harpers chp

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Gasification is a process that converts carbon based materials, such as wood, into a biogas composed mainly of carbon monoxide, carbon dioxide and hydrogen. The gasification process is achieved by reacting the material at high temperatures, without combustion and with a controlled amount of oxygen and/or steam. the gasification process

There are several different stages to the gasification process.

1. In the Drying zone, water within the fuel is vaporised. In the Burkhardt reactor (see right), this occurs at approximately 25 - 275C.

2. In the Pyrolysis zone, the fuel decomposes into gaseous volatiles and solid char. In the Burkhardt reactor, this occurs at approximately 350 - 650C.

3. In the Oxidation zone, a limited amount of combustion takes place. The gaseous volatiles and some of the char react with oxygen to primarily form carbon dioxide and small amounts of carbon monoxide. These combustion reactions provide heat for the subsequent gasification reactions. In the Burkhardt reactor, this occurs at approximately 700 - 1200C.

4. In the Reduction zone, the final gasification reactions take place as the char reacts with the water vapour generated in the drying zone to produce carbon monoxide and hydrogen. In the Burkhardt  reactor, this occurs at approximately 850 - 1100C.

The biogas produced by gasification can be combusted and, if it is clean enough, used as fuel for a gas engine. Thus, heat, mechanical or electrical power or, a combination of these, can be generated.

In the Burkhardt system, the fuel and oxygen are fed into the bottom of the reactor and form a fluidized bed within which the gasification process occurs. The outlet for the biogas product is at the top of the reactor. The composition of the biogas generated is shown below. The biogas is fed from the gasifier, via a gas line, through to a gas engine CHP unit which uses it to generate further heat and electricity.

The advantage of gasification is that using the biogas is potentially more efficient than direct combustion of the original fuel because it can be combusted at higher temperatures. As a result, the thermodynamic upper limit to the efficiency of the system defined by Carnot's rule is higher.

gas composition

Gasification is not a new technology, records of gasification experiments using coal and wood as the feedstock date back to the 17th century. During World War II, the shortage of petroleum led to an upsurge of interest in gasification and by 1945 many different types of vehicles had been adapted to run on biogas generated by gasification processes.

Although biomass gasification has a long history, it is also a chequered one. Many biomass systems have failed including several large heat network schemes in the UK. Many of the problems that have been encountered are due to problems with the feedstock. Biomass-based feedstocks are often naturally variable and a variable feedstock causes problems in the gasification process. Feedstock consistency and standard is essential if the process is to be well controlled.enplus a1 logo

It is for this reason that the Burkhardt gasifiers use high-quality wood pellets as feedstock. By only using EN Plus A1 wood pellets they ensure a consistent quality of feedstock for the gasification process and have thus overcome the traditional problems associated with biomass gasification.

Harpers CHP, Unit 2, North Road Industrial Estate, INSCH, Aberdeenshire, AB52 6XP

Email: | Phone: (01464) 821822