The company

Ti Tree Bioenergy is a joint venture between Veolia Environmental Services and JJ Richards. Ti Tree Bioenergy is managed by Veolia, who contribute all standard company practices, including strategies toward developing and operating sustainable environmental solutions into the joint venture.

The site

The Ti Tree Bioenergy Landfill Gas Power Station is a ‘best practice’ landfill bioreactor, developed from the outset to promote the degradation of waste and harness as much of the generated biogas for use in producing green electricity for export into the Queensland electricity grid.

At present, the site has the capacity to produce close to 3.3 megawatt (MW) of electricity, however this is planned to expand over the next ten years to approximately 10 MW. The current production is equivalent to producing enough renewable electricity to supply 2,500 homes every day, offsetting nearly three million tonnes of greenhouse gas over the next ten years.

Waste treatment rehab for Ipswich

As a result of the former days of open–cut mining in the region, the Ipswich area has been left scarred with large open-cut voids. In some cases, these open-cut mines are ideal for rehabilitation through backfilling with waste, predominantly from southeast Queensland.

At present, approximately two million tonnes of waste finds its way to the Ipswich industrial corridor for treatment or disposal.

The technology

The current power station is designed to accept six 1.1 MW Caterpillar generators to be individually housed in their own acoustic attenuated enclosures, operating independently of each other. However, there is additional area allocated should the system be able to achieve a greater capacity.

Currently, three of these reciprocating-type machines are generating electricity at 415 volts, which is passed through a 415/33 kV transformer embedded into the local high voltage distribution network via a former mine site primary electricity sub-station.

This type of generation is commonly known as ‘base load’ as it operates continuously. The fuel that powers these generators is produced through the degradation of waste and therefore is not always uniform in its consistency. The generators chosen by the client are selected for their proven integrity with dealing with this type of biofuel.

Gas treatment

In terms of gas treatment, little is required due to the monitoring and understanding of the waste that comes into the site. Waste is reviewed for its potential to be harmful or have long-term effects upon the mechanical integrity of the generators. However, the facility is fitted with gas chillers, which reduces the levels of harmful sulphides that could gradually reduce the life of the generators and increase life cycle costs.

Landfill gas is drawn from the gas field through approximately 3 km of high-density polyethylene (HDPE) pipe. The gas system itself is designed and operated in-house by Ti Tree Bioenergy, made up of a complex network of sub-systems in the waste.

As waste is not a uniform material and suffers with differential settlement, the system is designed and installed in horizontal and vertical planes allowing for a significant level redundancy in the sub-systems. If there should be any loss of the system through waste movement, operators are able to ‘target drill’ and install vertical risers to once again utilise the assets buried in the facility. This is an important part of maintaining life cycle costs and maximising gas yield from available waste, providing for the best return on the capital employed in the project.

Control philosophy: vacuum control

The gas pumps are a positive displacement lobe pump type, more commonly known as Roots blowers. This system has the Roots blowers controlled through a variable speed drive. The control philosophy is vacuum control, which means that as the vacuum in waste varies depending upon gas yield production, the variable speed controller responds to maintain a set point negative pressure.

Ti Tree Bioenergy believe this provides for a more consistent and reliable extraction over other methods currently employed in the industry as it reduces the potential for uncontrolled ingress of oxygen, which can have debilitating effects. Overall, it provides an increased generation factor and improved utilisation of the engines.

Managing gas temperature

Other critical aspects include management of the gas temperature. Gas typically emanates from degrading waste at 40°Celsius, however if it is drawn through black HDPE pipes that are exposed to sunlight it will gain temperature through the radiated heat due to the high emissivity co-efficient of plastic and the colour of the pipe. White plastic pipe can be purchased, but this has very limited impact when the gas is travelling at relatively low velocities in the pipe, such that ambient temperature has the greater impact.

The alternative is to bury the pipe in the ground or place an inert cover over the pipe. The depth to which these pipes are covered is important – too shallow and they insulate and retain heat, so depth of cover for the heat to be dissipated into dirt is critical. High temperature gas means that the engines will require greater volume to maintain calorific value which, in turn, causes the engines to run hot and inefficiently.

As a backup to loss of grid connection or total failure of generation capability, or even for the purpose of surplussing biogas, the facility employs two high temperature flare stacks that meet with the highest emissions standard – a 500 metres cubed per hour (m3/h) flare with a capacity of 2,500 megawatt thermal (MWth), and a 2,500 m3/h flare with a capacity of 10,000 MWth.

The reason for having two flares is that should an event arise where the plant will have to surplus gas at very low levels, Ti Tree Bioenergy will be able to achieve this and remain compliant with its emissions standards. The flare philosophy is that there is always sufficient capacity to destroy all gas. In the event of grid failure, these flares are supported by a standby power supply.

Waste decomposition: a timely issue

Ti Tree’s Bioreactor technology strategy significantly reduces the time required for the decomposition of waste. It provides for rapid stabilisation of waste in less than five years, much earlier than other typical landfill operations, and consequently allows for greater optimisation of the landfill airspace through enhanced settlement.

Energy purchase and supply

The electricity generated at this embedded biofuel power station is fed directly into the local distribution network, offsetting distribution losses in the network. Currently, Ti Tree Bioenergy trades the ‘black’ electricity produced with the National Electricity Market Management Company. The facility also benefits from Renewable Energy and Non-Fossil Fuel Obligation certificates.

In the event that the station is unable to send out electricity to the network, it then utilises electricity from the network to operate the landfill gas flares and other associated equipment.

The generator is a ‘base load’ type, meaning that it runs continuously throughout the year at its peak output and is registered as a market, non-scheduled generator.

The projected export of electricity is to increase from the current 15,000 megawatt hour per annum (MWh/a) up to 80,000 MWh by 2020; this figure is based around waste input and characterisation projections at the time of writing.

Success of the project to date

To date, Ti Tree Bioenergy has produced 28,000 MWh of electricity. The thermal efficiency of power station is 37.8 per cent.

“This a great achievement for the Ti Tree Bioenergy facility as it represents a milestone in the project conceived [over] many years previous. These types of projects are targeted at providing a sustainable solution to dealing with waste, employing best practice to do so,” says Chris Alexander, GM Resource Recovery Services, Ti Tree Bioenergy.