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Combustible gas production from sewage sludge with a downdraft gasifier
Energy Conversion & Management 42 (2001) 157±172
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Combustible gas production from sewage sludge with a downdraft gasi®er Adnan Midilli a,b,*, Murat Dogru b, Colin R. Howarth b, Mike J. Ling c, Teoman Ayhan a a Mechanical Engineering Department, Karadeniz Technical University, 61080 Trabzon, Turkey Department of Chemical and Process Engineering, University of Newcastle, Newcastle NE1 7RU, UK c Waste To Energy Ltd, Eyston, Sudbury, Suolk, CO10 7AH, UK
b
Received 4 October 1999; accepted 13 April 2000
Abstract Recently, sewage sludge has particularly become an important problem all over the world because of its harmful impacts on the environment and living beings. It should be converted to combustible gas or useful energy in order to remove all its negative eects and to contribute to a signi®cant portion of the power generation. In this study, combustible gas production from sewage sludge was experimentally investigated using a gasi®cation technique by a downdraft gasi®er. The amount of combustible gases, which are H2, CO, CH4, C2H2 and C2H6, was found to be 19±23% of the total produced gases in the operating conditions. The average calori®c value of the product gas was obtained between 2.55 and 3.2 MJ (N m3)ÿ1. However, its average energy quantity was estimated between 12.19 and 28.97 MJ hÿ1. Consequently, combustible gases were produced from sewage sludge by utilizing as a feedstock in a downdraft gasi®er. Henceforth, sewage sludge can be considered a renewable energy source for gasi®ers in order to produce thermal energy to help get rid of sewerage all over the world. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sewage sludge; Combustible gas; Gasi®cation; Combustion; Gasi®er; Energy
* Corresponding author. Tel.: +90-462-3772-960; fax: +90-462-3255-526. E-mail address: [email protected]; [email protected] (A. Midilli). 0196-8904/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 0 0 ) 0 0 0 5 3 - 4
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Nomenclature Fwet gas Fdry gas FDSS Fwater Qwet gas Q(STP)dry gas Mwet gas Mdry gas Pbfo To Tr Tbfo Cp(gas) R Xw (CV)dry gas (HHV)SS DHdry gas DEdry gas Ec(dry gas) Edry gas Etar Zhot gas Zcold gas Zraw gas XH2, CO , CH4
wet gas mass ¯ow rate, (kg wet gas) hÿ1 dry gas mass ¯ow rate, (kg dry gas) hÿ1 dry sewage sludge feed rate, kg hÿ1 water mass ¯ow rate in product gas, kg hÿ1 volumetric wet gas ¯ow rate at gasi®er system outlet, N m3 hÿ1 volumetric dry gas ¯ow rate in STP, N m3 hÿ1 molecular weight of wet product gas, kg mol molecular weight of dry product gas, kg mol pressure of product gas at box ®lter outlet, mm Hg gasi®er outlet temperature, K reference temperature, K temperature at box ®lter outlet, K average constant pressure speci®c heat capacity, kJ (kg K)ÿ1 gas constant (0.08205), m3 atm (kg mol K)ÿ1 molar ratio of water in product gas calori®c value of dry product gas, MJ (N m3)ÿ1 higher heating value of sewage sludge, MJ kgÿ1 sensible heat of dry product gas, MJ sensible energy of dry product gas, MJ chemical energy of dry product gas, MJ total energy of dry product gas, MJ energy of tar, MJ hot gas eciency cold gas eciency raw gas eciency molar ratios of hydrogen, carbon monoxide and methane gases in product gas
1. Introduction Recently, treatment and disposal of sewage sludge has become a problem of increasing urgency in industrialized societies because of an increase in its production, both from industrial and municipal sources. Sludge production in the EU is expected to increase at least by 50% by the year 2005 [1]. Nearly 1 million m3 yrÿ1 of sewage sludge dry solids are produced in the UK [2], 4.2 million m3 yrÿ1 in Switzerland, 50 million m3 yrÿ1 in the old Federal Republic of Germany [3] and 170,000 m3 yrÿ1 in Singapore [4]. Probably sewage sludge production will increase, since municipal wastewater treatment plants are continually being built to comply with environmental standards [5]. The costs of disposal and treatment may be 50% of the total wastewater treatment cost [6]. Many recent researchers, who are interested in sewage sludge, have mainly studied its
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pyrolysis since 1980 [7±14]. Also, some researchers investigated the drying properties of sewage sludge [15] and the production of fuel and chemical matter from sludge [16±18]. In addition to these, pre-treatment of municipal sewage sludge was applied for gasi®cation [19]. The ®rst partial study on sewage sludge gasi®cation was made in 1994 [5]. The properties of pyrolysis coke from sewage sludge and its possible applications [13] and combustion of sewage sludge with heat recovery [20] were also investigated. Certain methods of treatment and disposal of sewage sludge do exist, but they are not entirely satisfactory. Therefore, it is important to develop a technology for adequate treatment of sewage sludge in order to reduce the environmental problem and costs of treatment. It can be assumed that gasi®cation is a suitable technology because it reduces waste volume, removes toxic organic compounds and ®xes heavy metals in the resultant solid. The gasi®cation process converts any carbon containing material into a combustible gas composed primarily of carbon monoxide, hydrogen and methane, which can be used as a fuel to generate electricity and heat, and a little amount of these gases can be used to dry wet sewage sludge. Typical raw materials used in gasi®cation are coal, biomass, agricultural wastes, sewage sludge, and petroleum based materials. The product gas from these materials is a mixture of non-condensable gases, such as CO, CO2 and H2, and light hydrocarbons, such as CH4, C2H4, C2H6, C3H8 and C3H6. Their relative proportions depend on the pyrolysis reaction conditions, temperature, heating rate, fuel moisture content, particle size, composition of ambient atmosphere, pressure and vapor residence time [20±23]. If the product gas is to be used to produce electricity, it is typically used as a fuel in an integrated gasi®cation combined cycle (IGCC) power generation con®guration. Gasi®cation also adds value to low or negative feed stocks by converting them to marketable fuels and useful products. The feed stock reacts in the gasi®er with oxygen in air at high temperature and pressure in a reducing (oxygen starved) atmosphere. In this study, combustible gases were produced from sewage sludge by gasi®cation with a laboratory scale, ®xed bed, throated downdraft gasi®er. The amounts were obtained as 10.48% H2, 8.66% CO, 1.58% CH4, 0.72% C2H2 and 0.20% C2H6 on average in the operating conditions. It was clear that clean combustible gases can be produced from sewage sludge gasi®cation by a downdraft gasi®er, and sewage sludge would be utilized as a renewable energy source for gasi®ers to produce thermal energy to help get rid of sewerage all over the world.
