The combustion of wood fuel by cooking stove releases pollutants of particulate matter (PM), carbon monoxide (CO), oxide of nitrogen (NOx), volatile organic compounds (VOCs), and Polycyclic aromatic hydrocarbon (PAHs). The quantity of pollutants released is highly dependent on the moisture content, oxygen level, wood type and combustion temperature.
The pollutants from wood cooking stoves have detrimental effect on human health. The severity of health impact depends on the concentration of the pollutants and the exposure time. Examples of the health impact from the chemical in the wood smoke are skin and eye irritation, fatigue, headache and nausea. Some chemicals released during combustion of wood are carcinogenic. Hence, the pollution from wood fire combustion is of great concern.
In this report, pollution reduction techniques have been identified that will reduce indoor air pollution. The suggested strategies are the use of improved cooking stove, use of chimney stoves or smoke hoods and behavioral change by the people. The behavior change includes the use of dry wood, use of smaller size wood, use of lids on pots or use of pressure cookers during cooking and regular maintenance of stove. With these strategies, the indoor air pollution can be reduced, thereby decreasing the health hazards of the people.
The biomass has played an importance role in household energy sector, particularly in developing countries. According to the Kammen (2006), around 3 billions people in developing country energy source comes from wood, coal and charcoal. The average household energy consumption has been presented in Figure 1.
Figure 1. Household energy consumption (Douglas et al 1994)
Of this consumption, 50 percent is used for cooking purposes (Douglas et al 1994). The traditional cooking method in developing countries is by the wood cooking stove.
There are two cooking stoves which are broadly used, which are the three-stone stove and Household rocket stove. Three-stone stove is used by about 2.5 billions people, whereas another half million people used the household rocket stove (Nordica et al 2007). The thermal efficiency of the household rocket stove is better than the three stone stoves as it is properly insulation around the combustion chamber. The Improved Cooking Stove (ICS) is the new type of cooking stove which is designed with the purpose of addressing health problems; addressing deforestation issues, preserving the environment etc.
While the wood cooking stoves are a basic necessity to every household, its emission has been largely neglected in developing countries. Burning of wood results in the emitting of pollutants such as particulate matter (PM), carbon monoxide (CO), nitrogen dioxide (NOx), Volatile Organic Compounds (VOCs), and Polycyclic aromatic hydrocarbon (PAHs). The amount of the pollutant varies, depending on wood type, combustion temperature, stove design and the quantity of oxygen consumed during the combustion process (DEWHA 2005). All these pollutants have adverse impacts on human health
In this report, the type of air pollutants from the wood cooking stove will be identified and strategies addressed to reduce air pollutants indoors in homes of developing countries.
The objective of this report is to address the harmful air pollution released from the wood cooking stove and to identify pollution reduction strategies that reduce indoor air pollution thereby reducing risk to the human health.
Wood smoke typically contains a number of chemical, as presented in Table 1.
Table 1. Wooksmoke component (DEWHA 2002)
The research indicated that the pollutants from the cooking stove mainly arise from the incomplete combustion process. The pollutants from incomplete combustion process are of great concern due to their adverse effects on health. These pollutants are particulate matter (PM), Carbon monoxide (CO), Nitrogen oxide (NOx) and VOCs (NDIEMA et al 1998). In addition other pollutants emitted are Aromatic Volatile Organic such as Benzene, Toluene and Xylene (BTX), and Semi-volatile aromatic such as Polycyclic aromatic hydrocarbon (PAHs).
The PM emitted is of great concern when the wood burns. It is found that during burning about 81% of all PM comprise of fine particulate matter (PM2.5) (Torok et al 2000). These fine particles are so small that the respiratory system is unable to filter and remove it. Consequently, serious health effects associated take place such as the lung disease problems. The formation of PM2.5 in the lung decreases the capability of oxygen transformation to the blood stream. Simultaneously, it acts as the carrier by bringing lethal substance into the lung. These substances comprise of carcinogenic compound and toxic substance. Researches conducted by the EPA concluded that there is a direct relation between the amount of PM and the mortality rate. The mortality rate was found to increase by 6% for every 100 microgram of total PM. In addition, evidence from EPA indicates that there is the correlation between the death rate of diseases, such as cardiovascular disease, pneumonia, bronchitis and asthma, and the amount of PM in the air (EPA 1997).
