It is not clear to me what you mean by synergistic effects, but these comments might help.

Biomass and coal samples that are analyzed in the same furnace at the same time, which is common, can affect the results of one another. The most egregious example is ash from low sulfur biomass or low-sulfur coal chemically complexing with sulfur emissions from high-sulfur coal to form sulfates during the ashing process. This can produce an inaccurate high value for total sulfur in the ASTM sulfur analysis. There are other generally minor issues with carbonates and chlorides.

Larry L. Baxter Thanks for your response, after your reply I decided to perform the proximate analysis of different coal-algae blends, ranging from pure coal, 10-90; 20-80 ratios and so on, to pure algae. Experimental ash analysis results for coal-algae blends seem to give values that are higher than the calculated ones (after performing t-test). This observation has confused me considering my limited understanding about the complex reactions that you stated. Would you kindly help with a source where I can gain more understanding as to what really happens to the reacting species during the ashing process. Moving forward, I am thinking of determining the amount of sulfates in the ashes that I generate so as to ascertain whether is there a significant change in them. Also my question to your response, is the proportion of sulfates in ash significant to the extent that a change in the amount of sulfates can translate to a change in the ash content?

My coal has 4.70 moisture 14.98 ash 26.18 volatiles and 54.15% fixed carbon, algae has 9.81 moisture 9.49 ash 70.33 volatiles 10.83 % fixed carbon.