Full steam ahead

The chemical industry has been a major consumer of electricity since the 1930s, and for all this time, it's also been a user of combined heat and power plants.

The technology makes perfect sense - chemical plants invariably consume large amounts of steam, all year round, as the main method of providing heat to the process. And as long as you can use the steam, CHP represents one of the most efficient methods for generating electricity, producing five times as much wattage as conventional power plants from the same energy input.

BASF's Ludwigshafen complex has a head start on many chemical production operations when it comes to steam. The complex uses 2100 tonnes of steam every hour, but it is so large, and operates so many processes, that over half of its steam supply is generated by what the company calls the 'energy Verbund' -pooling the heat produced by exothermic processes.

There are also two power plants on the site, one belonging to the energy generator RWE, which opened in 1997, and BASF's own plant, now almost 40 years old. However, there is a serious shortfall in on-site power generation. 'The total electricity demand at the Ludwigshafen site comes to 710MW,' says Horst-Heinrich Bieling, energy director for the site. 'We produce a mere 15% of this ourselves, with most of the rest coming from the RWE power plant at the site - just under 40% - and from the national grid, more than 45%.'

As the existing plant is now considerably out of date, the company is taking the opportunity to build a new CHP plant, located on the bank of the River Rhine. Currently taking shape out of a nest of scaffolding and Portakabins, the plant will produce 650tonnes of process steam per hour and 450MW of electricity. 'As a result, the amount of steam generated at BASF will increase from 80 to 83%, while the amount of electricity we generate ourselves will leap from 15 to 63%,' Bieling says.

The new Ludwigshafen plant uses a modernised version of the typical cogeneration process, which uses natural gas to fire a boiler, raising steam to run a turbine. The new plant has three main components: gas turbines, waste heat boilers and steam turbines. Compressors draw in the air supply for combustion of the natural gas or liquid fuel from the surroundings, passing it through a set of filters and compressing it to 17bar pressure. This heats the air up to 300°C - the temperature at which it enters the combustion chambers.

Combustion provides a further heat boost, and the flue gases leave the chamber at 1100-1180°C and still under pressure. Expanding the gases into the gas turbine provides the power to drive both the air compressor and the gas turbine, which generates 0.58MWh of electricity. The temperature of the flue gases falls to 550°C during this step, and they shed a large part of their heat in the waste heat boiler, which generates high-temperature, high-pressure steam.

The flue gases, now at 100°C, are vented through a stack, and the steam passes through to the steam turbine, generating the plant's second stage of electrical power, amounting to 0.11MWh and cooling to 200-250°C in the process. This is the correct temperature for the complex's network steam supply, and the final stage of the plant's operations is feeding the steam into the network of pipelines that carries utilities and raw materials around Ludwigshafen.

In the conventional process, with no gas turbines, 1MWh of energy input from natural gas generates 0.14MWh of electricity and 1tonne/hr of steam. The new plant will burn 1.7MWh of gas and produce the same amount of steam, but will generate 0.69MWh of power.

The main difference between the CHP process and conventional power generation is in the temperatures that the gas turbines must withstand. A conventional turbine runs on steam at a maximum of 580°C, whereas the initial gas turbine of the CHP plant operates on flue gases direct from the furnace, at a blistering 1180°C. The metal parts of a conventional turbine would be reduced to a molten puddle at this temperature, so they must be protected and cooled.

The Siemens 3AGT turbines currently being installed at the plant use literal space-age technology to achieve this. The combustion chamber is lined with a ceramic material similar to that used in the tiles on the Space Shuttle, and a similar material jackets the precisely-machined turbine blades. Moreover, the blades are hollow, and a series of boreholes in the blades and the ceramic coating allows cool air to flow out of the blades and form a protective blanket over the surface. As a result of this, the inside of the blade is maintained at 400°C cooler than the outside.

The plant will generate power with a very high fuel efficiency - almost 85%, including the energy of the steam generated, compared with 58% for electricity generation plants. As technology advances, these efficiency figures are set to improve, Bieling predicts.

'Experts are working on the assumption that the 60% barrier in the CHP process is set to fall,' he says. New technologies currently being tested will permit efficiency increases of a further 4-5%, he adds, which means that CHP process will operate at 63%, and even the relatively inefficient coal-fired stations could run at 48-50%.