Full integration works at Seal Sands CHP23 April 1998
The industrial combined heat and power plant at BASF's Seal Sands chemical process plant on Teesside in the UK has many facets in common with the Kraftwerk Süd captive cogeneration plant at Ludwigshafen in Germany. Owned by the large local utility company in both cases, the plants are operated by BASF under contract to the owners, but Seal Sands has achieved a much higher level of system integration whilst maintaining operational stability.
The industrial combined heat and power (CHP) plant at BASF's Seal Sands chemical process plant on Teesside in the UK has many facets in common with the Kraftwerk Süd captive cogeneration plant at Ludwigshafen in Germany, albeit on a considerably smaller scale. Owned by the large local utility company in both cases, the plants are operated by BASF under contract to the owners – National Power for Seal Sands and RWE Energie for Kraftwerk Süd. The German plant burns natural gas from BASF's own subsidiary Wintershall, while the Seal Sands plant has a supply contract with BG Trading Ltd. for the first five years of operation. National power has agreed 15 year tariff terms with BASF for plant operation.
In close proximity to the Enron Teesside power plant and the new Phillips Petroleum CHP plant, Seal Sands CHP has been in full commercial operation since 1 November 1997 ready for the official opening on 2 April 1998. In terms of power plant technology, the smaller Seal Sands plant, with its 75 MW of electrical output and 65 te/h of process steam, is less ambitious than the 350 MW Ludwigshafen plant. But in terms of integration with the process plant and existing utility facilities as well as co-ordination of control responses with the process plant dynamics, the Seal Sands plant is probably considerably more complex.
The process plant, formerly owned by Monsanto, is composed of three main operations manufacturing the chemical intermediates Acrlonitrile, Adiponitrile and Hexamethylenediamine (HMD) which are used in the production of acrylic and nylon fibres as well a wide range of acrylic and nylon plastics for the domestic, engineering and motor industries.
The HMD is mainly shipped to Lugwigshaven for the production of the engineering plastics polymer Nylon 6.6.
Both plants testify to a remarkable history of radical savings in energy consumption as well as dramatic reductions in harmful emissions and greenhouse gases in particular by the use of on site cogeneration plants. Waste streams are recovered and recycled for conversion into energy and an active energy conservation programme has resulted in reduction of 70 per cent in the energy used per tonne of product compared with the mid-1970s.
In Germany the Kraftwerk Süd project has initiated a new trend of similar systems in a whole series of large chemical plants, car manufacturing plants and industrial food processing installations.
In the UK, this kind of on-site CHP plant has been a progressively growing market and a large number of projects have now been installed. Since the output range comes well under the limit for the current government moratorium on gas fired power plant installation, and strong government support is being extended to CHP in general since the Kyoto environmental summit, a further acceleration of this phenomenon is likely to be encountered.
The Seal Sands project
Capital investment in the new power plant, for which Balfour Beatty was the main contractor, was some $48 million. Considerable further investment was made in joint studies by National Power and BASF into the integration of the new plant with the existing utility systems, for which the highest possible levels of availability were paramount.
National Power financed, project managed construction, and also owns and manages the plant. BASF operates the plant on a day to day basis under a Partnership Energy Management agreement with National Power Cogen.
Construction commenced in March 1996 after a bid was selected from the four parties who tendered in February 1996. Not surprisingly, another strong contender in the bidding was a joint venture between RWE Energie of Germany and Northern Electric in the UK.
The energy supply contract had been signed in December 1995. Construction was completed in September 1997 with minimal intrusion on the production process, and commercial operation commenced on schedule in November 1997.
Of the 75 MW of electricity produced by the plant, BASF is contracted to take 55 MWe. The rest is traded by National Power on a merchant basis. A "CHP plant efficiency" of 65 per cent is claimed for the facility, which through the month of February 1998 turned in an availability factor of 100 per cent and also a load factor of 100 per cent
The CHP plant
There is nothing over ambitious in the power plant hardware. The prime mover is a 40 MW GE Frame 6B gas turbine built by the former John Brown Gas Turbines division of Kvaerner Energy at Clydebank. Peak output from this can be extended to 45 MW if necessary.
National Power already owns and operates a number of these trusty workhorses, as indeed does BASF who has one at the Mitte generating plant on a another part of the Ludwigshafen complex, and two at the Schwarze Heide complex. Steam injection is used purely to reduce NOx generation, not to augment power output. The gas turbine, which is directly coupled to a GEC Alsthom generator, receives gas at 70 bar NTS from the CATS high pressure gas terminal adjacent to the Enron terminal via a new high pressure line.
It also receives excess gas from a hydrogen reformer used to hydrogenate the exothermic AN6 reaction process. In addition, steam generated by the exothermic process is added to the steam supply bus for the LP turbine and process steam services.
