Renewable Energy 38 (2012) 1e9
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Review
Syngas production in downdraft biomass gasifiers and its application using internal combustion engines Juan Daniel Martínez a, b, Khamid Mahkamov c, *, Rubenildo V. Andrade b, Electo E. Silva Lora b Grupo de Investigaciones Ambientales, Instituto de Energía, Materiales y Medio Ambiente, Universidad Pontificia Bolivariana, Circular 1ra N 70 e 01, Bloque 11, Medellín, Colombia Núcleo de Excelência em Geração Termelétrica e Distribuída, Instituto de Engenharia Mecânica, Universidade Federal de Itajubá, Av. BPS 1303, Itajubá, Minas Gerais, Brazil c School of Computing, Engineering and Information Sciences, Northumbria University, Ellison Building, Newcastle upon Tyne, NE1 8ST, UK a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2010 Accepted 23 July 2011 Available online 19 August 2011
Biomass downdraft reactors, coupled with reciprocating internal combustion engines (RICEs), are a viable technology for small scale heat and power generation. This paper contains information gathered from a review of published papers on the effects of the particle size and the moisture content of biomass feedstock and the air/fuel equivalence ratio used in the gasification process with regard to the quality of the producer gas. Additionally, data on the parameters of producer gas, such as its energy density, flame speed, knock tendency, auto-ignition delay period and the typical spark ignition timing, are systematised. Finally, information on the typical performance of various diesel and spark ignition RICEs fuelled with producer gas is presented. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Biomass gasification Downdraft gasifier Producer gas Internal combustion engine
1. Introduction Gasification is a process of conversion of any solid or liquid carbon-based material (feedstock) into gaseous fuel through its partial oxidation with air, oxygen, water vapor or their mixture. It could also be defined as the thermo-chemical process limited to a partial combustion and pyrolysis [1e5]. This process can be considered as a thermo-chemical treatment which unlike the full combustion uses air/fuel ratios noticeably below the stoichiometric value. Such a deficit in the supply of the oxidation agent prevents the complete conversion of the carbon and the hydrogen present in feedstock into CO2 and H2O, respectively, and results in the formation of combustible components such as CO, H2 and CH4. In addition to those components, the producer gas also contains typical products of combustion, namely CO2, N2, O2 and H2O. Although the process takes place with a sub-stoichiometric amount of air, it is usual to find a low concentration of oxygen in the gasification products. Finally, hydrocarbons such as ethylene (C2H4) and ethane (C2H6) are also present in very small quantities in the producer gas. A detailed description of the thermo-chemical processes taking place during biomass gasification is presented in [1e4,6e11]. Currently, small scale electricity generation using biomass gasification is attracting increasing interest as a prospective way to * Corresponding author. E-mail address:
[email protected] (K. Mahkamov). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.07.035
provide remote districts with electrical power using local renewable fuels. An additional benefit in such a rural electrification mechanism is the possibility of the utilization of various organic wastes from the local industry and agriculture with a considerable CO2 emission reduction. In this context, downdraft gasification has the advantage of higher conversion efficiencies with a low rate of tar and particulate matter generation. Combustion properties of a product of biomass gasification (producer gas), such as its calorific value, flame speed and knock tendency, are often inferior to those of conventional hydrocarbon fuels, such as gasoline and natural gas. However, they are satisfactory for this gas to be used as fuel in RICEs or, in some cases, for gas turbines after an appropriate cleaning process [12]. For small scale applications biomass gasification in downdraft reactors has been studied extensively and currently is considered to be a mature technology [1,2,13,14]. Downdraft gasifiers are the most widespread reactors for small scale biomass and carbon conversion for a power generation using RICEs [13,15e17]. With the realization of the negative environmental and social effects caused by a rapid depletion of resources of natural gas and crude oil, research and development projects on electricity generation with biomass gasification have gained a new momentum. Results of theoretical and experimental investigations of downdraft biomass gasifiers are presented in a large number of publications. For example, the influence of the gasification process parameters such as equivalence ratio, biomass particle size, its moisture content, etc., on gas composition, heating value, yield,
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power output and process efficiency are studied by Jain and Goss [15], García-Bacaicoa et al. [18], Zainal et al. [19], Dogru et al. [20], Jayah et al. [21], Wander et al. [22], Lv et al. [23], Wang et al. [24], García-Bacaicoa et al. [25], Tiangco et al. [26], Yamazaki et al. [27], Sheth and Babu [28], Tinaut et al. [29] and Ryu et al. [30]. The aim of this work is to present a review of theoretical and experimental research undertaken on biomass gasification employing downdraft reactors with air as an oxidation agent and the application of producer gas in reciprocating internal combustion engines.
