We present our method of real-time rendering of grid-structured data. The method uses bicubic spline interpolation running directly on common current graphics hardware to map the values to a texture. It allows us to render isolines of the visualized data and dynamically change the visualization parameters. We have implemented the method and incorporated it into a system of real-time simulation and visualization of combustion processes in pulverized coal boilers. We have compared the visual quality of produced images with the previously used visualization methods.