Waste-to-energy saves money and boosts production



How can a large industrial complex convert something it is discarding into something of value? Or what technology can such a complex use to reduce energy costs and boost output? Glencore’s ferrochrome smelting plant found the answer to lie in the use of the waste gas produced during the smelting process to generate electricity.

The conversion from waste gas to electricity, however, has to be cost-effective for the concept to be viable. Glencore contracted Diesel Electric Services (DES), via an IPP process, to design, construct and operate a plant to convert waste gas, which was being flared, into electricity for the plant. According to the company’s Alain Bonfrer who spoke to Energize recently, the resultant embedded generation plant at Boshoek, just off the R565 near Rustenburg and about 160 km north-west of Johannesburg, provides up to
9 MW of additional electricity. It is owned, operated and maintained by DES with Glencore as the sole off-taker. All of the equipment was designed and built by DES inhouse, with the exception of the civil work, which was designed but built by a sub-contractor.

Gas composition

Although many gas-to-energy projects are in operation, this one is rather unique. Most gas-to-power plants use natural gas or landfill gas as a fuel source, but DES’s solution uses a dirty, wet and poisonous gas as its fuel source. The gas, which DES purchases from Glencore at a nominal amount, comes from the smelting process and is composed mostly of carbon-monoxide (80%) and hydrogen (5 – 10%) with some inert gases also being present.

This mixture of gases makes the project not just unique but downright dangerous. Safety is a major concern. The high CO content means that a number of precautions must be taken to protect people working on the plant. Working in this environment can be hazardous. The normal exposure limit to such a gas is usually about 200 ppm over an eight-hour period (a single breath of 4000 ppm is fatal). There are gas monitors all over the plant and every employee wears a CO sensor which emits an audible warning when the CO content in the ambient air exceeds safe limits. The employees are taught to take these beeps seriously and respond immediately by removing themselves from the environment.

Handling this gas poses many challenges. The hydrogen component makes the gas explosive and the CO is extremely poisonous. That is why the gas was previously flared as no alternate use was thought possible. However, in this project, the gas which is supplied by the ferrochrome plant is dirty, wet and of varying quality. It has to be conditioned before it can be used as a fuel source.

Fig. 1: Gas engine buildings with central gas line.

Conditioning process

This conditioning process includes drying, cleaning and filtering to remove particulates. The make-up of the gas is not changed. The hydrogen component in the gas can cause major problems such as pre-ignition in the engines, resulting in engine failure. The gas is therefore continuously monitored and when the hydrogen component exceeds 17% the plant is stopped.

Dust filtration is vital as dust clogs up everything. Although the furnace emits particulates no larger than 50 mg/m3 of dust, as limited by legislation, because the exhaust gas goes through a venturi scrubber, it can only catch a certain amount. Dust, finer than powder, still gets through and is in the gas supplied to DES’ embedded generation plant. This fine dust builds up in valve seals and prevents them from closing properly. To overcome this problem, DES installed a U-tube in the gas supply line. The only way to reliably seal is with water. The U-tube allows for a 2 m high head of water which reliably isolates the generation plant when needed.

The gas from the smelter is wet and dirty. The plant is 300 m away from the smelter and in that 300 m of pipe, the gas loses a lot of its heat and starts to condense. By the time it gets to us its fully saturated. It also loses pressure over that distance..

Halfway along the 300 m length is a booster station which belongs to Glencore which boosts the pressure to a maximum of 10 kPa. The equipment is housed in thirteen machine rooms: six on one side and seven on the other with the 600 mm gas pipe running between the two rows of buildings. DES is linked to Glencore’s SCADA system which enables the company to monitor furnace conditions remotely. Glencore in turn has access to our SCADA and can monitor power plant conditions.

Fig. 2: One of the gas engines.

Filtration and dehumidification

The filtration includes a reverse-pulse system which emits a jet of nitrogen from the inside which blows the dust off the outer surface of the filters, extending the time between filter changes dramatically. The filters comprise eight filter units with three elements per bank, and gives a reverse pulse at each one every 20 s.

Nitrogen, sourced from a nitrogen generator on site, is used instead of air so that oxygen is not added to the gas as this would create an explosive mixture and pose additional risks to the operation. The filters don’t work with wet gas and the dehumidification system gets clogged up easily.

One of the challenges the design team faced was that while one wants to filter dry gas, one also wants to cool clean gas. The system uses two dehumidification systems which dry the gas by first cooling several degrees below the dew point and then reheating, so that it reaches the filters in a dry state.

The two systems do not operate simultaneously. One operates until it gets clogged up – which only takes about two weeks – at which time the gas is routed to the other system. The first system is cleaned, filled with nitrogen and made ready for use for when the second system clogs up.

Fig. 3: Filtration plant.

Benefits

Electricity, which is generated from that gas, is sold to Glencore at about 95% of the ruling Eskom tariff. The 9 MW, generated by 13 machines, reduces the company’s auxiliary load at 6,6 kV on Eskom which saves Glencore money. The process of using the gas also helps to reduce pollution by not being released into the atmosphere. The sale of the gas, up to 10 000 Nm3/h, reduces Glencore’s emission responsibility.

The project was commissioned in April 2017 and has clocked up 50 000 operating hours between machines. The dirty fuel caused a problem when trying to source gas engines. Most manufacturers were unwilling to supply engines to run on this gas. Finally, DES found a Chinese manufacturer which was willing to sell machines for this project.

The plant was conceptualised by the client who contracted DES to design, build and operate it for Glencore. During negotiations, it was decided that DES would own the plant, with the result that the plant does not appear on Glencore’s balance sheet.

Advantages for the client

  • Electricity at 95% of Eskom’s tariff as well as a significant maximum demand saving.
  • Glencore’s maximum notified demand is 120 MW, but its furnaces can run at a higher capacity if there is more power available – this plant offers an additional 9 MW.
  • Not only Glencore getting electricity at a lower rate, but it can also benefit from being able run the plant at a higher maximum output as a result of this embedded generation plant.

Other projects

Although this is the first furnace gas project the company has done, it is not the first gas-to-power project it has undertaken. The first, a 12 MW plant, was completed in 2009 for ABSA’s towers in Johannesburg’s central business district, using natural gas supplied by Egoli Gas. The company still maintains that plant and has operators on site..

In 2010, the company installed two 1000 kW tri-generation units for MTN’s head office in Fairlands, near Roodepoort. The plant may soon be ready for its 60 000 hours service.

The company’s first biogas project was a small pilot. A second plant, in Worcester, ten times bigger than the first, has been running for 9000 hours per machine, and the same client recently broke ground for a new project which will be four times bigger than the Worcester one. At 6 MW, this might be the biggest biogas project in the country.

Licensing and regulations

DES is an independent power producer, registered with Nersa. Under the previous Act one had to get a ministerial approval, but now with the latest statement by Minister Jeff Radebe, a generation licence is not needed if the project entails using waste gas.

Send your comments to energize@ee.co.za

The post Waste-to-energy saves money and boosts production appeared first on EE Publishers.

Source: EE plublishers

More news