2. Calculations A list of the parameters for the fuel tested is summarized in Table 1. All the following equations were obtained using simple thermodynamic equations [24±26]. Wet gas mass ¯ow rate is one of the most important parameters to determine the amount of product gas from the sewage sludge. This value depends on the following parameters: amount of wet gas obtained from the gasi®er, pressure drop across the gasi®er system (outlet pressure of ®lter box), pressure of product gas at outlet (pressure across gasi®er system), temperature at gasi®er system outlet (temperature at ®lter box outlet) and molecular weight of the dry product gas. Wet gas mass ¯ow rate, Fwet gas, can be determined using Eq. (1) and assuming that the product gas is treated as an ideal gas
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Fwet gas
Wwet gas Pbfo Mwet gas RTbfo
1
where Qwet gas is the volumetric wet gas ¯ow rate at the gasi®er system outlet, Pbfo pressure of product gas at the box ®lter outlet, Tbfo temperature at the box ®lter outlet and Mwet gas molecular weight of wet product gas. Dry gas mass ¯ow rate, Fdry gas, depends on the wet gas mass ¯ow rate and the molar ratio of water in the product gas. Hence, Fdry gas Fwet gas 1 ÿ Xw
2
Here, Xw denotes the molar ratio of water in the product gas. Equating Eqs. (1) and (2), the water mass ¯ow in the product gas, Fwater, can be found as: Fwater Fwet gas ÿ Fdry gas
3
The dry gas volumetric ¯ow rate can be calculated assuming that the product gas is an ideal gas, and the dry volumetric gas ¯ow rate at STP (Standard Temperature Pressure), Qdry gas (STP), is given as: Fdry gas 273R Qdry gas STP 4 Mdry gas Here, Mdry gas de®nes the molecular weight of the dry product gas. The total energy of the dry product gas is equal to its sensible heat and its chemical energy from the gasi®er outlet temperature To 133±2838C to the reference temperature Tr 118C). The sensible heat of dry product gas, DHdry gas , is calculated as: DHdry gas Cp gas To ÿ Tr
5
where Cp is the average speci®c heat capacity at constant pressure. Hence, from Perry [24] for the average outlet temperature of the gasi®er, Cp gas 1:25 kJ (kg K)ÿ1. Using Eq. (5), the rate of sensible energy of dry product gas, DEdry gas , is calculated as:
Table 1 Parameters for the fuel tested Parameters
Comment
Wet gas mass ¯ow rate, Fwet gas (kg wet gas hÿ1) Dry sewage sludge feed rate, Fdry feed (kg hÿ1) Dry gas mass ¯ow rate, Fdry gas (kg dry gas hÿ1) Dry gas volumetric ¯ow, Qdry gas (STP) (N m3 hÿ1) Calori®c value of product gas (MJ (N m3)ÿ1) Hot gas eciency, Zhot gas (%) Cold gas eciency, Zcold gas (%) Raw gas eciency, Zraw gas (%) Total energy of dry product gas, Edry gas (MJ hÿ1)
Derived Derived Derived Derived Derived Derived Derived Derived Derived
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DEdry gas Fdry gas DHdry gas 10 ÿ3
161
6
The chemical energy of the dry product gas, Ec dry gas , can be de®ned, depending on Eq. (4), as follows: Ec dry gas Q STP dry gas CV dry gas
7
Here, (CV)dry gas denotes the calori®c value of dry product gas. Using Eqs. (6) and (7) the total energy of dry product gas, Edry gas, is calculated as follows: Edry gas DEdry gas Ec dry gas
8
The hot gas eciency depends on the hot gas energy and chemical energy of dry sewage sludge. The chemical energy of dry sewage sludge is based on the higher heating value of sewage sludge and dry sewage sludge feed rate. The dry sewage sludge feed rate depends on the wet sewage sludge feed rate and moisture content of the sewage sludge. Thus, the hot gas eciency is described as follows: Hot gas efficiency
Hot product gas energy 100 Chemical energy of dry sewage sludge
9
However, using Eqs. (7) and (9), the hot gas eciency, Zhot gas , can be written in mathematical form as follows: Zhot
gas
Ec dry gas DHdry gas 100 HHV SS FDSS
10
where (HHV)SS denotes the higher heating value of sewage sludge, and FDSS denotes the dry sewage sludge feed rate. Cold gas eciency depends on the cold gas energy and chemical energy of dry sewage sludge and is explained as follows: Cold gas efficiency
Cold product gas energy 100 Chemical energy of dry sewage sludge
11
However, using Eqs. (7) and (11), the cold gas eciency can be written in mathematical form as follows: Zcold gas
Q STP dry gas CV dry gas 100 HHV SS FDSS
12
If the gas is to be used directly in burners without any previous cleaning, it is possible to de®ne the raw gas eciency of the gas to include the tar in the gas. Thus, the raw gas eciency can be de®ned as Eq. (13), Zraw gas
Edry gas Etar 100 HHV SS FDSS
Here, Etar de®nes the energy of tar produced from sewage sludge or wet product gas.
13
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3. Experimental set-up and procedure In the down draft gasi®er, both the fuel and the gas ¯ow downwards through the reactor, enabling the pyrolysis gases to pass through a throated hot bed of char, which is supported by a grate. This results in cracking of most of the tars into non-condensable gases and water. Furthermore, air is admitted to the fuel bed through air intake nozzles, causing pyrolysis to charcoal and volatiles that partially burn as they are produced. The gaseous products of this `¯aming pyrolytic combustion' then consume the charcoal produced during the pyrolysis and are reduced to fuel gas. In this way, tar vapors are typically lowered to 0.1% of the total feed, whereas, in updraft or cross ¯ow gasi®ers, tar levels are higher than 0.1%. This gas is much more suitable for operation of clean gas burners, internal combustion engines, turbines, or to transport the product gas in pipelines. Other advantages are high char conversion, low ash carry over, quick response to load change and simple construction. Against these are the problems of higher gas outlet temperatures, diculty of scale up, ash fusion at high grate temperatures and moisture limitations (less than 25%). However, on balance, the lower tar levels and quick response to load and fuel changes make this type most suitable in a combined heat/power generation requirement, such as in a sewage sludge treatment plant where drying and power are required. 3.1. Apparatus The experimental system consists of a down draft gasi®er (10 kWe output), water scrubber, ®lter box, gas suction fan and a pilot burner. The gasi®er consists of four chemical and physical reaction zones, namely, drying, pyrolysis, oxidation and reduction zones from top to bottom. In terms of construction, this system is made of four sections, a fuel hopper, a gasi®er, an air feeding and an ash removal port [27]. In the drying zone, sewage sludge descends into the gasi®er, and moisture is removed using the heat generated in the zones below by evaporation. The rate of drying depends on the surface area of the fuel, the temperature dierence between the feed and the hot gases and the re-circulation velocity and relative humidity of these gases, as well as the internal diusivity of moisture within the fuel. Sewage sludge loses all of its moisture content in this zone. The temperature and the height of drying zone are about 70±2008C and 0.10 m, respectively (see Fig. 1). In the pyrolysis zone, the irreversible thermal degradation of dried fuel descending from the drying zone takes place using the thermal energy released by the partial oxidation of the pyrolysis products. The temperature and the height of the pyrolysis zone are 350±5008C and 0.17 m (see Fig. 1). In the oxidation zone, the volatile products of pyrolysis are partially oxidized in highly exothermic reactions, resulting in a rapid rise in temperature up to 11008C in the throat region. The heat generated is used to drive the drying and pyrolysis of sewage sludge and the gasi®cation reactions. The oxidation reactions of the volatiles are very rapid, and the oxygen is consumed before it can diuse to the surface of the char. No combustion of the solid char can, therefore, take place. Oxidation of the condensable organic fraction to form lower molecular weight products is important in reducing the amount of tar produced by a gasi®er. The
A. Midilli et al. / Energy Conversion & Management 42 (2001) 157±172 Fig. 1. Picture of experimental set-up. 1, Drying and pyrolysis zone temperature measurements; 2, oxidation zone temperature measurement; 3, oxidation zone; 4, reduction zone; 5, grate; 6, product gas outlet temperature measurement; 7, sample product gas taking; 8, pressure measurement of product gas; 9, waste water outlet; 10, fresh water inlet; 11, tar collection chamber; 12, wood chips; 13, coal; 14, rotameter; 15, tar and dust trap; 16, steel table. 163
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temperature and the height of the oxidation zone are approximately 1000±11008C and 0.12 m (see Fig. 1). In the reduction zone, the char is converted into product gas by reaction with the hot gases from the upper zones. The gases are reduced to form a greater proportion of H2, CO, CH4, C2H2 and C2H6. The combustible gases leave the gasi®er at an average temperature of 3008C and are loaded with dust, pyrolytic products (tar) and water vapor. Depending on the end use, it is necessary to cool and clean the gas in order to remove as much water vapor, dust and pyrolytic products as possible from the gas, especially if it is to be used in an internal combustion engine. The scrubber consists of water tank to re-circulate the spray water and a water spray packed bed cooling tower (see Fig. 1). The product gases leave the scrubber further cleaned and cooled to about 308C, and up to 80±90% of tar, dust, ¯y ash and condensate content has been removed. After scrubbing the product gas, it is further cleaned by the vertical ®lter box. The ®lter box has two layers (see Fig. 1). Wood and charcoal are chosen as the ®lter medium so that the contaminated ®lter can be recycled as fuel in the subsequent use of the gasi®er. To prevent excessive pressure drop over the box ®lter, the wood chips and charcoal are thoroughly sieved to remove any ®nes before they are placed in the ®lter trays. In the ®lter box, the charcoal tray occupies the lower tray, while the dry wood chips are on the upper tray. The remaining tar and condensate from the product gas are collected at the base of the box ®lter. Finally, a fan was utilized to draw the gases from the gasi®er (which operates at about 10 mm water gauge by regular pressure) and blow them into the burner or internal combustion engine. Apart from these, an auxiliary system consisting of U-tubes was used to take the product gas samples and qualify the amount of the combustible gases in the product gas. The product gas samples were monitored and collected for analysis by using three U-tubes. Three U-tubes were placed in series in a cold water container for trapping tar and moisture. The ®rst U-tube contained glass balls to provide a large surface area, and also, the second U-tube and third Utube contained silica gel and glass wool, respectively (see Fig. 2).