Another significant pollutant from the burning wood is carbon monoxide (CO). The CO is derived from incomplete combustion of the wood due to the insufficient amount of the oxygen. This condition is attributable to either inferior stove design or excess use of wood in the stove (NDIEMA 1998). The carbon dioxide generation can be represented by the three sequential equations (NDIEMA et al 1998).
As can be seen from the equation, the oxygen is initially reacted with the carbon in the wood and then CO is generated. After being produced, the CO reacts with the oxygen to form CO2 as is illustrated in eq.2. This step requires not only the sufficient oxygen but also the enough retention time. Because of the highly oxygen needs from eq. 1 and 2, there is deficiency of oxygen inside the stove. Consequently, the generated CO2 reacts with unburned wood and converted to CO (NDIEMA et al 1998).
Carbon Monoxide gas is colorless and odorless. It is a very dangerous gas as it is difficult to detect. The CO has an ability to attach itself to the hemoglobin in the blood which results in the loss of oxygen-carrying performance. The exposure to CO concentration of 200 ppm for 2-3 hours can cause the headache, fatigue and nausea, while the exposure to 400 ppm results in the fatality after three hour. There are evidence that indicates long term exposure of low concentration can contribute to heart disease and damage to the nervous system.
During the combustion of the wood, nitrogen oxide can be generated by two different processes. One is the result of the reaction between the nitrogen and oxygen in the air at high temperature from combustion. Another is that the nitrogen content in the wood in the form of ammonia, cyanide and hydro cyanide reacted with the oxygen in the air. Kituji (2001) found that most of the NOx generated is by the combustion of nitrogen in the wood and the oxygen in the air, because this reaction requires lower activation energy. The NOx formation can be represented by Figure 2.
Figure 2. NOx formation (Kituji et al 2001)
As can be seen from figure 2, there are two main reactions in NOx formation, which are gasification and oxidation. The NOx formed by the gasification arises from the nitrogen content in the wood and it is then changed by the oxidation reaction.
NOx causes numerous adverse health effects. At the concentration more than 0.5 %, it contributes to the eye and skin irritation. More seriously, in case of the concentration of more than 10%, the inhalation of NOx is capable of damaging the lung tissue (DEWHA 2001).
The most common VOCs generated by wood combustion are methane. Numerous parameters are associated with the methane generation during the wood combustion such as stove design, combustion temperature, chemical components in the wood and its size.
Methane is generated by the decarboxylation reaction of two substances. One is acetic acid (CH3COOH) and the other is the acetaldehyde (CH3CHO). The sources of these substances are from the wood composition such as cellulose, hemicelluloses and lignin.
In incomplete combustion, a number of VOCs which has higher molecular weight than methane are able to be generated. These VOCs are considered as non-methane hydrocarbon. Examples of non-methane hydrocarbon are shown in Figure 3. From the Figure 3, the three highest quantity of non-methane hydrocarbon is the Ethane, Ethyne, Benzene and Ethane, respectively.
Figure 3. The non-methane hydrocarbon from wood combustion (Ortega 2008)
The health effect of VOCs is dependent on the composition of VOCs, the concentration and the exposure time. Exposure to lower concentration can results in the irritation symptom, nausea, and the risk of damage to liver and central nervous system (DEWHA 2001).
3.4.1 Aromatic Volatile organic
The Benzene, Xylene and Toluene (BTX) are classified as the aromatic volatile organic. Of the total BTX in wood smoke, Benzene, Xylene and Toluene accounts for 40, 16.5 and 0.5 % respectively ( Hedberg et al 2002). The BTX emission is far less than CO, as can be seen in the Table 1. However in the poor ventilated area, it can have severe health impact as BTX can accumulate to concentration of unacceptable level.
