Thermal conversion of ligno-cellulosic agricultural waste inside a pyrolysis reactor and soil-storage of pyrolysis char is discussed as a low-tech negative emission technology. Cotton stalks are a typical agricultural residue with little economic value and there is no direct competition to food or feed provision. Currently, the stalks are often burnt on the fields. The investigated rotary kiln pyrolysis process has already been successfully applied at industrial scale before. In the investigated scenario, the pyrolysis char is returned to the soil for long-term carbon storage while the volatile pyrolysis products are used energetically. The steady-state process simulation environment IPSEpro was used to assess a virtual conversion plant. The mass- and energy flows are determined based on earlier measurements at a 500 kg/h test plant. The results show that 52.8% of the carbon contained in the biomass accumulate in the biochar, whereas 38% of the input energy can be exported as heat energy at temperature levels suitable for electricity generation or industrial heat supply. The pyrolysis char shows a low molecular O/C ratio of 0.07 and an H/C ratio of 0.26. The expected half-lives of biochar in soil are in the order of 1000 years for O/C ratios below 0.2. This makes the presented approach an interesting low-tech negative emission option. The predicted net negative emissions through stored carbon amount to 2.42 t CO2 per hectare and year. The overall CO2 emission avoidance effect can be increased if fossil fuel is substituted by the energy exported from the pyrolysis process.