Area: Plant Reliability Improvement Projects
An even more adventurous strategy for aging coal plants is eliminating coal altogether. Brian Reinhart, Study Manager at Black & Veatch Energy in Overland Park, Kansas, examined a variety of options including a switch from coal to natural gas.

He framed the discussion around a 250 MW subcritical pulverized coal-fired unit built in the late 70S  the type of site that is firmly in the EPAs crosshairs. This particular facility was challenged by the fact that the closest gas line with sufficient capacity was 60 miles away.

Reinhart covered several options such as conversion to natural gas only, natural gas and coal co-firing, a full emissions control retrofit to bring the unit into compliance, repowering the steam turbine to make it run as part of a combined cycle plant or replacing the entire plant with a new CC. Co-firing was dismissed rapidly as it is still regarded as a coal unit by the EPA, so regulatory issues remain.

Reinhart said that a more feasible long-term approach is either emissionscontrol retrofits or a combined cyclebased solution. A CC could entail retaining the steam turbine, condenser, control room and water treatment facilities and replacing the boiler.

The steam turbine would need to be repowered, said Reinhart. Although you would experience a derate of about 10% to 20% in steam output due to flow limitations in the steam turbine, by adding two F-class gas turbines you would boost net power by nearly three times.

On the downside, you would need about eight acres or more for these two GTs and heat recovery steam generator trains. But the end result would be a gain in efficiency, a drop in O & M and no need to perform heavy maintenance on pulverizers and other coal equipment. Further, by changing from 250 MW to 600 MW, the area would have to possess sufficient transmission margin.

Nick Zervos of the Thermal Engineering Group at Shaws Power Group was another pointing out the potential advantages of steam turbine repowering as an option for converting a coal site to a combined cycle plant. As it could cost hundreds of millions in regulatory compliance to keep an old coal boiler going, the owner may choose instead to retire the existing coal-fired boiler and put in one or more combustion turbines and HRSGs to repower the existing steam turbine, said Zervos.

He detailed the differences in steam systems in repowered plants, compared to compact Greenfield combined cycle plants. When a GT is added to run in CC mode, for instance, that can lead to long steam piping due to where the gas turbine is sited.

Zervos said that longer lines mean greater pressure and temperature losses. A mere 1°F drop in steam temperature can mean a big loss in revenue. So for long steam piping lengths, he advised plant managers to choose pipe insulation thicknesses carefully. And if at all possible, site the gas and steam turbines close together. One of the other consequences of long steam lines is more complicated warm ups for cold starts.

If there is no way around having the GT sited far away from the steam turbine, the plant will need adequate drain lines to avoid the accumulation of condensate, which can lead to water slug damage of the pipe. It is especially important to minimize the length of steam turbine bypass lines downstream of the bypass valves.

When a cold bypass pipe is suddenly filled with hot steam during a steam turbine trip event, condensate will form and be propelled along the pipe. The longer the line is downstream of the bypass valve, the greater will be the condensate accumulation and consequent transient loads applied at all changes in direction.

Ideally the gas turbine and HRSG will be selected to make as much steam as the steam turbine can swallow, said Zervos. Very often supplemental duct firing in the HRSG is appropriate for a good match with the existing steam turbine. You can also make modifications to the steam turbine to be able to handle more or less steam.

In addition to bringing in GTs and HRSGs, required modifications include condensate pumps, boiler feed pumps, a steam turbine bypass system with condenser spargers, steam drain tanks, and retiring feedwdater heaters.

For the steam path itself, modifications are needed on high pressure inlet blading and low pressure back-end blading. Alternatively, perform a complete steam path replacement.

It is often economical to reblade the whole steam turbine to keep it from acting as a bottleneck to the system, said Zervos. You also need a steam bypass system so that when the steam turbine trips, you dont need to trip the GT, too. This is especially important for multi- HRSG plants.