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Carbon neutral or a Trojan horse?

Complex models must be developed to determine just how green the seemingly environmentally friendly process of turning biomass into electricity really is, writes Penny Hitchin.

Miscanthus, also known as elephant grass, is a fast-growing source of biomass fuel<br>Credit: Drax
Miscanthus, also known as elephant grass, is a fast-growing source of biomass fuel
Credit: Drax

Utilities are looking for the holy grail: reliable baseload electricity derived from a sustainable, low carbon source and available around the clock, whatever the weather.

Biomass, despite the rapid growth in its use, is still not ticking all of these boxes. It has also recently received bad press from environmental and scientific agencies as they question whether it reduces greenhouse gas emissions compared with fossil fuels.

In November 2012, the UK's Royal Society for the Protection of Birds (RSPB), Friends of the Earth and Greenpeace called on the UK government to cancel plans to subsidise the burning of trees in coal power stations. The RSPB report 'Dirtier Than Coal?' says that generating power from typical conifer trees results in 49 per cent more emissions than burning coal, and calls on the government to withdraw public subsidy for generating from feedstock derived from tree trunks.

Binding climate change targets and government support for low-carbon energy are bringing about widespread use of biomass in electricity. Coal power stations are co-firing biomass, and dedicated biomass facilities are springing up. As the sector develops, so does understanding of the impact of the large-scale use of fuel made from recently living plant material. It is increasingly clear that the diverse forms of biomass come with different life-cycle carbon emissions and varying green credentials.Initial national policies were based on the assumption that biomass energy is carbon neutral. Biomass has been included in energy portfolios as an infinitely renewable energy source like wind and solar, so it has been eligible for the same support. But closer study of the net greenhouse gas benefits of burning biomass shows that a more complex model of carbon accounting is required. This should include factors relating to the type, source and treatment of the biomass, modelling of forest growth, transport of the biomass and timing of emissions and sequestration.

The Drax power plant will no longer be the UK's biggest polluter<br>Credit: Drax
The Drax power plant will no longer be the UK's biggest polluter
Credit: Drax

Burning biomass to generate a unit of electricity does release CO2. It initially liberates up to twice as much carbon dioxide as coal and up to four times as much as gas used to generate a similar unit of power. The idea of carbon neutrality arises because replacing the combusted vegetation with new trees will lead to the re-absorption of the CO2. The time it takes for a new plant to absorb the same amount of CO2 released during the harvest, transport and combustion of the felled plant is termed the carbon payback.

Biomass feedstocks

Biomass is available from sources such as:

  • conventional forestry management: thinning, felling and coppicing of sustainably-managed forests and trees
  • conventional agricultural crops and crops grown for use in energy generation. These include short-rotation coppice, such as willow and miscanthus (elephant grass), which can be grown on land unsuitable for food crops
  • biodegradable wastes and residues, including wood processing residues - for example, sawmill residues and parts of trees unsuitable for the wood industry; agricultural waste such as straw and husks; sewage sludge; animal manure; waste wood from construction; and food waste
  • algae, which is not yet viable on a commercial scale but could be an important source of liquid biofuel and solid biomass in the future

Biomass generation is booming on the back of climate change legislation and incentives, such as subsidies and FiTs designed to reduce greenhouse gas emissions.

A hydrolic lift tips a truck's load of wood chips at the Nacogdoches generating facility in Texas, US<br>Credit: Southern Company
A hydrolic lift tips a truck's load of wood chips at the Nacogdoches generating facility in Texas, US
Credit: Southern Company

These differ from country to country. As the sector becomes better developed, sophisticated distinctions may have to be built in to encourage the use of biomass that is environmentally and economically sustainable with an acceptable carbon payback period.

In May 2010 the US Environmental Protection Agency decided to include greenhouse gas emissions from biomass energy in its greenhouse gas permit programme. This would treat CO2 emissions from biomass generation and fossil fuels equally. But successful lobbying by the forestry industry led the EPA to defer implementation for three years while it considers how biomass emissions should be determined.

While the EPA ponders the issue, the state of Massachusetts is pressing ahead and implementing sustainability policies for biomass feedstocks. Regulations introduced in 2012 mean that in order to qualify for Renewable Energy Certificates (RECs), biomass generators in the state must use certified fuel in the form of residue or thinnings. An electronic Biomass Certificate Registry tracks and verifies the use of Biomass Fuel Certificates, and facilities must also achieve a 50-per-cent reduction in greenhouse gases over 20 years, compared with a gas-fired unit.

Around half of the targeted 20 per cent of energy that the EU wants to come from renewable sources will be generated by biomass, the bulk of it from wood, which the EU counts as carbon neutral.

In 2010 the European Commission made recommendations on sustainability criteria to be used by Member States in their biofuel policies. Environmentalists, scientists and others are pressing the EU to go further and introduce regulations on sustainability criteria for biomass.

