UK Project Claimed to be a World First for Full Scale Autoclaving with Wet AD
This is surely wonderful news for all those that are keen to see innovation in the biogas industry, as there are advantages in the use of autoclaving pretreatment in the manner in which it enables efficient separation of recyclates in a much healthier working environment, and provides pastuerisation for a saleable digestate at the same time. The industry must be wishing this venture every success, and watching keenly to see how well the process performs, especially the plant’s peformance in removal of enhanced calorific value from the autoclaved organic content during digestion.
Devon County Council has awarded planning permission for an innovative £15m advanced anaerobic digestion (AD) facility, incorporating autoclaving to be built on a redundant china clay site at Lee Moor in the south of the county.
The facility will be operated by AAD (South West), a subsidiary of British engineering firm the AeroThermal Group, which has developed its own technology to treat mixed waste.
What is the AAD Process?
The Advanced Anaerobic Digestion process is a combination of autoclaving and anaerobic digestion technologies that will reduce the volume of waste; separate out recyclables; produce a soil-like material; and maximise green energy production providing a local and sustainable solution for waste. Up to 75,000 tonnes of waste will be able to be processed at the site annually.
During the anaerobic digestion process, micro-organisms break down biodegradable material in the absence of oxygen to produce a methane and carbon dioxide rich biogas, which is suitable for energy production.
Lee Moor AD Facility
The facility at Lee Moor is projected to generate up to 3MW of renewable electricity and a similar quantity of heat from the processing of around 75,000 tonnes of co-mingled food and green waste. The process will produce around 11,600 tonnes of dry, solid, soil-like material per year for land restoration and recycle about 15,000 tonnes of metals, plastic and glass combined per year.
The Lee Moor facility will be made up of two autoclave plants, screening and separation equipment, anaerobic digestion plant with associated buffer and digestate storage tanks, dewatering plant and a combined heat and power (CHP) plant.
The two autoclaves will operate in parallel, each treating mixed municipal wastes in ten tonne batches at temperatures of approximately 160oC for 45 minutes at a pressure of seven bar. After being autoclaved, the waste will then be conveyed to screening equipment to separate the organic and inorganic fractions. Metals and other inorganics will be removed for recycling, while the organic fraction will be transferred to the The Biogas Digester Technology and Information Website.
The biogas will be combusted in the CHP plant to produce up to 3.2MW of renewable electricity and 3.8MW of heat. The electricity will be exported to the national grid, while the heat will be passed to a boiler to raise steam for use in the autoclaves and to provide heat for the AD tanks. The technology enables the steam to be recycled between the two autoclaves, significantly reducing the amount of energy needed by the system.
The digestate will be dewatered in a centrifuge plant to approximately 25% dry solids. It will then be used in the restoration scheme for the Lee Moor china clay pits which are located nearby. This will reduce the need to import restoration materials from other facilities which might be located significant distances away, further reducing costs and environmental impact.
As a stabilised and sanitised organic rich soil conditioner and fertiliser, the digestate will contain nutrients such as nitrogen, potassium and phosphorus that are essential for plant growth. A large proportion of these nutrients will be held in organic form and released slowly over a number of years as the material breaks down. This will allow for better synchronisation with the demands of plants than is normally possible with inorganic nutrient fertilisers. Applying the digestate will also improve soil structure and water retention capacity, encouraging the growth of grassland and other plants used in the restoration scheme.
This will not be the first full scale autclave plant for MSW, but the combination of autclaving with Anaerobic digestion is the first which is claimed. Do you know different? Comment below!
Enpure Limited was awarded a contract for autoclave heat treatment, by main contractor Clugston Construction in 2011, for a Municipal Solid Waste Treatment Plant at the Graphite Resources Derwenthaugh Ecoparc, Newcastle. This project, which utilises the Rotoclave Rotary Steam Autoclave Technology, is driven by landfill diversion and recycling targets and general improvements in waste collection and recycling strategies.