Текст 16. Recovery boilers

All paper is produced from one raw material: pulp. One of the most common methods used to produce pulp is the Kraft process, which consists of two related processes. The first is a pulping process, in which wood is chemically converted to

pulp. The second is a chemical recovery process, in which chemicals used in pulp- ing are returned to the pulping process to be used again. The waste liquid, from where chemicals are to be recovered, is called back liquor.

The largest piece of equipment in power and recovery operations is the re- covery boiler. It serves two main purposes. The first is to “recover” chemicals in the black liquor through the combustion process (reduction) to be recycled to the pulping process. Secondly, the boiler burns the organic materials in the blackliq- uor and produces process steam and supplies high pressure steam for other process components.

Black liquor is injected into the recovery boiler from a height of six meters. The combustion air is injected at three different zones in the boiler. The burning black liquor forms a pile of smelt at the bottom of the boiler, where complicated reactions take place. The smelt is drained from the boiler and is dissolved to form green liquor. The green liquor is then causticized with lime to form white liquor for cooking the wood chips. The residual lime mud is burnt in a rotary kiln to re- cover the lime. Energy released by the volatilization of the liquor particles in the recovery boiler yields a heat output that is absorbed by water in the boiler tubes and steam drum. Steam produced by the boiler is utilized primarily to satisfyheat- ing requirements, and to co-generate the electricity needed to operate the various pieces of machinery in the plant.

Текст 17. Bio-energy boilers

Renewable energy production is becoming a worldwide priority as countries strive for sustainable growth and better living conditions. Many countries (e.g. EU) have already set demanding targets to increase electricity production using bio- energy resources and have introduced attractive incentives to accelerate this pro- cess. Bio-energy solutions are based on a local fuel supply and thus provide price stability, a secure supply of heat and power, and also local employment. Biofuels are increasingly becoming locally traded commodities, which will further secure

fuel price stability and availability. At the same time, green certificates and emis- sion trading offer new opportunities for financing bio-energy projects.

Boilers combusting biofuels can be used to produce only electricity, but they are mostly used in combined heat and power (CHP) plants and district heating plants. These boilers are designed to operate on a wide variety of biofuels, includ- ing extremely wet fuels such as wood residues, wood chips, bark and sawdust. Smaller boilers use grate firing technology for biofuel combustion, while larger plants use fluidized bed combustion technology. Smaller grate fired plants for thermal heat production, (<10 MWth), have fire tube boilers, while larger ones are fitted with integrated water/fire tube boilers.

One of the world’s largest solid biofuel-fired circulating fluidized bed (CFB) boiler (550 MWth) has been built at Alholmens Kraft power plant at Pietarsaari, Finland. The CFB boiler with auxiliary equipment and the building was delivered by Kvaerner Pulping Oy and commissioned in autumn 2001.