Nacogdoches is one of the world's largest purpose-built biomass-fuelled power generation plants<br>Credit: Southern Company
Nacogdoches is one of the world's largest purpose-built biomass-fuelled power generation plants
Credit: Southern Company

In the UK, biomass electricity generators have been required to report annually on their performance against sustainability criteria for the biomass feedstocks they use. The reporting was intended to develop knowledge ahead of the introduction of European sustainability standards, which have yet to materialise.

The two sustainability criteria are:

  • a minimum 60-per-cent greenhouse gas lifecycle emission saving for electricity generation, using solid biomass or biogas relative to fossil fuel
  • restrictions on using materials sourced from land with high biodiversity value or high carbon stock (primary forest, protected areas, peatland and wetlands)

From April 2013, generators are required to meet the sustainability criteria in order to receive support under renewable obligations. The UK government says that further work is underway to include sustainable forest management criteria in future.

Calculating emissions

The RSPB report says that the payback period for electricity generated from using whole conifer trees is 80 years, but other studies come up with lower figures. The difference lies in the assumptions used in making life-cycle assessments.

Establishing the life-cycle emissions of biomass fuel is a complex calculation. It must take into account such factors as: location of the source, location of combustion, the type and derivation of the source material, treatment or preparation, and transport of the material.

If the biomass comes from an energy crop planted specifically to produce biomass fuel, then the calculation may need to incorporate a calculation of the indirect land-use change impact of the biofuel if net carbon loss occurs when forests and grasslands are cleared for food production that has been displaced by biofuel plantations elsewhere.

A 2012 report by the US Southern Environmental Law Centre - using woody biomass for a modelled expansion of power generation in the southeast of the US - found that it will take 35-50 years to provide carbon reduction benefit. Biomass Supply and Carbon Accounting for Southeastern Forests analysed 17 existing and 22 proposed biomass facilities in the states of Alabama, Florida, Georgia, North Carolina, South Carolina, Tennessee and Virginia. Based on current trends in using wood in large power plants and exporting fuel pellets to Europe, biomass energy in the southeast of the US is projected to produce higher levels of atmospheric carbon for 35-50 years, compared with fossil fuels. After that, it will result in significantly lower atmospheric levels as regrowing forests absorb carbon.Biomass from composted waste or agricultural residues is an efficient way of reducing carbon emissions, but removing crop residues such as straw may deplete the soil's carbon stock, with resulting increases in fertiliser and irrigation use, and lower yields. Similarly, over-enthusiastic removal of forest residues and stumps for energy production will negatively affect soil properties and biodiversity.

Transport and combustion

Practical problems associated with the use of biomass feedstocks relate to the high bulk volume, high transport costs and large storage capacities. High moisture content can lead to biological degradation and block in-plant transport systems in freezing temperatures. Variations in moisture content complicate plant operation and process control.

Biomass can be processed or densified by turning it into pellets or briquettes. This reduces the moisture content, the transport costs and storage volume and makes for easier handling. Increased energy density and more homogeneous composition result in better combustion control, which offers higher energy efficiency during combustion. The major disadvantage is the cost of the pelleting process, which increases the price of the end product and adds to its emissions profile.

Wood has been used for heat and light since the dawn of history, making it the oldest and most universal fuel. Industrialisation saw biomass superseded by large-scale use of fossil fuels, but the pendulum has swung. Biomass is undergoing a renaissance as an emerging renewable energy source, the technology surrounding which is capable of generating reliable, flexible baseload electricity.

Sustainable biomass requires sustainable forests. Large-scale generation needs large-scale supplies. The development of commercial biomass operations must lead to increased emphasis on sustainable forest management to produce fuel with a reasonable carbon payback period.

During a life-cycle of around 300 years, unmanaged forests emit almost as much CO2 as they absorb. If carbon that would be released into the atmosphere during the unmanaged phase of decay is put to use in the form of fossil fuel substitute, then a managed forest acts as a carbon sink. Looking to the future, the lifecycle assessment of forest to furnace will be part of any biomass generation scheme. The mechanisms used to support biomass generation will align with sustainable forest-management policies.

Biomass case studies

Drax, UK

The 4000 MW coal-fired power station at Drax in Yorkshire supplies around 7 per cent of the UK's electricity, and ranks as the country's largest emitter of carbon dioxide. When the station started generating in 1974 it was fuelled by coal from surrounding coal fields, but in 20 years these had all ceased production, and fuel is largely imported.

Drax is the second-largest power plant in Europe. It opted into the European Union's Large Combustion Plant Directive, allowing it to run without restriction. In April 2013 the carbon floor price came into effect at an initial charge of £16 ($24) on every tonne of CO2. Following in 2016 will be the Industrial Emissions Directive (IED), which will require coal plants to install selective catalytic reduction technology to remove NOx from flue gases.

Over the past ten years the Drax Group has been carrying out a lot of research and development work into co-firing biomass with coal. Following the UK government's Renewable Obligation Certificate (ROC) banding review announced in July 2012, which favoured boilers fully converted to run on biomass over co-firing, Drax decided to convert three of its six boilers to 100 per cent biomass. The company says that within five years this will halve its carbon footprint relative to today from 22 million tonnes (MT) to 11 MT of CO2.

