Why don't you first create cell vectors and then use xlswrite only one time at the end? For e.g.
filename = sprintf('EP3');
VarName = {'xV','Vent ratio',.. .. .. .. , 'F CH4'}; %1x12 cell vector containing Names
Units{1x12} = '';
Units(1:4) = {'[-]'};
Units(5:end) = {'[kmol/h]'};
%% same with the other lines too;
%% Then you make final cell vector to write in excel
for f = 1:N
sheetname = sprintf('%d °C',Temp(f)-273.15);
%% Write code to value cell vector
finalCell = [VarName;Units;Values];
xlswrite(filename,finalCell,sheetname,'A1'); % using only one time per loop
end
Finally, what I didn't understand from your code is:
xlswrite(filename,{'Conversion'},sheetname,'C1'); % first you write this
xlswrite(filename,X_matrix(f,:)',sheetname,'C1'); % then you change it with this in the same loop?
% I was thinking it should be 'C3', is it a typo?
