Plasma conversion is an alternative approach to the electrochemical and photochemical technologies searching for most efficient way to convert CO₂ into carbon monoxide (CO). The CO₂ plasma conversion investigated in recent years demonstrated conversion and energy efficiencies up to 50%. Commonly, these values are obtained at pressures lower than atmospheric pressure at about 100 mbar. In this work we present an investigation of the CO₂ conversion and the energy efficiency in a microwave plasma torch equipped with a nozzle of different diameters installed downstream of the plasma at varying distances to the microwave discharge, in the pressure range 100–900 mbar. The results obtained using the nozzle demonstrate an enhancement of conversion and energy efficiency, particularly at pressures close to atmospheric pressure and for lower CO₂ flows (SEI above 2 eV/molecule), bringing the values of conversion and energy efficiency close to those obtained at lower pressures. It is assumed that the fast cooling of the hot plasma gas with surrounding colder gas leads to reduction of CO recombination into CO₂ thus preserving the maximum conversion obtained. With a 2.5 mm nozzle at sub-atmospheric pressures, conditions with large pressure difference in the resonator and in the effluent are observed, which is accompanied by an additional increase in conversion. A likely reason for this is reduction of the recombination rates due to lowering of the pressure in the effluent and possible temperature reduction resulting from the supersonic expansion. Using the nozzle it is possible to significantly increase the performance of the plasma torch at 900 mbar, both in conversion and energy efficiency towards values achieved at 200 mbar, which is an important step towards industrial applicability of this technology.

等离子体转化是电化学和光化学技术的替代方法，可寻找将 CO ₂转化为一氧化碳 (CO) 的最有效方法。近年来研究的 CO ₂等离子体转化表明转化率和能量效率高达 50%。通常，这些值是在低于大气压力约 100 毫巴的压力下获得的。在这项工作中，我们对 CO ₂在 100-900 毫巴的压力范围内，微波等离子体炬配备了安装在等离子体下游与微波放电不同距离的不同直径喷嘴中的转化率和能量效率。使用喷嘴获得的结果表明转化率和能量效率提高，特别是在接近大气压的压力和较低的 CO ₂流量（SEI 高于 2 eV/分子）时，使转化率和能量效率值接近于在较低的压力。假设热等离子体气体与周围较冷气体的快速冷却导致 CO 再结合成 CO _{2 的}减少从而保持获得的最大转化率。在低于大气压的压力下使用 2.5 毫米喷嘴，观察到谐振器和流出物中具有大压差的条件，这伴随着转化率的额外增加。造成这种情况的一个可能原因是由于流出物中压力的降低和超音速膨胀可能导致的温度降低导致重组率降低。使用喷嘴可以显着提高 900 mbar 等离子炬的性能，在转换和能源效率方面均达到 200 mbar 的值，这是该技术向工业适用性迈出的重要一步。