Renewable energy from bioresiduals and waste: Designing a viable project in an overheated European market
Marcel Goemans
Ronald Tize
The waste-to-energy market is drawing increasing attention and investment in Europe. But areas of uncertainty remain. A key challenge for the industry is adopting an intergated approach that maximizes plant efficiency, ensures fuel flexibility and promotes effective supply chain management.
by Marcel Goemans and Ronald Tize
Recent changes in EU and national waste legislation and a growing focus on renewable energy have resulted in a very dynamic, if not overheated, market for waste-to-energy (WTE) and for biomass power and combined heat and power (CHP) projects. Equipment suppliers have full order books, capital costs are increasing and the market value for bio-residuals and pre-processed waste is expanding.
Under such dynamic conditions, project developers may encounter a range of difficulties in designing a viable project. For although the financial drivers are favourable – especially through green power and CHP credits provided by policy makers in several Member States – uncertainties in the market remain. These range from the ‘usual’ constraints such as planning permission and environmental permitting, to more recent issues such as equipment delivery times, rising fuel costs (or reduced tipping fees) and uncertainty about fuel availability. Minimizing these uncertainties is paramount to a successful project, which can be achieved with a methodical approach to optimization of the WTE and renewable energy process and its inputs.
New trends in energy production
Within the broader context of sustainable development, a state-of-the-art WTE facility can no longer be perceived as an advanced waste treatment or recycling centre, but instead it should be considered as a part of the renewable energy solution. With this in mind, municipal solid waste (MSW) and non-hazardous industrial wastes are not the only wastes being considered for WTE projects. Indeed, biowaste such as sludge from sewage treatment works, organic MSW, contaminated biomass streams and marginal bio-residuals (e.g. with high water or salt contents) are also considered as alternative or renewable fuels. Biowaste has the advantage that it is 100% renewable, hence a carbon-neutral source of energy. At the same time the market for renewable energy from clean biomass is booming.
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WTE in action: sludge from the Mersey Valley wastewater treatment plants is digested at about ten decentralized anaerobic digestion plants equipped with CHP engines.
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Digested sludge is used as fuel in the WTE plant at Shell Green. The integrated project produces over 10 MW of electrical power and was developed through a partnership between MWH and United Utilities.
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The current drive for renewable energy from solid residues results in an increased uncertainty about the availability and price of waste, clean biomass and contaminated bio-residuals during the lifetime of a project. The price for clean wood chips has increased substantially over the past years. Many waste streams that carried a negative cost in the past need to be purchased today; for example, the first projects where high strength wastewater is purchased from the producer have been reported. The pressures on the renewable fuel and waste, combined with premiums for green electricity or combined heat and power (ROC, Green Certificates, CHP Certificates) mean that the operating income for WTE plants comes increasingly from the sale of power and/or heat – up to 50% – instead of tipping fees.
How can a project developer design a viable renewable energy project in an unstable and fast-evolving market?
A project developer can address the market pressures on fuels by:
- increasing the fuel efficiency to reduce the amount of fuel needed per MW power generated
- including maximal flexibility in the design to be able to source different renewable fuels and/or implement plant modifications in the future
- ensuring active supply chain management to assure stable fuel supply at economical costs
- developing integrated and/or complementary projects to reduce the company’s dependency on fluctuations in the renewable fuels market.
Increased fuel efficiency
The search for ways to improve the overall plant efficiency has been driven by expected changes in EU legislation, premiums for renewable energy and increased fuel costs. The drive for more efficient WTE plants has resulted in developments in furnace control concepts, higher steam parameters, alternative steam cycles, more efficient recovery boilers and different flue gas cleaning processes, among others. Fuel efficiency for electricity-only plants has increased from about 20%–21% to 23%–26% for WTE 1,2 and from 26%–30% to 30%–33% for biomass power plants. Specific technology innovations include:
- higher steam parameters (2%–5% efficiency gain)
- reduced excess air ratios (0.5%–1% efficiency gain)
- lower boiler exit temperatures (1%–2% efficiency gain)
- lower condensate pressures (2%–3% efficiency gain)
- innovative Rankine cycles incorporating e.g. steam reheating (2%–5% efficiency gain) or external steam superheating (>10% efficiency gain)
- expert control systems for the combustion and flue gas cleaning process to obtain more stable operation, lower excess oxygen content, constant steam flow, higher throughput and lower emissions.
Efficiency gains resulting from these measures are, however, limited and partially off-set by increased corrosion risk, higher maintenance costs and/or reduced plant availability.1
Important gains in fuel efficiency are obtained when the project can be designed for combined heat and power (CHP) production. Heat can either be supplied to an external user – such as district heating, industrial plants or public infrastructure – or can be used to pre-dry the fuel to increase the calorific value. However, CHP does not always result in a higher return on investment (ROI): CHP and renewable energy incentive programmes, such as Green CHP Certificates in Belgium or Renewable Obligation Certificates (ROC) in the UK, appear not to favour reduced power output to provide heat (e.g. by supplying medium pressure steam to an external user). Instead, maximal power production with supply of low level waste heat often turns out to result in a higher return.
