One might be tempted to think that a cookstove that burns wood or some kind of biomatter (such as biomass pellets made from agricultural waste) is contributing to the overall impact of CO2 emissions in the atmosphere. However, this is a crude assumption that does not take into account the complexities of the alternatives. In fact, all forms of energy generation, storage and usage has some kind of impact. The objective is to find the forms that have the least impact and which are sustainable in the larger context of the ecosystem as a whole.
Ecosystem sustainability can be most simply defined as ‘activities in which the overall impacts are able to be absorbed by the system within which they occur’. A simple example of this is a village that cuts and uses wood for cooking. If the amount they use is less than the capacity of the forest to easily regenerate on a seasonal basis, then it would be sustainable. If the usage impacts the forest system such that it degrades over time, then it is not sustainable.
In order to reduce impacts, there are 2 key metrics. Quantity and quality. The volume of wood being harvested is the quantity, and there are certain amounts of regrowth each year that can be sustainably harvested. The quality is more nuanced but also not hard to see. If a tree is cut fully, then its ability to regrow or regenerate is impaired or lost completely. If several trees are coppiced by the same volume as the one fully cut tree, they will regenerate much faster. So in this way we can see that quantity is not the only metric that matters.
If a stove design caters to small sizes of wood, and small amounts of wood, it fosters a habit that is more sustainable than stoves that require larger volumes and accommodate larger sizes. If the stove caters to agricultural waste that is pelletised, then no trees are cut or coppiced at all. In this way the reduction in impact on forests is indisputable.
If the combustion technology of the stove is such that it produces very little in the way of smoke, then the emissions are few, and the trees that are left in the ground continue to store carbon in the most effective way we know. Agricultural waste that was anyway going to be burnt off and release great plumes of toxic smoke as well as CO2, being pelletised for fuel, is an incredibly effective way to reduce emissions on balance with the alternatives.