3.2. Experimental procedure The following procedures were performed during the experiments. Pre-weighted batches of sewage sludge are carefully loaded into the down draft gasi®er until the sewage sludge ®lled the gasi®er to a predetermined level. Clean and dry wood chips and charcoal for use as ®lters are placed in the respective trays in the box ®lter. The gas ¯ow meter is regulated to the required ¯ow rate. The air fan is switched on. The circulation pump (water scrubber pump) at the side of the water tank is turned on. The pilot lighter to ignite the product gas is lighted. The product gas ¯ow rate was measured by the gas ¯ow meter located after the suction fan. During the experiments, the samples were taken two times from the gasi®er outlet and water scrubber outlet to analyze the product gas and to determine the amounts of tar and condensate in the product gas. A gas chromatograph (Shimadzu GC-8A) is used to analyze the gas samples using the carrier gas as helium. The parameters for each column are presented in Table 2. The calibration was based upon the results of trial runs using a series of standard samples. These analyses were repeated for each product gas sample.
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Fig. 2. Picture of U-tube unit. 1, Glass balls; 2, connection pipe; 3, silica gel; 4, glass wool; 5, cold water container; 6, glass U-tube.
4. Results and discussion Sewage sludge gasi®cation experiments were performed in six runs in order to investigate the product gas quality and quantities. Using the data obtained from the runs, wet and dry gas mass ¯ow rates and also the amount of water in the product gas were calculated by using Eqs. (1)±(3) in order to provide a useful comparison between each run. The volumetric ¯ow rate of wet product gas was measured with a gas ¯ow meter. Drying, pyrolysis and oxidation zone temperatures of the gasi®er, and the water scrubber and box ®lter outlet temperatures were also monitored with the aid of R and K type thermocouples. The amounts of tar-dust and condensate in the product gas were determined before and after cleaning the product gas. The Table 2 Gas chromatograph parameters Parameter
Column 1
Column 2
Pressure (kPa) Gas velocity (ml/min) Detected gases Column packing
95 39.6 H2, O2, N2, CO, CH4 Molecular sieve
85 35.5 CO2, C2H2, C2H6 Chromosorb 101
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combustible gases from the sewage sludge were obtained as H2, CO, CH4, C2H2 and C2H6, and their amounts in the dry product gas were estimated in units of kg hÿ1 and as volume by volume percentage. The calori®c value and total energy of the produced gases were determined and evaluated based on the dry product gas volumetric ¯ow at STP. Hot, cold and raw gas eciencies of the dry product gas were estimated in order to investigate its usage in the CHP engines. During the experiments, pressure drops were measured as 0.10±0.80 mm Hg at the gasi®er outlet, 0.20±1.80 mm Hg at the water scrubber outlet and 2.15±4.60 mm Hg at the ®lter box outlet. It was found that they were small at the wet product gas ¯ow rate of around 8.145 N m3 hÿ1. Fig. 3 shows the variations of wet and dry product gas versus wet sewage sludge feed rate. Wet product gas samples were taken three times for each run from the gasi®er and box ®lter outlet in order to analyze them quantitatively and qualitatively. The wet product gas volumetric ¯ow rate was, for each run, continuously measured with the gas ¯ow meter during sewage sludge gasi®cation. Wet gas ¯ow rates were between 4.44 and 10.07 N m3 hÿ1. Using both Eq. (1) and the ¯ow rates of wet volumetric gas, wet product gas mass ¯ow rates were calculated to between 4.98 and 10.89 kg hÿ1, and also, dry product gas ¯ow rates were estimated to be between 4.88 and 10.56 kg hÿ1, depending on box ®lter outlet temperature and molecular weight of wet product gas, using Eq. (2). As shown in Fig. 3, it was observed that the amount of wet product gas increased by increasing the sewage sludge feed rate. However, the ¯ow rate of wet product gas was aected by char and ash from the sewage sludge. It was noticed that the ¯ow of wet product gas was restricted outside of the oxidation zone because of the fact that char and ash gathered on the grate, and sewage sludge adhered on the wall of the throat zone due to its high temperature and formed a clinker layer on the grate. Using Eq. (3), the water mass ¯ow in the wet product gas was estimated between 0.1 and 0.33 kg hÿ1,
Fig. 3. Variation of wet and dry product gas vs. wet sewage sludge feed rate.
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and it was found that the wet product gas contained a small amount of water at the outlet of the box ®lter. Fig. 4 presents the variations of wet gas temperatures in the gasi®er, water scrubber and box ®lter outlet versus its volumetric ¯ow rate. The product gas should have low temperatures (30± 708C) in order to be utilized in an engine. If the wet product gas has a temperature above 1008C, the tar in the composition of wet product gas can slag and plug the engine in the long run. So, the wet product was cooled and cleaned by the water scrubber and box ®lter, respectively. While the temperature of wet product gas was almost 2758C at the gasi®er outlet, it was between 20 and 308C at the water scrubber outlet and almost 16±208C at the box ®lter outlet. It was determined that the wet product gas still contained a small amount of tar and dust after the cleaning process because of low performance of the water scrubber. However, the remainder of tar and dust was collected by the box ®lter located after the water scrubber. Table 3 shows the contaminants in the wet gas composition. The amounts of tar-dust and condensate were determined before and after cleaning. During the experiments, it was observed that large amounts of tar-dust were deposited at the inlet and outlet of the circulation fan. During cooling of the gasi®er, tars from the product gas condensed inside the gasi®er and at the outlet of the fan and box ®lter. Because of the presence of tars in the product gas, problems were encountered in the measurement of the product gas ¯ow rate through the gas ¯ow meter. The tar content in the product gas condensed in the inlet manifold of the gas ¯ow meter proved to be particularly dicult to remove, and this factor limited the accuracy of the data collected in the ¯ow meter. Because of those reasons, the wet product gas should be analyzed to establish the harmful particulars in gas composition, damaging to the gasi®er system and the power engine. This is for future investigation. Wet product gas samples were ®rst taken from the outlet of the gasi®er before the water scrubber. In this stage, it was determined that according to the tar-dust and condensate
Fig. 4. Variation of water scrubber, box ®lter and gasi®er outlet temperatures vs. wet gas volumetric ¯ow rate.
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Table 3 Tar-dust and condensate in wet product gas before and after cleaning Wet product gas volumetric ¯ow (N m3 hÿ1)
Tar-dust before cleaning (gr (N m3)ÿ1)
Tar-dust after cleaning (gr (N m3)ÿ1)
Condensate before Condensate after cleaning (gr cleaning (gr (N (N m3)ÿ1) 3 ÿ1 m) )
10.07 9.18 8.14 8.14 6.22 4.44
6.37 5.60 5.50 5.47 8.38 7.52
1.64 1.55 1.47 1.39 1.96 1.60
142.50 172.8 144.10 163.25 97.39 166.85
23.47 29.22 21.08 26.82 28.98 26.10
analysis results, the wet product gas contained higher amounts of tar-dust and condensate at the increased rates of wet gas volumetric ¯ow before the cleaning processes. If wet product gas contains more than 0.5 g (N m3)ÿ1 of tar-dust before being introduced to the engine, it can damage some parts of the engine and decrease its performance. Therefore, it was passed through the water scrubber and box ®lter. After gas samples were taken from the box ®lter outlet, it was found that they contained lower quantities of tar-dust and condensate. Consequently, it was noticed that the amounts of tar-dust and condensate in the wet product gas were particularly reduced after the cleaning process, and the dirtiness ratio of wet product gas was decreased almost 25% using the water scrubber and box ®lter. The sewage sludge size used in the experiments was 3:5 1 0:5 cm. The average moisture content of the sewage sludge was 11.75%. The calori®c values of dry product gas were estimated using the IGT equation [28], shown as follows; ÿ CV dry gas 1:055 121XH2 119XCO 37:36XCH4 14 Assuming that the product gas was an ideal gas, the amounts of dry volumetric gas ¯ow at STP were calculated using Eq. (4), depending on the dry gas mass ¯ow rates. The amounts of combustible gases from sewage sludge are presented in Table 4. As shown in Table 4, the percentage volume of combustible gases was between 19 and 23% of the total dry product gas. At the ¯ow rate of total dry product gas of 10.56 kg hÿ1, the amount of total combustible Table 4 The amounts of combustible gases vs. total dry product gas Dry product gas ¯ow (kg hÿ1) Dry gas volumetric ¯ow (STP) (N m3 hÿ1) Calori®c value of gas (MJ (N m3)ÿ1) H2 ¯ow rate (kg hÿ1) CO ¯ow rate (kg hÿ1) CH4 ¯ow rate (kg hÿ1) C2H2 ¯ow rate (kg hÿ1) C6H6 ¯ow rate (kg hÿ1) Total combustible gas (%)
10.56 8.87 2.55 1.08 0.66 0.12 0.09 0.03 19.00
10.05 8.35 3.11 1.10 0.72 0.21 0.09 0.02 21.30
8.74 7.28 3.17 0.98 0.71 0.16 0.05 0.02 19.10
8.56 7.13 3.20 0.91 0.93 0.11 0.05 0.01 23.48
7.05 5.71 2.81 0.63 0.66 0.09 0.05 0.01 20.42
4.88 4.01 2.85 0.48 0.46 0.05 0.03 0.01 21.11
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gases was 19% of this quantity because the percentage ratio of N2 and CO2 was too much, almost 81%. However, it can be said that sewage sludge can not be appropriately gasi®ed at high sewage sludge feed rates. Fig. 5 shows the variation of energy of the product gas with dry gas mass ¯ow rate. The increase in the gas heating value with dry sewage sludge volumetric ¯ow rate was due to an increase in the useful heat fraction that favors the production of more combustible gases. The higher heating value of sewage sludge was determined as 17.14 MJ kgÿ1 by the Mahler Adiabatic Bomb Calorimeter. Using Eqs. (5)±(8), the amounts of energy of the dry product gas were calculated. The dry product gas energy is based on the gasi®er outlet temperature, reference temperature (Tr = 118C), dry gas volumetric ¯ow, dry gas mass ¯ow and calori®c value of the dry gas. Gasi®er outlet temperature and the fractional volume compositions of the combustible gases have more in¯uence on the energy quantity of the dry product gas. Using all these parameters, the total energy quantities of dry product gas were obtained between 12.19 and 28.97 MJ hÿ1. As shown in Fig. 5, the energy of dry product gas increased with an increased amount of dry product gas, and this regular increase continued until it reached a value of 10.05 kg hÿ1. However, it was noticed that the energy quantity of the dry product gas decreased after this value because of the change of calori®c value of the dry product gas. This change arises from the fractional volume compositions of the combustible gases. Fig. 6 contains the variations of product gas eciency with wet product gas mass ¯ow rate. The dry product gas eciency was investigated using three energy indicators, hot, cold and raw gas eciency. The hot gas eciency depends on the hot gas energy and chemical energy of the sewage sludge and was calculated using Eq. (10). The cold gas eciency is based on the cold gas energy and chemical energy of the sewage sludge and was determined using Eq. (12). If the product gas is to be used directly in burners without any previous cleaning, it is possible to de®ne the raw gas eciency of the product gas to include the tar in the gas. Thus, the raw gas eciency was calculated using Eq. (13).
