As more industrial companies look to cogeneration to meet their heat and power needs, a new contract to for a CHP plant for the UK’s first bioethanol plant has been awarded to Aker Kvaerner.

By Alan Kidner Aker Kvaerner Engineering Services Ltd, UK

As the expansion of the biotechnology market continues alongside an increased confidence in the petrochemical industry, so the need to secure production of additional steam and power within a petrochemical complex has become increasingly important.

£20 million CHP plant by Aker Kvaerner at Wilton Power Station in 2004
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Indeed, increased confidence and expansion within numerous industries in the UK have led to a number of new power-related projects. A good example of this is the Wilton International site on Teesside, an integrated petrochemical complex in the north east of England. Aker Kvaerner is currently undertaking a new project on this particular site to increase its steam and power generating capability.

Aker Kvaerner is designing and building a new combined heat and power (CHP) plant for SembCorp Utilities UK, a subsidiary of the Singapore-based SembCorp Utilities. The £36 million ($74 million) project, of which Aker Kvaerner’s contract value is £26 million, will supply the recently announced Ensus bioethanol plant, the UK’s first bioethanol plant, and is SembCorp’s third major investment in four years.

Delivering an integrated solution

Under the contract, Aker Kvaerner will provide all engineering, procurement and construction services to deliver an integrated power plant. The plant is composed of one Frame 6 B gas turbine supplied by GE Energy and a single pressure HRSG supplied by Istroenergo Group A.S. (IEG), with the balance of plant equipment requirements being designed and procured through Aker Kvaerner. The project will be implemented over a target 20-month programme, culminating in an operational start date of early October 2008.

The plant is designed to supply a nominal 86 te/h of high pressure (HP) steam at the client’s terminal point at an ambient temperature of 10 oC in the unfired operating mode or a nominal 79 te/h of intermediate pressure (IP) steam at the terminal point at an ambient temperature of 10 oC in the unfired operating mode.

The plant is also designed to operate in supplementary fired mode. In this mode, the steam produced is approximately 100 te/h of HP steam at the terminal point, or 162.5 te/h of IP steam at the terminal point at an ambient temperature of 10 oC. The steam from the HRSG will be conditioned and exported at the following operating conditions: high pressure at 96 bara, 320 oC and intermediate pressure at 18 bara, 300 oC.

The plant is electrically rated nominally at 42 MW at the generator terminals at an ambient site temperature of 10 oC, with the generated power at 11 kV being exported into the client’s existing electrical distribution system via a step up electrical transformer to a 66 kV rating.

The plant is to be an island site, within a boundary fence and is located on a plot within the Wilton International site. It will predominately be an unmanned installation with an expected availability of 97-98 per cent after the first year of operation. It will operate 365 days per year, 24 hours a day, limited only by periods of defined unavailability. The plant is not provided with any “black start” capability or the ability to be operational while disconnected from the grid.

Instrumentation philosophy

To achieve the expected availability levels, in certain instances, the redundancy of instrumentation is critical to the plant operation in terms of process or safety criticality. In these instances the instrument philosophy has followed the typical criteria as noted below:

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Process critical – an instrument is defined as process critical if failure of the instrument prevents generation of steam at the required connection point at the defined temperature, pressure and flow rate.

Safety critical – an instrument is defined as safety critical, if a failure of the instrument would compromise personnel safety.

Protection critical – an instrument is defined as protection critical, if a failure of the instrument would compromise plant integrity.

Non-critical – an instrument is defined as non-critical, if a failure of the instrument does not compromise any of the above.

The implementation of this philosophy is carried out across the balance of plant equipment and in a number of instances within the sub-contractors equipment.

Plant design

The plant comprises of a number of main equipment items as noted in the tinted box below:

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Meeting the need for steam

The operation of the plant will include a number of interfaces with utility systems, to provide both on and off plot, and also connections into the existing steam distribution systems. The interfaces are created by integrating a number of field instrumentation signals and plant permissive signals into the overall plant control system.

The steam distribution systems are integrated into the client’s steam networks providing the additional requirements, which are determined by the selection of the appropriate operating mode – load control, flow control or pressure control, and the desired steam distribution – HP export only, IP export only or a combination of HP/IP steam export. The demand for steam is critical to the overall integration.

Process flow schematic with heat and mass balance
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Thus, the design provides the client with the capability to enable the supplementary fired burners early, thereby providing the response to the end user demand with a typical rate of increase from minimum fired burner condition to maximum fired burner condition within six minutes.

Safeguarding and protection

The safeguarding of the steam distribution network is undertaken in a safety PLC provided as part of the main plant control system. The safeguarding is implemented in fail-safe architecture with additional mechanical protection of the systems being provided by “full rated” capacity relief valves.

Additional safeguarding and protection systems are provided by the gas turbine and the HRSG, providing a number of protection layers. The safeguarding and protection of the gas turbine are undertaken by a Mark VIe Speedtronic control system provided by GE Energy. The boiler and burner safeguarding system is implemented by a safety PLC provided by the supplementary burner supplier Maxon.

To provide the required plant control hierarchy, each proprietary control system is integrated into the central plant control system. This provides all necessary control actions, safeguarding, protection and overall plant monitoring. A dedicated operator workstation is provided for plant monitoring and alarm annunciation within the existing power station control room, while critical plant data is provided to the client via a dedicated communication link to the existing on-site power station distributed control system.

Work is beginning immediately on the CHP plant, which is expected to be operational by the end of 2008. The project is likely to sustain around 85 jobs during the construction period. The success and knowledge of the original CHP plant built for SembCorp at Wilton in 2004 and an excellent relationship were key factors in this new contract being awarded to Aker Kvaerner.