Cogeneration CHP, Equipment, Equipment & Technology

Flexible cogeneration for chemicals plant

Issue 4 and Volume 12.

Support from the government of Flanders in Belgium has helped the Essent-owned Inesco cogeneration plant to successfully supply reliable steam to an Ineos site, while also exporting electricity to the North-West European power market, as J Th G M Eurlings writes.

Cogeneration is all about the capture and creation of synergy. Generally, the combined generation of heat and power cuts fuel consumption and consequently emissions of greenhouse gases. Nevertheless, higher specific investment costs, low-off peak power prices and periods of low heat demand need to be addressed. Thanks to government support (cogeneration certificates) and a fit for purpose power plant design, the Essent-owned Inesco cogeneration plant has successfully supplied all required utilities since September 2007.

On its production site in Antwerp, Belgium, Ineos Oxide operates Europe’s largest single ethylene oxide plant. Apart from ethylene oxide, Ineos also provides utilities and services to third parties on their site. The total site requires some 350 tonnes/h of steam at different pressure levels. Part of the steam demand is covered by the exothermal ethylene oxide (ETOX) process; the balance is produced by utility boilers.

The Ineos site currently hosts 14 companies, many of them using ethylene oxide as feedstock. The site economics are improved by this co-hosting, driven by economies of scale and the integration of heat.

View of the twin gas turbine and heat recovery steam generator (HRSG) sets
View of the twin gas turbine and heat recovery steam generator (HRSG) sets

THE FLEMISH COGENERATION SUPPORT MECHANISM

Since 2006, Flanders has supported high quality cogeneration initiatives. The introduction of a supplier obligation has created a demand for cogeneration certificates; if no certificates can be handed over, a penalty must be paid per certificate. Operators of CHP plants can apply for cogeneration certificates and create a supply. The demand/supply mechanism results in a cogeneration certificate price, which represents an additional cash flow for the cogeneration plant operator.

The cogeneration certificates are only awarded to high quality cogeneration plants. The amount of awarded certificates is calculated as the avoided fossil fuel using predetermined reference efficiencies for the production of power and steam. In this way any improvement to the fuel efficiency is rewarded three times: lower fuel costs, lower carbon costs, greater cogeneration certificate revenues.

THE INESCO COGENERATION PROJECT

In late 2003, Ineos faced an increasing steam demand and vast utility investment costs. The opportunity was taken to investigate the possibilities for a combined heat and power (CHP) plant. Hans Casier, CEO of Ineos Oxide, said the power company Essent was picked for several reasons.

‘Essent convinced us in many ways: a solid technical concept providing flexibility, best economic terms and a broad expertise in the erection and operation of cogeneration facilities in a chemical environment,’ said Casier.

Essent’s design and engineering activities started by listing the Ineos utility requirements (steam, boiler feed water, demineralized water).

A survey of the existing utilities revealed the need to replace the demineralized water facility entirely. Furthermore, one of the existing three boilers needed to be decommissioned along with its associated deaerator. The remaining two boilers were earmarked for further use (as backup) as well as some boiler feed water systems and a fuel oil tank including fuel oil logistics.

Operational window of the Inesco cogeneration plant
Figure 1. Operational window of the Inesco cogeneration plant

CHP DESIGN

A CCGT-CHP plant was selected, based on a 2+1 line-up and two 43 MW Siemens SGT-800 gas turbines. The heat recovery steam generators (HRSGs) are dual pressure (75 bar/500°C and 3.5 bar/240°C) and supplementary fired with a mixture of natural gas and site off gases. The chosen plant design allows for:

  • staged commissioning: first the CCGT, second the CHP unit;
  • flexible power dispatch down to even no steam export; the CHP plant can run stand alone as a 125 MW CCGT at 52% fuel efficiency;
  • peaking power: the power output in the cogeneration mode can be increased by ‘filling’ the steam turbine with duct firing steam – so that up to 38 MW of peaking power can be produced at an incremental fuel efficiency of 37%;
  • dynamic support of the HHP-steam grid of Ineos: the system pressure is actively supported by a steam buffer (steam is produced by the HRSGs at 50–75 bar where as the steam pressure of the Ineos system is around 43 bar) and the steam turbine controller actually controls the steam pressure. In exceptional situations the steam turbine can even be cannibalized (steam export has higher priority than power production).

Project economics were further improved by:

  • combining the condensate polishing plant with the polishers of the demineralized water unit (reversed osmosis and polishing beds based on ion exchange);
  • combining the cooling tower blow down system with the waste water discharge of the demineralized water unit;
  • introducing one integrated closed cooling water system to cool the packages, instead of separate fin-fan air coolers per package;
  • installing a water/water heat exchanger in the gas receiving station to heat the natural gas with surplus heat from the Ineos return condensate;
  • limiting the steam export to two pressure levels (HHP and LP) here by making a costly bi-directional LLP-steam connection obsolete.

OPERATIONAL RESULTS

In the first full commercial year (2008), the Inesco cogeneration plant performed outstandingly at a power availability of over 92%, including a planned shutdown of both gas turbines. The improved water steam chemistry has resulted in an approximately 50% decrease of the dosing of boiler feed water and steam drum chemicals, resulting in a corresponding decrease of salt equivalent into the river Schelde.

Figure 1 displays operation last year. The graph plots operating points (power output versus steam export) based on hourly values. It can readily be seen that the cogeneration plant operates at three power levels while supplying steam anywhere between 30 and 160 t/h:

  • minimum load (gas turbines);
  • nominal load (gas turbines);
  • peak load (nominal load gas turbines, peak load steam turbine).

As the vast majority of operating points lies within the CCGT area (with or without duct firing) the addition of a condensing steam turbine to the design proves its value.

Since the start of granting cogeneration certificates in April 2008, the operation of cogeneration has focused on the design point (nominal load gas turbines, minimum load steam turbine and duct firing) at which the fuel efficiency (or the amount of cogeneration certificates) is maximized. The relative saving of primary energy (fuel) is in the design point as high as 18%.

CONCLUSION

A fit-for-purpose design and government support has enabled the Essent-owned Inesco cogeneration plant to supply reliable steam to the Ineos site while competing successfully on the North-West European power market.

J Th G M Eurlings is the manager, Conceptual Design, with Essent, the Netherlands www.essent.nl

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