Autothermal two-stage gasification of low-density waste-derived fuels

ARTICLE IN PRESS

Energy 32 (2007) 95–107 www.elsevier.com/locate/energy

Autothermal two-stage gasification of low-density waste-derived fuels Stefan Hamel, Holger Hasselbach, Steffen Weil, Wolfgang Krumm Universita¨t Siegen, Institut fu¨r Energietechnik, Paul-Bonatz-Str. 9-11, D-57068 Siegen, Germany Received 2 December 2005

Abstract In order to increase the efficiency of waste utilization in thermal conversion processes, pre-treatment is advantageous. With the Herhof Stabilats process, residual domestic waste is upgraded to waste-derived fuel by means of biological drying and mechanical separation of inerts and metals. The dried and homogenized waste-derived Stabilats fuel has a relatively high calorific value and contains high volatile matter which makes it suitable for gasification. As a result of extensive mechanical treatment, the Stabilats produced is of a fluffy appearance with a low density. A two-stage gasifier, based on a parallel-arranged bubbling fluidized bed and a fixed bed reactor, has been developed to convert Stabilats into hydrogen-rich product gas. This paper focuses on the design and construction of the configured laboratory-scale gasifier and experience with its operation. The processing of low-density fluffy waste-derived fuel using small-scale equipment demands special technical solutions for the core components as well as for the peripheral equipment. These are discussed here. The operating results of Stabilats gasification are also presented. r 2006 Elsevier Ltd. All rights reserved. Keywords: Pyrolysis; Combustion; Fixed bed; Fluidized bed; Hydrogen; Synthesis gas; Fuel feeding

1. Introduction Gasification is by no means a new technology. As early as in the mid 19th century, ‘town gas’ obtained from the gasification of coal was used first for illumination. This was followed by heating, then as a raw material for the chemical industry and more recently for power generation. With the decline of ‘town gas’, caused by the widespread introduction of natural gas and electricity, gasification became a specialized niche technology [1,2]. The search for a more efficient use of existing energy sources and raw materials, initiated by the exploitation of fossil fuels and the release of greenhouse gases, has led to a major renaissance of gasification technology. Nowadays, gasification is regarded as becoming the main technology for the thermochemical conversion of biomass to energy or synthesis gas. Moreover, gasification offers an attractive alternative to the thermal treatment of solid waste in Corresponding author. Tel.: +49 271 740 2634/2633; fax: +49 271 740 2636. E-mail address: [email protected] (W. Krumm).

0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.03.017

incineration plants [3]. The number of different uses for product gas clearly indicates the flexibility of gasification and therefore allows it to be integrated into several industrial processes as well as to be used with power generation systems. One of the most difficult aspects of waste as feedstock, whether for gasification or for incineration, is its heterogeneous nature. The broad variability in chemical composition, water and ash content, heating value and the presence of a number of substances, such as sulphur, chlorides or metals, affects the performance of thermal conversion processes. Waste composition is influenced by numerous local conditions, such, for example, as population structure and local waste separation and recycling regulations. In addition, the composition of local waste is subject to major seasonal or even daily variations. For these reasons, feedstock preparation plays an important role in any thermal waste conversion approach. A number of different approaches for waste prepreparation are known, mostly involving mechanical shredding, removal of metals and drying. A promising approach to improve homogeneity and to reduce the amount of contaminants, which has already

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been successfully demonstrated, is a mechanical–biological pre-treatment of waste. An accurate and efficient separation of inert materials such as glass, minerals, ferrous and non-ferrous metals usually requires several mechanical shredding and conditioning stages. The result is a lowdensity fluffy material which does not necessarily meet the requirements of the fuel-feeding system or the gasifier itself. In order to avoid extensive upgrading of the fluff, either by further grinding to reduce its size or by pellet forming to increase size and density, careful selection and design of the gasification reactor and its peripheral equipment must be ensured. The purpose of this paper is to describe the experience gained in the design, construction and operation of a twostage laboratory-scale gasifier for processing fluffy lowdensity Stabilats fuel derived from waste which has been mechanically and biologically pre-treated [4,5]. The main design parameters of the laboratory-scale plant were defined in co-operation with the manufacturer of Stabilats. The most important requirement, which influenced the design and the scale of the entire plant, was to process Stabilats as produced by the manufacturer, i.e. without any further treatment. In the following section, the joint development of the laboratory-scale gasifier and the specific challenges presented by the small scale of the reactor and their solution are discussed in detail. The results from different gasification runs using Stabilats are presented. 2. Gasification concepts Gasification technology permits the production of gas for a variety of applications such as fuels and chemicals. The type of application for product gas is in practice limited by the gas quality of the gasification process [6]. To meet high gas quality requirements for product gas use, most research activities are focused on gas cleaning by primary measures within the gasifier, such as reactor design, optimization of operating parameters and addition of catalytically active materials. Secondary gas cleaning measures include improvement of downstream clean-up such as hot gas clean-up and catalytic tar conversion. Basically, gasifiers can be categorized in several reactor design groups such as ‘single-stage’ and ‘multi-stage’ arrangements. 2.1. Single-stage gasifiers The aim of a ‘single-stage’ gasifier is to convert organic substances entirely in one reactor. Depending on the type of operation, different gasification agents such as oxygen, air and/or steam are supplied. The most commonly used gasification technologies for ‘single-stage’ processes are fixed bed, fluidized bed and entrained flow reactors. Fixed bed or moving bed gasifiers are often countercurrent flow systems [7]. The fuel is fed in at the top of the gasifier and the gasification agent is injected near the