Exposure to benzene at low level around 700-3,000 ppm for 5-10 minutes can cause the symptoms of headaches or even result in unconsciousness.
Long-term exposures to benzene results in disorder of blood component as Benzene have a negative influence on the red blood cell tissue formation. Severe, continuous long-term exposure can lead to the blood cancer known as leukemia. In addition, Benzene is harmful to the reproductive organ (ATSDR 2007).
The health impacts of Toluene are tiredness, memory loss at low concentration exposure. At higher concentration it can result in fatality (ATSDR 2001).
Inhalation or contact with Xylene causes health problem. In case of exposure to high concentration of Xylene, long-term or short-term, the nervous system is affected (ATSDR 2007).
PAHs also known as Semi Volatile organic are generated during incomplete combustion of wood. It can be found in the form of gaseous or particle. Domestic household area is the significant emitter of the PAHs. The evidence indicated that PAHs in domestic area accounted for 100-10,000ng/m3, whereas the concentration of PAHs in traffic area and smoking area, represented 20 ng/m3 and 20-100 ng/m3, respectively.
The wood combustion is likely to generate more types of the PAHs than any other solid fuel such as coal and rice hull. Oanh et al (1999) reported that around 18 PAHs was detected in the wood smoke while only 14 PAHs in the coal smoke. Also, the emission factor of PAHs from the wood is relatively high at around 13.4 mg/kg compared with 6.5 mg/kg of coal. Therefore, the wood smoke is considered as the major source of PAHs among the solid fuel.
An example of PAHs in wood smoke is mainly Benzo pylene (BaP). During the wood combustion in the kitchen area, it is found that BaP has the highest concentration, accounting for 0.61 µg/m3. The BaP is considered as carcinogenic. Research indicates that the long-term exposure to BaP by inhalation can lead to the formation of tumor in the lung. Besides, the threat to the reproductive system is also reported by the disorder of fetal formation.
Reducing indoor air pollution needs to be done in the following ways:
- Smoke reduction by changing the characteristics of stoves and using improved better stoves
- Smoke removal by use of chimneys, smoke hoods, flues, increasing ventilation
- Behavior change of people such in using dried fuelwood, use of lids on pots, use of pressure cookers, good maintenance of stove, chimneys etc.
Traditional method of cooking
The traditionally method of cooking is on a three stone stove. It requires three stones of equal height on which a cooking pot is put over a fire. Traditional cooking stoves have efficiency of less than 10% (CrtNepal 2005). This inefficiency means that more wood is required to be burned which result in more pollutants released.
Because traditional stoves have low combustion efficiencies they release large amount of products of incomplete combustion such as particulate matters, carbon monoxide, and hydrocarbons (Naeher et al., 2007). Smoke that is vented indoors causes various health aliments such as respiratory problem, eye infection etc.
Types of Improved Cooking Stoves (ICS)
There are two types of Improved Cooking stoves which is the fixed type and the portable type. The purpose of initiating improve cooking stove programs in developing countries can have priorities such as health, energy saving or preservation of forest.
Improved Cooking Stoves
Incomplete combustion is the main reason for emission of pollutants. Better design of stoves with adequate supply of air leads to completeness of combustion which results in increased efficiency. Improved combustion efficiency of stove will result in reduction of smoke and harmful emission. ICS stoves have higher efficiency of 15 -25% as compared to the traditional stove (CrtNepal 2005).
Improved cooking stove not only reduce fuel consumption but also the total pollution load. The emission of all air pollutants per unit of useful heat was found to decrease with the increase in stove efficiency (S.C. Bhattacharya et. al, 2002). As efficiency of stove increase there is decrease in emission of pollutant as shown in Figure 4.
