The iron and steel industry is the second largest user of energy in the world industrial sector and is currently highly dependent on fossil fuels and electricity. Substituting fossil fuels with renewable energy in the iron and steel industry would make an important contribution to the efforts to reduce emissions of CO2. However, different approaches to assessing CO2 emissions from biomass and electricity use generate different results when evaluating how fuel substitution would affect global CO2 emissions. This study analyses the effects on global CO2 emissions when substituting liquefied petroleum gas with synthetic natural gas, produced through gasification of wood fuel, as a fuel in reheating furnaces at a scrap-based steel plant. The study shows that the choice of system perspective has a large impact on the results. When wood fuel is considered available for all potential users, a fuel switch would result in reduced global CO2 emissions. However, applying a perspective where wood fuel is seen as a limited resource and alternative use of wood fuel is considered, a fuel switch could in some cases result in increased global CO2 emissions. As an example, in one of the scenarios studied, a fuel switch would reduce global CO2 emissions by 52 ktonnes/year if wood fuel is considered available for all potential users, while seeing wood fuel as a limited resource implies, in the same scenario, increased CO2 emissions by 70 ktonnes/year. The choice of method for assessing electricity use also affects the results.