Overview Biomass Ash in Gasification System

The gasification of biomass materials was initially used commercialy in the 1940’s as a source of gaseous fuels for internal combustion engines, over the next 30-40 years, a number of small and medium sized biomass gasification units were introduced for heat and power generation applications.

With the increasing interest in the development of renewable energy technologies over the past 20 years or so, there has been a significant increase in the technical interest in biomass gasification processes, particularly in North America and in Europe.

History

Based on scale of operation and the product gas quality requirements, which operate under very different condition;

Biomass Gasification Type

• Fixed bed and moving bed systems, which tend, for biomass, to be at the smaller end of the scale, from a few kWth up to around 10 MWth, principally for the production of heat, but also for small scale power generation.•Fluidised bed gasifier, generally operate in the MW range, and are generally of two types,i.e. bubbling fluidised bed (BFB) gasifiers and circulating fluidised bed (CFB) gasifiers. In the main, the CFB units tend to be towards the larger end of the scale, i.e. above around 15 MWth•Entrained flow gasifier are generally larger units for power generation and operate at very high temperatures, in excess of 1200 C and for very short residence times. These are normally fired with fossil fuels, and are co-fired with biomass materials at relatively low co-firing ratios.

Updraft fixed bed gasifier

Updraft moving bed gasifier

Scheme

Bubbling fluidized bed gasifier

circulating fluidized bed gasifier

Entrained flow gasifier

The fusion or partial fusion of the ash components can result in the formation of ash agglomerates in fixed bed and fluidised bed reactors, leading to poor air distribution and difficulties with ash removal from the system.

The formation of ash deposits on the furnace and heat exchanger surfaces,

The carryover of particulate material and of nitrogen, sulphur and chlorine-containing species, along with condensable organic species, into the heat exchangers and gas clean-up systems, and

The utilisation/disposal of the solid residues from the plant.

Ash-related issues and problems

A. Fixed Bed GasifierBecause the temperatures in the oxidation

zone can be up to around 1200 C or so, particularly in updraft gasifiers, there’s potential for significant ash fusion to occur in these system. The succesful operation of fixed bed reactors depends to a large extent on maintenance of a good bed structure, good air and gas distribution through the bed and the effective operation of the ash take-off system

Biomass ash fusion behaviour and slag deposition in gasification systems

Fluidised Bed GasifierThe fouling of furnace and heat exchanger surfaces, and

the formation of ash agglomerates and the subsequent defluidisation of the bed, are key processes issues, and these have been fairly extensively studied at laboratory, pilot plant and industrial scale.

-For the biomass ashes rich in silica or using a silica bed material, the risks of bed agglomeration were high, with significant melting of the ash temperatures at around 800 C, i.e. at normal bed temperatures for fluidised bed gasification system for biomass materials,

-For the biomass ashes rich in CaO or when processes using a CaO-rich bed material, the risks of bed agglomeration at normal bed temperatures were significantly lower.

In practice, the key to avoiding bed agglomeration is the establishment of good fuel and air distributors, and operation at a bed temperature that is appropriate for the chemistry and fusion behaviour of the fuel ash, and bed material. It is also apparent that circulating fluidised beds, which operate at significantly higher fluidisation velocities, are generally less prone to bed agglomeration problems than are bubbling beds.

Entrained flow gasifier.More commonly,biomass and waste material have been co-fired in

large coal-fired integrated gasification combined cycle (IGCC) plants and significant technical work has been done on the ash-related impacts. The majority of the entrained flow gasification systems for coal fire dry milled fuel or aqueous slurries, and are designed to operate at very high temperatures and for short residence times. The pre-milled biomass materials are normally mixed with the fossil fuel feed material. A portion of the ash is retained within the reactor and is drawn off at a slag tap at the bottom. The fly ash carried over from the reactor normally comprises small molten ash particles entrained in the syngas.

The fouling of heat exchangers and other plant components has been one of the most common problem areas with gasification systems for biomass at industrial scale. The syngas produced from the gasification of biomass materials can have significant levels of both inorganic and organic volatile species, which can condense on cooled surfaces in the syngas coolers and gas cleaning system. This has been a significant issue in industrial plants, both in dedicated biomass gasification system and in the co-gasification of biomass with coal in large gasifier, and has been the subject of intensive study over the past few years. The deposition mechanisms and the design of the syngas coolers had been seriously underestimated by the plant designers and that these issues should be the subjects of significant further study.

The fouling of heat exchangers and other components in biomass gasification plants

The Major undesirable constituents of the syngas are :-Particulate materials entrained in the syngas-Low and high molecular weight organic compounds

(tars),-H2S, COS and other sulphur gas,-NH3, HCN, and ther nitrogen-containing species,-Other impurities, including HCl, and volatilised alkali

metals and heavy metals.Fabric and ceramic filters, and electrostatic precipitator,

are capable of particulate collection efficiencies are capable of particulate collection efficiencies appropriate to the utilisation of the syngases in gas engine and gas turbine applications.

The removal of ash and other particulate from the syngas

-As with the ash residues from combustion processes, the preferred route for utilisation of the gasifier residues is to recycle them directly to the land on which the fuel was produced, or utilise the residues as low grade fertiliser.

-Biomass gasifier and boiler system generates two major solid discard streams:

a. The biomass/waste gasifier bottom ashb. The boiler filter ash

The utilisation and disposal of the solid residues from biomass gasification system

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