Fuelwood species characterization and development of pollutant emission factors for fuelwood in Limpopo Province, South Africa

Abstract

Approximately 2 billion people worldwide continue to depend primarily on traditional energy sources such as fuelwood, charcoal, agricultural residues, and animal dung to meet their household energy needs. The burning of fuelwood for cooking and heating, particularly in poorly ventilated kitchens, poses significant risks to human health due to prolonged exposure to indoor air pollutants such as CO₂, CO, NO, SO₂, and particulate matter (PM). Although numerous studies have examined solid fuel characterisation globally, limited attention has been given to the physicochemical properties of predominantly used fuelwood species, particularly in Sub-Saharan Africa (SSA). The majority rural households in Limpopo Province continue to depend primarily on fuelwood as their main energy source for cooking and heating. However, the province still lacks comprehensive and reliable emission inventories on fuelwood consumption patterns and the associated emission levels. Furthermore, rural communities in Limpopo Province continue to face growing air quality concerns associated with residential fuelwood burning, underscoring the importance of developing reliable baseline emission inventories to inform effective air quality management and policy interventions. This study was therefore undertaken to address this knowledge gap within the study area. Moreover, the reliance on default pollutant emission factors (EFs) derived from European, North American, and Asian studies for residential fuelwood burning in South African air quality and climate change modelling has introduced significant uncertainties into emission inventories. This highlights the urgent need for locally developed pollutant EFs specific to residential fuelwood combustion. The present study aimed to determine the physical and chemical properties of commonly used and preferred fuelwood species for cooking and heating, and to develop EFs for CO, CO₂, NO, SO₂, and particulate matter (PM₁₀) from residential fuelwood combustion in Limpopo Province. The first objective examined the socio-economic factors influencing household energy choices. The second objective involved characterising the predominantly preferred fuelwood species used for cooking and heating. The third objective estimated diurnal, seasonal, and annual fuelwood consumption rates, as well as the associated gaseous and PM₁₀ emissions from domestic fuelwood burning, using default emission factors for the selected pollutants. Finally, the fourth objective determined pollutant EFs for residential fuelwood combustion using controlled laboratory chamber measurements. During the survey, questionnaires were administered in all selected villages to gather information from participants. A chi-square test was applied to assess the relationship between household energy choices and the socio-economic characteristics of respondents. In addition, Cramer’s V symmetric measure was used to evaluate the strength and direction of the association between these variables. A laboratory experiment was conducted to determine the physicochemical properties of the predominantly preferred fuelwood species. Household fuelwood consumption was monitored over two seasons—summer (2022) and winter (2023)—using the weight survey method in accordance with standard protocols. Furthermore, laboratory-based experiments were carried out to quantify flue gas and PM emissions from residential fuelwood combustion. The findings of the study indicate that fuelwood remains the dominant source of energy for cooking and heating across all the surveyed villages. A total of twelve tree species were identified as the primary sources of fuelwood: Ozoroa paniculosa, Senegalia afra, Dichrostachys cinerea, Colophospermum mopane, Senegalia burkei, Peltophorum africanum, Berchemia zeyheri, Combretum molle, Brachylaena rotundata, Vachellia tortilis, Combretum apiculatum, and Combretum imberbe. The study further revealed that key socio-economic factors influencing household energy choices included income level, employment status, and the age of the household head, among other variables. The gross calorific values (GCV) of the studied species ranged from 14.27 to 19.11 MJ/kg, with only slight variation among them. However, a higher GCV does not necessarily imply superior fuel quality when environmental considerations are taken into account. Among the evaluated species, S. afra was identified as the most suitable fuelwood, followed by O. paniculosa, C. mopane, V. tortilis, and D. cinerea. The average daily household fuelwood consumption was estimated at 14.62 ± 4.5 kg per household (3.11 ± 0.78 kg per capita). At this consumption level, the corresponding average daily emissions were as follows: CO₂ (25.22 ± 7.82 kg; 5.36 ± 1.35 kg per capita), CO (1,424.62 ± 441.52 kg; 302.95 ± 76.33 kg per capita), NO (6.46 ± 2.0 kg; 1.37 ± 0.35 kg per capita), SO₂ (2.24 ± 0.69 kg; 0.48 ± 0.12 kg per capita), and PM₁₀ (195.17 ± 60.49 kg; 41.50 ± 10.46 kg per capita). These results demonstrate the considerable volume of emissions generated from daily residential fuelwood use. The average CO EF values ranged from 54.8 ± 10 g/kg for C. imberbe to 139 ± 20 g/kg for O. paniculosa. Corresponding CO₂ EF values varied between 1207 ± 600 g/kg for S. afra and 1522 ± 30 g/kg for O. paniculosa. The average NO EF values ranged from 0.37 ± 0.2 g/kg for C. imberbe to 2.6 ± 2 g/kg for D. cinerea, while SO₂ EF values varied from 0.48 ± 0.5 g/kg for Combretum apiculatum to 16 ± 20 g/kg for S. afra. In addition, the average PM₁₀ EF values ranged from 3.35 ± 2.28 g/kg for S. burkei to 69.98 ± 71.41 g/kg for S. afra. Overall, average laboratory EFs determined for CO₂, CO, NO, SO₂, and PM₁₀ were 1389 ± 149, 87.04 ± 22.54, 1.22 ± 0.63, 5.16 ± 4.9, and 21.54 ± 19.70 g kg⁻¹, respectively. These findings underscore the importance of conducting comprehensive assessments of woody biomass materials prior to their selection as energy sources. Such evaluations can contribute to reduced atmospheric emissions. It is further recommended that the provincial government, in collaboration with relevant traditional leaders, introduce electricity subsidy programmes in the studied villages to reduce reliance on fuelwood. Additionally, priority should be given to the use of tree species that produce lower levels of pollutant emissions as preferred fuelwood sources. Keywords: Biomass fuel; Fuelwood species; Rural households; Socio-economic factors; Residential fu