Hrvoje Medarac is Scientific / Technical Project Officer at the JRC's Energy Technology Policy Outlook Unit. His main interests are cogeneration power plants and the use of biomass for energy purposes. Hrvoje graduated with degrees in mechanical engineering and economics and holds a PhD in economics. He gained his professional experience in the private sector, with the Croatian national energy authority and at the JRC.
Biomass in the form of log wood has traditionally been used for heating purposes since the discovery of fire and even now it is a very important energy source. In the year 2012, almost 54% of total renewable energy in the EU-28 came from biomass, while in the heating and cooling sector the share was 86%. The amount of biomass used for heating/cooling purposes is expected to increase by 24% between 2012 and 2020, while its share in the renewable energy mix is going to decrease slightly due to faster proliferation of other renewable sources1. When looking at the primary energy level, the expectations are that by 2020 biomass could contribute 13% of the EU primary energy demand2.
At national level, the highest amounts of biomass are used in large countries like Germany and France or countries with long heating season like Sweden and Finland. In order to reach their 2020 targets, most Member States will have to increase the use of biomass for heating and cooling purposes but some, such as Estonia, Austria and Slovenia, have already reached their targets, as indicated in the map above.
At household level, biomass for heating purposes is traditionally used in stoves where log wood or briquettes are fired to generate heat in a decentralised way at typically low efficiency between 10% and 30%3. Besides stoves, small scale boilers can also use similar types of fuel for small, household central heating systems. These systems can usually also use smaller sized fuels like pellets or wood chips, which enable automatic feeding. In recent years, with the development of modern condensing wood pellet boilers, the efficiency of these systems has increased to almost 90%.
Middle-sized centralised systems dedicated to heat generation in small networks use fuels which enable automatic feeding, like pellets or wood chips, and usually use hot water boilers to generate heat at the level of up to 90% efficiency. In wood industries it is not uncommon to find fire-tube steam boilers which generate saturated steam for industrial use and provide heat for heating purposes by the means of heat exchangers.
Larger district heating systems and industrial plants fuelled with bio-origin fuels usually use cogeneration technologies. In the case of solid fuels, like straw, forestry biomass, municipal solid waste or sewage sludge, water-tube steam boilers are often used, usually with a grate incineration system and sometimes with fluidised bed incineration. The steam turbine - generator set is used to convert energy from steam to electricity, while either bleeds or back-pressure outlet are used to generate heat in the form of steam. The use of condensing instead of back-pressure turbines increases operational flexibility and makes it possible to increase the profitability of a plant by selling more electricity to the grid. The efficiency of these plants directly depends on the way the plant is operated: with high heat generation and low electricity generation the overall efficiency may exceed 80%, while in the case of low heat generation and high electricity generation the efficiency may drop below 30%.
For smaller wood industries with significant heat demand throughout the year, organic Rankine cycle (ORC) cogeneration has also been proven to be a competitive technological option. These plants are usually designed as a combination of an oil biomass boiler where the energy from biomass is used to heat thermal oil, a heat exchanger where the energy from thermal oil is transferred to the organic medium which evaporates, and a turbine - generator set where the electricity is generated. The organic medium is then condensed in the condenser and the heat generated from this process can be used as commercial product. The overall energy efficiency of these plants at nominal capacity is higher than in steam Rankine cycle plants, but the problem is lower flexibility in operation. Another issue with these plants is that both thermal oil and the organic medium are flammable media operating at high temperatures, which is why the size of these plants is usually below 10MWel, but as the technology develops the size is increasing.
Another technology which is not widespread, but can be found in smaller scale systems, is the externally fired hot air gas turbine. This technology consists of a combustion chamber with a heat exchanger where the energy from biomass is transferred to hot air. Hot air at a temperature of between 800°C and 900°C enters the gas turbine – generator set where the electricity is generated and the exhaust air at the temperature 600°C enters the combustion chamber where the biomass is incinerated. Flue gases from biomass incineration leave the first heat exchanger at a temperature of between 300°C and 400°C, after which this heat can be used by means of another heat exchanger in a secondary circuit. For wider use of this technology it would be necessary to reduce capital costs.
Besides the direct use of dry solid biomass, another option which is commonly used is the production of gas from biomass. Dry solid biomass and municipal solid waste are suitable inputs for gasification processes, where the main product is syngas. Wet agricultural biomass, residues from farms and food industries and residues from waste water treatment systems and sewage systems are suitable input material for anaerobic digestion plants where decomposition of biodegradable material is carried out in faster and controlled conditions. The result of this process is biogas. In normal conditions, anaerobic digestion is a natural process which happens to all material of biological origin and for this reason can be found also on landfill sites, where the gaseous product is called landfill gas. Biogas and landfill gas consist mainly of methane (CH4), while syngas also contains significant amounts of carbon-monoxide (CO). These gaseous fuels are used in internal combustion engines to generate electricity, and useful heat is collected from exhaust gases, engine cooling system and engine oil cooling system, which makes these CHP plants very efficient.
Several processes allow for the production of liquid bio-fuels, which can be used for heating or cogeneration purposes. However these fuels are most often used as biofuel in the transport sector.
When cooling is needed, absorption (COP between 0.5 and 2.2) or adsorption (COP 0.5-1.5) systems can be used to convert the available heat for cooling purposes. In the EU, 4.8% of the final energy for used heating and cooling is dedicated to cooling, with large differences across Member States, from 2.4% in Sweden to nearly 50% in Malta. Most of this cooling is produced by traditional mechanical compression systems, often electricity driven. When renewable or waste heat is available, thermal cooling by absorption or adsorption are interesting options.
2Source: Ruiz Castello P, Sgobbi A, Nijs W, Thiel C, Dalla Longa F, Kober T, Elbersen B, Hengeveld G: The JRC-EU-TIMES model. Bioenergy potentials for EU and neighbouring countries; EUR 27575; Luxembourg (Luxembourg): Publications Office of the European Union; 2015; JRC98626
3Source: Lacal Arantegui R, Jaeger-Waldau A, Bocin Dumitriu A, Sigfusson B, Zubi G, Magagna D, Perez Fortes M, Moss R, Lazarou S, Baxter D, Scarlat N, Giuntoli J, Moro A, Padella M, Kousoulidou M, Vorkapic V, Marelli L, Steen M, Zucker A, Moya Rivera J, Bloem J, Gutierrez Moles C, authors, Carlsson J, Vellei M, editors; 2013 Technology Map of the European Strategic Energy Technology Plan (SET-Plan) - Technology Descriptions; EUR 26345; Luxembourg (Luxembourg): Publications Office of the European Union; 2014; JRC86357