## 3 Results and Discussion
### Policies and Standards Linked with the Energetic Use of Agricultural Waste in Ecuador
For the rural sector of low- and middle-income countries, flame-curtain kilns and modern biomass cooking stoves (e.g., top-lit updraft stoves, TLUDs) have emerged as alternatives for biochar production [55]. Flame-curtain kilns are low-complexity alternatives to produce biochar that are mostly adapted to the use of forest residues as feedstock. Flame-curtain kilns are inverted pyramid shaped kilns or deep-cone bowls that could be made of steel and as a simple soil pit, consisting of a conically shaped hole in the ground. Unlike retorts and converters, flame-curtain kilns reduce the waste of residual biomass during the initial heating process [56]. However, it is difficult to recover the heat generated during the carbonization process in flame-curtain kilns for use in drying and other processes (Figure 4). Several models of TLUDs, in contrast, are well adapted to efficiently use various types of agricultural waste [57]. However, the yield of biochar is usually too low to supply more than a single farm or to sell it as charcoal (0.5 to 1 kg per batch). Probably, the case of bigger community TLUDs [58] that produce up to 10 kg of biochar per batch would be more convenient, given the quantity and type of agricultural waste produced in these farmers' cooperatives and our selection criteria (Table 1). Nevertheless, the typical batch operation of TLUDs will result in an intermittent supply of thermal energy, and the drying operations in these processing centers would need to be adapted accordingly. The simplicity of flame-curtain kilns and TLUDs facilitates local construction and implementation of pyrolysis facilities in rural areas. However, they allow for little or no monitoring and control (e.g., for temperature) during the carbonization process. Accordingly, the properties and quality of the resulting biochar can be uncertain and highly variable, breaching Criteria 2 and 6 (Table 1). Although the physicochemical characteristics of biochar produced in TLUDs and flame-curtain kilns can be in accordance with international standards for safe application in soils [59], some studies of biochar that was produced at low temperatures in simple carbonization kilns have also found high concentrations of polycyclic aromatic hydrocarbons and heavy metals, which can threaten soil health [60; 61]. As an alternative, auger-type reactors and rotary kilns are industrial models that can guarantee energy self-sufficiency (Figure 4), traceability, quality, homogeneity, and replicability of the physicochemical properties of the produced biochar in agreement with the selection criteria (Table 1) [62; 63]. Auger-type reactors and rotary kilns allow for continuous operation and facilitate the productive use of the heat that is generated during the carbonization process [64]. A tentative design of the pyrolysis process integrated into the farmers' cooperatives allows part of the heat that is generated during the carbonization of coffee husks to be used to replace diesel and LPG in the mechanical drying process (Figure 4). Following the selection criteria (Table 1), the burning of pyrolysis gas should lead to energy self-sufficiency. The required monitoring to guarantee the biochar quality and homogeneity is also facilitated. The combustion of pyrolysis gases is of major importance to meet Criteria 1 and 3-6 (Table 1). One concern, however, is that the combustion of gases with high tar content as the pyrolysis gases has been linked with increased particulate matter (PM) emissions, namely, PM\({}_{10}\), PM\({}_{2.1}\), and nitrous oxide emissions [65]. The control of these pollutants requires the implementation of complex processes of combustion and flue gas treatment, such as flameless oxidation (FLOX) processes, electrostatic precipitators, bag filters, or cyclones [66]. Therefore, the production of high-quality biochar in auger and rotary kiln reactors results in elevated initial investment costs (>500,000 USD) and high maintenance costs.