Fuel supply chain management
A second step to achieve a viable project is to maximize fuel flexibility in order to reduce dependency on the volatile biomass market. Fuel flexibility is achieved both by technical means (discussed below) and strategic partnerships. Developers and renewable energy producers are actively managing – or even entering into – the fuel supply chain. This can be achieved by assuring long-term supply contracts, or by partnering with a waste management company or owners of important biomass streams such as farmers’ organizations and large agro-industrial entities. Partnerships with waste management companies have the additional advantage that the partner has ample experience to play on the spot-market for waste and typically owns several waste separation and pre-treatment centres.
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MWH is currently the engineering partner of Thenergo for the ‘Gallus’ renewable energy project in Flanders, Belgium. Biowastes such as chicken litter and manure are pre-dried in different drying hubs using waste heat from CHP plants and/or industrial processes. The pre-dried biowaste will be used as fuel in Thenergo’s new 20 MW power plant (www.thenergo.eu)
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Developing synergies between different renewable energy projects can be an interesting alternative or complementary approach to partnerships. Synergies can range from including several fuel pre-processing centres in the overall WTE project, to designing complementary renewable energy projects. For example MWH designed a biowaste WTE project for Thenergo using pre-dried digested sludge and biowaste. Sludge originates from Thenergo’s anaerobic digestion with CHP plants. Waste heat from the CHP engines and industrial processes is used to pre-dry sludge. Biowaste is pre-dried in several fuel processing hubs in parallel with the main biomass fired WTE facility. In these hubs, waste heat from industrial processes is used to pre-dry marginal biomass streams such as chicken litter, manure or organic sludge streams.
Design of WTE installations to fire biowaste
Technical measures to improve fuel flexibility focus on the selection of the technologies as well as provisions for future modifications to the technical installations. These provisions can range from additional fuel-feeding systems to provisions for CHP in a power plant. Flexible design of a WTE plant includes balanced decisions on fuel-feeding systems, firing systems, boiler design, flue gas treatment process and steam cycle design, among others.
Feeding and combustion systems
Fuel flexibility is an important aspect for waste- and biomass-fired power plants. Many installations are designed to accept different fuels (e.g. wood and straw) or non-homogeneous fuels (e.g. MSW or poultry litter). It is widely known that fuel efficiency and plant operation are optimal when firing a homogeneous and constant fuel mixture. This can be obtained through in-line mixing (e.g. different biomass streams) or off-line mixing (e.g. MSW in waste bunker). Also, the risk associated with certain fuel characteristics – such as high salt contents – can be reduced by mixing with cleaner fuels, specific wastes (e.g. tyres for sulphur content) and/or addition of additives (e.g. lime).
Table 1 provides a comprehensive list of possible fuel feeding and firing technologies. Ram feeders provide the highest flexibility towards physical dimensions of the fuel. However, they result in higher capital costs and usually are combined with more expensive firing systems (e.g. reciprocating grates). The size distribution requirements for the fuel are more limiting for spreader stoker systems than for ram feeders. This may result in a higher fuel cost because of the requirement for off-site or on-site fuel preparation (shredder, sieve, metal removal). However, spreader stokers can be combined with less expensive firing systems (e.g. vibrating or chain grates, fluidized bed systems). Mono-fuel systems such as cigar burners for straw bales have a very limited market share.
Boiler design
The boiler design is a critical part of the technology selection process and includes selection of the steam parameters, boiler configuration and flue gas exit temperature.
The vast majority of WTE plants in Europe operate at steam parameters of about 40 bar and 400°C, resulting in net efficiencies of 20%–21%. For biomass-fired power plants or CHP plants, these steam conditions are typically 60–100 bar and 425–575°C resulting in net efficiencies of 26%–30%. A lot of development efforts in WTE are devoted to increasing the steam parameters for WTE plants to those common for biomass power plants, while for pure biomass power plants pressures of 100 bar and more are being envisioned. However, these efforts are not without risk. Higher steam pressures – with or without higher superheating temperatures – will result in increased corrosion risks, especially in the superheaters, and thus a risk of reduced plant availability and increased maintenance costs. Higher steam parameters will also result in a higher capital investment cost, partially due to additional corrosion protection measures. Although biomass is renewable, it is not necessarily a clean fuel, containing significant amounts of sulphur, halogens and alkalis, resulting in high temperature corrosion and fouling risks in the radiation section of the boilers. Manure, chicken litter, straw, rice husks and some energy crops have all been reported to cause corrosion in the high temperature section of the boiler or difficult-to-remove salt and ash deposits on superheater tubes. In addition, combination of different fuels can either increase or decrease this phenomenon.