Figure 4. Emission factors of CO, CO2, CH4, TNMOC and NOx in g (useful MJ)-1 against efficiency (% )( S.C. Bhattacharya et. al, 2002)
A study by Holley F. Ried and Kirk Smith highlighted that an average women in rural Nepal spends about 90% of the cooking period within 2 minutes of the cooking stove for a period of about 5 hours a days on average. The study then correlated the finding with a health study that indicated the exposure of fuelwood smoke in poorly ventilated conditions increase the risk of young children suffering from Acute Respiratory Infections (ARI) by 100 to 400%. The study found that mean personal exposure to total suspended particles (TSP) was 3.92 mg/m3 and CO concentration was 380 ppm in traditional cooking stoves. With the introduction of the improved cooking stove (ICS) the mean exposure to TSP was 1.13 mg/m3 and CO concentration was 67 ppm. The result indicated that with the use of ICS, there was a reduction of TSP concentration by 71% and CO concentration by 82%. The impact of reduction in TSP and CO concentration resulted in decrease in people suffering from Acute Respiratory Infection (Holly F. Reid, Kirk Smith and Bageshowri Sherchan cited by Raut. A.K).
Removal of smoke from indoors can be by the use of chimney or smoke hoods. They are both very effective in reducing the pollution load indoors.
The two systems are different in that the chimney stove is a fully enclosed system, in that smoke has nowhere to go but through the chimney outside. In the smoke hood, the hoods are constructed around the cooking fire and direct the smoke through the chimney vent in the roof to the outside.
A smoke and health project was undertaken in Kajiado, Kenya in 2001 as a result of major health issues faced by the local community of about 50 households. As a direct result of burning fuelwood, the indoor air quality was extremely poor that caused serious health problems amongst the people, particularly women and children that spending most of their time in the kitchen. Some of the health problems complained by the villages related to the smoke were painful and irritable eyes, respiratory infections, ear infections, chest pain, difficulty problems, respiratory problems, frequent headaches, giddiness, and malaise. It was found that the daily concentration of TSP to be 5526 μg/m3 in Kajiado which is excessively above the US Environmental Protection agency acceptable TSP concentration level of 50 μg/m3 annually. This means that the TSP concentration was still 100 times greater in Kajiado when comparing daily rates against the annual acceptable rates in the US. With the introduction of smoke hoods, statistical analysis shows that there was a reduction in TSP in the house by 75% and CO concentration by 77%. The result was the improvement in health with people reporting reduction in aching eye, fewer headaches, fewer coughs, less malaise, no chest pains etc. Other benefits of the project was reduced drudgery with less soot indoors thereby allowing for easier washing of clothes, easier to light fire due to circulation for air flow. In addition, cleaner indoor air environment also meant more opportunity for income generation as women are able to work (e.g. sewing) indoor when weather was unsuitable outdoors, reduction in time wasted due to ill-health, and prestige for the household who entertain guest. (Doig A, Gitonga S and Bruce N, 2001). With the success of this project other community from other area, seeing the benefit from smoke hoods would also be eager to set up the smoke hood, even if they had to pay a small price. Given that in developing countries, huge amount of government’s resource are going into health sector, the involvement of government in subsidizing smoke hoods would go a long way in a win-win situation with reduction of health expenditure by the government for its people and at the same time improving the quality of lives of the people.
Better ventilated rooms by opening the window will ensure improved combustion of stove as well as reduce indoor pollutants. Ventilation can be improved by enlarging the size of the window or opening eaves spaces.
Another crucial issue in reducing indoor air pollution is for the exit location of the vent ducts to be outside the house. There have been cases where the vent ducts had ended in the attic, thereby resulting in the pollutant being re-circulated in the house, thereby increasing the concentration of harmful pollutants indoors.
Burning of wood releases smoke. The amount of smoke depends on how dry the wood is and how efficient the burning is. These factors are in the hands of individual households and hence require behavior change on their part.
Using dry wood
It is recommended that clean dry wood that have been cut, split and stacked for over 6 months prior to burning be use. More moisture content of wood would result in decrease efficiencies of stoves. This is due to the fact that with higher moisture content of wood, higher fraction of heat released during combustion is required to evaporate the moisture.
