The amount of food waste due to the product expiration date is growing globally each year. Although the expired food loses its nutritional and safe edible value, it still offers great energy conversion value. In this study, expired pistachios were pyrolyzed and gasified in a semi-batch reactor at temperatures of 873–1223 K. The gases components of the produced syngas were analyzed using a micro-gas chromatograph for the syngas yield, and gases mass flowrates as well as the energy of each component in the syngas were calculated. CO2 consumption from the gasification reaction at different temperatures was also evaluated. Experimental results showed that the syngas yield and syngas energy from pyrolysis and CO2-assisted gasification increased with the in-reaction temperatures. Higher reaction temperature resulted in a shorter reaction time for the evolution of the peak value of the syngas mass flowrate. During pyrolysis, the increase in temperature from 873 to 1223 K enhanced syngas yield by 8.6 times from 1.42 kJ/g to 13.62 kJ/g. However, during the CO2-assisted gasification, syngas energy increased from 5.43 kJ/g to 17.27 kJ/g in the temperature range of 973–1173 K. The CO2 consumption in the gasification of pistachio samples enhanced with the increase in reaction temperature. The mass of CO2 consumption at 1223 K was 0.67 g/g, which was 138 times higher than that of 0.005 g/g at 973 K. Furthermore, at the same temperature (1223 K), the syngas yield from gasification was 1.3 times higher than that from pyrolysis. Thus, higher temperatures promoted the reaction rate of gasification processes as well as the consumption of greenhouse gas (CO2). The CO2-assisted gasification technology is an effective pathway to convert expired food into clean sustainable energy.