| Boiling Point No. 02 - Special Edition April 1991 |
by Dilip R Ahuja, of TATA Energy Research Institute (TERI), India 1990
If future programmes are to achieve their intended societal objectives and satisfy consumer requirements, research on designing improved stoves with lower emissions is critical. Integrated research will also be required on other related aspects, such as measurements of emissions from various stove-biofuel combinations, and on developing improved procedures, evaluation and dissemination. The multiple benefits that can accrue from these programmes make continuing and increased investment of efforts worthwhile. Although on a global basis biomass accounts for about one-seventh of energy consumed, it is dose to being the only source of energy for over two billion of the world's poorest people.
Several studies have reported that the current global extraction of biomass resources exceeds the natural rate of regeneration. It is this difference between annual growth and use that exacerbates the greenhouse effect and contributes to global warming. Later in this paper, this contribution is shown to add anywhere between 12% to the warming caused by human additions to greenhouse gas emissions. The causes of deforestation are many and vary from region to region, including clearing for agricultural lands, shorter cycles of fallow periods in shifting agriculture, timber extraction, fuelwood and fodder harvesting etc. In Guatemala, for instance, of the 1% of land area degraded annually by deforestation, only about a quarter is attributable to firewood use in stoves.
The increase in oil prices, however, made more remote the hope of a transition to these fuels for a substantial portion of the world's population and in fact in places there has been a return to traditional fuels. In densely populated areas, past unsustainable use of firewood has created a scarcity that forces people to turn to even poorer biomass materials: twigs, leaves, agricultural residues and animal dung. Because modem fuels can be burnt more completely and at higher efficiencies than traditional, solid biomass fuels, using modem fuels could possibly reduce the emissions of CO2 in the short term, depending on the extent to which the source of biomass was being regenerated. In the longer term from a greenhouse perspective, clean, efficient and entirely sustainable use of biofuels (upgraded! would be preferable to increased use of fossil fuels.
This paper traces the evolution of biofuel-burning cookstove technology over the last four decades as newer societal concerns are addressed and incorporated in stove designs. These concerns include the desire to reduce human exposures to smoke, deforestation and most recently the emissions of air pollutants including greenhouse gases. Besides carbon dioxide, biomass combustion is a source of CH4, N2O, CO and NOX. Both the contribution of these stoves to anthropogenic greenhouse warning and the potential of improved stoves to reduce their emissions cost-effectively are shown to be substantial. Other areas of research required for effective implementation of programmes that achieve societal objectives while satisfying consumer demands are also identified.
Traditional stoves come in a bewildering variety of designs and materials and have evolved to suit local fuels and diets. They perform a multitude of functions that were not considered by the early 'improved' stove designers and promoters. They are constructed and repaired with locally available materials, cost little or nothing, accept multiple fuels depending upon their seasonal availability, and without appreciable decline in fuel economy. They accept pots of different sizes; their power output can be varied to perform different baking, frying, boiling or simmering tasks. Because biofuels require frequent tending, the open firebox provides visual feedback and enables the cook to perform other chores. In some regions the fire may be the only source of light in the room; in others the smoke is used for curing food or fumigating the thatched roof. In cold and highland regions the open fire provides warmth and a convenient locale for socializing. Failure of the newer designs to meet those dimensions has hampered their acceptance.
While the traditional stoves have evolved from the threestone over thousands of years, modem, systematic attempts at improving traditional stoves date from the early 1950s. The earliest designs merely provided chimneys (or flues) for the removal of smoke from the cooking area. They were generally high-mass, constructed on-site with sand clay mixtures. The promoters' claims of large fuel savings were disproved by standardized and field testing as testing procedures were developed. Two design features tend to increase thermal losses and reduce energy efficiency: the high mass and the poorly controlled dimensions of the channels for the gas to pass around the pots.