Fig. 5. Variation of dry product gas energy vs. dry product gas mass ¯ow rate.
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Fig. 6. Variation of gas eciency vs. wet product gas mass ¯ow rate.
It was observed that the product gas eciency increased with an increase of wet product gas mass ¯ow rate; however, after the wet product gas mass ¯ow rate of 10.15 kg hÿ1, it decreased based on the composition of the wet product gas.
5. Conclusions Combustible gas production from sewage sludge was performed with a down draft gasi®er. It was concluded that: . Combustible gases can be produced from sewage sludge utilized as a feedstock in a down draft gasi®er. . The product gas contained carbon monoxide, carbon dioxide, hydrogen, methane, trace amounts of higher hydrocarbons, such as ethane and ethylene, water, nitrogen and various contaminants. . The gas composition was in¯uenced by the feed composition, water content, reaction temperature and the extent of oxidation of the pyrolysis products. . The product gas formed by gasi®cation was contaminated by ash, char and ¯uid bed materials if the water scrubber and box ®lter were insucient to cool and clean the product gas from the sewage sludge. . The amounts of tar-dust and other contaminants were dependent on the feedstock. . The down draft gasi®er produced the product gas with a low calori®c value of approximately 3.8 MJ (N m3)ÿ1 by using sewage sludge as feed stack for the gasi®er. . Sewage sludge could be considered a renewable energy source for gasi®ers in order to produce the thermal energy to get rid of sewerage. Among the renewable energy sources available today, sewage sludge appears to be able to
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contribute to the world's growing energy needs in an environmentally responsible manner. One of the most interesting and pollution free bioconversion systems is the production of combustible gas from sewage sludge by a gasi®cation process. The development of gasi®er systems to produce energy from sewage sludge will have a favourable impact on the economies of the UK and Turkey by creating new markets for energy companies. Acknowledgements The authors wish to thank the Chemical and Process Engineering Department, University of Newcastle, UK, for providing the facilities for this study and would also like to thank Waste to Energy Ltd for ®nancial support. References [1] Malik AA, Crove N. Energy from sewage sludge Ð start of a new era. UK: Entec, 1988. [2] Hall JE. Recent developments in sludge disposal and use. Chemistry and Industry 1993;6:188±91. [3] Gruter H, Matter M, Oehlmann KH, Hicks MD. Drying of sewage sludge is important step in waste disposal. Water Sci Tech 1990;22(12):57±63. [4] Lu GQ, Low JCF, Liu CY, Lua AC. Surface area development of sewage sludge drying pyrolysis. Fuel 1995;74(3):344±8. [5] Bacaicoa PG, Bilbao R, Uson C. Sewage sludge gasi®cation: ®rst studies. University of Zaragoza, Spain. 1994; Project: 286-23. [6] Winkler M. Sewage sludge treatments. Chemistry and Industry 1993;7:217±23. [7] Urban DL, Antal MJ. Study of the kinetics of sewage sludge pyrolysis using DSC and TGA. Fuel 1982;61:799± 806. [8] Kaminsky W, Kunumer AB. Fluidized bed pyrolysis of digested sewage sludge. Journal of Analytical and Applied Pyrolysis 1989;16:27±35. [9] Stammbach MR, Kraaz B, Hagenbuche R, Richarz W. Pyrolysis of sewage sludge in a ¯uidized bed. Energy and Fuels 1989;3:255±9. [10] Bellmann U, Kummer AB, Ying Y, Kaminsky W. In: Ferrero GL, Maniatis K, Buekens A, Bridgewater AV, editors. Pyrolysis and gasi®cation. London and New York: Elsevier Applied Science, 1989. p. 190±4. [11] Dumpelmann R, Richarz W, Stammbach MR. Kinetic studies of the pyrolysis of sewage sludge by TGA and comparison with ¯uidized beds. The Canada Journal of Chemical Engineering 1991;69(4):953±63. [12] Koch J, Kaminsky W. Pyrolysis a re®nery sewage sludge. Science and Technology 1993;46:323. [13] Ramphorst MP, Ringel HD. Pyrolysis of sewage sludge and use of pyrolysis coke. Journal of Analytical and Applied Pyrolysis 1994;28:137±55. [14] Conesa JA, Marcilla A, Prats D, Pastor MR. Kinetic study of the pyrolysis of sewage sludge. Waste Management and Research 1997;15:293±305. [15] Bretchel H, Eipper H. Improved eciency of sewage sludge incineration by preceding sludge drying. Water Sci Tech 1990;22(12):169±276. [16] Lowe P, Boutwood J. In: Hall JE, editor. Alternative uses for sewage sludge. Oxford: Pergamon Press, 1989. p. 277±84. [17] Boocock DBG, Konar SM, Makay A, Cheung PTC, Liu J. Fuels and chemicals from sewage sludge 2: The production of alkanes and alkenes by the pyrolysis of triglycerides over activated alumina. Fuel 1992;71(11):1291±7. [18] Konar SM, Boocock DGB, Mao V, Liu J. Fuels and chemicals from sewage sludge 3: Hydrocarbon liquids from the catalytic pyrolysis of sewage sludge lipids over activated alumina. Fuel 1994;73(5):642±6.
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A. Midilli et al. / Energy Conversion & Management 42 (2001) 157±172
[19] McMahon MA, Khan MR, Albert C, McKeon R. Pretreatment of municipal sewage sludge for gasi®cation. Abstract of papers of the ACS 1991;II:22. [20] Verkooijen AHM. Combustion of sewage sludge with heat recovery. Resources and Conversation 1987;14:257± 72. [21] Stone RJ, Ekwue EI, Clarke RO. Engineering properties of sewage sludge in Trinidad. J Agric Engng Res 1998;70:221±30. [22] Littlewood K. Gasi®cation: theory and application. Prog Energy Combust Sci 1977;3:35±71. [23] Bridgwater AV. The technical and economic feasibility of biomass gasi®cation for power generation. Fuel 1995;74(5):631±53. [24] Perry RH, Green GW, Malony JO, Perry's chemical engineers' handbook. International student edition, 6th ed. McGraw-Hill, Singapore. [25] Cengel YA, Boles MA. Thermodynamics: an engineering approach. 2nd ed. USA: McGraw-Hill, 1994. [26] Himmelblau DM. Basic principles and calculations in chemical engineering. 5th ed. USA: Prentice-Hall, 1989. [27] Midilli A, Dogru M. Gasi®cation of sewage sludge with a downdraft gasi®er. Research project, University of Newcastle, UK. Submitted to Waste to Energy Ltd, UK, May 1999. [28] Reed TB, Markson M, The SERI high pressure gasi®er. Proceedings of biomass to methanol specialist workshop, SERI/OP: 234±1540, Colorado, USA.