bottom. The fuel particles move slowly down the reactor and react with the gases moving upwards. The fuel passes through different reaction zones where various processes such as drying, pyrolysis and oxidation take place. The maximum temperature at the bottom is normally 930–1430 1C [8]. Another type of fixed beds are co-current gasifiers [9]. In a co-current fixed bed gasifier, the fuel and the gasification agent move in the same direction, whereby the fuel must pass successively through the drying, pyrolysis, oxidation and reduction zones. The advantages of counter-current gasifiers are fewer restrictions on fuel moisture content and fuel particle size. Thus no special fuel preparation is generally required and a wide range of biomass types can be used as feed material. By comparison, co-current gasifiers produce a better quality gas but place strict requirements on fuel properties. Fluid bed systems allow a more efficient gasification due to the elimination of hot spots inside the reactor. They are suitable for various types of feedstock and can be scaled up to relatively large plant sizes. Whilst fixed bed gasifiers are used in the low-capacity range of up to a few MWth, fluidized bed reactors are typically applied in the range over 5 MWth [10]. However, they are more expensive to build and need better gas cleaning due to the high particulate content in the product gas. Unlike fixed bed reactors, fluidized beds have no defined reaction zones. The conversion of fuel particles and the secondary reactions of pyrolysis products take place in the same reaction volume. Tar conversion can be supported by the introduction of catalytically active bed materials. The large amount of solid material in the fluidized bed serves as heat storage and its permanent mixing induced by bubble movement results in a uniform temperature distribution. The maximum bed temperature is determined by the ash softening point and is usually in the range of 815–1040 1C [8]. Entrained flow gasifiers require pulverized fuels. The fuel particles are introduced into the steam/oxygen feed and gasified at a residence time of a few seconds. Gasifiers can be operated at lower temperatures to maintain ash as a dry solid, or at temperatures well above the ash fusion point in the slagging mode so that ash is removed as molten liquid [11]. Operation at these high temperatures makes it possible to produce a product gas which is virtually free of tar and oil. Entrained flow gasifiers have the ability to handle practically any fuel as feedstock, provided they are ground to the right, i.e. small, size. Although entrained flow gasifiers may seem attractive for the production of a clean, tar-free synthesis gas for chemical applications, no method of size reduction has yet been found to mill the fibrous biomass materials to a satisfactory size [1]. However, most of the ‘single-stage’ gasifiers for solid biomass and waste can be used if the requirements placed on product gas quality are low, as is the case for direct thermal gas use such as co-combustion of hot raw gas in coal boilers [12] or use as fuel gas in a cement process [13]. To improve the gasification process, modern and advanced gasification technologies separate the drying,

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devolatilization, gasification and combustion reaction zones. These ‘multi-stage’ processes enhance process efficiency and product gas quality by combining several reaction zones under consideration of various fuel characteristics such as high reactivity, low ash and sulphur content and high volatile matter. 2.2. Multi-stage gasifiers Various ‘multi-stage’ processes are currently under development or already in operation. The high volatile amount of biomass, which is released rapidly as gaseous substances during pyrolysis, is taken into account in numerous reactor concepts by spatial subdivision of the fuel conversion steps. This makes it possible to influence and optimize the operating parameters in each conversion step. These concepts can be categorized as ‘single-line’ or ‘double-line’ processes. ‘Single-line’ processes use only one main stream of mass through a number of reactors which are arranged in series. ‘Double-line’ processes divide the mass stream into at least two partial streams which pass through parallel-arranged reactors. In Fig. 1(a), a two-stage ‘single-line’ gasification process according to Ref. [14] is shown. The fuel is dried and pyrolysed in the first stage in an indirectly heated pyrolyser. The pyrolysis products are subjected to partial oxidation by air in a narrow zone between pyrolyser and char gasifier. The product gas has to pass the hot char bed which leads to substantial tar cracking and results in low tar content in the product gas. Other two-stage single-line concepts are, for example, the combination of a counter-current and a co-current fixed bed gasifier [15,16]. A three-stage single-line configuration entitled Carbo-Vs has been successfully demonstrated, details can be found in Ref. [17]. Another gasification approach is to divide the mass stream into at least two partial streams which are processed in several parallel-arranged reactors. The heat needed for gasification can be produced separately by using air for combustion without affecting the gas quality of the gasification reactor, see Fig. 1(b). Combustion and

