## 3 Results and Discussion
### Gas Emission Factors  
Importantly, methane emissions were below quantification levels (i.e., <5.5 g kg\({}^{-1}\) biochar) for grass, maize cobs, and grass/twigs (Table 2). For sesame stems, methane emissions were detected only during two short positive methane detections of 253 and 1097 g kg\({}^{-1}\) biochar lasting around 10 to 30 s halfway through the pyrolysis process and during final quenching, respectively. Over the whole pyrolysis cycle, this value would increase the average methane emissions from <5.5 to 17 g kg\({}^{-1}\) biochar. Delaying quenching by a few minutes might have resulted in complete pyrolysis of the sesame stems at the surface, and this could have avoided the second and highest spike in methane emissions. Low methane emission factors were mostly in accordance with earlier reported values for dry (15% moisture) twigs (0-3.6 g kg\({}^{-1}\)) and lower than those for twigs with 25% moisture (28.5 g kg\({}^{-1}\)) [7]. Much higher methane emission factors were measured for coffee husks (179 g kg\({}^{-1}\); Table 2) due to the flame curtain being quenched by the smaller particle size of this feedstock. The grass biomass was lifted by poles upon addition to the kiln to avoid disruption of the flame curtain, and through this approach, the pyrolysis of pure grass biomass was found not to result in detectable methane emissions. Maize cobs also had to be added at a low rate to avoid quenching the flame curtain, highlighting the importance of keeping the flame curtain intact. Methane emissions of 179 g kg\({}^{-1}\) biochar, as observed in this coffee husk trial, correspond to around 4.5 kg CO\({}_{2}\)-eq kg\({}^{-1}\) biochar (Table 2), exceeding the approximately 2.0-2.5 kg CO\({}_{2}\)-eq sequestered by biochar amendment to soil [33]. The data show that many different biomasses can be pyrolyzed with low to zero methane emissions, except for finely grained feedstocks such as coffee husks and likely rice husks and cashew nut shells as well. Furthermore, quenching is a part of the cycle that is especially prone to methane emissions. First, after the last feedstock addition, quenching needs to be delayed until after the flames have disappeared, which is a signal of pyrolysis completion. Second, quenching has to be fast and effective to avoid methane emissions when there is no more flame curtain to combust potential syn-gases from the last stages of the pyrolysis process. Emission factors for CO (between 21 and 82 g kg\({}^{-1}\) biochar; Table 2) were close to those previously reported for dry twigs in Kon-Tiki kilns (52 g kg\({}^{-1}\)[7]; 3-24 g kg\({}^{-1}\)[8]; 101 g kg\({}^{-1}\)[19]). CO emissions were higher for maize cobs (70-90 g kg\({}^{-1}\)) than for grass (\(p\) = 0.02), sesame stems (\(p\) = 0.0007), and grass/twigs (\(p\) = 0.04; all 30-40 g kg\({}^{-1}\)), probably because the conversion of CO to CO\({}_{2}\) is less efficient for maize cob as a denser feedstock than grass, sesame stems, and sticks, which allow for better mixing between oxygen and the emitted syn-gases. CO emissions were lower than those from traditional kilns (351 g kg\({}^{-1}\)[28] and retort kilns (148 g kg\({}^{-1}\)[28]) for all biomass types and comparable to those from Top-Lit Updraft Kilns (TLUDs; 94 g kg\({}^{-1}\)[28]). CO emission factors could not be quantified for coffee husk pyrolysis due to sensor failure. This led to maximum deviations of <3% in the overall carbon balance (CO being <3% of total gas emissions) and a slight (<3%) uncertainty in the other emission factors reported for this feedstock. In general, the large interquartile ranges in all emission factors do not reflect a lack of data but a high variability of gas emissions during operation, which is caused by variations in burning conditions during individual runs.