2. Downdraft gasifiers The main advantage of this type of reactor is the lower tar concentration in the producer gas, which is very important for the durability of RICEs. The lower tar concentration is due to gas passing through a high temperature zone (the combustion zone), which enables the cracking of the tars formed during the gasification process. According to Bhattacharya et al. [31], tar concentrations in the producer gas during biomass gasification in a moving bed are in the range between 10 and 100 g/m3 and from 50 to 500 mg/m3 for downdraft and updraft gasifiers, respectively. Other advantages of the downdraft gasifier are the high char conversion and the lower ash carry over since gases pass through the charcoal bed allowing its filtration and catalysis and a quick response to any load change. The downdraft gasifier implementation is limited to small capacities and according to Reed and Das [17] there are difficulties in obtaining the homogeneous distribution of air in reactors with large diameters so preventing the scale-up of this type of gasifier. The largest downdraft gasifiers which exist have the power output in the range from 1.5 MWt to 5 MWt. For reactors with a throat section, Beenackers [13] recommends a maximum capacity of 1 MWe. Others disadvantages of downdraft gasifiers are the potential difficulties with ash fusion and the necessity to have feedstock with a moisture content less than 25% [20]. Additionally, the fuel to be gasified needs to have an adequate particle size in order to sustain a certain biomass consumption rate (or a chemical reaction rate), as well as to maintain an acceptable pressure drop inside the reactor without the formation of preferential channels (bridging). The recommended maximum particle size to be used in the Imbert downdraft gasifier is equal to one-eighth of the reactor’s throat diameter [20]. Downdraft gasifiers which can be used with RICEs can be categorized as open and close top designs, respectively. The open top design (or stratified) configuration, see Fig.1 has an open top, forcing air (by suction) to move downwards homogeneously throughout the gasifier in order to prevent hot spot formations. The homogeneous airflow also reduces inefficiencies in the thermo-chemical process taking place in the reactor, as well as a possibility of the formation of preferential channels and internal bridges. The stratified downdraft gasifier demonstrates high versatility and relatively high efficiency in operation with solid fuels of poly-dispersed nature, such as rice husk of small particle size and low density. A number of authors have highlighted the ratio of the biomass mass flow rate and the reactor area, called the specific rate of gasification, as an important optimization and scaling variable. Jain and Goss [15] found this parameter value to be optimal at 192.5 kg/(h,m2) for a rice husk gasification reactor with an internal diameter of 152 mm at 58% cold efficiency. Tiangco et al. [26] found this ratio to be 200 kg/ (h,m2) for a similar rice husk gasifier with a 300 mm internal diameter at 60% cold efficiency. Singh et al. [32] in experiments with cashew nut shells found the optimum value of the specific rate of gasification to be 167 kg/(h,m2) at 70% gasification efficiency.
Fig. 1. Gasifier with open top.
The closed top gasifiers have two different designs, namely one with a conventional downdraft with a straight cylindrical reactor as shown in Fig. 2 and one with a throat in the reactor core, see Fig. 3, also called Imbert gasifier [17]. The throat in the second design plays an important role in reducing the tar concentration in the producer gas. In such gasifiers air is introduced just above the throat and this creates a highly uniform temperature field and better mixing conditions [13]. However with the increase of gasifier dimensions, some low temperature zones appear in the throat zone resulting in a rise of tar content in the producer gas [13]. In
Fig. 2. Conventional downdraft gasifier.