gasification reactors are kept separate and are only interconnected by heat transfer. In principle, a pyrolysis stage is necessary to split the fuel into gas and char. To provide the heat necessary for autothermal operation, the char or part of the pyrolysis gas must be oxidized outside the pyrolysis reactor [18]. A ‘double-line’ plant concept with heat production by char combustion is the Fast Internal Circulating Fluidized Bed (FICFB) process developed at the Technical University of Vienna, Austria [19]. The endothermic gasification of the fuel takes place in a stationary fluidized bed connected via a chute to the combustion chamber which is operated as a circulating fluidized bed. The char particle combustion heats up the bed material. The hot bed material is separated by a cyclone and fed back as a heat carrier to the stationary fluidized bed. A plant with a thermal capacity of 8 MWth is in operation in Gu¨ssing, Austria [20]. A similar principle has been realized with FERCO’s SILVAGASs process which consists of two circulating fluidized beds (CFB) [21]. The first CFB is used for pyrolysis and partial gasification with steam. The second CFB is used to combust the remaining char from the gasification CFB. Another ‘multi-stage’ ‘double-line’ process is the socalled ‘Staged Reforming’ [22] which is also based on the spatial separation of pyrolysis and heat production. The input material is fed into the pyrolysis unit, heated up and pyrolysed in contact with a hot metallic or mineral heat carrier. The coke and heat carrier thus produced are separated and the cold heat carrier is fed into a heat exchanger and heated up again using flue gas from pyrolysis coke combustion. Raw gas hydrocarbons are cracked with steam in a reforming unit to form carbon monoxide and hydrogen. Another two-stage parallel-arranged gasifier presented in this paper is the Herhof-IPV-Verfahrens. A brief description of the process principle is given in the following section, further details of the design, construction and initial operating results are discussed in the subsequent chapters. Two-stage parallel gasification

Two-stage gasification Gasifier Pyrolysis gas

Drying & pyrolysis

2. Stage

Raw gas

Tar- & char gasification

Fuel

Char

Heat

(a)

Steam

Gasifier 1. Stage

Fuel

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Gasification agent

Drying & pyrolysis Char

Ash

Heat

Air Char combustion

(b) Fig. 1. Two-stage single-line and two-stage double-line gasification concept.

Raw gas

Ash

Flue gas

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2.3. The Herhof-IPV-Verfahrens The Herhof-IPV-Verfahrens is characterized by the parallel arrangement of a fixed bed and a bubbling fluidized bed reactor (see Fig. 2). The fuel is fed directly into the fixed bed reactor filled with hot bed material. Through its contact with the hot bed material, rapid drying and pyrolysis of the fuel occurs. The volatiles released, including tars, pass through the hot ash layer above the fuel feed and leave the reactor at the top. Adding steam in the upper part of the bed enhances tar conversion by catalytic and/or thermal cracking in the presence of the hot ash particle surface. The remaining char and the bed material move towards the bottom of the gasification reactor and are transported by a screw conveyor into the fluidized bed combustor. The char combustion leads to a heat-up of the fluidized bed material which is then discharged towards the gasification reactor by a fluidized loop seal. The energy required for pyrolysis and gasification in autothermal plant operation is generated by char combustion. The combustion and gasification reactors are connected by circulating solid material, ensuring separation of gasifier raw gas and combustor flue gas. Thus a

dilution of the product gas by nitrogen from the ambient air required for combustion can be avoided. Downstream of the gasification reactor, the remaining tars and other impurities are separated from the product gas by a gas scrubber and an electrostatic precipitator. The water/tar/ dust mixture generated in the gas cleaning can be burnt directly in the fluidized bed combustor to ensure economic operation of the plant. 3. Fuel characteristics 3.1. Waste treatment for Stabilats production The first step after the delivery of the residual domestic waste is its preconditioning with a shredder and a first separation of ferrous metals. The shredded residual waste is then kept in a composting box for 7 days. During this time, a forced air flow through the waste creates optimum conditions for microbial respiration and biological breakdown of organic matter. Warm moist air removed from the boxes is passed over a heat exchanger, enabling the condensate to be captured, cleaned and used within the process. The air passed through the boxes is re-circulated

Electrostatic precipitator

Scrubber

Product gas

Fly-ash, tar, water

Raw gas

Flue gas Fixed bed gasification reactor (GR)

Bed material

Bubbling fluidized bed combustor (BFB)

Condensate to analysis Fuel Steam

Fuel particle Raw Drying Pyrolyzing

Bed material + Coke

Coke

Ash Air Fig. 2. The two-stage parallel-arranged gasifier designed for the processing of waste-derived fuels.

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which significantly reduces the volume of exhaust air emissions. The heat produced during this biological conversion is used in the plant for waste drying [5]. The prerequisite for the subsequent separation of the waste mixture into various fractions is moisture removal. During several downstream process steps, different materials are sorted in order finally to obtain Stabilats, plastics (as an option), ferrous and nonferrous metals, batteries, glass (with high separation efficiency regarding colour) and other inert substances (mixture of ceramics, stones and porcelain). The dry consistency of Stabilats permits easy storing and makes it suitable as a fuel for industrial processes. A picture of Stabilats is shown in Fig. 3. The bulk density of the sample displayed is ‘as produced’, usually in the range of 150–250 kg/m3. A compaction into pellets is possible if required. However, further pelletizing should be avoided for economic reasons and the Stabilats preferably used ‘as produced’. The statistical average composition is summarized in Fig. 4. 3.2. Pyrolysis of Stabilats For the intended processing of Stabilats in a two-stage gasifier, pyrolysis behaviour is critical for the process design. The pyrolysis conditions and thus the resulting pyrolysis products determine the distribution of energy into the gaseous phase and char. A certain amount of char is

Fig. 3. Stabilats sample.

