Drying technologies for an integrated gasification bio-energy plant

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Abstract

Forced drying of the biomass feedstock is nearly always necessary in an Integrated Gasification Bio-energy Plant (IGBP), and a dryer can represent the highest capital cost item in the pre-treatment section of such a plant. Despite this, there has been relatively little attention paid to the selection and performance of such processes. This review first considers the general requirement for feedstock drying in an IGBP. Brief discussion follows of the theory of evaporative drying, and of the classification of dryer types. The characteristics of biomass feedstocks and IGBP’s of relevance to the drying process are then discussed. Suitable dryer types for an IGBP are then identified and described in detail, with performance data for the drying of biomass feedbacks provided where available.

Introduction

Feedstocks for an Integrated Gasification Bio-energy Plant (IGBP) are required in the form of loose particulate solids, derived typically by chipping either woody biomass such as residues from forestry operations or purpose-grown short rotation coppice (SRC), or herbaceous biomass such as purpose-grown miscanthus or reed canary grass. Such material can have moisture contents ranging from 15 to over 60% (wet basis, w/b) at the point of harvest, depending on the material type, the growing location and the time of year. A value of 50% w/b is often quoted as typical for woody material at harvest under European conditions [1], whereas a herbaceous crop such as miscanthus might be expected to have a content of nearer 30% w/b or less under similar conditions [2].

Natural drying may occur during storage, the degree again depending on the material type and location, the form the material is stored in, the initial moisture content and whether the store is in the open or enclosed. In the present case where crops would be harvested annually in the winter but would be required on a continuous basis at the conversion plant, up to a full year’s storage may be necessary. At the end of the summer, a woody crop might have dried naturally from a moisture content of 55% w/b to as low as 30% w/b [3]. However, during operation in the spring, the storage period will have been far shorter and the moisture content correspondingly higher. The conversion plant might therefore expect to receive feedstock with moisture content in the range 30 to 60% w/b in the case of woody material, and 15 to 40% w/b in the case of herbaceous material.

Biomass gasification processes require the time-averaged moisture content of the feed to be in a certain range, depending on the technology, although sizeable variations in moisture content from particle to particle are not likely to be a problem provided the feed is well mixed. Conventional downdraft gasifiers tend to require close control of moisture content somewhere in the range 10 to 20% w/b [4], [5] for reasons of temperature control—any higher and the reactor temperature is too low, any lower and slag formation becomes a problem. Updraft gasifiers on the other hand can tolerate average moisture contents of up to 50% w/b [4] because of the effective pre-drying that takes place. In all gasifier types, however, an increase in the average moisture content of the feed results in a greater energy requirement for evaporation and a correspondingly lower reaction temperature, which in turn results in a poorer quality product gas with increased levels of tar and a lower overall conversion efficiency (as illustrated in Fig. 1, from model calculations of an idealised gasifier operating under equilibrium conditions). The moisture content of the product gas will also be greater, presenting an increased waste water treatment burden if this water has to be removed prior to use of the gas in an engine or turbine.

In an IGBP therefore, it is usually necessary to dry the feed to some extent at some point between reception and delivery to the reactor.

Forced or assisted drying requires a supply of energy which can be large, and the provision of suitable equipment which can be expensive. It is therefore clearly in the interests of the plant designer to attempt to maximise the efficiency of the drying process while at the same time minimising the capital outlay. Unfortunately greater efficiency is often associated with greater capital cost. The impact of the drying operation on overall plant efficiency can however be reduced by integration, making use of surplus energy streams within the process. If there are competing uses for those energy streams, as is the case in combined heat and power (CHP) schemes, then a further dimension is added to the optimisation process necessary to specify the extent of drying required and the means to achieve it.

There are many methods of drying available, reflecting the wide range of drying tasks encountered in industry. Often, a number of different technologies would be suited to the drying of a particular material, and the final choice is made after careful consideration of operational and economic factors specific to the application. This is certainly true of IGBP feedstocks. The present review is concerned with drying technologies where water is removed by evaporation. Mechanical de-watering techniques (e.g. rams, centrifuges) are available, but are designed for very wet materials such as slurries and pastes which are easily compacted or deformed, and they cannot generally achieve moisture contents below about 55% w/b [6]. Even considering evaporative techniques alone, however, the range of options is vast, and classification of evaporative dryers is a substantial task in itself (see Section 3). It is important therefore in any review to define scope at an early stage.

Considering that the dryer can represent the highest capital cost item in the pre-treatment section of an IGBP [7], there has been relatively little attention paid to the selection and performance of the drying process. This review therefore seeks to give a brief general discussion of drying theory and evaporative dryer technologies, and then to describe and evaluate in detail the technologies available for drying of IGBP feedstocks.

The main purpose here is to promote the efficient integration of drying into an IGBP, so as to improve overall efficiency and reduce cost. The review therefore limits itself to evaporative drying technologies for loose particulate solids of a size suitable for either a fixed or a fluid bed gasifier (the most common types), where the primary source of energy for the dryer is thermal energy obtained from some other point in the IGBP process. This energy can be and usually is supplemented by some electrical energy in the drying process, usually in modest quantities but sometimes substantially as with those steam-drying technologies using electrically-driven mechanical vapour re-compression (MVR), in which the outlet steam is compressed to a higher temperature and then used to heat the steam drying medium via a condensing heat exchanger [8].

Section snippets

The evaporative drying process

Here a brief overview of evaporative drying theory is given, with particular reference to drying of loose particulate solids. Emphasis has been given to aspects which are of relevance in the understanding of why dryers are designed in the way they are, and on what basis they should be selected. For greater detail the reader is referred to the many texts in the literature dealing with the theory of drying [9], [10], [11], [12], probably the most relevant of which is that of Keey [12].

The

Dryer classification

Many different classifications of dryers have been developed in attempts to rationalise what is a complex diversity of technologies, and simplify the process of dryer selection. Most classifications begin with major criteria which are usually either aspects of the drying process, usually mode of operation (batch or continuous) and mode of feed heating (conductive, convective or other), or the form of the feed (e.g. particle, sheet, block, paste, etc.), or both. There may then be further minor

Biomass properties relevant to drying

Biomass as a feedstock for an IGBP has a number of characteristics which place constraints on the selection of drying technology. These include size, density and friability, moisture properties, the effects of temperature with regard to emissions and fire risk, and the tolerance to different gaseous environments.

Suitable technologies for an integrated gasification bio-energy plant

An initial selection of dryer technologies may be made using the classification given in Table 1. As was pointed out in Section 3, the material category corresponding to IGBP feedstocks is ‘granular, crystalline or fibrous solids’. Small batch technologies, vacuum technologies, those technologies requiring good free-flow characteristics and those technologies not based on conductive or convective heat transfer may be eliminated, for the reasons given in 4.1 Biomass properties relevant to drying

Conclusions

  • 1.

    The drying of biomass feedstocks to gasification is usually desirable and sometimes essential, in order to ensure satisfactory gasifier operation and improve product gas quality.

  • 2.

    Key determinants in the choice of dryer are cost, capacity range, available sources of heat, alternative uses of that heat, avoidance of excessive material temperatures to prevent thermal degradation, avoidance of fire or explosion hazards.

  • 3.

    Dryer types suited to integrated gasification bio-energy plants are perforated

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