The forests would be spared, because making wood pellets requires far fewer trees than wood fires and charcoal. Customers would gain a reprieve from ailments related to smoke from cooking, including cataracts, heart disease and respiratory ailments that, in many countries, kill more people than malaria, H.I.V. and tuberculosis combined. Worldwide, close to 4 million people die prematurely each year from ailments linked to air pollution from cooking, according to the World Health Organization.

Rwandans in rural areas — and, eventually, across Africa and South Asia — would be freed from the time-sucking drudgery of having to look for wood. People in cities, who rely on charcoal, could switch to cheaper wood pellets, using the savings to buy health care, food and school uniforms.

In much of the developing world, initiatives aimed at sparing the environment tend to pit the livelihoods of poor people against the protection of natural resources. Peasants in the Amazon are supposed to stop hacking away at forests to clear land for crops so the rest of the planet can benefit from a reduction in carbon emissions. Yet in Inyenyeri’s designs, the everyday concerns of poor households are aligned with environmental imperatives, because people prefer to cook with the stoves.

At 66, with sunburned cheeks and intense blue eyes, Mr. Reynolds can at times sound like the latest white man come to save Africa. “It’s just outrageous that we have three billion people still cooking in the Stone Age,” he says. “It’s entirely solvable.”

He touts his ability to connect with customers, the women who do the cooking in Rwanda, though even after a decade living in the country he does not speak the local language, Kinyarwanda. He sits on the dirt floors of villagers’ homes and speaks English slowly and loudly, exaggerating each syllable. Rwandan women trust him, he says, because he married one. He hands them his phone and displays the proof — pictures of his wife, Mariam Uwizeyimana Reynolds, 32, and their two fresh-faced boys, Terry Toulumne Reynolds, 6, and Marc Booth Reynolds, 3.