Our government is starting to limit NO_x emissions from cement kilns and the regulation, as I heard, was more severe than in the European countries. I think the best way for reducing NO_x emission with no further cost is to use fuels that contain the lower nitrogen content, especially coal and residual fuels. Am I correct?

The NO_x emissions arise from two sources: (i) the nitrogen in the fuel as you say, and (ii) "thermal" NO_x from the breakdown of atmospheric nitrogen at the high temperatures in the burning zone. Usually the thermal NO_x is much higher than the fuel NO_x and the best way to reduce NO_x emissions is to reduce this thermal NO_x. Low NO_x burners reduce the temperatures in the flame and produce less NOx by using less primary air. Low NO_x calciners burn some of their fuel in a depleted oxygen environment to cause NOx degradation by the following reaction: NO + CO -> 0.5N₂ + CO₂. These are the best way sto reduce the NO_x emissions of the kiln.

How can we determine the maximum capacity of a gas conditioning tower ?

The sizing of the tower is determined by a heat balance which tells you the amount of water which would need to be evaporated to realise a certain fall in temperature of the gases entering the tower. The results of this heat balance are then used with the Ideal Gas Laws which allow you to calculate the contraction in volume of the gases caused by the reduction in temperature. This contraction creates a negative pressure. This then needs to be compared with the negative pressure rating of the tower.