Biomass and biofuels have received a great deal of science, engineering and media attention, and the debate surrounding the sustainability of these fuel sources is ongoing. Professor John Oakey, Head of the Energy Technology Centre at Cranfield University, UK, reports on the Institute’s Energy from Biomass and Waste conference, held on 11 March in London, UK.

There is wide belief that it is common sense to recover energy (heat and/or electricity) from biomass and biomass-containing waste where it is feasible to do so.

The disposal of these potential fuels in landfill produces methane, a much more potent greenhouse gas (GHG) than CO2. While some of this is captured and used in gas engines, much will escape to the atmosphere. The increasing tax pressure to divert waste from landfill through the Landfill Directive is leading to greater interest in mass-burn waste incineration.

The UK’s Environmental Knowledge Transfer Network has energy from waste as its top priority and, through the development of a business case, has estimated that 34Mt of carbon emissions could be offset.

However, despite the considerable research funding and support measures in the UK (Non-Fossil Fuel Obligation, Renewables Obligation Certificates [ROCs], etc.), the penetration of the technologies developed has been limited, with modest levels of energy recovery, rather than higher efficiency plants.

Exceptions are dedicated systems using single materials such as wood chip, straw and chicken litter, where the plants have been optimised for the specific fuel properties, and for which ROCs are available. Conversely, for mixed municipal solid waste (MSW), the difficulty in directly measuring its biomass content has limited access to funding credits. However, legislation was updated from April to enable easier access to grants. Fifty per cent of MSW can now be ‘deemed’ to contain renewable material without direct measurement.

Engineering a change

 

The technical, economic and regulatory challenges facing developers and researchers in designing and deploying plant systems were highlighted during an IOM3 conference on Energy from Waste and Biomass, held on 11 March in London, UK.

Speakers explored state-of-the-art energy conversion technologies (anaerobic digestion, pyrolysis, gasification and fuel cells). Of the options, anaerobic digestion is widely employed in the sewage industry to generate biogas, while a number of gasification plants are operating in the UK.

Several talks also reviewed the growing range of biomass being exploited from different sources of waste – sewage sludge, MSW and agricultural – and other sources (micro-algae), as well as the advanced processing of these to produce liquid biofuels and improved solid fuels.

Separation of biomass materials from MSW, and their preparation for use as fuels (as pellets or other forms) is improving as sorting at the source increases to enable recycling – although much of the potential biomass fuel is currently diverted into composting.

The high variability of biomass feedstocks, whether available singly (straw), or as mixtures (as separated from MSW), has led to a range of technical challenges. These include the need to ‘over-design’ the plant to accommodate varying properties, such as drying requirements, and to tackle the high risk of corrosion from aggressive species in biomass and waste fuels. There is a demand for protective coatings to solve these problems.

Language barrier?

As well as technical and economic challenges, non-technical issues were highlighted at the event.

Three approaches to classifying energy from waste plants have been considered, based on the fuel’s energy content, the European Court of Justice’s definition of recovery or disposal, and the efficiency of the plant. The Waste Framework Directive 2008/98/EC attempted to clarify the situation, but problems remain with calculating plant efficiency.

Also, difficulties occur with defining renewable fuels from waste sources – for example, biogas from anaerobic digestion of sewage sludge is treated as a waste rather than a by-product, and so is fully regulated under Environmental Planning Regulations.

Essentially, even with the low efficiency of existing waste incineration plants, they still produce less fossil fuel derived CO2 per MWe than natural gas combined cycle power plants. The use of biomass and waste in decentralised power plants therefore offers a real contribution to meeting the UK’s emission targets. Real effort is still needed, however, to change attitudes so that the UK can benefit from higher levels of renewable energy from these dependable fuel sources.

>

Author : John OakeyMaterials World Magazine, 01 Jun 2009