Flame curtain kilns produce biochar from dry biomass with minimal methane emissions.

Flame curtain kilns have emerged as the preferred biochar technology for smallholders but reported methane emissions (30 g kg biochar) have impeded carbon certification. Here, for flame curtain kilns we show almost no methane (0-3.6 g kg biochar) emissions for dry (<15 % moisture) feedstock consisting of twigs and leaves. Wet feedstock (>40 % moisture) however generated significant methane (>500 g kg biochar), underscoring that feedstock preparation is decisive for the carbon balance. Even for dry feedstock, both aerosol and CO emissions were significant (21-82 and 40-118 g kg biochar, respectively). The data demonstrate that certification of low-tech biochar made from dry twigs and leaves should not be objected to on the grounds of methane. Careful selection of feedstock and potential after-combustion of the syn-gases are probably needed to avoid CO and aerosol emissions. More data are needed on methane emissions of other dry feedstocks.