Case Study: Combined Diesel Particulate Filter and SCR System

We all know that sometimes a visible problem is a sign of a deeper issue. Such was the case for a Thames Water pumping station in Greenwich, which used two diesel-driven pumps as a backup to electric pumps. Unfortunately, the two diesel engines used were built in the 1980s, from a time before emissions standards were applied to diesel engines. As a result, action needed to be taken.

Black smoke at the pumping station

Authorities were alerted to the issue when local residents complained of black smoke whenever the pumps were operating. At a meeting between Thames Water, Greenwich Borough Council, and Blackthorn it was determined that although the black smoke was a problem, the pumps also needed to be brought in line with the forthcoming Medium Combustion Plant Directive (MCPD). The reduction in emissions required was dramatic: a 90% drop in NOx would need to be achieved, and Blackthorn also agreed to reduce carbon monoxide by 90% and diesel particulate matter by 95%.

The Greenwich pumping station is a listed Victorian building, which presented an added challenge. Since regulations prevented any changes to the exterior of the building, all the new emissions-control technology needed to be installed inside, where space was limited. This required the new components to be custom-designed and to match up to the existing exhaust system perfectly at either end. An added complication was that no cutting or welding was permitted on site, meaning that parts needed to be fabricated accurately off-site and then simply bolted into place.

In view of the need to get the design right first time, a team of surveyors were employed to create a 3D model of the existing equipment inside the building using lasers. The new components were then designed in 3D and inserted into the model to check that they fitted, before conventional drawings were produced.

DPF and SCR

Two wall-flow diesel particulate filters were fitted to each diesel engine. These trapped soot which was then oxidised with the help of a catalytic coating. The filters relied on ‘passive regeneration’ which means that the exhaust heat alone was required to oxidise the soot. The catalytic coating was also able to oxidise carbon monoxide. Blackthorn installed a system to monitor exhaust pressure upstream of the filters to provide a warning if they began to get clogged.

Downstream of the diesel particulate filters, Blackthorn added Selective Catalytic Reduction technology to tackle the issue of high NOx levels. This works through the injection of a “reductant” into the exhaust gases (in this case a solution of urea) which breaks down into ammonia.

The mixture of exhaust gas and ammonia is then passed through a special kind of catalytic converter which converts the NOx into nitrogen and water. Since the engines were quite old, they did not provide any load signal, therefore fuel-flow sensors were installed so that the dosing of the reductant could be calibrated based on the fuel flow.

Installation

The old exhaust system needed to be removed and the new one installed, and this work was carried out by an experienced Thames Water contractor. Many of the items were large and heavy, and the task was complicated further as everything entering or leaving the building had to go up a set of steps and through a narrow doorway. Once all the metal parts were in place, the wiring and plumbing connections were made and bespoke insulation jackets fitted.

Commissioning

Since the engines in question are normally only used in periods of extreme rainfall, Thames Water switched off the electric pumps which are normally used so that the level of wastewater increased on the inlet side of the pumping station. The first diesel-powered pump was then started, and once it was up to temperature, the NOx emissions were monitored and at the same time, the reductant dosing was adjusted.

Since excess reductant turns back into NOx, there is a point where the reductant injection rate corresponds with the lowest NOx emissions, and this needed to be found for a variety of load conditions. As the head of wastewater on the inlet side of the pump reduced, the load on the engine increased, enabling a graph to be created with engine load on one axis and reductant injection rate on the other. This graph was then saved to memory in the control panel of the dosing system. It was found that NOx reduction of up to 95% could be obtained, which was in excess of the target of 90%. Exhaust backpressure was also checked and found to be acceptable.

Current Situation

Since the installation of the new exhaust technology, there have been no further complaints about black smoke, and Greenwich Pumping Station is now compliant with the Medium Combustion Plant Directive well ahead of schedule.