Cover photo: Example of an 18 MW biomass power plant in Aberdeen, Washington State, USA.
PNG Biomass grows up to 10 million seedlings a year in nurseries in the Markham Valley in Papua New Guinea. They will generate 30 MW of electrical power by growing the seedlings into trees, processing the timber logs into woodchips, and burning them in a biomass power plant.
The seedlings are planted over 16,000 hectares of plantations to grow for 5 to 8 years to mature into trees. While the trees grow they absorb from the atmosphere Carbon Dioxide through the process of photosynthesis. Mature trees are harvested, debarked, and loaded as tree logs onto trucks. Tree logs are then transported to a centrally located power plant site.
Tree logs are processed on site into wood chips to fuel our biomass power plant. Wood chips are channelled into the combustion chamber through a conveyor system and burned to heat water in the boiler. The burning wood chips release stored carbon dioxide back into the atmosphere, remaining ash is used as plantation fertiliser and for road construction.
Under high temperatures of the heat, the water in the boiler becomes high pressure steam that is used to spin the turbine, and the turbine drives the generator.
The generator creates electricity which flows into the grid to power residential, commercial, government, and small industry customers.
Our biomass power generation carbon cycle is neutral. The amount of carbon absorbed by growing trees equals the amount released through burning the biomass wood chips. But by establishing plantations on underutilised grasslands, PNG Biomass has an overall positive effect, as each tree is a small additional carbon sink.
The combustion of solid fuels has been the mainstay of steam and power production for over 150 years. It is only relatively recently that oil and gas were available in sufficient quantities to justify their use in large power stations. Coal overtook wood in its use because of the much higher energy density but wood fired boilers are still in widespread use worldwide (including countries such as Indonesia, Philippines and Fiji), and especially where sustainable wood sources are available. The burning of biomass is therefore not a new technology, but in recent years combustion technologies have been designed to make best use of this sustainable resource.
PNG Biomass will combust woodchip fuel in its biomass power plant using conventional steam cycle technology and use well-tested and trialled travelling-grate biomass boiler technology. The plant is designed to combust woodchips by conveying and blowing the woodchips onto a travelling grate within the boiler.
The spent steam from the turbine is condensed in a condenser cooled with water from a cooling tower using grey water, extracted under license, from local boreholes and treated process-effluent. The condensate is then recycled to the boiler.
Hot flue gas is passed through a mechanical cyclone and then cleaned further in an electrostatic precipitator to remove ash and particulates in the gas stream, prior to venting to the atmosphere via a tall (40m) stack. Ash and any effluent sludge will be collected and used as a plantation fertiliser.
There has been a development of combustion technology and boiler design to achieve the combined goals of higher efficiency and ability to burn fuels set in a regulatory framework which demands ever reducing emission levels.
Fixed-bed or grate combustion is the traditional technology for burning solid fuels. There are different grate technologies which are primarily distinguished by the type of grate movement (horizontal, vibrating, travelling, rotating grate) and by the type of fuel feed mechanism (spreader stoker or underfeed). PNG Biomass will use a travelling grate with a spreader feed system.
The fuel burns out over a certain time releasing its energy into the flue gas. With continuous fuel feeding, combustion will occur in various sections of the grate. Primary air supply is divided into sections to be able to adjust the specific air amounts to the specific air requirements of the zones. The speed of the travelling grate system is dependent on the furnace temperature which is constantly monitored.
From the grate, the semi-combusted gases are led into the main combustion chamber, where secondary air is added. This leads to complete burn out of the combustion gases. The hot flue gasses then pass through to the superheater, evaporator and economiser section of the boiler. The steam is used to mechanically turn a turbine, which in turn spins a generator to produce electricity.
The biomass boiler and turbine are the key elements in making an efficient and reliable power plant. Other equipment referenced (and called balance of plant) include an electrostatic precipitator (used for taking ash out of the gas stream), and cooling towers to cool the steam after passing through the turbine and so be able to reuse the specially processed water.
Key construction considerations include:
Key to the success of building the power plant will be the work of approximately 300-400 local tradespeople from PNG who will be undertaking all the civil and building works, installing all the equipment, ducting and pipe work and electrical and controls installations and finally operating the plant.
The PNG Biomass power plant will be operated by about 30-40 staff – most will be employed locally from PNG. Staff will include operators – running the plant via computer controlled systems, mechanical and electrical technicians and fuel handling and plant maintenance roles.