The second generation of stoves followed the energy crises of the 1970s. The focus was on improving efficiency with the assumption that wood was the fuel of choice. Typically, efficiency was improved by controlling excess air in the combustion zone.
In many cases and for a variety of reasons, improved stove programmes did not do well. Their evaluations revealed that stoves with enclosed fireboxes were unable to cook the local diet and removed visual feedback and light; the flues interfered with the structural integrity of the dwelling and in cold areas also exhausted the heat from the room; inappropriately installed and maintained flues proved to be fire hazards; the cooks were unable to manipulate simultaneously the firebox and the flue dampers to keep the fire going. Inattention to dimensional accuracies resulted in much lower operational efficiencies than achieved by designers in laboratories. Rear ports rarely functioned as designed and designs optimized for wood did poorly with other biofuels. Excessive attention to targets and lack of follow-up or evaluation resulted in misuse, non-use or destruction of the stoves once they were installed. Some evaluations found no significant savings in fuel consumption or that the new designs used more wood than did the traditional stoves. Reductions in smoke exposures were reported in some programmes, but not in others.
Sobered by the extraordinary difficulties encountered, stove researchers and funding agencies have reamed some lessons but only after installation of millions of stoves. There are maybe half a dozen examples of successful dissemination efforts. Notable dissemination programmes are in place in West Africa and in Kenya and in Karnataka, India. Successful designs are backed up by sound principles of heat transfer, are targeted to a particular region (generally where cooking fuel is traded), require no substantial behavioural modification from users and are provided with follow-up support. In Africa, improved charcoal-burning stoves have proved popular, elsewhere other fuels are preferred. Charcoal making, however, introduces its own losses at the manufacturing and transport stages. By far the largest stoves programme has been in China, but the Chinese experience in this regard is poorly documented in the English scientific literature.
Fuel savings with improved stoves under field conditions have proved to be invariably lower than those predicted from laboratory water boiling tests of efficiency. This is especially true for multi-port designs. If the fuel savings observed in the laboratory were directly transferable to the field, an improved stove with 30% efficiency would result in a 50% fuel saving when it replaced a traditional stove with 15% efficiency. Most programme evaluations have reported marginal fuel savings, though greater savings should be possible as programmes improve with experience. Variations in critical dimensions, tolerances and usage practices tend to blur the distinctions between improved and traditional stoves. These naturally affect mud-lined stoves more than they do ceramic or metal ones.
Figure 2 is a simplified schematic of the biofuel cooking cycle and some of its major implications. Highlighted are those areas that will benefit from an infusion of more research effort.
The earliest concern was merely the exhaustion of the smoke from the cooking area; this was followed by the concern to reduce fuelwood consumption and deforestation; to these must be added the need to reduce the emissions of greenhouse gases and other pollutants.
More studies of adverse health effects of biofuel smoke are required. While these by themselves would not help to reduce exposures, confirmation of adverse effects would provide a compelling case for more efforts in this entire field. Moreover, developing country kitchens that bum biofuels may be the last few sites left of nonoccupational high exposures, and provide a reasonable opportunity to establish causality between exposures to air pollutants and adverse health effects.
It seems that a 2-3 year longitudinal study of the relationship between exposures to biofuel smoke and the incidence and severity of acute respiratory infections (ARI) in young children before and after the introduction of stoves that effectively reduced exposures would be most likely to produce results that are scientifically sound. In developing countries, ARI are responsible for nearly a third of childhood deaths under five years of age.
Special global climate concerns
While there are many other reasons to pursue the development of unproved cookstoves, from a global climate perspective there are three basic questions that need to be answered.
What is the magnitude of the contribution of the household biofuel cooking energy system (HBCES) to the anthropogenic emissions of greenhouse gases and risk of global warming?
What is the likely impact that programmes specifically designed to improve the performance of HBCES can make on reducing the risk of global climate change?
Do these programmer achieve this risk reduction in a cost-effective manner as compared to the other options available?
