## 3 Results and Discussion  
### Biochar Production Process  
Figure 1 shows the temperatures recorded in the TLUD combustion chamber during charring, for each biochar prepared (BB1, BB2 and BB3 for bamboo and WB1, WB2 and WB3 for pinewood). Each observation period lasted 60 min, including the holding time depended on the activation of the fire inside the chamber. These temperatures were recorded with thermocouples placed in the space above the charring material in the TLUD combustion chamber. At first sight, it is apparent that, with pinewood, temperatures over 500 \({}^{\circ}\)C were reached within the first 20 min and maintained nearly until the end of the charring period. Meanwhile, the bamboo data show comparatively lower temperatures as well as lower reproducibility in the patterns and the overall temperatures. BB1 showed the most delayed heating process; temperatures rose above 260 \({}^{\circ}\)C only during the last 25 min, while BB2 and BB3 reached 500 \({}^{\circ}\)C much earlier. In all cases, overall temperatures recorded at 18 cm from the bottom were lower, possibly due to admixture of secondary air. It is not clear what may have caused the differences in the temperature patterns observed in the BB essays, as the sample feedstock was the same and was prepared in a single batch. Figure 1: Pattern of temperatures of the triplicate charring lots during production of two biochars made of _Guadua_ bamboo (BB), (**a**) BB1, (**c**) BB2 and (**e**) BB3, and pinewood waste (WB), (**b**) WB1, (**d**) WB2 and (**f**) WB3. Yields of the biochars ranged from 16% to 24% (Figure 1), similar values to those (10-30) previously reported by [PERSON] [31]. No statistically significant differences between the groups were found (\(p=0.0972\)). In TLUD stoves, the heat required to drive pyrolysis is provided by combustion of the gases emitted by the heated material. A single load of biomass is placed in the combustion chamber and lit at its top, so the pyrolysis front advances downwards, driven by the heat produced by combustion of gases in and above the front. According to [PERSON] et al. [35] the air supply drives the temperature gradient inside the TLUD, so total gasification can be reached within few minutes, depending on the incoming air flow. Partial combustion of the charred material also takes place, since primary air passes from the bottom of the material to its top. Secondary air enters above the biomass and mixes with the partially combusted gases to complete combustion. The temperatures recorded at thermocouple II, in a higher position above the feedstock, were generally lower, evidencing the entrainment of secondary air. Charring can be considered complete for most of the feedstock once the production of combustion gases seems to have ceased, taking the lack of a flame for evidence. The temperature that the pieces of biochar undergo at their surface is that of the surroundings, but due to short charring times, the maximum temperature of the interior of the pieces could be lower if the feedstock pieces are large enough. A limited control of the reaction conditions is believed to produce variability in quality of the biochars [31], mostly depending on the amount of air entering the chamber [35]. However, the TLUD scheme necessarily leads to charring times that vary from less than 10 min, for the bottom material, to nearly the whole charring period, for the material at the top, thus producing an inherently and necessarily heterogeneous biochar anyway. Charring temperature and time can have significant repercussions on surface chemistry and macroporosity of a char. However, composition of the feedstocks can have an influence on the process itself, and therefore on its results. Above 120 \({}^{\circ}\)C, organic materials begin to undergo some degree of thermal decomposition (pyrolysis), first losing chemically bound moisture. Hemicelluloses start to be pyrolyzed from 200 to 260 \({}^{\circ}\)C, cellulose at 240-350 \({}^{\circ}\)C, and lignin at 280-500 \({}^{\circ}\)C [13].