Inert materials (stones, glass,metal) < 1 wt.-%

Material of fossil sources (textiles, vulcanized rubber, composite materials, etc.) about 25wt.-%

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required for autothermal operation. The external char combustion has to provide sufficient energy to facilitate pyrolysis and gasification. In the case of low char yields, part of the Stabilats can be fed directly into the fluidized bed to support heat generation. In the case of high char yields, the gas yields are affected which results in lower cold gas efficiency. Table 1 presents the elemental and proximate analysis of Stabilats compiled on the basis of different references. Fig. 5 shows experimental results on char yields from Stabilats pyrolysis at different temperatures and carried out with different equipment. The results published by Ref. [23] were obtained in an externally heated batch-fed fixed bed retort with an initial amount of Stabilats of 250–300 g for each experiment. The heating rates were comparably slow at some 3 K/min with the aim of producing a maximum char yield. Pyrolysis experiments in a batch rotary kiln, which was also externally heated, were performed by Ref. [24] using an amount of 1000 g for each run. The heating rates of individual particles introduced into an already formed bed inside the rotary kiln were some 550 K/min for wall temperatures of 900 1C. For our own pyrolysis experiments, small amounts of some 15 g were used for each data point. The sample was divided into eight portions of 1–2 g, each placed in a coking crucible and exposed to the designated temperature in a muffle furnace for 7 min. The procedure follows the determination of the volatile matter content of solid fuels (900 1C, 7 min) according to DIN 51720. The range of data points for the char yields obtained is displayed in Fig. 5 within the grey area, the average is depicted as a dashed line. Firstly, the expected decrease of char yield with increasing temperature is obvious. Secondly, the influence of sample size is indirectly visible. Whilst the data points of Ref. [24], who used an amount of 1000 g, show less deviation, our own results obtained from small sample sizes show a large range of remaining char yields (grey area). This effect may be explained by heterogeneous fuel composition. The composition of Stabilats can be considered homogeneous compared to the original waste source and can, of course, be called homogeneous when large quantities are processed. However, with decreasing sample size, waste-derived material appears to be of a heterogeneous nature. It is no longer possible with Materials from biomass sources (paperboard/paper, textiles, wood, organic compounds) about 65 wt.-%

Plastics about 9 wt.-%

Fig. 4. Main constituents of Stabilats, as determined in long-term statistical analysis [4].

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Table 1 Comparison of different published compositions of Stabilats Reference

[23]

Water content (raw) Ash content (dry) Volatile matter (dry) C (daf) H (daf) O (daf) N (daf) Lower heating value (dry)

[24]

[25]

Own results

No. 1

No. 2

No. 1

No. 2

Range

Average

10.50 33.10 57.00 59 7.1 30 — 14,900

12.7 26.30 64.40 59 7.1 30 — 18,700

24.0 20.6 71.1 57.6 7.7 32.1 1.9 18,250

9.4 27.5 66.9 59.2 8.1 28.8 1.9 16,770

— — — 47.63–54.43 6.80–8.84 36.74–51.71 0.95–2.72 13,000–15,700

11.5 17 — 48.99 7.48 40.82 1.50 14,300

15 23.5 — 58.34 7.97 30.80 1.80 —

Mass% Mass% Mass% Mass% Mass% Mass% Mass% kJ/kg

0.7 Literature data Retort [23] Rotary kiln [24]

Char yield in kg/kg(daf)

0.6

Own results Average Range

0.5 0.4 0.3 0.2 0.1 Feedstock: Stabilat® 0.0 300 400 500

600

700

800

900

1000

Pyrolysis temperature in °C s

Fig. 5. Char yields from Stabilat

pyrolysis, comparison of literature data and own results.

decreasing sample size to ensure a representative material mixture with the original particle size. This is also reflected in the pyrolysis results in Fig. 5. 4. Design, construction and operation of the gasifier The laboratory plant, with a maximum thermal power of 150 kW, consists of many parts other than the core reactor components, such as extensive peripheral equipment for the supply of auxiliary energy, gasification agent, pressurized air for pneumatic devices, handling of product gas and control and measurement systems. Since the peripheral equipment is usually regarded as state-of-the-art, the following description focuses on the newly developed main components which had to be carefully designed for the task in question. 4.1. Stabilats feeding system The current feeding system performs several tasks which are slightly different to those of a full-scale plant. In the case of the laboratory-scale equipment, the general task similar to that of a full-scale plant is to feed a defined