The conversion requires an investment of nearly £700 million, of which half will be spent on equipment installations and modifications at the site, including fuel delivery and storage facilities with the other half required for upstream supply chain infrastructure and IED compliance

The first converted unit is undergoing commissioning, with the second planned to come on stream in 2014, and the third two years later. Peter Emery, production manager at Drax told Power Engineering International, "Since the beginning of April we have been running our first biomass converted unit.

We are into a period of phased commissioning of both the boiler modifications and, later in the year, the storage and distribution systems. Our first unit is currently utilising our existing receipt, storage and boiler delivery systems, which were initially commissioned for biomass co-firing in 2010. Our new receipt, storage and boiler delivery systems will be phased in over the coming months and we plan to have them fully operational by around November of this year."

Developing a brand new supply chain to ensure deliveries of adequate quantities of biomass pellets has been a challenge. Biomass is more expensive and bulky than coal and costs more to transport inland. Drax is actively encouraging the development of a local supply of miscanthus, but this will provide only a fraction of the amount of fuel needed.

Drax has looked to North America for its major fuel contracts. It has invested in two pellet production plants in the USA, in Mississippi and Louisiana, and has signed long-term supply contracts with Pinnacle Renewable Energy to provide pellets of pine wood from western Canada, and with the US companies Enviva, Green Circle and Plum Creek.

Drax contracts cover fuel for first unit - 2 Mt for the 2013/14 ROC year, with arrangements for fuel for the subsequent units under negotiation. More than 3 MT will be required for the 2014/15 ROC year.

Developing the infrastructure to deliver the biomass to the plant has involved investment in ports, shipping and rail. Agreements are in place for the expansion of the UK east coast ports of Tyne, Hull and Immingham. Onward rail transport will be via purpose-designed biomass rail wagons, of which the first 50 are being fabricated in Derby to a Drax design. The wagons will enable trains to carry up to 50 per cent more than current trains and provide efficient loading and unloading with full weather protection. Four new storage domes are being prepared to house the biomass on site.

The biomass plant will largely rely on imported fuel, transported from North America, but Drax believes that the fuel will be sustainable in both environmental and economic terms, despite the distance.

Matt Willey from Drax told PEi: "Biomass is an essential part of the renewable energy mix because it is a low-cost, low-carbon and reliable renewable. Sustainable biomass of the sort Drax uses is independently verified as sustainable, including habitat and biodiversity protection, and delivers significant CO2 reductions relative to fossil fuels, even accounting for the CO2 emitted in the processing of the biomass and transporting it from forest to furnace."

Nacogdoches, US

Biomass Magazine says the USA has 180 biomass plants, which contribute 1.2 per cent of US generation.

One of the largest purpose-built biomass-fuelled electricity generation plants is at the Nacogdoches Generating Facility in Texas. US utility Southern Power's $500 million wood-fired biomass bubbling fluidised bed (BFB) plant there uses non-merchantable wood biomass materials sourced locally.

This includes wood waste from logging and milling, and urban wood waste from clearing, tree trimming and pallets. The plant uses around 1 million tonnes of fuel each year - which represents an average of 160 truck loads per day. All of the fuel can be procured within a 120-km radius of the plant.

Alongside the BFB at the plant are a condensing steam turbine generator with an evaporative cooling tower, a wood fuel handling system and auxiliary support equipment. Finnish boiler technology specialist Metso supplied the BFB boiler and began the design and construction of the generating facility boiler island in 2009. The project is designed to produce 117 kg/s of steam, and to generate 100 MW of renewable electricity.

The plant has a 20-year contract with Austin Energy, which serves the City of Austin and has a goal of meeting 35 per cent of its energy needs from renewable sources by 2020. Approximately 100 service contracts have been created for operating and maintaining the plant and fuel supplies.

Polaniec, Poland

Poland's incentive schemes for the development of renewable energy make it one of Europe's major growth markets for the biomass industry, and developers are commissioning innovative installations here.

GDF Suez's new 200 MWe biomass unit at Polaniec, one of the country's largest power stations, started operating in November 2012. The plant is powered by the world's largest and most advanced biomass-circulating fluidised-bed (CFB) boiler, supplied by Foster Wheeler.

CFB technology is capable of fully firing a wide range of biomasses and other fuels, including those derived from agricultural crops. The plant will use up to 20 per cent agricultural material. Agro biomass can be produced much faster and on a larger scale than wood-based biomasses, offering improved sustainability of power.

The Polaniec CFB can achieve a net plant efficiency of over 36 per cent, based on the fuel's lower heating value.

Poland produces about 35 million tonnes of wood waste and 14 million tonnes of agricultural waste annually. The Polaniec biomass plant is likely to use up to 890,000 tonnes of wood chips and 222,000 tonnes of agricultural waste annually.

Penny Hitchin is a freelancer writer, who specialises in energy matters.

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