## 2 Materials and Methods
### Emission Measurements  
Gases analyzed were CO\({}_{2}\), CO, CH\({}_{4}\), NO\({}_{\omega}\), and aerosols (\"smoke\", Total Suspended Particles (TSP), derived from PM\({}_{10}\)). Instrumentation, carbon mass balance calculations, and statistical analyses were carried out as described in previous work [7; 19; 28]. As in those works, a Microtector II 6460 (GfG Instrumentation, Ann Arbor, MI, USA) was used to analyze carbon dioxide (CO\({}_{2}\)) and methane (CH\({}_{4}\)) by infrared sensors and NO by an electrochemical sensor. CO\({}_{2}\) had a detection limit of 0.1%, CH\({}_{4}\) of 0.005% (0.1% of the Lower Explosive Limit of 5%), and NO of 1 ppm (0.0001%). To account for potential emissions related to methane concentrations below the instrument limit of quantification (LOQ), we  
* from Braga, Costa [23], \({}^{b}\) from Braga, Costa [23], \({}^{c}\) from Braga, [PERSON] [23] and [PERSON] and [PERSON] [24]. \({}^{c}\) from [PERSON], [PERSON] [25]. \({}^{d}\) from [PERSON], [PERSON] [26]. \({}^{e}\) not determined as this feedstock was pyrolyzed during a run with maize cobs, as it proved impossible to pyrolyze pure coffee husk in a flame curtain kiln. \({}^{f}\) from [PERSON], [PERSON] [27]. \({}^{g}\) from [PERSON], [PERSON] [19]. \({}^{h}\) from [PERSON] and [PERSON] [24]. \({}^{i}\) from [PERSON], [PERSON] [7].  
\end{table}
Table 1: Moisture contents (%), biomass weight in (kg dry weight, dw), biochar mass out (kg), biochar mass yield (%), feedstock C contents (%), biochar H, N contents (%), biochar H/C ratios, biochar pH.  
assigned a discreet value of LOQ/2 (0.0025%). Data points recorded as <LOQ were hence reported as an EF of <5.5 g kg\({}^{-1}\) biochar. The instrument converted nitric oxide (NO) to total nitrogen oxides (NO\({}_{\text{x}}\)) by applying a conversion factor of 1.05, thus assuming 95% of NO\({}_{\text{x}}\) consists of NO [29]. As with methane, to account for potential emissions related to NO\({}_{\text{x}}\) concentrations <LOQ, a discreet value of LOQ/2 (0.05 ppm) was assigned. Data points recorded as <LOQ were hence reported as an EF of <0.006 g kg\({}^{-1}\) biochar.  
Carbon monoxide (CO) was analyzed with a Kigaz 300 flue gas analyzer (Kimo Instruments, Le Val-Saint-Germain, France) by internal jacket-type electrochemical sensors, with a detection limit of 1 ppm (0.0001%). For CO values above 8000 ppm, the instrument internally dilutes the gas stream to detect concentrations of up to 50,000 ppm.  
Particles in the form of PM\({}_{10}\) were analyzed with a Thermo Scientific PDR-1500 (Thermo Fisher Scientific, Franklin, MA, USA) instrument with photometric detection of particles (limit of detection 0.1 mg/m\({}^{3}\)).  
Non-methane volatile organic carbon (NMVOC) emissions were only measured at a few data points for some feedstocks before sensor failure during fieldwork. NMVOC has earlier been shown to constitute <0.1% of total carbon emitted and <1% of all products of incomplete combustion (all gases except CO\({}_{2}\)) [7; 19; 28]. Therefore, NMVOC data are omitted from this study.  
To calculate the emission factors of the kilns, the carbon balance method was utilized [28; 30; 31]. In this method, only the emission ratios between the gases are measured, without the need to register the absolute mass of gases emitted. Instead, this mass is calculated by performing a carbon balance between the biomass entering the process and the biochar produced. From ten to twenty single-point ratios, time-weighted average values were calculated. Net molar component-to-CO\({}_{2}\) emission ratios for the measured gases and TSP (from PM\({}_{10}\)) for the flame curtain runs were calculated for CO, CH\({}_{4}\), TSP, and NO\({}_{\text{x}}\). These ratios were used to calculate the emission factors in g per kg biochar produced. Two-sample t-tests with unequal variance were used to test for the effects of feedstock type on gas emission factors. Median values and interquartile ranges were reported to avoid data bias due to the high inherent variability of the measurements.