Diesel engines over 560 kW are not present in huge numbers, and for this reason, when a retro-fit diesel particulate filter system is required for a large engine we normally treat it as a special project. As with all diesel particulate filters, one of the most important considerations is how the filter is going to be cleaned (or ‘regenerated’). The options are ‘passive regeneration’ which requires an appropriate exhaust temperature, or ‘active regeneration’, which uses an external source of energy to heat up the filter. Many systems rely on a combination of ‘active’ and ‘passive’ regeneration strategies.
The exhaust temperature, and the extent to which this varies, will depend on what the engine is used for. Locomotives, ships, pumps and prime-power generators tend to have high and predictable exhaust temperatures, and therefore passive regeneration may be adequate. Standby generators on the other hand often have unpredictable load factors and therefore exhaust temperatures, which means that there needs to be a back-up plan to prevent the filter becoming blocked.
In some critical applications (e.g. standby generators in hospitals) it may be advisable to only filter the exhaust gas during routine testing so that in a mains-failure situation there is no danger of the filter becoming blocked. Another option is to install two filters in parallel and then use a motorized valve to divert the exhaust gas from one to the other. This approach enables the filter which is ‘off-line’ to be actively regenerated, and therefore the exhaust gas can be filtered under any operating conditions.