2.1.2. Fuels

The chemical composition of both considered fuels was specified by the furnace operator. Natural gas available in the facility contains the following combustible species: 97.97% methane, 0.76% ethane, and 0.38% other hydrocarbons (by volume)—the rest consists of carbon dioxide, nitrogen, and oxygen (<0.1%). NG is fed to the furnace at 20 ◦C (293 K).

The alternative fuel is generated by the located on-site fluidized bed gasifier, in which air is used as a gasification agent. The gasifier produces up to 1 MW power in gaseous fuel (ca. 0.7 MW on average). The lower heating value of a typical syngas produced by this type of gasifiers fluctuates in the range of 3–7 MJ/m<sup>3</sup> <sup>N</sup> [18]. As one of the adopted objectives to achieve higher efficiency of the co-firing system is to preserve the initial high temperature of syngas, it was assumed that the SG produced by the gasifier is not cooled down on the way to the furnace, and it enters the dedicated burners at 600 ◦C (873 K). Hot syngas is not cleaned from tars—SG temperature should not drop below 350 ◦C (633 K) to avoid tar condensation and contamination of the installation. The amount of energy contained in tars (below 0.04 MJ/m<sup>3</sup> <sup>N</sup> of SG, as tars concentration in the considered syngas is lower than 1 g/m3 N) is insignificant for the course of the heat treatment.

Presence of species with concentrations lower than 0.5% vol. was neglected. The composition of the gasification product is variable over time and estimated average values were chosen for the calculations (Table 1). Lower heating values for natural gas and synthesis gas are 50.05 and 4.52 MJ/kg, respectively. The fuels were considered in a dry state.


**Table 1.** Chemical composition of natural gas (NG) and syngas (SG) adopted for simulations.

Despite the content of CH4, CO, and CO2, syngas is regarded as carbon-neutral because of its biomass origin. This means carbon dioxide emission factor for SG is 0 kg CO2/GJ, instead of 135 kg CO2/GJ as its gas composition would indicate. NG emission factor is 55 kg CO2/GJ.
