Sustainable waste management
UK. The recent Sustainable waste management seminar, organised by the Bulk Materials Handling Committee of the Institution of Mechanical Engineers at their London headquarters, covered a broad range of topics, providing an insight into strategies for dealing with waste and the options available for its use as a resource. 
  The past decade has been a period of upheaval for the waste sector as it tries to achieve a fine balance between environmental aspiration and fiscal reality, coping with changing legislative priorities and demands whilst implementing technological solutions that will stand the test of time. The substantial costs associated with facilities that collect, process, store, handle and convert waste also mean that there is a high price to pay if the final result is not fit for purpose.
• The keynote address by Dr Alan Whitehead MP , Chair of the All-Party Parliamentary Renewable and Sustainable Energy Group, and Co-Chair of the Associate Parliamentary Sustainable Resources Group, emphasised parliament’s consensus of the inevitable move towards a low carbon economy, where the assumptions that made in the past are no longer applicable.
  Worldwide targets on emissions and a legally binding Climate Change Act are putting the UK in the global driving seat in terms of having a legally binding long term framework to cut carbon emissions. The result will be aggressive decarbonising of the economy, said Dr Whitehead. And how efficiently we deal with waste will be a key element of this policy. 
  • Richard Woosnam, Business Development Manager, Orchid Environmental Ltd, started the municipal solid waste section of the seminar with a paper on Integrated energy parks - an opportunity not to waste. 
  Waste-to-energy schemes are springing up around the country, part of the solution for dealing with the mountains of unwanted material that we generate on a daily basis. 
  Orchid is prime mover in this market, but Woosnam feels that a more positive approach towards waste is called for - examining what it is and what it isn’t. One example of this positive thinking is Orchid's partnership with Merseyside Waste Disposal Authority in developing a new facility at Huyton, a pilot waste treatment and recycling plant designed to divert significant volumes of household waste from landfill, converting household and commercial waste into refined renewable biomass fuel products and recovering the mixed recyclable materials. 
  Using low temperature technology, known in the trade as mechanical heat treatment (MHT), the facility converts waste into a renewable biomass fuel, or refuse derived fuel (RDF), which can then be used as a sustainable alternative to fossil fuels. Although the plant was initially designed with just municipal solid waste in mind, process enhancements have allowed the facility to deal with both commercial and trade wastes and, in the first few months of operation, has considerably reduced the volume of waste going to landfill.
   One pressing need, said Woosnam, is the need to define RDF as a tradable commodity and not treat it as just waste, a decision that undoubtedly would help the industry move forward. 
• Allan Barton, Director, Global Leader, Resource and Waste Management, Arup, provided an imaginative paper entitled Sustainable resource and waste management and utilising the embedded energy in waste. 
  “If we have got to the stage where we need a technological solution, it’s already too late”, he commented. What is required is a major shift in thinking, a behavioural change, where the engineer’s role is to design waste right out of products at the start and not put the planet’s resources beyond reuse. This involves a combination of sustainable resource and waste management and an ability to utilise the energy value that is inherent in much waste. 
  In a world where there is a strong correlation between waste and GDP, Barton cited the case of China which has an increasing waste problem. Arup, a company with a reputation as an innovator in many areas of construction, is currently engaged in projects in Hong Kong where the waste problem runs at 1mt per year and Dubai where there is a proposal to deal with 2mt per year.
  Barton emphasised the mantra of sustainability - development that meets the needs of the present without compromising the ability of future generations to meet their own needs by keeping all resources recycling around. Even if we manage to get this right, he said, there will still be a fraction of waste material which will ultimately have to be disposed of. 
  Arup has been involved in a variety of waste-to-energy schemes that utilise grate incineration, fluidised bed incineration, gasification, mechanical biological treatment and anaerobic digestion. Because of the high costs associated with these projects he questioned the reasoning in dealing with household and industrial waste separately. Why not together? 
• Looking at the more practical aspects of handling waste and getting it to where it is needed, Eddie McGee, Technical Director, Ajax Equipment Ltd, presented a paper entitled Keep plugging away. This dealt with the specific problems associated with feeding pyrolysis systems. 
  In these facilities material blockages or plugs can easily form due to the variable nature of the feedstock, its badly flowing properties and the inclusion of foreign matter. Added to this are the complications of the pyrolysis process itself, operating at an elevated temperature, where air ingress must be avoided at all costs, a considerable challenge for any continuously operating equipment that has to feed it. 
  McGee says that for these instances, Jenike’s classical bulk solids flow theory may be inappropriate and it is more sensible to have a more tolerant design ethos. Testing material to determine bulk density, shear strength, wall friction, permeability, particle size, presence of lumps, particle shape and moisture content are a requirement for any project, as all affect flow and influence the formation of plugs. However, problems arise when trying to bring all this conflicting information together when predicting how the feedstock is going to behave in practice. 
  Spider Diagrams are a help, says McGee, a tool that he has developed to gain an insight into a material's likely behaviour. By plotting each bulk property on a series of axes, a graphical representation can be built up for a particular material to discover its range of flow attributes. Armed with this ‘picture’, the hopper or feeder design can be undertaken with more confidence, says McGee. It is also essential to have an experienced supplier on board, he adds. 
• Mechanical heat treatment of municipal solids waste (MSW) at 3NRG’s Bridgend facility formed the focus of a paper by Michael Geary, business development director, 3NRG. 
  The Bridgend plant employs MHT (Mechanical Heat Treatment) utilising a novel closed loop process which is said to have provided the lowest capital and operating cost relative to other alternatives such as MBT (Mechanical Biological Treatment) or mass burn incineration. 