J.D. Martínez et al. / Renewable Energy 38 (2012) 1e9
3
Fig. 3. Imbert gasifier.
accordance with García-Bacaicoa et al. [18], optimal conditions exist if the ratio of the biomass consumption rate and the area of the throat is between 0.05 and 1 kg/(s,m2). The gasification process consists of several stages (drying, pyrolysis, combustion and reduction) and the relevant equations describing the chemical reactions in each of these stages are presented in a number of publications, see for example [1,2,23]. The height of the reduction zone is very important for obtaining high quality producer gas. The gasifiers with a short reactor length do not provide a sufficient residence time which is necessary for the conversion of the biomass and this results in a lower operational efficiency. García-Bacaicoa et al. recommend in [18] that the ratio of the volume of the reduction zone and the area of the reactor’s throat should be greater than 0.5 m3/m2. According to Jayah et al. [21], a greater reactor length may increase its operational efficiency, but this also results in an increase in the manufacturing cost. A further reduction of the tar concentration in the producer gas used in RICEs can be achieved in downdraft gasifiers with a double stage air supply, see Fig. 4. This type of reactor has been studied in detail in the Combustion Gasification and Propulsion Laboratory of the Indian Institute of Science (IISc) and in Thailand in the framework of the Energy Program of the Asian Institute of Technology (AIT). Currently, an extensive investigation of such gasifiers is being undertaken at the Federal University of Itajubá in Brazil (UNIFEI). The downdraft gasifier of the IISc is an open top reactor in which the first stage of the air supply is provided at it stop, wherein the feedstock is fed into the reactor. The second stage of the air supply occurs at the oxidation zone level where along with oxidation of a part of the char the volatiles are released into the upper zone of the reactor. The downdraft gasifiers being investigated at AIT and UNIFEI are of the closed top design. The first air supply stage is located near the top of the reactor where the feedstock is partially oxidized and the thermal energy is generated. This is needed for the drying and pyrolysis phases occurring above the combustion zone. The second air supply stage is in the middle of the reactor, more precisely, in the oxidation zone where the tar decomposition into lighter compounds takes place. Bhattacharya et al. stated in [33] that the gasifier of the AIT also
Fig. 4. Downdraft gasifier with double stage air supply.
could be coupled with a fixed bed charcoal gasifier in order to decrease the tar concentration during the warm-up period. The designs described above are also known as two-stage gasifiers. A different design of a two-stage gasifier exists, named Viking, and developed by researchers at the Technical University of Denmark (DTU). The Viking gasifier consists of two separate reactors where the pyrolysis and char gasification take place. Between the pyrolysis and gasification processes air is supplied to the partially oxidized products of pyrolysis. This results in the reduction of the tar content in the volatiles and in the generation of thermal energy for the endothermic char gasification. Since the partially oxidized pyrolysis products pass through the char bed in the char gasification reactor, the tar content is further reduced by a factor of 100 [34]. Despite a comparatively low content of tar in the producer gas generated by downdraft gasifiers this should be further reduced using water scrubbers or special condensers in order to satisfy the requirements for the quality of gas used as fuel in RICEs. 3. The governing parameters in the gasification process to affect the quality of syngas fuel The specifics of reactor design and flow patterns of air and biomass particles within a gasifier strongly influence the quality of syngas fuel generated in the gasification process. For a specific gasifier design there are two major variable parameters which are used to maintain an acceptable level of the quality of syngas. These are the equivalence ratio and the superficial velocity. 3.1. Equivalence ratio (ER) In the gasification process it is the ratio of the actual air volume supplied per kg of biomass fuel and the volume of air which is necessary for stoichiometric combustion of the above amount of
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J.D. Martínez et al. / Renewable Energy 38 (2012) 1e9
biomass fuel. Typical values of ER for biomass gasification vary between 0.2 and 0.4. According to a number of studies, ER is one of the most important variables in the gasification process in fixed/ moving and fluidized bed reactors [11,20], which affects the quality of syngas produced. In accordance with García-Bacaicoa et al. [25], Yamazaki et al. [27] and Tinaut et al. [29] the amount of air fed into downdraft moving bed gasifiers controls the biomass consumption rate. The stoichiometric air/fuel ratio in cubic meters (at normal conditions) per kg of biomass can be expressed in terms of the chemical composition of the fuel and its typical value is between 5 and 6 Nm3/kg when firewood is used. 3.2. Superficial velocity (SV) A superficial velocity, SV, is defined as a ratio of the syngas production rate at normal conditions and the narrowest cross sectional area of the gasifier. A number of authors have indicated that SV influences the gas production rate, the gas energy content, the fuel consumption rate, the power output and char and tar production rates. It is independent of reactor dimensions, allowing a direct comparison of gasifiers with different power outputs. Yamazaki et al. [27] reported a case with a good performance of the gasifier when a low tar content in producer gas and high efficiency were obtained for SV values of about 0.4 Nm/s. Low values of SV result in a relatively slow pyrolysis process with high yields of char and significant quantities of unburned tars. On the contrary, high values of SV cause a very fast pyrolysis process, formation of a reduced amount of char and very hot gases in the flaming zone. However, such high SV values may significantly decrease the gas residence time in the gasifier, resulting in lower efficiencies in the tar cracking processes. 4. Output parameters of biomass gasifiers The main output parameters in the gasification process in moving bed reactors are the producer gas composition, its calorific value, thermal power of the gasifier, gas yield and the thermochemical process efficiency.
