# Experimental Comparison of Diesel and Crude Rapeseed Oil Combustion in a Swirl Burner
## 1 Introduction
An overview of HRF utilization by fellow researchers and their key findings is shown in Table 1. The conclusion is the following: stable combustion is possible up to a few times ten percent HRF content, but more concentrated blends usually result in either unstable operation or excessive pollutant emission. However, novel concepts offer seamless operation. The key challenges of HRF combustion are summarized in the following. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fuel (Blends)** & **Platform** & **Key Findings** & **References** \\ \hline waste cooking oil & industrial & Neat WCO combustion is unstable. 20\% WCO + & \\ (WCO) and diesel & burner & 80\% diesel oil blend provides a stable flame. & [15] \\ \hline pyrolysis bio-oils & industrial & Pyrolysis bio-oil combustion is possible with low & \\  & burner & emissions; however, a system upgrade & [16] \\ \hline sunflower oil (SFO) & micro gas & 15\% and 30\% SFO–diesel oil blends provided & \\ and diesel & turbine & smooth operation with increased CO and & \\  & & NO\({}_{\mathsf{X}}\) emissions. & \\ \hline liquefied wood & micro gas & Stable combustion and energy generation is & \\ and ethanol & turbine & possible, but CO and unburnt fuel emission & [18; 20] \\ \hline WCO and kerosene & stove burner & Stable combustion up to the investigated 50\% & \\  & & blend; however, CO and NOx emissions & \\  & & slightly increase. & \\ \hline soybean and algae oils & atmospheric & A novel atomizer design allowed stable operation & \\  & burner & and low emission without fuel preheating. & [22; 23] \\ \hline tire pyrolysis oil (TPO) & diesel engine & Neat TPO leads to unstable combustion. Blends & \\  & and diesel & show lower NOx and higher CO and & \\  & & HC emission. & \\ \hline C-heavy oil and crude & & Note that CJO is lighter fuel than C-heavy oil. & \\  & & NO\({}_{\mathsf{X}}\) emission and wall heat flux were both & [25] \\  & & decreased in the case of a 79 MW utility boiler. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Literature survey of heavy renewable fuels (HRF) utilization. The very first process in liquid fuel combustion is atomization. Highly viscous liquids show a significantly lower tendency to breakup [26]. To overcome this issue, there are three ways. The first one is using water-oil emulsion, which features micro-explosions when heated above the boiling point of the water [27]. The resulting very fine spray evaporates quickly, and the presence of H\({}_{2}\)O molecules near the fuel also helps in reducing the NOx emission by lowering the concentration of atomic oxygen [28]. The nearly double heat capacity of H\({}_{2}\)O compared to CO\({}_{2}\) and N\({}_{2}\) lowers the adiabatic flame temperature as well, prolonging the maintenance intervals. Nevertheless, the higher H\({}_{2}\)O concentration promotes hot corrosion of steel parts downstream. The downside of H\({}_{2}\)O addition is that a water purification plant is required to avoid the deposition of minerals inside the combustion chamber, leading to decreased heat transfer and contamination issues. A highly practical solution is the use of a unique atomizer [29]. Nevertheless, the traditional way is preheating the fuel to an adequate temperature range [30]. This solution is even more economical if the waste heat of combustion can be used for this purpose. Since there is no latent heat to cover, this versatile solution was also used in the present case. Atomization characteristics will be discussed in Section 2.2, based on an earlier work [31], which focused on the spray measurement of various liquids, including both the presently investigated diesel and crude rapeseed oils in a wide parameter range. The second process is evaporation. Fuels with low volatility are almost impossible to ignite under atmospheric conditions, hence it is a common practice that the warmup and shutdown procedures of the combustion chamber are performed by using a conventional light fuel [18], such as diesel oil in transportation and natural gas in power generation.