www.elsevier.com/locate/enconman
Combustible gas production from sewage sludge with a downdraft gasi®er Adnan Midilli a,b,*, Murat Dogru b, Colin R. Howarth b, Mike J. Ling c, Teoman Ayhan a a Mechanical Engineering Department, Karadeniz Technical University, 61080 Trabzon, Turkey Department of Chemical and Process Engineering, University of Newcastle, Newcastle NE1 7RU, UK c Waste To Energy Ltd, Eyston, Sudbury, Suolk, CO10 7AH, UK
b
Received 4 October 1999; accepted 13 April 2000
Abstract Recently, sewage sludge has particularly become an important problem all over the world because of its harmful impacts on the environment and living beings. It should be converted to combustible gas or useful energy in order to remove all its negative eects and to contribute to a signi®cant portion of the power generation. In this study, combustible gas production from sewage sludge was experimentally investigated using a gasi®cation technique by a downdraft gasi®er. The amount of combustible gases, which are H2, CO, CH4, C2H2 and C2H6, was found to be 19±23% of the total produced gases in the operating conditions. The average calori®c value of the product gas was obtained between 2.55 and 3.2 MJ (N m3)ÿ1. However, its average energy quantity was estimated between 12.19 and 28.97 MJ hÿ1. Consequently, combustible gases were produced from sewage sludge by utilizing as a feedstock in a downdraft gasi®er. Henceforth, sewage sludge can be considered a renewable energy source for gasi®ers in order to produce thermal energy to help get rid of sewerage all over the world. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sewage sludge; Combustible gas; Gasi®cation; Combustion; Gasi®er; Energy
* Corresponding author. Tel.: +90-462-3772-960; fax: +90-462-3255-526. E-mail address: [email protected]; [email protected] (A. Midilli). 0196-8904/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 0 0 ) 0 0 0 5 3 - 4
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Nomenclature Fwet gas Fdry gas FDSS Fwater Qwet gas Q(STP)dry gas Mwet gas Mdry gas Pbfo To Tr Tbfo Cp(gas) R Xw (CV)dry gas (HHV)SS DHdry gas DEdry gas Ec(dry gas) Edry gas Etar Zhot gas Zcold gas Zraw gas XH2, CO , CH4
wet gas mass ¯ow rate, (kg wet gas) hÿ1 dry gas mass ¯ow rate, (kg dry gas) hÿ1 dry sewage sludge feed rate, kg hÿ1 water mass ¯ow rate in product gas, kg hÿ1 volumetric wet gas ¯ow rate at gasi®er system outlet, N m3 hÿ1 volumetric dry gas ¯ow rate in STP, N m3 hÿ1 molecular weight of wet product gas, kg mol molecular weight of dry product gas, kg mol pressure of product gas at box ®lter outlet, mm Hg gasi®er outlet temperature, K reference temperature, K temperature at box ®lter outlet, K average constant pressure speci®c heat capacity, kJ (kg K)ÿ1 gas constant (0.08205), m3 atm (kg mol K)ÿ1 molar ratio of water in product gas calori®c value of dry product gas, MJ (N m3)ÿ1 higher heating value of sewage sludge, MJ kgÿ1 sensible heat of dry product gas, MJ sensible energy of dry product gas, MJ chemical energy of dry product gas, MJ total energy of dry product gas, MJ energy of tar, MJ hot gas eciency cold gas eciency raw gas eciency molar ratios of hydrogen, carbon monoxide and methane gases in product gas
1. Introduction Recently, treatment and disposal of sewage sludge has become a problem of increasing urgency in industrialized societies because of an increase in its production, both from industrial and municipal sources. Sludge production in the EU is expected to increase at least by 50% by the year 2005 [1]. Nearly 1 million m3 yrÿ1 of sewage sludge dry solids are produced in the UK [2], 4.2 million m3 yrÿ1 in Switzerland, 50 million m3 yrÿ1 in the old Federal Republic of Germany [3] and 170,000 m3 yrÿ1 in Singapore [4]. Probably sewage sludge production will increase, since municipal wastewater treatment plants are continually being built to comply with environmental standards [5]. The costs of disposal and treatment may be 50% of the total wastewater treatment cost [6]. Many recent researchers, who are interested in sewage sludge, have mainly studied its
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pyrolysis since 1980 [7±14]. Also, some researchers investigated the drying properties of sewage sludge [15] and the production of fuel and chemical matter from sludge [16±18]. In addition to these, pre-treatment of municipal sewage sludge was applied for gasi®cation [19]. The ®rst partial study on sewage sludge gasi®cation was made in 1994 [5]. The properties of pyrolysis coke from sewage sludge and its possible applications [13] and combustion of sewage sludge with heat recovery [20] were also investigated. Certain methods of treatment and disposal of sewage sludge do exist, but they are not entirely satisfactory. Therefore, it is important to develop a technology for adequate treatment of sewage sludge in order to reduce the environmental problem and costs of treatment. It can be assumed that gasi®cation is a suitable technology because it reduces waste volume, removes toxic organic compounds and ®xes heavy metals in the resultant solid. The gasi®cation process converts any carbon containing material into a combustible gas composed primarily of carbon monoxide, hydrogen and methane, which can be used as a fuel to generate electricity and heat, and a little amount of these gases can be used to dry wet sewage sludge. Typical raw materials used in gasi®cation are coal, biomass, agricultural wastes, sewage sludge, and petroleum based materials. The product gas from these materials is a mixture of non-condensable gases, such as CO, CO2 and H2, and light hydrocarbons, such as CH4, C2H4, C2H6, C3H8 and C3H6. Their relative proportions depend on the pyrolysis reaction conditions, temperature, heating rate, fuel moisture content, particle size, composition of ambient atmosphere, pressure and vapor residence time [20±23]. If the product gas is to be used to produce electricity, it is typically used as a fuel in an integrated gasi®cation combined cycle (IGCC) power generation con®guration. Gasi®cation also adds value to low or negative feed stocks by converting them to marketable fuels and useful products. The feed stock reacts in the gasi®er with oxygen in air at high temperature and pressure in a reducing (oxygen starved) atmosphere. In this study, combustible gases were produced from sewage sludge by gasi®cation with a laboratory scale, ®xed bed, throated downdraft gasi®er. The amounts were obtained as 10.48% H2, 8.66% CO, 1.58% CH4, 0.72% C2H2 and 0.20% C2H6 on average in the operating conditions. It was clear that clean combustible gases can be produced from sewage sludge gasi®cation by a downdraft gasi®er, and sewage sludge would be utilized as a renewable energy source for gasi®ers to produce thermal energy to help get rid of sewerage all over the world.