With higher moisture content the flame temperature is reduced and hence the rate of heat transferred to the cooking pot and hence more wood would be needed to cook.
An investigation done by Bhattacharya, Albina and Khaing in 2002 concluded that increase in moisture content results is an increase in CO emission. The increase in CO emission appears to be due to the lowering of gas phase reaction rates at reduced temperature caused by high moisture content.
With high moisture, more wood would be required for cooking for a longer period of time which would result in the release of more CO pollutant.
Figure 5 Emission factor at different moisture content (Bhattacharya et al 2002)
Use of dry wood would result in more complete combustion and therefore less indoor pollutant.
Smaller size of fire wood used
Burning smaller pieces of wood that are stacked loosely will burn more efficiently. The wood blocks size also influence the CO emission as emission factor was found to reduce as the fuel size was reduced. The decrease in emission of CO can be attributed to the intensification of the combustion process, which was indicated by increased burn rate, when the fuel size was reduced. As a result of rising temperature level inside the stove promotes complete combustion, and so less amount of incomplete combustion products are produced (Bhattacharya et al 2002).
Figure 6. Emission factors at different size of wood fuel (Bhattacharya et al 2002)
Using of lids during cooking
Keeping the lids on pots during cooking will reduce cooking time for preparing conventionally prepared meals. By this simple action of decreasing cooking time, we also reduce the amount of energy required and therefore less use of fuelwood and hence less pollutant released.
Use of Pressure Cookers
Pressure cookers are more beneficial at higher altitude. This is due to the fact that at higher altitude the boiling point of water decreases.
Figure 7. Boiling Point of Water varying with altitude (Vickie, 2008)
The use of pressure cookers reduces the time for cooking. As the vessel does not allow for liquids to escape below the preset pressure what happens is that the boiling point of water increases as the pressure increases. This results in the rise in temperature before boiling. With higher temperature, food can be cooked faster and hence requires about 1/3 the energy required as required by conventional cookers (Midwest Energy Inc, 2008). Less energy requirement means that less wood is burned and hence less pollutants being emitted from cooking stoves.
Regular maintenance of stove and chimney/hood are crucial for the efficient performance of the stove. Proper maintenance will ensure that the vent ducts are not blocked, thereby allowing for the pollutants to exit the house.
Given that most of the people using the wood fuel stoves are from developing countries with little or no skills, it is essential that the maintenance of stoves, hoods or chimneys be easy. For example in Kenya, the hoods required for dismantled during cleaning. As it was difficult no maintenance was done which resulting in decrease in efficiency of the hoods. Due to this problem, modifications of hoods were done so that there was no need for dismantling the hoods during cleaning thereby allowing for easy maintenance. The clean hood helps to draw air into the stove resulting in proper combustion and at the same time allows for smooth gas flow thereby removing pollutants from the house (Doig A et. al 2001).
Wood fire cooking stove will continue to be one of the source of energy for cooking food especially in developing countries. While change to clearer fuel such as LPG, Natural gas etc. would result in cleaner indoor air condition, people are not able to afford this energy. Hence use of fuel wood as energy source will continue.
The main pollutants from the wood fuel are of particulate matter (PM), carbon monoxide (CO), oxide of nitrogen (NOx) and volatile organic compounds (VOCs).
The release of high concentration of harmful pollutants indoor causes headache, fatigue, nausea, eye, nose, throat and skin irritation, and results in major damage to lung tissues, central nervous systems, etc.
The concentration of indoor air pollutants can be reduced by the use of
- improved cooking stoves,
- use of smoke hoods, chimney stoves,
- better behavioral practices such as using dried wood, use of smaller size fuel wood, use of lids during cooking, use of pressure cooker to reduce cooking time and
- Regular maintenance of the stove, chimney and hood.
With these approaches, the indoor air pollution can be reduced thereby decreasing the risk of health hazards to the people.