Title : Combustion performances of advanced cooking stoves using woody and herbaceous pellets as fuel
Abstract:
In Africa, biomass is primarily used in the form of wood and charcoal for heating and cooking (WBA, 2020). South of the Sahara Desert, wood fuels and charcoal account for more than 90% of the traditional biomass used in energy production (Smeets et al., 2012). In 2019, Africa was responsible for 65% of the global production of charcoal (WBA, 2020). The population in Zambia relies on charcoal. It is the only option for those without access to electricity. Approximately 85% of the rural population and 55% of the urban population use charcoal as their main source of energy (Matkala et al., 2015). The demand for charcoal has led to deforestation on the order of 250,000–300,000 hectares per year (Day et al., 2014). The production is done in primitive kilns where only about 10–20% of the energy present in logs is utilized (Kassam, 2012). Continuous and prolonged exposure to volatile particles and CO-emissions from cooking causes an estimated 4.5 million deaths each year (WHO, 2019). One solution is to convert to sustainable advanced cooking stoves with biomass pellets as the fuel.
A testing facility has been designed to analyze the combustion of pellets in cooking stoves at Karlstad University. The facility includes measurements of carbon monoxide, nitrogen oxides, particles, as well as the temperature and velocity of outgoing flue gases. Throughout the entire combustion process, fuel consumption of the stove and water temperature are monitored. The test unit and its design satisfy most of the requirements outlined in ISO 19867-1, making it suitable for conducting tests on the combustion of various types of pellets in different cooking stoves.
The study investigated the impact of wood pellets, straw pellets, alfalfa pellets, elephant grass pellets, as well as pellets made from coconut shells, rice husk and Zambian pine, on combustion emissions from cooking stoves. Results indicate that higher moisture and ash content lead to increased emissions of CO and particles. Ash content also impacts NOx-emissions and reduces thermal efficiency, while moisture content significantly affects ignition. Rice husk and straw pellets exhibit both the highest moisture content and ash content, resulting in high emission factors for CO and particles per kg of fuel. The study further reveals that excessive turbulence within the combustion chamber can result in elevated CO- and particle emissions during ignition and extinguishing. Conversely, achieving a more uniform velocity distribution in the chamber can reduce emissions of CO and NOx, particularly during extinguishing. However, it is essential to avoid excessive turbulence during ignition and extinguishing, especially in stoves where airflow is regulated by a single fan.
To enhance the performance of cooking stoves, it is advisable to produce pellets from biomass with low ash and moisture content, like wood. This ensures efficient combustion, leading to higher efficiency and reduced emissions, with mitigating health risks and environmental impact. If more herbaceous materials, like elephant grass and rice husk, should be used then the stoves need improved design regarding the primary and secondary air flow especially during ignition and extinguishing.
Audience Take Away Notes:
- The results will help in deciding types of raw materials suitable for pelleting and cooking.
- The findings can help in setting up new research and development goals regarding the transition from cooking with charcoal to cooking in highly efficient pellet stoves.
- Understanding the differences of combustion performances among different raw materials can help designers of cooking equipment to optimize their stoves.
