Biomass-Fired CHP Plants

Second volume Seventh issue April 2005 i Bioenergy research From straw to gas 1 From straw to gas Danish Fluid Bed Technology is Since 1999, the development of the far ahead with the development technology has been supported by a of a new technology in order to grant of DKK 10 million from the Dan- Standards for biofuels 3 ish Energy Authority, and the part of transform the more difficult the PSO programme administered by types of biomass into combusti- Eltra. Apart from Danish Fluid Bed ble gas. This makes it possible to Technology, researchers from the Tech- Biomass-fired choose between several differ- nical University of Denmark, as well as CHP plants 4 ent types of fuel and also to re- Danish energy companies Elsam, Force duce the maintenance costs at and Rica Tec, have taken part in devel- oping this new technology. the major power plants. The gasification plant is based on a Six million cubic metres so-called circulating fluid bed, which By Torben Skøtt works at relatively low temperatures of of biogas are lost 6 around 700°C. There are two chambers Gasification is not a new invention, but in the plant: In the first chamber, the not until recently have researchers straw is heated up to 650°C; As there is found out how to treat the more unman- no oxygen inside, the straw does not Criticism of new biogas ageable fuels such as straw and live- burst into flames, but is instead trans- knowledge centre 7 stock manure. formed into 80 percent gas and 20 per- It is the company Danish Fluid Bed cent ash-containing particles of coke. In Technology which spearheads the de- the next chamber, the coke particles are velopment of the technology. For a transformed to gas, leaving a small From straw to ethanol 8 number of years, they have moved from amount of ash, which may partly be re- laboratory scale to a 50 kW test plant used as fertilizer. Both chambers con- and on to a 500 kW plant, which was tain sand that circulates between them, recently tested very successfully at the which thus helps secure a stable tem- Technical University of Denmark. perature level. u Gasification "The tests were carried out using bales of straw from Kolindsund on Djursland. Here, the soil contains a lot of salt and consequently, a high content of alkaline salts, including chlo- rine, which will normally make a boiler plant corrode within a short period of time. Furthermore, the straw from Kolindsund has an ash content of 12 percent, whereas straw from other parts of the country normally contain only 4-5 percent ash." "We have consciously chosen the worst possible fuel. Power plants cannot analyse every load of fuel they re- ceive, so they must be sure the plant will function using a wide range of dif- photo: danish fluid bed technology ferent fuels," explains Peder Stoholm. photo: danish fluid bed technology The gasification plant during prepara- At the smaller 50 kW plant, they have The gasification plant during testing at tions at the Technical University of successfully degassed both straw, wood, the Technical University of Denmark. Denmark. chicken manure and pig manure. How- ever, wood has only been the subject for u The gasification plant produces a one trial, as it may be utilized more easily burn it in a boiler, instead of burning hot, tarry gas, which is suitable for in many other energy plants. the fuel directly in the boiler. large power plant boilers, where coat- But there is a purpose to all this. ing and corrosion often pose problems Many advantages Firstly, you avoid problems with slag- when the biomass is combusted di- At first, it may seem troublesome to ging, corrosion and ruined environ- rectly in the boiler. The gas, however, first transform the fuel to gas and then ment plants, which cost the power cannot directly be used for engine fuel because of the high tar contents. The so-called two-step gasifier, which we How the gasifier works mentioned in the latest issue of Bioenergy Research, is more suitable Primary Secondary Pulverized fuel is added to the bottom cyclone cyclone for this job. Gas of the pyrolysis chamber, where it is If the gasifier is to be used for en- heated up to 650°C. As there is no gines, it is necessary to remove the oxygen inside, the straw does not high tar contents from the gas. This is burst into flames, but is transformed not a simple process, but it is techni- into 80 percent pyrolysis gas and 20 Pyrolysis gas percent coke. A stream of circulating cally possible. In small engines, the Coke residue two-step gasifier will probably be more sand particles takes the coke parti- suitable, whereas for engines of several cles with it, after which they are sepa- MW, the fluid bed gasifier will be the rated by a primary cyclone and re- more obvious choice, as it is easier to circulated to the bottom of the pyroly- increase in scale. sis chamber through a reactor, which transforms coke to gas. Ash The worst straw in the world By gasifying the coke in a separate For the past few months, researchers chamber it is possible to keep pro- have carried out two tests using straw cess temperatures low, so the ash in the 500 kW plant and at the mo- does not melt. The ash, including al- ment, they are planning to carry out a kaline salts and phosphor, can then series of tests using livestock manure be separated, resulting in a gas which as fuel. does not cause slagging and corro- Fuel sion. The nutritious ash may then be "We chose the worst straw in the 650 °C 730 °C world for the two tests, and it still went Air reused as fertilizer and probably also well," says Peder Stoholm, managing as a source of potassium and phos- director of Danish Fluid Bed Tech- phor in the production of fertilizer. nology. Pyrolysis Fluid bed 2 Bioenergy research no. 7 • April 2005 Gasification plants a lot of money every year. Sec- ondly, you can utilize different and thus also cheaper fuels, such as grain residue and sewage. Thirdly, the gasification technology will make it easier to reuse the ash from the power plants, instead of burning coal and straw in the same boiler as it is done today. According to Peder Stoholm, the gasification plant may also be a cheap alternative to the many biogas plants, because nearly the entire process takes place within a few seconds instead of weeks. He imagines that the manure is separated at the farms, after which the solid fraction is transported to a com- munal gasification plant. The transport costs will be limited, as only the solid photo: danish fluid bed technology fraction will be transported, and it will The 50 kW plant at the Technical University of Denmark. At this plant, tests on the be possible to reuse the contents of gasification of wood chips, straw, pig manure and chicken manure have been per- phosphor and potash of the manure, formed. which are concentrated in the ash. However, biological gasification are to be reduced or removed alto- considerable export of both know- also offers a number of advantages, so gether. how and plants. Many of the world's another possibility is to only gasify the resources of biomass are difficult to phosphor-containing fibre residue from Great export opportunities utilize, especially for the production of the biogas plants. Following the successful tests, re- electricity. This is the case with rice Existing legislation does not allow searchers have now started planning an straw and cotton waste; and the increas- the burning of livestock manure. Re- upgrade to 5-10 MW. The gasifier is to ing problems with livestock manure and cently, however, an interdepartmental be upgraded to a size of 100 MW, al- sewage are definitely not just a Danish committee has been set up, which, by 1 lowing it to supply gas for an entire phenomenon. In many cases, the fuels June, must present a draft on whether power plant block. simply lie and rot, and are thus contrib- this legislation is to be changed, in- Peder Stoholm predicts that the tech- uting to the greenhouse effect as well cluding whether the duties on burning nology may open up the possibility of a polluting the water environment. n BioNorm concluded Liquid biofuels Solid biofuels The EU-financed research project In December 2002, the European Com- The Danish standardization commit- BioNorm has now been concluded. mittee for Standardization, CEN, ap- tee for solid biofuels has been very Originally, it was the idea that the pointed a task group, BT/WG 149, to active within the European work BioNorm results were to be used in introduce the need for standardization on standardization, and has influ- connection with upgrading the technical of alternative liquids and gaseous fuels. enced many exisisting and future standards to European standards after The task group has just published a standards. three years. Already today, many of the report which concludes that there is a The European task group for the results are being used in the technical need for standardization in this field. standardization of solid biofuels, standards which have just been pub- This applies to fuels for motorized CEN/TC 335, has a total of 30 subject lished, or will be soon. vehicles as well as stationary plants. matters. The first standards have now However, there are still quite a few Denmark is one of the few countries been published.

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