amount of fuel into the reactor at a rate as steady and uniform as possible. At the same time, the feeding system has to seal the reactors against environment and serve as fuel storage in order to supply fuel for about 30 to 60 min. During design of the system, the advantages and disadvantages of fully automated fuel-feeding apparatus were discussed in detail. Since the focus of the project was the development and testing of the gasification reactor, the decision was ultimately taken to build first a partly manual feeding system with the option of full automation in the future. The main reason for this was that in the case of full automation, a scale-down of the entire conveying and storage devices would be necessary. Although the manufacturer has extensive experience in the design and operation of large-scale plants for the production, handling, storage and conveying of Stabilats, the prevailing opinion was that problems were to be expected from a scale-down to feeding rates of some 10–30 kg/h. The aim of the solution identified is to feed Stabilats manually in a cycle of about 30 min from barrels or bags into the lock hopper which simply consists of a charge hopper (a in Fig. 6) and a lock hopper (c in Fig. 6) between

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Fuel a Lock hopper

Seal of the slide valves

b

Flange I

c

N2+ air

d

N2

open

close

Slider e N2

II Cooling water

f

g

Water

Water Fuel

Water Water

Expansion joint

Fig. 6. The Stabilats feeding system.

two pneumatic access slide valves (b+d in Fig. 6). This is a well-known principle which has proven to be reliable for feeding solids into pressurized reactors [26]. After placing the fuel in the lock hopper, it was flushed with nitrogen before being released into a vertical bin (e in Fig. 6) below. This bin serves as fuel storage for the fuel screw feeder (g in Fig. 6) which is mounted at the bottom. To ensure the continuous transportation of Stabilats, a paddle mixer (f in Fig. 6) was additionally installed above the screw conveyer to prevent blockage or bridging. The completely cooled screw, comprising separate cooling of the conveying tube jacket, the double central tube and the screw wings (see detail II in Fig. 6), forces the Stabilats into the fixed bed reactor. A considerable part of the Stabilats is made up of different types of plastics with low softening temperatures and also of substances with comparably low starting temperatures for pyrolysis (see Fig. 4). Therefore extensive cooling of the screw is necessary to prevent melting and pyrolysis in the feeding system, especially inside the conveying tube. It is known that many of the operating problems experienced in plants for processing waste, refuse-derived

fuels or biomass are caused by a mismatch of the internal transport system to the fuel properties. A particular problem, called bridging, which causes interruptions in the material flow, is described as the phenomenon where a particulate solid fuel forms a stable structure across an opening. Several parameters are known to increase particulate fuel’s tendency to bridge over openings. This includes particle shape and size distribution, depth of the fuel bed over an opening and moisture content. Investigations for different biomasses indicate that especially the proportion of long and hook-shaped particles influences significantly the tendency to form bridges [27,28]. Increasing moisture content usually accentuates this influence. In the current task of handling refuse-derived fuel, not only the presence of long particles is seen as a cause of conveying difficulties but also their concurrence with large amounts of soft and low-density fluffy matter. Furthermore, the small diameters of hopper and auger make handling difficult. In order to avoid problems with the hopper system, the diameter from the charge hopper down to the screw conveyor is enlarged. Before selecting a pneumatic slide

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valve, the clear diameter of the opening has to be carefully checked. The best results for the flow through the slide valves were achieved when choosing not the nominal size according to the pipe diameter but the next standard size. In turn, the clear diameter is large enough to be able to extend the pipe length of the hopper beyond the connection flange so that the pipe ends as closely as possible above the slider (see detail I in Fig. 6). This avoids rough edges on the one hand and on the other hand prevents, as far as possible, contact of the solids with the sealing of the slider. The bin below the lock hopper is also designed so that the diameter is enlarged in flow direction towards the paddle mixer. The paddle mixer prevents the formation of bridges and ensures the continuous filling of the screw feeder with fuel. Experience clearly showed that the paddle mixer is the most important component for ensuring reliable material flow. The positioning of the hopper bin directly either on a small or a large diameter screw feeder without a paddle mixer was not successful. After a couple of minutes, a bridge formed over the auger. This simple gravity-driven movement of the material downwards onto the screw was obviously not effective enough to fill the screw. Visual inspections indicate that the fluffy bulk is highly elastic and tends to spring back from the auger so that the screw wings were unable to force the material into the conveying tube. Feeding was only possible once the paddle mixer was installed, but the conveying characteristics were ultimately improved by designing the auger with a conical shape at the end. This affects the way in which the paddle mixer pushes the material onto the conical part of the auger which then forces the material in the conveying direction as it rotates. 4.2. Gasification reactor The gasifier is designed as a cylindrical reactor with a narrow cone above the fuel feeding point. The subsequent enlargement of the diameter downwards aims to achieve better mixing and heat transfer between the hot heat carrier and the Stabilats introduced and thus lead to better fuel conversion. A cooled screw conveyor forces the Stabilats directly into the bulk of hot bed material, see Figs. 6 and 2. Through its contact with the hot bed material, drying and pyrolysis take place, the gas moves in counter-current flow to the bed material and leaves the reactor at the top. The bed material, together with the remaining coke, is discharged at the bottom by another cooled screw feeder directly into the fluidized bed combustion reactor. The mixture of coke and bed material is cooled down on its way downwards and has a temperature of about 500–600 1C at the bottom. Therefore in order to minimize heat loss, the cooling of the screw conveyor is dimensioned so that the steel of the screw is just protected against high temperatures and no further unnecessary cooling of the solid occurs. The solid material flow inside the gasifier is driven by gravity. The mean solid fuel residence time inside the