  The scheme ensures that energy is obtained from the biomass and that recyclables are recovered. Importantly, the plant is eligible for ROCs (Renewables Obligation Certificates), part of Government strategy to source an increasing proportion of electricity from renewable sources. 
• The afternoon session on biowaste was led by Terry Coleman , Science manager: Waste, resources, remediation and technologies, Environment Agency, whose paper on Renewable energy recovered by different municipal waste processes and their greenhouse gas contributions covered a study into waste processes which produce electricity from: food; paper; wood; refuse derived fuel (RDF); and residual municipal waste. 
  All the available options were looked at including: incineration without energy recovery; incineration with recovery of electricity only; dedicated electricity generation; gasification; MBT to produce RDF for incineration; MBT to produce SRF (Solid Recovered Fuel) for dedicated industrial use; and anaerobic digestion and burning biogas for electricity generation. The project was an immense undertaking and in the final analysis findings were limited to food and wood waste. 
  Recovering energy from food waste can be achieved in two ways: either without separation and burning in an energy from waste (EFW) scheme or separating material and subsequently passing it through an anaerobic digester. The study determined that EFW produces over twice as much renewable energy as anaerobic digestion after taking into account the fuel consumption involved. 
  Wood waste, the other area identified in the findings, already regarded as being in its separated form, can be dealt with by: EFW; burning in a dedicated power plant; or gasifying and burning. Findings indicated that burning in a dedicated power plant and gasification both produce around 50 per cent more renewable energy than EFW.
• Frans Lamers, senior consultant, KEMA Consulting, presented a paper on Opportunities for bio-energy projects - relations with waste , outlining the waste-to-energy experience from a European perspective. 
  The EU has ambitious bio-energy targets, setting a figure of 20 per cent renewable energy by 2020, a 50 per cent bio-energy contribution and a 100 per cent increase in biomass in the 2003 - 2010 period. Bio-energy is heavily promoted, but questions remain as to whether this market growth can be accommodated, suggested Lamers.
  Is there enough biomass at a reasonable price, can hardware be supplied by industry in time and is there enough support from national governments - a lot of unanswered questions. Lamers said that a 'total' approach is required involving close cooperation with end users, a combination of reduced energy usage, renewable energy/bio-energy and smart fossil energy measures. 
  Lamers cited the differing experiences of EU countries. For instance, Germany had a high feed in tariff for all bio-energy plants, but since 2007, only fresh biomass plants are subsidised, with the waste wood market completely saturated. Countries such as Sweden and Finland have a lot of experience with bio-energy because of the need for combined heat and power plants. Other EU countries were only now beginning to address the problem, said Lamers.
• Paul Cowell, mechanical engineer with Hanson Cement’s Ribblesdale Works outlined Biomass as a fuel in the cement industry . The energy intensive nature of cement has driven Ribblesdale to investigate alternative fuels such as shredded tyres and MBM (Meat and Bone Meal) in conjunction with pulverised fuel. 
  MBM, increasingly used in cement kilns as an environmentally sustainable alternative to coal, is normally pneumatically conveyed from road tankers to purpose built silos and extracted using swept screw conveyors and subsequently conveyed to the kiln calciner. Isolation valves are a key part of the set up to allow material to be blown into the kiln only when the temperature inside has reached a set level. This is vitally important as cement plants of this kind are regulated under the Waste Incineration Directive as co-incineration plants. Operating conditions such as gas temperatures and residence times are therefore critical in meeting specific emission requirements.
• Storage and handling of solid biofuels was the subject of a presentation by John Pethullis , director and general manager, Portasilo Ltd. 
  Most biomass materials need to be stored under cover to control moisture and microbial degradation and Pethullis listed the factors that need to be taken into account including: usage rate/storage life/delivery method to determine type and size of store; odour and dust hazards; biomass materials requiring sealed containment; control of temperature, humidity, ventilation and storage time to prevent self heating; gentle handling of pelletised material to prevent breakage and dust release; and prevention of dust explosions. In addition, there are the inherent handling and storage problems in dealing with a material that has a wide range of physical properties.  
  The key to success, said Pethullis, is "for the client to work closely with the equipment supplier", obvious you might think, but often sadly lacking on many projects. Again, it is essential that the biomass supplier has experience with similar products to help define limits for the design, said Pethullis. 
• The final paper of the day was given by Professor Mike Bradley, Manager, Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, and entitled Waste Materials - How do you know what to expect, and how to handle it. 
  Bradley, a seasoned campaigner for good practice in the field of solids handling and processing, quoted the Rand Report (1990) which determined that 60 per cent of solids processing plants never achieve satisfactory operation, and that average cost overruns on novel solids processing systems are in the region of 110 per cent, more than twice the original estimate! 
  The basis for success said Bradley is to recognise the characteristics of the material, a fundamental but often unresolved point. Bradley said that the trouble with waste is that, unlike other products, it is not made to any specification and can potentially fall into one or a combination of three handling categories: Class 1 - consisting of rounded free flowing particles; Class 2 - comprising rounded particles of a cohesive nature: and Class 3 - a flaky/stringy particle group with extreme shapes. 
  Bradley explained that it is essential to take on board the level of variability in a material, realise that materials with the same name can behave differently, and use characterisation techniques on many samples. Some pieces of equipment may even require more material characterisation than others to gain a better understanding of the flow characteristics. In the final analysis, any procurement process should take into account the whole life costs of equipment and not just the initial capital cost, he said.
• Further details of future IMechE events can be obtained at: www.imeche.org