Bo Sander

Pretreatment of straw for power production by pyrolysis and char wash

Abstract Co-firing of straw and coal in existing pulverised coal-fired boilers is an option for biomass based power generation. However, the high chlorine and potassium content of straw may cause problems. Experiments with co-combustion of straw and coal in power plants have shown that when a moderate amount of straw is applied (up to 20% on a thermal basis), the most serious problems are deactivation of SCR catalysts applied for NO reduction and deterioration of the fly ash quality caused by potassium. To prevent these problems a pretreatment process is required in which the heating value of the straw is supplied to the boiler without introducing potassium into the furnace chamber. A pretreatment process based on pyrolysis and char wash was investigated. Straw is pyrolysed at moderate temperatures at which the potassium is retained in the char. Potassium and residual chlorine are then extracted from the char by water, and char and pyrolysis gases may be co-fired with coal. Fundamental laboratory studies and technical investigations were conducted to evaluate the pretreatment concept. The investigations indicate, that the low temperature pyrolysis of straw can be performed in a circulating fluid bed reactor applying only straw and straw char as bed material. With a bed temperature of approximately 550°C no significant amount of potassium is released to the gas phase. Applying a counter current moving bed to the char potassium extraction with water will probably be advantageous.

Removal of K and Cl by leaching of straw char

Abstract A pretreatment process to remove potassium from straw fuel may be based on pyrolysis followed by char wash. The straw is pyrolysed at moderate temperatures at which the potassium is retained in the char. Potassium and residual chlorine are extracted from the residual char by water. Char and pyrolysis gases may then be used in a conventional boiler without problems due to the high straw potassium content. To evaluate this pretreatment process knowledge about the char wash process is needed. In this study wheat straw chars were experimentally investigated in the laboratory to determine the extraction time of potassium and chlorine during char wash with water. The influence of particle size, water temperature, straw type, potassium content in water and pyrolysis conditions were investigated. Based on the experimental data a mathematical model describing the extraction, was developed. The laboratory experiments showed that three fractions of potassium in the straw reacted differently: 35–58% of the char potassium was dissolved very fast, followed by a secondary slow potassium release that was strongly influenced by particle sizes, water temperature, char type and water KCl content. The residual 5–10% of the char potassium remains in the char and could not be removed with pure water.

Deposition and Corrosion in Straw- and Coal-Straw Co-Fired Utility Boilers

Department of Chemical Engineering, Technical University of Denmark Building 229, DK-2800 Lyngby, Denmark Phone: +45 45 25 28 83, Fax: +45 45 88 22 58, E-mail: ff/hpn/paj/lah/hl/kdj@kt.dtu.dk Midtkraft I/S Power Company, Studstrup Power Station, DK-8541 Skodstrup, Denmark. Phone: +45 86 99 17 00, Fax: +45 86 99 37 20 E-mail: pbh@midtkraft.dk,kha@kt.dtu.dk (1) Currently with Rockwool International A/S, DK-2640 Hedehusene, Denmark. (2) Currently with SunChemical A/S, DK-4600 Koge, Denmark. Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark. Phone: +45 38 14 20 00, Fax: +45 33 63 39 89 E-mail: hss@geus.dk Currently with Danfoss A/S, DK-6430 Nordborg, Denmark. Faelleskemikerne, I/S Fynsvaesrket, Havnegade 120, DK-5000 Odense C, Denmark. Phone: +45 65 90 44 44, Fax: +45 65 90 38 12 Faelleskemikerne, ElsamProjekt A/S, Kraftvaerksvej 53, DK-7000 Fredericia, Denmark Phone: +45 79 23 33 33, Fax: +45 75 56 44 77 E-mail: bos/nh@elsamprojekt.dk Elkraft A.m.b.a., Lautruphoj 5–7, DK-2750 Ballerup, Denmark Phone: +45 44 66 00 22, Fax: +45 44 65 61 04