The smokeless performance of flame curtain pyrolysis kilns has been somewhat over-idealized. In this study, a small-scale experiment was conducted using a 30 L iron kiln to produce biochar from nine wood species and five straw species. Flue gas emissions from all feedstocks were visually observed and photographically documented. Comprehensive analyses were carried out on the resulting samples, including determination of uncarbonized materials, proximate analysis, and characterization of pH, electrical conductivity (EC), and water-holding capacity (WHC), which are relevant to the utilization of biochar in farmland. The results showed that all tested woody feedstocks achieved smokeless combustion in the iron kiln. In contrast, flue-cured tobacco stalk and rape straw frequently emitted smoke and even experienced flame extinction, resulting in failed biochar production. This confirmed that problematic feedstocks such as rape straw and flue-cured tobacco stalk compromise the smokeless advantage of flame curtain pyrolysis kilns. Notably, under conditions of low biochar production, the fixed carbon content of flue-cured tobacco stalk biochar exceeded 50%, and the biochar yield surpassed 10%. Therefore, elucidating the combustion inhibition mechanisms and eliminating these inhibiting factors would not only provide a theoretical foundation for processing straw biomass in flame curtain kilns, but also enable full realization of their smokeless potential in handling straw materials.