Inductor ripple current

Which project are you talking about and about which circuit? How do you expect people to help you, when your quesion itself is not clear 

Because Inductor works when changes in current occur. During these changes, a circuit is not an ideal one, it is much more complex. Therefore, to tolerate with the peak current condition, a larger value inductor is used.

The word CCM and DCM is just a meaningless word if you do not include the term "Switching frequency".  As told you earlier that the inductor works only when there is a change in the flow of current, a switch always changes its state to control the current flow through the inductor. Therefore if the switch turns OFF or ON in a situation where the inductor current flow set to a static level (No changes) in the switching cycle is called as discontinuous conduction mode and if the current flow through an inductor never gets into a static level it is called continuous conduction mode. Therefore, CCM and DCM have a boundary condition of switching frequency or how the switch changes its state and what is the condition of current change inside the inductor.

Zero is not static in Electronics. There is nothing called Zero in the practical world. The Inductor current never gets zero until or unless the power of supply is disconnected physically. However, again its depends on the used components, depends on the switching cycle (As it is CCM), depends on the inductor's value and many other crucial factors.

Ripple current ratio is usually defined as the peak-to-peak value of the inductor current at high line, divided by the maximum load. Their answers typically vary, from a value of 10% up to perhaps 30%. A traditional value of inductor current ripple is 10%, and you will find this in several.

Ripple specifically ripple voltag in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform after rectification.

As well as these time-varying phenomena, there is a frequency domain ripple that arises in some classes of  filter and other signal processing networks. In this case the periodic variation is a variation in the insertion loss of the network against increasing frequency. The variation may not be strictly linearly periodic. In this meaning also, ripple is usually to be considered an incidental effect, its existence being a compromise between the amount of ripple and other design parameters.