.NET based workflow engine with BPMN support

I need to design a solution for a public portal built on SharePoint Server 2013 (on-premises) to support a number of business processes. The customer requires that the workflow solution used for this has full support for BPMN standards and reference models.

Something where a user can design a workflow and export it to standard BPMN, and import it to a workflow engine that support this.

From what I know, SharePoint workflows don’t support all of the BPMN activities and neither does K2, for example.
I’ve been told solutions like Activiti (in Java) and Bizagi (Java and .NET), support it.
Do you know and have you used any other .NET-based workflow engines that support BPMN?

Thank you!