectively. This means that, over the short term, PIC emissions from biofuel combustion account for almost one-quarter of the GWP of energy use in Asia. Biofuel combustion is clearly not greenhouse-gas neutral, even under conditions of sustainable harvesting. Table 7 presents country contributions (Gg-CE) calculated on a 20-year GWP basis. In comparison with Table 4 it can be seen that the contribution of the PIC emissions is much enhanced. In India, for example, the PIC releases represent more than 51% of the total GWP from biofuel combustion. In China the value is 46%. Throughout the Indian sub-continent (especially in Bang-

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Table 7 Global warming potential (20-year) of biofuel combustion in 1990, by species (Gg-CE) Country

CO2

CO

CH4

NMHC

Total

Bangladesh Bhutan Brunei Cambodia China Hong Kong India Indonesia Japan North Korea South Korea Laos Malaysia Mongolia Myanmar Nepal Pakistan Philippines Singapore Sri Lanka Taiwan Thailand Vietnam Asia

11 524.9 393.1 21.5 1826.5 227 905.7 51.1 163 649.9 27 227.0 1995.7 6236.6 802.4 779.1 2421.0 483.9 6178.0 4653.6 16 279.8 7137.7 0.0 2860.2 130.5 9171.3 12 198.0 503 927.5

6335.1 183.2 9.9 853.7 119 082.2 26.6 81 163.8 12 754.4 868.1 3248.6 372.6 359.1 1212.3 252.5 2882.7 2170.4 8027.6 3516.3 0.0 1356.3 74.3 4567.5 5666.9 254 983.5

4043.6 96.8 4.4 420.2 54 971.4 11.0 58 355.0 5506.6 349.8 1412.4 156.2 180.4 519.2 268.4 1443.2 1128.6 6080.8 1493.8 0.0 567.6 33.0 1949.2 2811.6 141 803.2

1998.0 50.4 1.2 214.8 23 691.6 4.8 32 497.2 2637.6 180.0 584.4 73.2 93.6 224.4 160.8 745.2 591.6 3451.2 660.0 0.0 261.6 12.0 849.6 1449.6 70 432.8

23 901.6 723.5 37.0 3315.2 425 650.9 93.5 335 665.9 48 125.6 3393.6 11 482.0 1404.4 1412.2 4376.9 1165.6 11 249.1 8544.2 33 839.4 12 807.8 0.0 5045.7 249.8 16 537.6 22 126.1 971 147.0

ladesh and Pakistan) biofuel contributions are high, due to the extensive use of dried animal waste with its high PIC emission factors (Table 2). Country-level, global-warming mitigation strategies that omit biofuel combustion and focus only on fossil-fuel combustion are clearly missing a major portion of the problem. Fig. 3 presents per capita 20-year GWP for biofuel combustion in each Asian country. This can be compared with Fig. 2, in which contributions are calculated on a weight (Gg-C) basis. Countries that emit large amounts of PIC, especially CH4 and NMHC, rank higher on the 20year GWP scale than on the carbon emission scale. For instance, when examined in terms of kg-C per capita, Mongolia is the fourth highest emitting country, but when adjusted for GWP it ranks first on a 20-year GWP time horizon with over 500 kg-CE per capita. 4. Conclusions It is clear from our analysis that the emissions of carbonaceous greenhouse gases from the combustion of biofuels in Asia are considerable. Because of the inefficiency of combustion and the high GWP of the resulting products, these emissions are even more important when global

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Fig. 3. National per capita global warming potentials (20-year) from biofuels in 1990, by chemical species.

warming potentials are examined. As an additional consideration, the large quantities of CO and NMHC released will perturb the atmospheric chemistry, especially as it relates to the formation of tropospheric ozone. If biofuels cannot be phased out in the short term, then it will be important to improve the combustion efficiencies of domestic stoves and cookers, which will reduce the formation of PIC species, and to harvest biofuels sustainably. There are two other important reasons for improving combustion efficiency in typical biofuel stoves. The first is the negative health effects associated with high concentrations of PIC found in homes that use biofuel stoves and cookers. The potentially deadly effects of CO are well known—to the extent that many American households now have CO detectors to warn of high levels of this poisonous gas—but even chronic exposure to low doses of CO can have adverse health effects in the long term. Other PIC, especially those present as inhalable particles, are also very dangerous to human health. It is believed that “...human particulate exposures from biomass use could be responsible for something more than one-half of total global exposure” ([21], p. 5). These PIC also contribute to the development of pneumonia and respiratory diseases in children living in developing countries. Not only do PIC represent very real health risks to the women and children consistently exposed to them in domestic environments, they also signify energy inefficiency. Incomplete combustion represents lost energy and heat, increasing the amount of fuel required to cook a meal or heat a home and thereby increasing total emissions. In fact, Smith states that the results of his pilot study in Manila [2] indicate that “...the loss of energy represented by the PIC from biomass-fired cookstoves is roughly 1% of total human energy use and could approach 10% for some countries” ([21], p. 5). Many attempts have been made to increase the efficiency of heat transfer in biofuel stoves (and thereby reduce the consumption of biofuels). One method for accomplishing this is to limit the air supply to the fuel. Unfortunately, decreasing air supply also increases the smoldering time of the fuel, thereby increasing PIC production [2,9]. The negative greenhouse-gas, health, and energy

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effects of increased PIC production make it vital that heat-transfer efficiency in biofuel stoves is not increased at the expense of combustion efficiency. Several programs aimed at promoting efficient cook stoves in the developing world have already been introduced [22]. Our data show that the assumption that biofuel combustion is neutral in terms of global warming potential is erroneous. Under present-day conditions (the prevalence of biofuel stoves with low combustion efficiencies and the unsustainable harvesting of biofuels), biofuel combustion most definitely contributes to global warming and has other ill effects. Recognizing the significance of biofuel combustion to greenhouse-gas emissions is an important step in the development of a solution to the problem of traditional fuel use in Asia and to the more general question of finding appropriate energy resources for a vast and rapidly changing continent.

Acknowledgements The authors wish to express their thanks to K. Smith (University of California at Berkeley) and J. Zhang (Environmental and Occupational Health Sciences Institute) for helpful advice in the preparation of this paper. This work was partially supported by the US Department of Energy under contract W-31-109-Eng-38 and partially by the National Aeronautics and Space Administration under Interagency Agreement S-92591-F with the US Department of Energy. The opinions expressed herein are those of the authors themselves and should not be construed as representing the official positions of Argonne National Laboratory, the US Department of Energy, or the National Aeronautics and Space Administration.

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