2. Calculations A list of the parameters for the fuel tested is summarized in Table 1. All the following equations were obtained using simple thermodynamic equations [24±26]. Wet gas mass ¯ow rate is one of the most important parameters to determine the amount of product gas from the sewage sludge. This value depends on the following parameters: amount of wet gas obtained from the gasi®er, pressure drop across the gasi®er system (outlet pressure of ®lter box), pressure of product gas at outlet (pressure across gasi®er system), temperature at gasi®er system outlet (temperature at ®lter box outlet) and molecular weight of the dry product gas. Wet gas mass ¯ow rate, Fwet gas, can be determined using Eq. (1) and assuming that the product gas is treated as an ideal gas
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Fwet gas
Wwet gas Pbfo Mwet gas RTbfo
1
where Qwet gas is the volumetric wet gas ¯ow rate at the gasi®er system outlet, Pbfo pressure of product gas at the box ®lter outlet, Tbfo temperature at the box ®lter outlet and Mwet gas molecular weight of wet product gas. Dry gas mass ¯ow rate, Fdry gas, depends on the wet gas mass ¯ow rate and the molar ratio of water in the product gas. Hence, Fdry gas Fwet gas 1 ÿ Xw
2
Here, Xw denotes the molar ratio of water in the product gas. Equating Eqs. (1) and (2), the water mass ¯ow in the product gas, Fwater, can be found as: Fwater Fwet gas ÿ Fdry gas
3
The dry gas volumetric ¯ow rate can be calculated assuming that the product gas is an ideal gas, and the dry volumetric gas ¯ow rate at STP (Standard Temperature Pressure), Qdry gas (STP), is given as: Fdry gas 273R Qdry gas STP 4 Mdry gas Here, Mdry gas de®nes the molecular weight of the dry product gas. The total energy of the dry product gas is equal to its sensible heat and its chemical energy from the gasi®er outlet temperature To 133±2838C to the reference temperature Tr 118C). The sensible heat of dry product gas, DHdry gas , is calculated as: DHdry gas Cp gas To ÿ Tr
5
where Cp is the average speci®c heat capacity at constant pressure. Hence, from Perry [24] for the average outlet temperature of the gasi®er, Cp gas 1:25 kJ (kg K)ÿ1. Using Eq. (5), the rate of sensible energy of dry product gas, DEdry gas , is calculated as:
Table 1 Parameters for the fuel tested Parameters
Comment
Wet gas mass ¯ow rate, Fwet gas (kg wet gas hÿ1) Dry sewage sludge feed rate, Fdry feed (kg hÿ1) Dry gas mass ¯ow rate, Fdry gas (kg dry gas hÿ1) Dry gas volumetric ¯ow, Qdry gas (STP) (N m3 hÿ1) Calori®c value of product gas (MJ (N m3)ÿ1) Hot gas eciency, Zhot gas (%) Cold gas eciency, Zcold gas (%) Raw gas eciency, Zraw gas (%) Total energy of dry product gas, Edry gas (MJ hÿ1)
Derived Derived Derived Derived Derived Derived Derived Derived Derived
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DEdry gas Fdry gas DHdry gas 10 ÿ3
161
6
The chemical energy of the dry product gas, Ec dry gas , can be de®ned, depending on Eq. (4), as follows: Ec dry gas Q STP dry gas CV dry gas
7
Here, (CV)dry gas denotes the calori®c value of dry product gas. Using Eqs. (6) and (7) the total energy of dry product gas, Edry gas, is calculated as follows: Edry gas DEdry gas Ec dry gas
8
The hot gas eciency depends on the hot gas energy and chemical energy of dry sewage sludge. The chemical energy of dry sewage sludge is based on the higher heating value of sewage sludge and dry sewage sludge feed rate. The dry sewage sludge feed rate depends on the wet sewage sludge feed rate and moisture content of the sewage sludge. Thus, the hot gas eciency is described as follows: Hot gas efficiency
Hot product gas energy 100 Chemical energy of dry sewage sludge
9
However, using Eqs. (7) and (9), the hot gas eciency, Zhot gas , can be written in mathematical form as follows: Zhot
gas
Ec dry gas DHdry gas 100 HHV SS FDSS
10
where (HHV)SS denotes the higher heating value of sewage sludge, and FDSS denotes the dry sewage sludge feed rate. Cold gas eciency depends on the cold gas energy and chemical energy of dry sewage sludge and is explained as follows: Cold gas efficiency
Cold product gas energy 100 Chemical energy of dry sewage sludge
11
However, using Eqs. (7) and (11), the cold gas eciency can be written in mathematical form as follows: Zcold gas
Q STP dry gas CV dry gas 100 HHV SS FDSS
12
If the gas is to be used directly in burners without any previous cleaning, it is possible to de®ne the raw gas eciency of the gas to include the tar in the gas. Thus, the raw gas eciency can be de®ned as Eq. (13), Zraw gas
Edry gas Etar 100 HHV SS FDSS
Here, Etar de®nes the energy of tar produced from sewage sludge or wet product gas.
13
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3. Experimental set-up and procedure In the down draft gasi®er, both the fuel and the gas ¯ow downwards through the reactor, enabling the pyrolysis gases to pass through a throated hot bed of char, which is supported by a grate. This results in cracking of most of the tars into non-condensable gases and water. Furthermore, air is admitted to the fuel bed through air intake nozzles, causing pyrolysis to charcoal and volatiles that partially burn as they are produced. The gaseous products of this `¯aming pyrolytic combustion' then consume the charcoal produced during the pyrolysis and are reduced to fuel gas. In this way, tar vapors are typically lowered to 0.1% of the total feed, whereas, in updraft or cross ¯ow gasi®ers, tar levels are higher than 0.1%. This gas is much more suitable for operation of clean gas burners, internal combustion engines, turbines, or to transport the product gas in pipelines. Other advantages are high char conversion, low ash carry over, quick response to load change and simple construction. Against these are the problems of higher gas outlet temperatures, diculty of scale up, ash fusion at high grate temperatures and moisture limitations (less than 25%). However, on balance, the lower tar levels and quick response to load and fuel changes make this type most suitable in a combined heat/power generation requirement, such as in a sewage sludge treatment plant where drying and power are required. 3.1. Apparatus The experimental system consists of a down draft gasi®er (10 kWe output), water scrubber, ®lter box, gas suction fan and a pilot burner. The gasi®er consists of four chemical and physical reaction zones, namely, drying, pyrolysis, oxidation and reduction zones from top to bottom. In terms of construction, this system is made of four sections, a fuel hopper, a gasi®er, an air feeding and an ash removal port [27]. In the drying zone, sewage sludge descends into the gasi®er, and moisture is removed using the heat generated in the zones below by evaporation. The rate of drying depends on the surface area of the fuel, the temperature dierence between the feed and the hot gases and the re-circulation velocity and relative humidity of these gases, as well as the internal diusivity of moisture within the fuel. Sewage sludge loses all of its moisture content in this zone. The temperature and the height of drying zone are about 70±2008C and 0.10 m, respectively (see Fig. 1). In the pyrolysis zone, the irreversible thermal degradation of dried fuel descending from the drying zone takes place using the thermal energy released by the partial oxidation of the pyrolysis products. The temperature and the height of the pyrolysis zone are 350±5008C and 0.17 m (see Fig. 1). In the oxidation zone, the volatile products of pyrolysis are partially oxidized in highly exothermic reactions, resulting in a rapid rise in temperature up to 11008C in the throat region. The heat generated is used to drive the drying and pyrolysis of sewage sludge and the gasi®cation reactions. The oxidation reactions of the volatiles are very rapid, and the oxygen is consumed before it can diuse to the surface of the char. No combustion of the solid char can, therefore, take place. Oxidation of the condensable organic fraction to form lower molecular weight products is important in reducing the amount of tar produced by a gasi®er. The
A. Midilli et al. / Energy Conversion & Management 42 (2001) 157±172 Fig. 1. Picture of experimental set-up. 1, Drying and pyrolysis zone temperature measurements; 2, oxidation zone temperature measurement; 3, oxidation zone; 4, reduction zone; 5, grate; 6, product gas outlet temperature measurement; 7, sample product gas taking; 8, pressure measurement of product gas; 9, waste water outlet; 10, fresh water inlet; 11, tar collection chamber; 12, wood chips; 13, coal; 14, rotameter; 15, tar and dust trap; 16, steel table. 163
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temperature and the height of the oxidation zone are approximately 1000±11008C and 0.12 m (see Fig. 1). In the reduction zone, the char is converted into product gas by reaction with the hot gases from the upper zones. The gases are reduced to form a greater proportion of H2, CO, CH4, C2H2 and C2H6. The combustible gases leave the gasi®er at an average temperature of 3008C and are loaded with dust, pyrolytic products (tar) and water vapor. Depending on the end use, it is necessary to cool and clean the gas in order to remove as much water vapor, dust and pyrolytic products as possible from the gas, especially if it is to be used in an internal combustion engine. The scrubber consists of water tank to re-circulate the spray water and a water spray packed bed cooling tower (see Fig. 1). The product gases leave the scrubber further cleaned and cooled to about 308C, and up to 80±90% of tar, dust, ¯y ash and condensate content has been removed. After scrubbing the product gas, it is further cleaned by the vertical ®lter box. The ®lter box has two layers (see Fig. 1). Wood and charcoal are chosen as the ®lter medium so that the contaminated ®lter can be recycled as fuel in the subsequent use of the gasi®er. To prevent excessive pressure drop over the box ®lter, the wood chips and charcoal are thoroughly sieved to remove any ®nes before they are placed in the ®lter trays. In the ®lter box, the charcoal tray occupies the lower tray, while the dry wood chips are on the upper tray. The remaining tar and condensate from the product gas are collected at the base of the box ®lter. Finally, a fan was utilized to draw the gases from the gasi®er (which operates at about 10 mm water gauge by regular pressure) and blow them into the burner or internal combustion engine. Apart from these, an auxiliary system consisting of U-tubes was used to take the product gas samples and qualify the amount of the combustible gases in the product gas. The product gas samples were monitored and collected for analysis by using three U-tubes. Three U-tubes were placed in series in a cold water container for trapping tar and moisture. The ®rst U-tube contained glass balls to provide a large surface area, and also, the second U-tube and third Utube contained silica gel and glass wool, respectively (see Fig. 2).