gasifier, from fuel feed down to the bottom, is designed to be 20–30 min, depending on the desired solid circulation rate. The amount of circulating solid heat carrier material is about 10 kg/kgfuel for the experiments presented here. The residence time of the released volatiles through the hot bed material layer is in the range of 2–4 s. Due to drying and pyrolysis reactions inside the gasifier, the hot bed material cools down and internal temperature zones are formed. The temperature level at the top of the gasifier is up to 920 1C, depending on fluidized bed operation temperature, and reaches about 500–600 1C at the bottom of the gasifier. The addition of steam into the conical part above the fuel feed is carried out by a ring line and several nozzles mounted along the perimeter. Steam serves on the one hand as a reactant to support tar conversion and gas phase reactions and on the other hand effects a continuous loosening of the bulk solid. In the upper zone of the gasifier, the hot bed material layer, in combination with the added steam, supports tar cracking before the raw gas produced is piped at the top of the gasifier into a gas-cleaning unit. The gas is analysed in a gas analysis device and combusted in a post-combustion unit without any further utilization. 4.3. Fluidized bed combustion reactor The task of the fluidized bed is to burn the char from the gasifier and simultaneously to ensure the transportation of bed material in an upward direction. The coke/bed material mixture is fed with a temperature of some 500–600 1C from the fixed bed gasifier into the fluidized bed. The feeding point is in the lower part of the fluidized bed above the air distributor. The preheated air entering the fluidized bed oxidizes the char, which mainly consists of carbon, and the bed material is heated up as a result of exothermic combustion reactions. The operation is carried out in bubbling fluidization mode, thus a simple overflow was chosen to adjust the bed height and to discharge the bed material towards the gasification reactor. In order to be able to investigate experimentally the influence of relevant parameters and their interaction, the plant design must be as flexible as possible in order to take into account different operating parameters. In the current case, the fluidized bed is influenced directly or indirectly by numerous factors. The properties of the char remaining from pyrolysis, such as heating value, yield and reactivity, are expected to be in a wide range due to the heterogeneous nature of the feedstock. The char yield, which depends mainly on fuel properties and feed rate but also on pyrolysis conditions, determines the amount of required combustion air. The fluidized bed temperature is a result of heat release from char combustion, temperature and flow rate of the primary air itself, temperature and flow rate of the bed material fed from the gasifier and of the reactor heat loss. Air flow rate and resulting fluidized bed temperature in combination with geometric design determine

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the subsequent fluidization patterns. The desired and possible hydrodynamic fluidized bed operation mode decides finally about the allowed and realizable range of the above-mentioned parameters. Since some of the parameters are not known a priori and others fluctuate considerably, it is not possible to define the optimal operation point. The design of the air distributor is important for flexible operation. During operation, the gas distributor must ensure that the gas is distributed on entry as evenly as possible across the fluidized bed. When the gas flow is stopped, the solids must be prevented from raining through the distributor into the plenum. In most small-scale studies, porous plate or perforated plate distributors are used. The advantage is that they are inexpensive and easy to manufacture and normally ensure good gas distribution for a wide range of operating parameters [29]. For largescale applications, especially with difficult operating conditions such as high temperature or a highly reactive environment, tuyere designs are used. A detailed comparison concerning design, construction, advantages and disadvantages of all types of gas distributors can be found in Refs. [29–31]. It must furthermore be taken into account in the current case that inert material particles of larger size and particles unsuitable for fluidization (e.g. metal) will most probably be fed in with the fuel. The possibility for a discontinuous discharge of solids at the bottom of the fluidized bed must therefore be taken into account in the design. The air distributor realized is displayed in Fig. 7. A nozzle-type design with a comparably high specific pressure drop was built in order to ensure that air distribution is as uniform as possible over a wide air flow and to prevent a backflow of solids into the plenum. At the same time, large inert particles have the opportunity to settle between the nozzles. Fluidized bed Nozzles

Inclined plane

Solid discharge Air

Plenum Bed material Fig. 7. Design of air distributor, solid discharge and plenum.