Contribution of the biofuel system to greenhouse warming
The estimation of the magnitude of contribution of the HBCES to anthropogenic emissions involves pooling together information from three different scientific subliteratures - those on deforestation and carbon cycles; on global air pollution from biomass burning; and from household energy consumption surveys. Neither all net deforestation nor all biomass burning is attributable to the HBCES.
The stove itself is a source of CO2, CO, CH4, N2O and NOX. Carbon dioxide from stoves contributes to the greenhouse effect only to the extent that the firewood is harvested on a non-sustainable basis. For the other greenhouse gases, emissions from all biofuel combustion in stoves must be accounted. A first attempt at the estimation of the range of contribution to global warming from CO2, CO, CH4 and N2O is made in Table 1. If we were to take into account also the second-order effects due to NOX, it is certainly possible that the HBCES contributes close to 296 of the anthropogenic emissions contributing to global warming.
Figure 3 is the part of household biofuel cooking energy system that needs to be studied for determining contribution to possible global climate change. The flows of carbon and the relative emissions of greenhouse gases must be estimated for both the current system and changes expected with the introduction of new designs. Part of carbon not burned or ploughed under or used as fertilizer is sequestered.
The least certain parameter in this calculation is the estimation of the contribution of fuel use in stoves to deforestation. I have conservatively assumed this to be one-eighth; larger estimates (up to one-third) can be found in the literature.
Potential to reduce the risk of global climate change
Doubling the total efficiency of existing cookstoves is technically feasible and a realistic target. If this can be achieved without simultaneously increasing the emission factors for other greenhouse gases (see below), the impact of the HBCES on global warming could be reduced to 1% from its current contribution of 2%. To achieve more accurate estimates of current contribution and expected savings, the magnitude of the flows (and expected carbon sequestering) and emissions from each of the fuel cycles of the HBCES (Figure 2) need to be estimated. Because the newer designs have tended to improve overall stove efficiency by improving heat transfer efficiency at the expense of combustion efficiency, it is possible that the emissions of CO and CH4 (both products of incomplete combustion) could increase with existing 'improved' designs.
Widespread introduction of the current crop of high efficiency stoves, while reducing the total emissions of oxides of carbon per cooking task, will change the ratio of CO2:CO emitted. This ratio (on a mass basis) for traditional stoves is close to 10:1, whereas for more efficient stoves this could be reduced to 5:1, reflecting the more complete combustion in traditional stoves. While CO is not a radiatively interactive gas, however, by competing for hydroxyl ions, its presence affects the concentration of methane and ozone in the troposphere. Moreover, it is relatively short lived in the atmosphere being oxidised to carbon dioxide.
The household biofuel cooking energy system makes a significant contribution (2%) to the anthropogenic emissions of greenhouse gases and this contribution can be halved at a cost (1.2 c/kg - C) that is lower than two of the other promising alternatives, namely electricity end-use efficiency improvements (7.6 c/kg-1 C) and plantation programmes (1.8-3.7 c/kg-1 C).
Improved cookstove programmes thus far have shown mixed results. High efficiency stoves that no one uses, or effectively low efficiency stoves that everyone uses do not save fuel or conserve carbon. We need a co-ordinated and integrated research programme that will help ensure that stoves have desirable characteristics and that they perform in homes as well as they do in laboratories. Research is required in the following areas:
• designing stoves that have inherently low emissions of several air pollutants and have high efficiency; simultaneous measurements of efficiency and emissions of greenhouse gases from different stove-fuel combinations, and a study of how emissions and effciency vary with design parameters and with each other; development of fuel-specific use cycle tests for stoves that take into account local use of stoves;
• longitudinal studies of the relation between exposures to biofuel smoke and the incidence and severity of adverse health effects before and after the introduction of improved stoves; and detailed exploration of the contribution of the household biofuel cooking energy system to the greenhouse effect and the potential of improved stove programmes to alleviate the risk of global climate change..