4.1. Producer gas composition and calorific value The producer gas composition depends mainly on the temperature in the reactor, which in its turn is influenced by the ER value. Additionally, concentrations of CO, H2 and CH4 in the producer gas are also controlled by the kinetics of the chemical reactions occurring in the gasification process. Therefore the type of the oxidizing agent used for gasification has a considerable influence on the calorific value of the producer gas. CO and H2 concentrations reach a maximum value as ER increases and then the concentration of these useful components decreases due to the combustion intensification at higher ER values. With the rise of ER the CO2 and N2 concentrations also increase in the producer gas, as observed by Sheth and Babu [28]. Air as an oxidizing agent produces syngas with relatively high concentrations of nitrogen and this results in a lower calorific value which usually does not exceed 6 MJ/Nm3. In this case the producer gas is classified as poor quality fuel gas. A typical gas composition from biomass gasification in a downdraft reactor with air used as an oxidizing agent is as follows: 15e20% of H2, 15e20% of CO, 0.5e2% of CH4, 10e15% of CO2 and the remaining part is made of N2, O2 and CxHy. On the other hand, if oxygen or water steam or a mixture of both these is used, then the concentration of combustible components is significantly increased and the calorific value rises to 18 MJ/Nm3 [35]. 4.1.1. Yield This parameter is usually used to measure the producer gas specific production in cubic meters per mass of feedstock supplied to the system. In both fluidized bed and moving bed reactors, the yield is directly proportional to the ER variation [19,28,36] and to the residence time of the gases in the reduction zone [18]. The ash content in the biomass also has a significant influence and limits the producer gas yield. Typical yield values for wood gasification in a downdraft reactor are between 2 and 3 Nm3/kg (Table 1). 4.1.2. Efficiency The gasification efficiency or the conversion efficiency depends on the type of biomass used, its particle size, the ER value and the
Table 1 Design characteristics of downdraft gasifiers and experimental results published in open literature. Biomass
Diameter (mm) Reactor
Wood chips
1000
Rice husk
152 203 244 343 600 450 920 270 350 440 30 250 310
Wood chips Hazelnut shells Rubber wood Sawdust Pine wood blocks Wood chips Rice husk Wood chips Wood waste
Throat 500
200 135 100 n.a 350 70 150
Height of reactor (m)
ER
Combustion zone temperature ( C) c
2.5
1.66
n.a n.a n.a n.a 2.5 0.81 1.15 1.1 1.3 2 n.a 1.05 1.1
0.40 0.39 0.40 0.41 0.287 1.51 e 1.9c 0.26 0.28 1.3c 1.5c 0.32 0.205
n.a: not available. a Higher heating value. b Lower heating value. c Air/Fuel ratio in Nm3/kg. d Dry, inert free. e Air/Fuel ratio (Nm3/kg), fuel is dry, ash free. f Lower heating value at 25 C. g At 25 C. h Dry basis.