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The solids slide over the inclined plane into the discharge pipe which is emptied manually by a slide valve. The hot solids fall into a sealed box and are collected until the end of the experiment. 4.4. Loop seal Many chemical or energy-related processes such as combustion or gasification recirculate solids and use preferably non-mechanical solid flow devices. In the valve mode of operation, the solid flow through the nonmechanical device can be controlled by the amount of fluidization gas, whilst in automatic mode, solid flow corresponds to the natural circulation rate of the process. A comprehensive overview of different standpipes and non-mechanical valves and their operation is given by Ref. [32]. In the current laboratory-scale plant, hot bed material is transported between the two reactors and the mixing of flue gas and product gas is avoided by using a loop seal. A loop seal is essentially a variation of the well-known seal pot [32]. Like the seal pot, it comprises a standpipe and a fluidized bed section. Hot bed material leaves the fluidized bed combustor via an overflow into the standpipe of the loop seal. The bed material from the standpipe enters the steam fluidized bed section of the loop seal from the side before it is discharged towards the gasification reactor. To ensure the separation of the flue gas from the combustion and the product gas from the gasifier, first of all the pressure in the freeboard of both reactors has to be kept at the same level. In fact, there are slight pressure fluctuations in both reactors. In the bubbling fluidized bed combustor, these are mainly caused by bubble activity and in the gasification reactor by gas generation which is not always uniform. To enhance the pressure sealing of the loop seal, the height of the solid packing in the standpipe can be increased. A pressure measurement below the air distributor of the fluidized bed combustion reactor provides a signal which is proportional to the bed height. A steady-state pressure level is achieved during operation with constant bed height. Stopping the loop seal fluidization interrupts the transport of solids towards the gasification reactor and, as a consequence, the height of the fluidized bed in the combustor rises. The increase of bed height in the fluidized bed combustor becomes apparent through the increasing pressure across the bed (see Fig. 8). To increase the efficiency of the gas separation, the steam injection for loop seal fluidization was carried out discontinuously. When stopping loop seal fluidization, first the height of the solid packing increases until the standpipe is completely filled and then the fluidized bed height in the combustor increases. After the addition of steam into the loop seal recommences, the transport of the solids continues and the previous pressure level is reached again. The discontinuous operation of the loop seal is shown in Fig. 9. Although solid temperatures of 900 1C are reached in the fluidized

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bed combustor, the recorded pressure indicates a reliable operation of solid flow.

5. Gasification results The gasifier at the University of Siegen (see Fig. 2) was operated using Stabilats and wood as feedstock. A plot of product gas concentrations during operation with Stabilats fuel is shown in Fig. 11. In the laboratory plant, nitrogen was used to inert the lock hopper fuel feeding system. Nitrogen content in the dry product gas was in the range of 3–7 vol% as a result of the inerting procedure. To be able to compare the experimental results from different runs the gas concentrations are related to dry and N2-free basis. Both reactors were heated up simultaneously for the start-up using preheated hot air to reach operation temperature. Solid circulation was adjusted to ensure uniform heat-up of both reactors and bed material. Once the operating temperature had been reached, air was replaced by nitrogen in the pyrolysis reactor until the measured oxygen concentration was almost zero. Fuel feeding then commenced, followed by replacement of nitrogen with steam after a certain period of time, see

4.5. Bed material The task of the bed material is to transfer heat produced in the combustion reactor to the gasification reactor. The hot bed material is transported, as described earlier, by a fluidized loop seal into the gasifier, providing the heat needed for drying, pyrolysis and gas phase and tar conversion reactions. For the start-up, silica sand with a particle size between 0.2 and 0.6 mm was used as initial bed material. As the experimental run time progresses, the original silica sand is slowly replaced by fuel ash. Fig. 10 presents the size distribution of initial silica sand and bed material after some 40 h of operation. It appears that the size distribution shifts towards larger particle sizes due to the accumulation of fuel ash. The diameter of the average mass of bed material shifts from 0.277 to 0.385 mm. So far, the change in particle size has not noticeably affected the operating characteristics of the fluidized bed, loop seal and moving bed.

100 0.5

Pressure in hPa

600

Cumulative amount in kg/kg

800

pBelow distributor BFB

400

200

Steam into loop seal ON

10-1

10-2 Initial silica sand 10-3

Bed material after 40 h experiment

ON BFB

05:55

10-4

06:05

06:15

Steam

0.01

0.1

Run time in hh:mm

Fig. 10. Particle size distribution of initial silica sand and bed material after 40 h of operation.

1.000

5

800

4

600

TBFB

pBelow distributor BFB

TGR crackzone

Steam into loop seal

3

400

2

200

1

0 09:15

10

Diameter in mm

Fig. 8. Effect of discontinuous loop seal operation on bubbling fluidized bed combustor.

Temperature in °C Pressure in hPa

1.0

09:20

09:25

09:30

09:35

09:40

Run time in hh:mm Fig. 9. Discontinuous operation mode of the loop seal.

0 09:45

Steam in kg/h

0 05:45

OFF

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Concentration in vol.-% [dry + N2 free∗]

S. Hamel et al. / Energy 32 (2007) 95–107

60 50

N2 inerting of gasification reactor

Steam addition (replacing N2 with steamin gasification reactor)

105

Run no.: Fuel: mfuel:

TS10 ® Stabilat 8.31 kg/h

Steam to fuel ratio:

0.44 kg/kg

H2

40 = 920 °C

TBFB

30

TGR crackzone = 860 °C

Start of fuel feed

CO2

20

CH4

O2

10 0 01:00

CO

∗N is used for inerting the lock hopper fuel feeding system 2

02:00

03:00

04:00

05:00

06:00

07:00

08:00

Run time (hh:mm) Fig. 11. Product gas concentrations during operation of the Herhof-IPV-Verfahrens with Stabilats.