3.2. Experimental procedure The following procedures were performed during the experiments. Pre-weighted batches of sewage sludge are carefully loaded into the down draft gasi®er until the sewage sludge ®lled the gasi®er to a predetermined level. Clean and dry wood chips and charcoal for use as ®lters are placed in the respective trays in the box ®lter. The gas ¯ow meter is regulated to the required ¯ow rate. The air fan is switched on. The circulation pump (water scrubber pump) at the side of the water tank is turned on. The pilot lighter to ignite the product gas is lighted. The product gas ¯ow rate was measured by the gas ¯ow meter located after the suction fan. During the experiments, the samples were taken two times from the gasi®er outlet and water scrubber outlet to analyze the product gas and to determine the amounts of tar and condensate in the product gas. A gas chromatograph (Shimadzu GC-8A) is used to analyze the gas samples using the carrier gas as helium. The parameters for each column are presented in Table 2. The calibration was based upon the results of trial runs using a series of standard samples. These analyses were repeated for each product gas sample.
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Fig. 2. Picture of U-tube unit. 1, Glass balls; 2, connection pipe; 3, silica gel; 4, glass wool; 5, cold water container; 6, glass U-tube.
4. Results and discussion Sewage sludge gasi®cation experiments were performed in six runs in order to investigate the product gas quality and quantities. Using the data obtained from the runs, wet and dry gas mass ¯ow rates and also the amount of water in the product gas were calculated by using Eqs. (1)±(3) in order to provide a useful comparison between each run. The volumetric ¯ow rate of wet product gas was measured with a gas ¯ow meter. Drying, pyrolysis and oxidation zone temperatures of the gasi®er, and the water scrubber and box ®lter outlet temperatures were also monitored with the aid of R and K type thermocouples. The amounts of tar-dust and condensate in the product gas were determined before and after cleaning the product gas. The Table 2 Gas chromatograph parameters Parameter
Column 1
Column 2
Pressure (kPa) Gas velocity (ml/min) Detected gases Column packing
95 39.6 H2, O2, N2, CO, CH4 Molecular sieve
85 35.5 CO2, C2H2, C2H6 Chromosorb 101
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combustible gases from the sewage sludge were obtained as H2, CO, CH4, C2H2 and C2H6, and their amounts in the dry product gas were estimated in units of kg hÿ1 and as volume by volume percentage. The calori®c value and total energy of the produced gases were determined and evaluated based on the dry product gas volumetric ¯ow at STP. Hot, cold and raw gas eciencies of the dry product gas were estimated in order to investigate its usage in the CHP engines. During the experiments, pressure drops were measured as 0.10±0.80 mm Hg at the gasi®er outlet, 0.20±1.80 mm Hg at the water scrubber outlet and 2.15±4.60 mm Hg at the ®lter box outlet. It was found that they were small at the wet product gas ¯ow rate of around 8.145 N m3 hÿ1. Fig. 3 shows the variations of wet and dry product gas versus wet sewage sludge feed rate. Wet product gas samples were taken three times for each run from the gasi®er and box ®lter outlet in order to analyze them quantitatively and qualitatively. The wet product gas volumetric ¯ow rate was, for each run, continuously measured with the gas ¯ow meter during sewage sludge gasi®cation. Wet gas ¯ow rates were between 4.44 and 10.07 N m3 hÿ1. Using both Eq. (1) and the ¯ow rates of wet volumetric gas, wet product gas mass ¯ow rates were calculated to between 4.98 and 10.89 kg hÿ1, and also, dry product gas ¯ow rates were estimated to be between 4.88 and 10.56 kg hÿ1, depending on box ®lter outlet temperature and molecular weight of wet product gas, using Eq. (2). As shown in Fig. 3, it was observed that the amount of wet product gas increased by increasing the sewage sludge feed rate. However, the ¯ow rate of wet product gas was aected by char and ash from the sewage sludge. It was noticed that the ¯ow of wet product gas was restricted outside of the oxidation zone because of the fact that char and ash gathered on the grate, and sewage sludge adhered on the wall of the throat zone due to its high temperature and formed a clinker layer on the grate. Using Eq. (3), the water mass ¯ow in the wet product gas was estimated between 0.1 and 0.33 kg hÿ1,
Fig. 3. Variation of wet and dry product gas vs. wet sewage sludge feed rate.
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167
and it was found that the wet product gas contained a small amount of water at the outlet of the box ®lter. Fig. 4 presents the variations of wet gas temperatures in the gasi®er, water scrubber and box ®lter outlet versus its volumetric ¯ow rate. The product gas should have low temperatures (30± 708C) in order to be utilized in an engine. If the wet product gas has a temperature above 1008C, the tar in the composition of wet product gas can slag and plug the engine in the long run. So, the wet product was cooled and cleaned by the water scrubber and box ®lter, respectively. While the temperature of wet product gas was almost 2758C at the gasi®er outlet, it was between 20 and 308C at the water scrubber outlet and almost 16±208C at the box ®lter outlet. It was determined that the wet product gas still contained a small amount of tar and dust after the cleaning process because of low performance of the water scrubber. However, the remainder of tar and dust was collected by the box ®lter located after the water scrubber. Table 3 shows the contaminants in the wet gas composition. The amounts of tar-dust and condensate were determined before and after cleaning. During the experiments, it was observed that large amounts of tar-dust were deposited at the inlet and outlet of the circulation fan. During cooling of the gasi®er, tars from the product gas condensed inside the gasi®er and at the outlet of the fan and box ®lter. Because of the presence of tars in the product gas, problems were encountered in the measurement of the product gas ¯ow rate through the gas ¯ow meter. The tar content in the product gas condensed in the inlet manifold of the gas ¯ow meter proved to be particularly dicult to remove, and this factor limited the accuracy of the data collected in the ¯ow meter. Because of those reasons, the wet product gas should be analyzed to establish the harmful particulars in gas composition, damaging to the gasi®er system and the power engine. This is for future investigation. Wet product gas samples were ®rst taken from the outlet of the gasi®er before the water scrubber. In this stage, it was determined that according to the tar-dust and condensate
Fig. 4. Variation of water scrubber, box ®lter and gasi®er outlet temperatures vs. wet gas volumetric ¯ow rate.
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Table 3 Tar-dust and condensate in wet product gas before and after cleaning Wet product gas volumetric ¯ow (N m3 hÿ1)
Tar-dust before cleaning (gr (N m3)ÿ1)
Tar-dust after cleaning (gr (N m3)ÿ1)
Condensate before Condensate after cleaning (gr cleaning (gr (N (N m3)ÿ1) 3 ÿ1 m) )
10.07 9.18 8.14 8.14 6.22 4.44
6.37 5.60 5.50 5.47 8.38 7.52
1.64 1.55 1.47 1.39 1.96 1.60
142.50 172.8 144.10 163.25 97.39 166.85
23.47 29.22 21.08 26.82 28.98 26.10
analysis results, the wet product gas contained higher amounts of tar-dust and condensate at the increased rates of wet gas volumetric ¯ow before the cleaning processes. If wet product gas contains more than 0.5 g (N m3)ÿ1 of tar-dust before being introduced to the engine, it can damage some parts of the engine and decrease its performance. Therefore, it was passed through the water scrubber and box ®lter. After gas samples were taken from the box ®lter outlet, it was found that they contained lower quantities of tar-dust and condensate. Consequently, it was noticed that the amounts of tar-dust and condensate in the wet product gas were particularly reduced after the cleaning process, and the dirtiness ratio of wet product gas was decreased almost 25% using the water scrubber and box ®lter. The sewage sludge size used in the experiments was 3:5 1 0:5 cm. The average moisture content of the sewage sludge was 11.75%. The calori®c values of dry product gas were estimated using the IGT equation [28], shown as follows; ÿ CV dry gas 1:055 121XH2 119XCO 37:36XCH4 14 Assuming that the product gas was an ideal gas, the amounts of dry volumetric gas ¯ow at STP were calculated using Eq. (4), depending on the dry gas mass ¯ow rates. The amounts of combustible gases from sewage sludge are presented in Table 4. As shown in Table 4, the percentage volume of combustible gases was between 19 and 23% of the total dry product gas. At the ¯ow rate of total dry product gas of 10.56 kg hÿ1, the amount of total combustible Table 4 The amounts of combustible gases vs. total dry product gas Dry product gas ¯ow (kg hÿ1) Dry gas volumetric ¯ow (STP) (N m3 hÿ1) Calori®c value of gas (MJ (N m3)ÿ1) H2 ¯ow rate (kg hÿ1) CO ¯ow rate (kg hÿ1) CH4 ¯ow rate (kg hÿ1) C2H2 ¯ow rate (kg hÿ1) C6H6 ¯ow rate (kg hÿ1) Total combustible gas (%)
10.56 8.87 2.55 1.08 0.66 0.12 0.09 0.03 19.00
10.05 8.35 3.11 1.10 0.72 0.21 0.09 0.02 21.30
8.74 7.28 3.17 0.98 0.71 0.16 0.05 0.02 19.10
8.56 7.13 3.20 0.91 0.93 0.11 0.05 0.01 23.48
7.05 5.71 2.81 0.63 0.66 0.09 0.05 0.01 20.42
4.88 4.01 2.85 0.48 0.46 0.05 0.03 0.01 21.11
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gases was 19% of this quantity because the percentage ratio of N2 and CO2 was too much, almost 81%. However, it can be said that sewage sludge can not be appropriately gasi®ed at high sewage sludge feed rates. Fig. 5 shows the variation of energy of the product gas with dry gas mass ¯ow rate. The increase in the gas heating value with dry sewage sludge volumetric ¯ow rate was due to an increase in the useful heat fraction that favors the production of more combustible gases. The higher heating value of sewage sludge was determined as 17.14 MJ kgÿ1 by the Mahler Adiabatic Bomb Calorimeter. Using Eqs. (5)±(8), the amounts of energy of the dry product gas were calculated. The dry product gas energy is based on the gasi®er outlet temperature, reference temperature (Tr = 118C), dry gas volumetric ¯ow, dry gas mass ¯ow and calori®c value of the dry gas. Gasi®er outlet temperature and the fractional volume compositions of the combustible gases have more in¯uence on the energy quantity of the dry product gas. Using all these parameters, the total energy quantities of dry product gas were obtained between 12.19 and 28.97 MJ hÿ1. As shown in Fig. 5, the energy of dry product gas increased with an increased amount of dry product gas, and this regular increase continued until it reached a value of 10.05 kg hÿ1. However, it was noticed that the energy quantity of the dry product gas decreased after this value because of the change of calori®c value of the dry product gas. This change arises from the fractional volume compositions of the combustible gases. Fig. 6 contains the variations of product gas eciency with wet product gas mass ¯ow rate. The dry product gas eciency was investigated using three energy indicators, hot, cold and raw gas eciency. The hot gas eciency depends on the hot gas energy and chemical energy of the sewage sludge and was calculated using Eq. (10). The cold gas eciency is based on the cold gas energy and chemical energy of the sewage sludge and was determined using Eq. (12). If the product gas is to be used directly in burners without any previous cleaning, it is possible to de®ne the raw gas eciency of the product gas to include the tar in the gas. Thus, the raw gas eciency was calculated using Eq. (13).