Possible solutions for corrosion control include measures to reduce the flue gas temperature before entering the superheater sections, such as installation of protective evaporators, flue gas recirculation or co-current superheaters. Installation of corrosion resistant refractory linings (e.g. with high alumina content) and Inconel cladding in critical areas has also been used with success. These technical measures are compatible with proven boiler design concepts and have a typical payback period of less than three years.
Alternatively, steam can be superheated to higher temperatures in external superheaters. External superheaters are placed in a separately fired section, not exposed to the contaminated flue gases of the main plant. Natural gas and cleaned biogas are viable fuels to fire external superheaters. External superheaters are best placed in the exhaust gases of a gas turbine or gas engine in order not to jeopardize overall plant efficiency targets. High efficiency biomass plants firing fuels with high salt concentrations – such as straw – can also be equipped with superheaters designed as wear parts, an approach also taken recently in some WTE plants, such as the plant in Amsterdam3. The latter option is only economically viable when electricity prices are higher than €80 /MWh.1
A lower exit temperature for the flue gases results in a higher thermal efficiency of the boiler since the stack losses are minimized. However, lower flue gas temperatures may result in an increased corrosion risk of down stream flue gas cleaning equipment. Safe operating parameters are above 140°C, depending on the acid load. Dew point corrosion can be reduced by in-situ lime injection in the furnace (especially suitable in fluidized bed furnaces). In CHP applications, condensation of flue gases after the flue gas cleaning plant – e.g. for district heating – is a viable way to reduce stack losses to an absolute minimum.
Most of the biomass-fired boilers for power plants and combined heat and power plants are vertical, while most WTE plants use tail-end boilers. Tail-end boilers are very suitable for applications where efficient intermediate dust removal is required. The disadvantages are the slightly higher investment cost and headers in permanent contact with flue gases. Vertical boilers are typically less expensive and have a less complex natural circulation. The disadvantage is the lack of intermediate dust removal options. The final design should be selected in close collaboration with the owner, balancing capital cost with operational cost, plant flexibility and plant availability.
Flue gas treatment
The selection between flue gas cleaning systems is determined by several parameters, such as pollutant loads and removal efficiencies, temperature of the flue gases and disposal options for ashes, residues and water. Boiler exit temperatures are minimized in order to increase plant efficiencies. As a result, many new plants operate with full dry or full wet systems, while semi-wet systems are no longer the optimal choice (energy is destroyed in the semi-wet scrubbing process). In general, the low dust emissions require a high efficiency electrostatic precipitator (ESP) or a baghouse filter, making the wastewater-free dry scrubbing processes the installation of choice for many biomass-fired combined heat and power plants.
In order not to reduce the overall plant efficiency, NOx should be reduced by primary measures (flue gas recirculation, combustion control measures). In case primary measures cannot achieve the required NOx emission levels, a selective non-catalytic reduction (SNCR) deNOx can be considered. Selective catalytic reduction (SCR) deNOx systems have the advantage that low NOx and dioxin emissions can be obtained, but suffer the disadvantages of additional pressure drops (power), energy needs (flue gas reheating) and operational costs (catalyst). These disadvantages can be minimized when the catalyst can be integrated in the boiler or de-dusting stage, or when a combined SNCR/SCR deNOx system is installed.
Conclusions
An integrated approach to biomass WTE project development – including plant efficiency, fuel flexibility, supply chain management and synergy between renewable energy projects – has been successfully implemented by MWH at several locations.
Maximal fuel flexibility will significantly reduce the exposure to the long term uncertainty on the alternative fuel market. However, fuel flexibility comes at a cost, since the chemical and physical characteristics of a fuel have a large impact on the design of the combustion and fuel handling equipment. Higher plant efficiencies often result in increased corrosion risks and associated maintenance and availability concern. Overall plant efficiencies are dramatically improved when combined heat and power is feasible.
Supply chain management can be achieved by partnerships with biomass producers and waste management companies and developing several fuel drying and pre-treatment satellites – e.g. anaerobic digestion with CHP and thermal drying – for the biomass WTE plants.
Marcel Goemans is Manager of the MWH Centre of Excellence for Waste Management and Renewable Energy, Brussels, Belgium.
Website: www.mwhglobal.com
Ronald Tize is Senior Process Engineer at the MWH Centre of Excellence for Waste Management and Renewable Energy
References
1. B. Adams, E. Prasman, R. De Proft, B. Van Renterghem, The Quest for Higher Electricity Recovery Rates for MSW or RDF Power Plants. When to Choose which Option, International Exhibition and Conference for Energy from Waste and Biomass, Bremen, Germany, 2007.
2. O. Gohlke and A. Seitz, Innovative Approaches to Increase Efficiency in WTE Plants – Potential and Limitations, ISWA World Congress, Amsterdam, The Netherlands, 2007.
3. M. Van Berlo, Waste is innovation: Amsterdam’s high efficiency waste fired power plant, ISWA World Congress, Copenhagen, Denmark, 2006.
This article is on-line. Please visit www.waste-management-world.com
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