New Zealand has a goal to achieve carbon zero emissions by year 2050. Fossil fuels are still the main energy sources for steam boilers which are important elements in the process heat sector. Therefore, it is quite important to decarbonise the process heat sector by using low carbon energy sources, such as green hydrogen, grid sourced hydrogen, blue hydrogen, hydrogen blend, wood pellets and electricity. In this research, a comparative analysis of these energy sources is conducted to find out the difference between fuel options in terms of carbon emission reduction, conversion efficiency and the levelised cost of energy. The outcome is that wood pellet boiler system has the best performance with the lowest carbon emission factor from energy delivered to plant, high overall energy conversion efficiency, and low levelised cost of energy. Further analysis was conducted to find out how the real-time prices of grid electricity impact the levelised cost carbon emissions of energy for grid sourced hydrogen systems. It found that levelised cost of energy of grid sourced hydrogen is less than the one of natural gas boiler system, when spot electricity prices are less than 37.98 NZD/MWh with carbon price at 70 NZD/tCO2-e. Also, grid sourced hydrogen brings more carbon emissions if it is generated by using the real-time grid electricity with higher prices. The real-time carbon emissions factor and annual carbon emission factor affect the annual amount of carbon emissions and carbon costs when applied to real world case studies of steam boiler systems. The total annual amount of carbon emissions calculated using the average annual carbon emissions factor from the official source was less than that calculated by real-time carbon emission factor for the case studies considered. That would have implications for the carbon costs paid for by companies. Correlations among the electricity spot price, active lake storage and real-time carbon emissions factor of grid electricity was also found. Low carbon emissions factor of grid electricity occurred most often when the electricity spot price was low and high active lake storage level were available.