According to the law, there are four possibilities of divorce. Apart from the consensual and the contested divorce, there is the divorce for other reasons (among them the divorce without culpability of the spouse, e.g. due to illness) and the divorce due to the dissolution of the common household since at least three years. The prerequisite is that the marriage has broken down irretrievably.
Furthermore, a marriage can be dissolved or declared void. You can plead nullity if for example one of the spouses was not marriageable according to the law. The annulment of a marriage serves the purpose of dissolving a marriage which was contracted under misleading circumstances. A typical case would for example be marriage fraud by a marriage-impostor.