n.a n.a n.a n.a n.a 1000 1025 1000 900 1108 1460 1000 900 1050
Gas composition (%) CO
H2
CH4
26.5 22.1 n.a n.a n.a n.a n.a 16.8 20.2 19.48 25.53 9.4 n.a 19.48 22
7.0 13.4 n.a n.a n.a n.a n.a 14.12 18.3 18.89 28.93 14.8 n.a 18.89 14
2.0 2.9 n.a n.a n.a n.a n.a 1.70 1.1 3.96 6.82 1.2 n.a 3.96 0.1
Heating value (MJ/Nm3)
Yield (Nm3/kg)
Power (kW)
Cold efficiency (%)
Ref
5.06b 5.59b,d 3.91f 4.02f 4.00f 3.98f 5.19a 4.55a n.a 6.32a 4.76 3.8b 4.2a 6.32a 6.34a
1.44 1.86 2.13g 2.10g 2.17g 2.22g n.a 1.97 n.a 1.99h n.a n.a n.a n.a 1.62
448.04 765.15 8.20 14.83 21.40 43.89 44.93 9.17 n.a n.a n.a n.a n.a n.a 7.38
48.77h 69.42h 58.11 58.78 60.44 61.49 76.68 51.53 n.a 62.5h n.a n.a 60 62.5 55
[2] [3]
[4] [5] [6] [7] [8] [10] [11] [12] [13]
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reactor’s design. The gasification efficiency, usually determined on the lower heating value basis, can be calculated in two different ways and is defined as the hot or cold efficiency. The hot efficiency is calculated as the ratio of the total energy in the producer gas (sensible and chemical) and the chemical energy in the feedstock (the heating value). The cold efficiency calculations account only for the heating value of the producer gas and neglect the value of the sensible heat. In order to compare the gasification efficiencies, the majority of authors consider the cold efficiency value in order to avoid the uncertainty related to the calculations of the sensible heat of producer gas discharged from the reactor since the high temperature of this gas is very often not the objective in the gasification process. Typical values of the cold efficiency for biomass gasification in a downdraft reactor are between 50 and 80%, see Table 1. Overall, the performance parameters of the biomass gasification in a downdraft reactor, namely the producer gas composition, its calorific value, the yield and the efficiency of the conversion process, depend on such physicalechemical properties of the feedstock as the moisture content and the particle size [19,21]. Also there is the influence of process parameters such as the equivalence ratio which determines the temperature levels. Finally, the performance is affected by a number of design features of the reactor, such as the locations of the air inlets, the volume of the gasification zone [19] and the grate’s design [18]. Table 1 presents main design dimensions and test results of biomass gasification in downdraft reactors both with and without throat, as reported by a number of authors. As it can be seen in Table 1, the low heating value and the process cold efficiency for this type of reactor is around 4e6 MJ/Nm3 and 50e70%, respectively. The typical average temperature in the combustion zone is around 1000 C. The equivalence ratio varies between 0.2 and 0.4. In gas engine applications a typical safe turndown ratio of a downdraft gasifier, which is ratio of the maximum and minimum flow of producer gas, is 3. Any further increase of the turndown ratio compromises the quality of syngas in terms of the tar content and negatively affects the durability of the engine. 5. Reciprocating internal combustion engines fuelled with producer gas RICEs are traditionally used with downdraft gasifiers and significant research has been performed on studying and improving the operation of RICEs fuelled by producer gas. The quality of producer gas as a fuel is considerably poorer compared to gasoline and natural gas. Hence engines require certain design modifications to be carried out in order to be able to run on producer gas. Spark ignition and diesel engines fuelled with artificial gas with a quality similar to that of producer gas were studied by Muñoz et al. [37]. The operation of engines was also investigated using real producer gas obtained in biomass gasifiers and the results were described by Wang et al. [24], Sridhar et al. [38], Shashikantha et al. [39], Ramachandra [40], Bhattacharya et al. [31], Uma et al. [41], Henriksen et al. [34], Ramadhas et al. [42], Ramadhas et al. [43] and Banapurmath and Tewari [44]. The above publications present data obtained in experimental investigations on the engine’s brake power, torque, efficiency, power de-rating, emissions, exhaust temperature and knock tendency, taking into account the influence of the air/fuel equivalence and the compression ratios. Alongside the experimental research, extensive theoretical investigations have also been performed using various modelling tools on the operation of RICEs fuelled by producer gas. Thus Lapuerta et al. [45] employed a chemical equilibrium model which took into consideration 28 species to calculate the producer gas composition as a function of the biomass-to-air ratio and its
