Summary of Main Report for USEPA: November 1, 1998

GREENHOUSE GASES FROM SMALL-SCALE COMBUSTION DEVICES IN DEVELOPING COUNTRIES. Phase IIa: Household Stoves in India

by

Kirk R. Smith, R. Uma, V.V.N. Kishore, K. Lata, V. Joshi, Junfeng Zhang, R.A. Rasmussen, and M.A.K. Khalil

Early in the decade, a pilot study funded by EPA was conducted in Manila, Philippines, to measure the concentrations of a range of greenhouse gases (GHG) from cookstoves burning biomass, charcoal, kerosene and liquefied petroleum gas. Based on intriguing results, a more comprehensive study to characterize the emissions of non-CO₂ gases and other pollutants from cookstoves using different solid, liquid, and gaseous fuels was undertaken in China and India in a project organized by East-West Center (EWC) and funded by EPA. The study focuses on more than two dozen of the most common fuel/stove combinations in each nation. Since these countries contain nearly half of all stoves in developing countries, the stoves in this study represent a large fraction of the combinations in use world-wide.

In this report we describe the methodology and results of the study undertaken in India. The monitoring took place using a hood-type monitoring system in a simulated kitchen built at the Gual Pahari Campus of the Tata Energy Research Institute (TERI), just outside New Delhi. Laboratory analyses took place at TERI and at the Oregon Graduate Institute of Science and Technology.

Introduction

Small-scale combustion devices, such as household stoves, although individually small, are numerous and thus have the potential to contribute significantly to inventories of GHG, particularly in those many developing countries where household use is a significant fraction of total fuel use. In addition, the simple stoves in common use in such countries do not obtain high combustion efficiency, thereby emitting a substantial amount of fuel carbon as products of incomplete combustion (PIC) - such as carbon monoxide (CO), methane (CH₄), and total non-methane organic compounds (TNMOC) - as well as carbon dioxide (CO₂ ). This is true for fossil fuels, such as coal and kerosene, but seems particularly important for unprocessed biomass fuels (animal dung, crop residues, wood), which make up the bulk of household fuel use in developing countries.

It is estimated that biomass combustion contributes as much as 20-50 percent of global GHG emissions. Though the major fraction of the emissions is from large-scale open combustion associated with permanent deforestation, savannah fires, and crop residues, combustion in small-scale devices such as cookstoves and space-heating stoves also releases a significant amount of GHG. A more accurate estimation of emissions from biomass combustion would require better estimates of GHG emission factors from different types of biomass combustion as well as estimates of the amounts burned.

The emissions of non-CO₂ greenhouse gases from small-scale combustion of biomass are not well characterized, but are known to be different from open large-scale combustion, such as forest and savannah burning, which have been the focus of more research. Emissions from other fuels as commonly used in developing-country households are also not well known. Therefore extensive measurements of emission factors for GHG from a range of fuels and combustion devices would lead to removing some of the uncertainty in the estimates of total emissions from biomass combustion and also will provide a baseline database to understand the potential for reduction in GHG emissions due to various mitigation measures, such as fuel switching, in the household sector.

The pilot study conducted in Manila indicated that the emission factors for CH₄, CO, and TNMOC from the combustion of wood and charcoal in cookstoves are high. In the case of wood combustion, the analysis also revealed that the global warming commitment (GWC) of the non-CO₂ GHG - CO, CH₄, and TNMOC - may in some circumstances rival or exceed that from CO₂ itself. In addition, the study seemed to indicate that in some instances substitution of biomass by fossil fuels, such as kerosene and gas, could be considered as means to lower GWC, even when the biomass fuel is harvested renewably. If verified, these would have important implications in setting energy and global-warming policies.

We report here on the results of testing 28 fuel/stove combinations using 8 biomass fuels, kerosene, LPG, and biogas, all in common Indian use, for a range of GHG and other emissions (CO₂, CO, CH₄, TNMOC, N₂O, TSP, SO₂, NO_x) while simultaneously being monitored for fuel use, thermal efficiency, and other parameters.

