Overview

New research shows that advanced biomass stoves used with biomass pellets may offer a pathway to clean energy for communities where electricity or gas are not yet viable. The study was conducted by North Carolina State University in Gisenyi, Rwanda to assess the potential climate and health benefits of an advanced biomass stove (the Mimi Moto forced-draft, pellet-fed gasifier) during household use.

*This research was funded by the Clean Cooking Alliance and the Climate and Clean Air Coalition.

Key takeaways

  • The potential climate and health benefits of the pellet stove approach those of gas stoves. 
  • Compared to cooking over a three-stone fire, the pellet stove reduced emission rates by approximately 90% or more for most measured pollutants (black carbon, particulate matter, and carbon monoxide).
  • The climate impacts of the pellet stoves are negligible when the pellets are produced from renewable sources, such as gathered wood or agricultural waste, and when using renewable energy to power pellet production.
  • Although the pellet stove exceeded the particulate matter emissions target for the World Health Organization indoor air quality guidelines, it neared the interim target, offering drastic improvement over traditional stoves, and the pellet stove met the guideline for carbon monoxide emissions.
  • Stove maintenance and customer support programs are critical for ensuring that stoves perform as designed once they are off the shelf and in customers’ homes.

Summary

Up to 58% of global black carbon emissions come from household cooking, heating, and lighting, and the majority of those emissions originate in African and Asian countries. Recognizing clean cooking as a critical solution for combatting climate change, many governments and investors are backing transitions to modern clean fuels such as electricity or LPG (liquid petroleum gas). Yet both require large investments in infrastructure and carry high up-front costs, making them out of reach for many consumers in the short, or even medium term. Improved biomass stoves can potentially offer a viable solution for those consumers, but little data exists on the emissions reduction potential of advanced biomass technologies outside of a laboratory setting.

As part of an effort to understand how the various stove and fuel combinations available on the market perform during household use and how these various technologies may contribute to achieving universal access to clean cooking solutions by 2030, the Alliance awarded Dr. Andy Grieshop at North Carolina State University a research grant to determine the emissions performance of the Mimi Moto forced-draft gasifier stove when used with pellets in homes.

Why the researchers chose Rwanda:

In Rwanda, 98% of the population relies on firewood and charcoal for cooking. This reliance on firewood and charcoal contributes to forest degradation. Over 52% of wood harvested for firewood or charcoal production in Rwanda is non-renewable and 84% of Rwanda’s total climate damaging emissions come from household cooking. While the limited access to clean cooking poses a significant challenge, there are efforts underway to find solutions that fit the Rwandan context. One example is Inyenyeri, a social business that utilizes a bartering model for clean cooking. Inyenyeri provides a forced-draft, pellet-fed, gasifier stove (Mimi Moto) for free to consumers who are willing to pay for or exchange gathered firewood for a supply of pellets. Pellets are a processed biomass fuel that burns more efficiently due to the consistent shape and size.

What the researchers did:

The study assessed real-time emissions of carbon dioxide (CO2), carbon monoxide (CO), organic and elemental carbon (OC, EC), black carbon (BC), and fine particulate matter (PM2.5) from households cooking in urban and rural homes in Gisenyi, Rwanda. Emissions were measured in 22 randomly selected homes,14 were randomly selected from Inyenyeri customers using the Mimi Moto, and 8 from the local community (4 three-stone fire and 4 charcoal). Uncontrolled cooking tests (UCTs) were conducted during two seasons November/December 2017 and May/June 2018 in order to characterize the potential climate and health co-benefits of the forced-draft, pellet-fed gasifier stove. In total the researchers conducted 91 UCTs (59 pellet tests; 16 wood; and 16 charcoal).

What they found:

The researchers found the pellet stove reduced most pollutant emissions by approximately 90% or more compared to a three-stone fire. The climate impacts of pellet stoves are estimated to be at least 10 times less than charcoal stoves, and negligible when the pellets are produced from a renewable source, such as agricultural waste or sustainably harvested wood, and with electricity from renewable sources. The results further indicate that the pellet stoves can offer high potential co-benefits for health—the pellet stove reduced PM2.5 emission rates by 97% compared to wood, and 88% compared to charcoal; and reduced CO emissions by 87% compared to wood, and 95% compared to charcoal.

The pellet stove has not been without criticism, particularly for its inconsistent performance. Some users find it difficult to light or refuel correctly, resulting in the stove producing emissions on par with a three-stone fire. Over the course of the 59 conducted tests of the pellet stoves, around 10% included these heavy smoke events. This finding suggests that stove maintenance and customer support programs are important for ensuring that stoves perform as designed. In the case of the Mimi Moto, the researchers recommend that to achieve peak performance great care should be taken during refueling, towards the end of cooking, and that pellet char should be properly disposed of and not left to smolder.

For the first time, this research indicates that a biomass stove used with biomass pellets can approach the emissions performance, and potential climate and health impacts, of a gas stove. These findings have far reaching implications for governments, investors, product developers, and for the clean cooking sector at large.

Sector Implications                                                                    

  • Measuring impact: This study provides the first published emissions factors from the Mimi Moto stove under household use conditions. This data is a critical input into climate and energy planning models as well as for estimating program/project impacts.
  • The fuel matters: Pellets lead to enormous benefits compared to using collected wood in a gasifier stove. The researchers note that a consistently sized and shaped fuel (such as pellets) could greatly improve the emissions performance of an already advanced stove design.
  • Affordability: Pellet stoves may offer a more affordable option relative to LPG or electricity with the right business model or program.
  • Suite of clean cooking options: High-performing biomass stoves, such as the pellet-stove, should be considered a viable step for transitioning to cleaner energy, especially for those that cannot afford the more modern fuels (e.g. LPG and electricity).
  • Consumer follow-up: Companies, and governments sponsoring clean cooking programs, should consider the importance and sustainable implementation of maintenance and customer support programs.
  • Scale-up: Companies interested in the Mimi Moto stove and pellet production model should consider that scaling to meet customer demand requires a consistent biomass supply, industrial-scale equipment and facilities for pellet production, reliable power, and infrastructure for fuel distribution. The researchers also emphasize the need for an effective means to promote the complete adoption of the technology.

“Our study shows that with this combination of stove, fuel and business model, Inyenyeri has hit on a biomass-based system with enormous and exciting potential to provide air pollution reductions and climate benefits. The key challenge now is to show that this approach can be scaled up in Rwanda and expanded to communities around Africa and the world.” – Dr. Andrew Grieshop

Links to further reading and citations

Implementation and Scale-Up of a Biomass Pellet and Improved Cookstove Enterprise in Rwanda https://www.sciencedirect.com/science/article/pii/S0973082618302801

The Carbon Footprint of Traditional Woodfuels https://www.nature.com/articles/nclimate2491

Black Carbon Emissions Trends http://www.ccacoalition.org/ru/slcps/black-carbon