A simple single pressure, unfired 65 te/h heat recovery steam generator (HRSG) was supplied by Standard Fasel Lentjes, with a steam drum outsourced to a Polish manufacturer. The steam output from this supplements the output of three existing boilers to provide a complex network of steam services with maximum flexibility and back-up provision.
The AN6 plant, although it consumes considerably less steam than the AN5 plant which Monsanto took back to Texas City, and a great deal less steam than the now dismantled AN4 plant, still takes a full 100 te/h of steam at 32 bar to get it going on start-up. It also consumes a lot of electricity to drive large air compressors to initiate the process until the exothermic reaction commences.
The utility boilers can be fired by either gas or oil, and substantial quantities of natural gas are consumed by the hydrogen reformer.
The condensing 35 MW steam turbine (38 MW nameplate rating), takes some 126.7 te/h of high pressure steam at 63 bar from the combined outputs of the HRSG (65 te/h) and the three existing utility boilers (61.7 te/h) but it can also be operated on steam from either. A further 78.6 te/h of relatively high pressure, 32 bar, steam may be taken from the HP process steam main which takes steam either to the process or from the process depending on the operating mode. Further pass-out points from the HP/IP turbine supply up to 40.2 te/h at 10.5 bar of steam to the medium pressure process steam main, while 60 te/h of LP steam at 2.7 bar is tapped off from the LP turbine entry to the LP process steam main. Some 12.4 te/h of steam is tapped off from the HP process steam main for NOx suppression in the gas turbine.
The turbine was supplied by ABB in Nuremburg to be directly coupled to a Brush generator. A plumeless hybrid cooling tower from Balcke Dürr extracts heat from the underslung condenser.
No black start facilities have been incorporated because the load shedding requirements would have been an undesirable complication in terms of system integration and control. A Detroit Diesels power unit is used to run up the gas turbine.
To integrate such a complex mixture of systems and still maintain operational stability and ultimate availability of energy services, mathematical modelling of the entire complex was critical. The dynamics of the utility systems had to be 100 per cent compatible with the process plant dynamics without adverse interactions at any of the 13 design cases. These had to take into account responses to trips, outages of local public services, initiation and changes in control functions.
A joint study carried out by BASF and National Power took over a year complete, and it has now been extensively tested since the start of commercial operation. The fact that steam turbine trips have been encountered without any impact on the production process tends to indicate that the exercise has been successful. The experience also shows the advantages of close co-operation between an industrial operator who knows more than anyone else about his own plant, and a very experienced power plant operator who knows how to integrate utility services in a cost effective manner.
The entire system is out of necessity controlled from the main BASF control room via digital bus networks. Since the BASF plant is controlled through a Fisher Rosemount RS3 distributed control system (DCS) installed in December 1995, this system had to be extended to take in the CHP plant systems in a systems integration exercise, and this has been simply and effectively executed with a small local control room adjacent to the plant housing the GE Speedtronic 5 control system for the gas turbine and the ABB Turloop system for the steam turbine.
Since this is where much of the complexity of controlling and optimising a highly integrated and diverse set of plant equipment is monitored, it is impressive that so much can be handle by such a modest facility.
Operation and maintenance
The gas turbine is essentially run base loaded except during exceptional operating conditions. The steam turbines are generally run to produce as much electricity as possible depending on the steam demands of the process plant and boiler availability.
The generation plant is too small and specialised to be called on for frequency control dispatch, so depends on the power trading skill of National Power Cogen's personnel.
Maintenance planning is different to that of a purely utility power plant, although this function is every bit as critical to economic performance. Since the chemical plant operators possess much of the maintenance skill and experience needed to operate the power plant, and the continuing high availability of the power plant is of critical interest to their operations, most of the maintenance functions fall on the BASF personnel.
These have more practical experience of the implications for the 13 different operating regimes of the process pant of the maintenance planning strategy.
This means that National Power has effectively off-loaded from their business plan one of the more critically expensive cost components of investment in independent power plant.
Three levels of maintenance regimes are stipulated in the maintenance regime:non-intrusive routine repairs routine unplanned outages major overhauls and upgrades.
The first two categories are the responsibility of BASF, the third category is carried out by the equipment suppliers.
The current financial commitment by National Power Cogen is approximately $380 million, having signed 13 CHP contracts to date totalling around 500 MW of electrical and 750 MW of heat capacity. Seven other plants are currently operating including sites at Lancaster University, ChiRex in Northumberland, Aylesford Newsprint in Kent, Albright and Wilson's chemical works in the West Midlands and in Cumbria, Fort James's paper mill in South Wales and Lindsay Oil Refinery in North Lincolnshire.
The company is also building plants at Dow Corning in South Wales, at Rolls- Royce's Derby works and at the Phillips Petroleum site at Seal Sands adjacent to the BASF plant, and at the Esso Refinery at Fawley in Hampshire.
This could be just the tip of the iceberg.