Fig. 5. Variation of dry product gas energy vs. dry product gas mass ¯ow rate.
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Fig. 6. Variation of gas eciency vs. wet product gas mass ¯ow rate.
It was observed that the product gas eciency increased with an increase of wet product gas mass ¯ow rate; however, after the wet product gas mass ¯ow rate of 10.15 kg hÿ1, it decreased based on the composition of the wet product gas.
5. Conclusions Combustible gas production from sewage sludge was performed with a down draft gasi®er. It was concluded that: . Combustible gases can be produced from sewage sludge utilized as a feedstock in a down draft gasi®er. . The product gas contained carbon monoxide, carbon dioxide, hydrogen, methane, trace amounts of higher hydrocarbons, such as ethane and ethylene, water, nitrogen and various contaminants. . The gas composition was in¯uenced by the feed composition, water content, reaction temperature and the extent of oxidation of the pyrolysis products. . The product gas formed by gasi®cation was contaminated by ash, char and ¯uid bed materials if the water scrubber and box ®lter were insucient to cool and clean the product gas from the sewage sludge. . The amounts of tar-dust and other contaminants were dependent on the feedstock. . The down draft gasi®er produced the product gas with a low calori®c value of approximately 3.8 MJ (N m3)ÿ1 by using sewage sludge as feed stack for the gasi®er. . Sewage sludge could be considered a renewable energy source for gasi®ers in order to produce the thermal energy to get rid of sewerage. Among the renewable energy sources available today, sewage sludge appears to be able to
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contribute to the world's growing energy needs in an environmentally responsible manner. One of the most interesting and pollution free bioconversion systems is the production of combustible gas from sewage sludge by a gasi®cation process. The development of gasi®er systems to produce energy from sewage sludge will have a favourable impact on the economies of the UK and Turkey by creating new markets for energy companies. Acknowledgements The authors wish to thank the Chemical and Process Engineering Department, University of Newcastle, UK, for providing the facilities for this study and would also like to thank Waste to Energy Ltd for ®nancial support. References [1] Malik AA, Crove N. Energy from sewage sludge Ð start of a new era. UK: Entec, 1988. [2] Hall JE. Recent developments in sludge disposal and use. Chemistry and Industry 1993;6:188±91. [3] Gruter H, Matter M, Oehlmann KH, Hicks MD. Drying of sewage sludge is important step in waste disposal. Water Sci Tech 1990;22(12):57±63. [4] Lu GQ, Low JCF, Liu CY, Lua AC. Surface area development of sewage sludge drying pyrolysis. Fuel 1995;74(3):344±8. [5] Bacaicoa PG, Bilbao R, Uson C. Sewage sludge gasi®cation: ®rst studies. University of Zaragoza, Spain. 1994; Project: 286-23. [6] Winkler M. Sewage sludge treatments. Chemistry and Industry 1993;7:217±23. [7] Urban DL, Antal MJ. Study of the kinetics of sewage sludge pyrolysis using DSC and TGA. Fuel 1982;61:799± 806. [8] Kaminsky W, Kunumer AB. Fluidized bed pyrolysis of digested sewage sludge. Journal of Analytical and Applied Pyrolysis 1989;16:27±35. [9] Stammbach MR, Kraaz B, Hagenbuche R, Richarz W. Pyrolysis of sewage sludge in a ¯uidized bed. Energy and Fuels 1989;3:255±9. [10] Bellmann U, Kummer AB, Ying Y, Kaminsky W. In: Ferrero GL, Maniatis K, Buekens A, Bridgewater AV, editors. Pyrolysis and gasi®cation. London and New York: Elsevier Applied Science, 1989. p. 190±4. [11] Dumpelmann R, Richarz W, Stammbach MR. Kinetic studies of the pyrolysis of sewage sludge by TGA and comparison with ¯uidized beds. The Canada Journal of Chemical Engineering 1991;69(4):953±63. [12] Koch J, Kaminsky W. Pyrolysis a re®nery sewage sludge. Science and Technology 1993;46:323. [13] Ramphorst MP, Ringel HD. Pyrolysis of sewage sludge and use of pyrolysis coke. Journal of Analytical and Applied Pyrolysis 1994;28:137±55. [14] Conesa JA, Marcilla A, Prats D, Pastor MR. Kinetic study of the pyrolysis of sewage sludge. Waste Management and Research 1997;15:293±305. [15] Bretchel H, Eipper H. Improved eciency of sewage sludge incineration by preceding sludge drying. Water Sci Tech 1990;22(12):169±276. [16] Lowe P, Boutwood J. In: Hall JE, editor. Alternative uses for sewage sludge. Oxford: Pergamon Press, 1989. p. 277±84. [17] Boocock DBG, Konar SM, Makay A, Cheung PTC, Liu J. Fuels and chemicals from sewage sludge 2: The production of alkanes and alkenes by the pyrolysis of triglycerides over activated alumina. Fuel 1992;71(11):1291±7. [18] Konar SM, Boocock DGB, Mao V, Liu J. Fuels and chemicals from sewage sludge 3: Hydrocarbon liquids from the catalytic pyrolysis of sewage sludge lipids over activated alumina. Fuel 1994;73(5):642±6.
172
A. Midilli et al. / Energy Conversion & Management 42 (2001) 157±172
[19] McMahon MA, Khan MR, Albert C, McKeon R. Pretreatment of municipal sewage sludge for gasi®cation. Abstract of papers of the ACS 1991;II:22. [20] Verkooijen AHM. Combustion of sewage sludge with heat recovery. Resources and Conversation 1987;14:257± 72. [21] Stone RJ, Ekwue EI, Clarke RO. Engineering properties of sewage sludge in Trinidad. J Agric Engng Res 1998;70:221±30. [22] Littlewood K. Gasi®cation: theory and application. Prog Energy Combust Sci 1977;3:35±71. [23] Bridgwater AV. The technical and economic feasibility of biomass gasi®cation for power generation. Fuel 1995;74(5):631±53. [24] Perry RH, Green GW, Malony JO, Perry's chemical engineers' handbook. International student edition, 6th ed. McGraw-Hill, Singapore. [25] Cengel YA, Boles MA. Thermodynamics: an engineering approach. 2nd ed. USA: McGraw-Hill, 1994. [26] Himmelblau DM. Basic principles and calculations in chemical engineering. 5th ed. USA: Prentice-Hall, 1989. [27] Midilli A, Dogru M. Gasi®cation of sewage sludge with a downdraft gasi®er. Research project, University of Newcastle, UK. Submitted to Waste to Energy Ltd, UK, May 1999. [28] Reed TB, Markson M, The SERI high pressure gasi®er. Proceedings of biomass to methanol specialist workshop, SERI/OP: 234±1540, Colorado, USA.