Fig. 11. During the following 5 h (displayed in Fig. 11), the operating parameters were kept constant. Gas sampling was carried out continuously after the scrubber unit. The scrubber unit consists of a quench cooler which cools the raw gas by injecting water. The cooled product gas subsequently moves through a column filled with Pall rings in counter-current flow to the washing liquid. Entrained fines and condensed tars are collected in the liquid which was analysed after each experiment. The gas sample was dried for analytical purposes, i.e. to detect the main gas components. A time-averaged hydrogen concentration of about 45 vol% (dry and N2-free gas) was achieved in this particular run. The cumulated gas production was measured during the experiment for several time intervals of some 10 min. The dry gas yields obtained were in the range of 0.56–0.7 m3 (std. dry) per kg Stabilats for run No. 10 (Fig. 11). The composition corresponds to an average lower heating value (LHV) of the producer gas of some 13:3 MJ=m3ðstd:;dryþN2
freeÞ ; one third of natural gas LHV. In Fig. 12, the time-averaged concentrations from several runs with Stabilats are plotted over the temperature in the pyrolysis zone near the fuel feeding port. After first start-up of the equipment, the gasifier was operated systematically from low to higher temperatures. The preferred operating point is in the range of high temperatures where a maximum gas yield and high hydrogen content can be obtained. The effect of temperature in the pyrolysis zone of the gasifier on gas composition and heating value is shown in Fig. 12. The evaluation of all the experimental data obtained to date substantiates the repeatability of the measured values. One run usually takes about 24 h from start-up of the cold plant to shutdown, during which the period of fuel operation was usually in the range of 6–8 h. Part of the equipment, e.g. that needed for the solid circulation such as loop seal, bed material screw and fluidized bed, worked for the whole run time, whilst the equipment concerned with

Lower Heating Value (LHV) gas in MJ/m³ (std., dry + N2 free∗) 16

Concentration in vol.-% (dry + N2 free∗)

14 Exp. Data

Trend LHV gas

50

12 Meas.

Trend H2 CO CH4 CO2

40

30

20

10

Fuel: Steam/Fuel:

Stabilat® 0.15 - 0.5 kg/kg

∗N

2 is used for inerting the lock hopper fuel feeding system

0 600

700 800 Temperature pyrolysis zone in °C

900

Fig. 12. Concentrations of gas produced in the processing of Stabilats.

fuel feeding was only involved during the fuel operation period of each run. However, the results showed that the equipment components worked as planned for an overall time of approximately 450 h including some 100 h of fuel operation. The next targets are:



to increase fuel operation time which is mainly a question of the availability of scientific staff;

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to vary further operating parameters in order to determine their influence on gas quality; to use different bed materials in order to improve gas quality.

[5] [6]

An important step in the optimization of the process is to be expected from the installation of a tar measurement system to analyse the raw gas directly after the gasification reactor. To date, tar yields and compositions are detected only by analysing the quench water and washing liquid. Tar values so far obtained give only a lump-sum yield. That means all tars released during the experiment, whether from unsteady state in start-up or from parameter adjustments, are summarized in the lump-sum amount. Therefore this information is not detailed enough and unsuitable for analysing the influence of varying operating parameters or different bed materials. 6. Summary

[7]

[8] [9]

[10] [11]

The development of gasification processes for waste-derived fuels tends to result in ‘multi-stage’ systems in which the parallel-arranged approaches offer high potential and several advantages. In general, parallel-arranged generation of heat by combustion of char and spatially separated devolatilization and gasification prevents mixing of flue gas and raw gas and therefore provides an undiluted producer gas. In the Herhof-IPV-Verfahrens presented here, the twostage parallel arrangement is realized by coupling a fixed bed gasifier and a fluidized bed combustor. The forced fuel feed directly into the fixed bed reactor offers several advantages which enhance the processing of low-density fluffy fuel such as the investigated Stabilats. The setting up of a technical-scale plant for processing low-density fuel required specific solutions for several plant components. Initial experience and results from gasification runs demonstrate the successful operation of all core components. Sufficiently good gas qualities with high hydrogen contents have already been achieved.

[12]

Acknowledgement

[19]

The authors gratefully acknowledge the financial support of the German Federal Environmental Foundation (DBU) and the financial and technical support of HerhofUmwelttechnik GmbH. References [1] Higman C, van der Burgt M. Gasification. Amsterdam: Elsevier; 2003. [2] Belgiorno V, De Feo G, Della Rocca C, Napoli RMA. Energy from gasification of solid wastes. Waste Manage 2003;23:1–15. [3] Malkow T. Novel and innovative pyrolysis and gasification technologies for energy efficient and environmentally sound MSW disposal. Waste Manage 2004;24:53–79. [4] Ramthun F. Thermal Conversion of Secondary Fuel/Stabilats in Mono-/Co-combustion (Thermische Umsetzung von Ersatzbrenn-

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