RESULTS

Basically confirming and extending the preliminary results in the Manila pilot work, the database developed in this study shows that solid biomass fuels are typically burned with substantial production of PIC. In addition, as has often been shown in the past, biomass stoves usually have substantially lower thermal efficiencies than those using liquid and gaseous fuel. As a result, the total CO₂ and PIC emissions per unit delivered energy are considerably greater in the biomass stoves. In general, the ranking follows what has been called the "energy ladder" from lower to higher quality fuels, i.e., emissions decrease and efficiencies increase in the following order: dung>crop residues >wood >kerosene>gas. There are variations, however, depending on specific stove designs.

Table 1 shows the results aggregated by fuel, i.e., averaged over all the stoves tested with each fuel. The fuels are roughly ranked by average nominal combustion efficiency (NCE), which is defined as the airborne fraction of fuel carbon released as CO₂. Note the low NCE for the biomass fuels. Indeed, some individual fuel/stove combinations had NCEs of less than 80%, meaning that over one-fifth of the fuel carbon was diverted into PIC.

The GWC of the fuel/stove combinations depends on which PIC gases are included in the calculations and whether the wood and root fuels are considered to be renewably harvested (dung and crop residues are always considered so). When using the full set of GHG, essentially all biomass fuels, except biogas, have substantially higher GWC_(non-ren) (global warming commitment - here on a 20-year horizon) per standard meal than any of the fossil fuels tested. This is because of the low combustion and thermal efficiencies of biomass stoves, even improved ones, compared to the liquid and gaseous fuels. In the case of GWC_(ren), a few of the wood and root stoves are comparable to the kerosene stoves, and two wood stoves actually do better than LPG.

If GWC only from CO2, CH₄, and N₂O is considered (basic set of GHG) several of the dung and crop residue stoves have GWCs comparable to kerosene and LPG. In the case of GWC_(ren), however, 15 of the biomass stoves are comparable to or lower than the fossil-fuel stoves.

Interestingly, however, biogas, which in India is made mostly in household anaerobic digesters, is by far the best of all, with only some 10% of LPG GWC and more than a factor of 200 less than the most GWC-intensive solid biomass fuel-stove combinations. Figures 1 and 2 show the results by fuel. Note the large reduction in GWC represented by biogas compared to directly burning the dung, from which it is made.

For a complete analysis, the GWC of the rest of the fuel cycles should be included as well. The fossil fuels, for example, will have GHG releases at the oil well, refinery, and transport stages of the fuel cycle. Biogas will lose some of its apparent lead because of methane leaks from the digester and piping. Nevertheless, it is clear that the database confirms some of the preliminary counter-intuitive conclusions of the Manila pilot study, i.e., that in some circumstances a switch from solid biomass fuels, even if renewably harvested, to kerosene can be recommended for the purpose of reducing GHG emissions. It would appear that this is true in essentially every case for a switch to LPG. The remarkable performance of biogas is due to its double advantage of being a gas when burned and renewable when harvested and indicates a huge potential for processed biomass fuels to reduce the GWC of cooking.

Basically, three conclusions can be drawn:

--Even if renewably harvested, many biomass fuel cycles are not GHG neutral because of their substantial production of PIC.

--To be GHG neutral, not only must biomass fuel cycles be based on renewable harvesting, they must have close to 100% combustion efficiency, which most do not in their current configurations in India.

--In the processed form of biogas, however, biomass seems to offer the opportunity of providing a renewable source of household energy with extremely low GWC.

From these measurements it seems that CH₄ emissions from small-scale biomass combustion in India may be about 3.2 GT, roughly twice as high as previous estimates. It is thought that Indian biomass stoves represent about 27% of the global total. Thus, if the distribution of stove types globally is similar to India’s, it could be expected that biomass stoves produce globally about 12 GT of CH₄ annually, which is equivalent to about 12% of the estimated global emissions from all harvesting and combustion of fossil fuels.

Table 1. Efficiencies and Emissions For Major Fuels Used in Indian Household Stoves (unweighted averages by fuel of the 3 separate tests for each of the 28 fuel-stove combinations examined)

Notes:

Overall efficiency = combustion efficiency x heat transfer efficiency = percentage of chemical heat in fuel that enters the pot.

Nominal combustion efficiency = a rough estimate of combustion efficiency = the fraction of airborne carbon emissions that are released as carbon dioxide.

Figure 1 (above)

Figure 2 (above)

Index