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thermo-chemical parameters, such as the adiabatic flame temperature, the heat release and the auto-ignition period. A quasidimensional two zone combustion model was employed to describe the working process of a spark ignition engine taking into account the influence of the important combustion parameters. In conclusion the authors in [45] proposed recommendations on engine design modifications, including an increase in the compression ratio. Rakopoulos et al. [46] used a zero-dimensional, multi-zone thermodynamic combustion model for the analysis of the effect of the spark ignition timing on the engine’s performance. The model was calibrated using experimental data and applied for calculations of a multi-cylinder, four-stroke natural gas engine running on synthetic gas fuel. The above engine was equipped with a turbocharger with an after-cooling system. Tinaut et al. [16] used a two zone combustion model for the description of the working process of an engine and concluded that the main parameter defining the performance of the engine is the calorific value (per unit volume) of the stoichiometric air/producer gas mixture. The above model determines the fraction of the mass burnt, the variation of the pressure and temperature over the cycle and values of the engine’s efficiency and of its indicated mean pressure. Using a parameter named as the Engine Fuel Quality (EFQ) the authors estimated the magnitude of the power de-rating in the engine fuelled by the producer gas. To use the producer gas obtained in the process of biomass gasification for the electricity generation in both diesel and spark ignition engines it is necessary to ensure that the quality of gas is sufficiently high in terms of tar and particulates content to maintain the reliable engine’s operation and to provide an adequate durability of major engine components, such as the valves, the combustion chamber, the piston, etc. Table 2 presents the main substances contained in producer gas that limit its application in RICEs. Hasler and Nussbaumer [47] indicate that the allowed particle and tar concentration in producer gas for satisfactorily operation of the internal combustion engine must be less than 50 mg/Nm3 and 100 mg/Nm3, respectively. The gas quality requirements described in the literature should be interpreted with caution since the type of the engine used in tests and its design features play an important role [47]. In some cases a satisfactorily engine operation was observed at higher tar concentration levels than those indicated above. It was reported in [38] that producer gas was used in standard diesel engines in the dual-fuel mode operation and that diesel fuel savings up to 85% had been obtained. However, in the case described above, the power produced by the engine cannot be considered as achieved entirely by the utilization of only a renewable energy source. The specific design feature of that engine was its capability to operate on diesel oil in the case when biomass was not available or when a malfunction occurred in the operation of the gasifier. Diesel engines have certain advantages such as their higher efficiency due to a greater compression ratio which usually varies between 12 and 24 [48], their better durability and, in some cases, the lower maintenance compared to spark-ignition engines. Generally, design modifications required in diesel engines in order to make these machines run on 100% producer gas include the installation of additional equipment incorporating spark ignition and air-gas mixing systems. In spark ignition and diesel engines producer gas and air are usually mixed in an intake collector and then the air-fuel mixture now ready for combustion enters the cylinders of the engine. Test results obtained using diesel engines running on low calorific gas produced in a biomass gasifier are presented by Sridhar et al. [38], Shashikantha et al. [39] and Ramachandra [40]. The possibility of running RICEs in the dual-fuel mode of operation with a considerable reduction in the consumption of diesel oil was demonstrated in a number of publications, such as by Wang et al. [24], Ramachandra
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Table 2 Presence of pollutants in producer gas and used controlling mechanisms. Pollutant
Source
Possible problems
Control mechanism and/or mitigation
Particulates
Ash, bed material
Alkali metals (sodium, potassium in the ash). Nitrogen compounds (NOx, NH3, HCN)
Ash
Erosion, agglomeration and fouling. Environmental pollution Corrosion
Sulfur and chlorine compounds (HCl, H2S) Tar (complex hydrocarbon mixtures).
Reaction of sulfur and chlorine contained in the feedstock Low temperatures in the process, considerable amount of volatile in the feedstock
Filtration, gas cleaning (scrubber) Cooling, condensation, filtration, adsorption. Treatment with substances of basic character, use of pure oxygen in the process Cleaning, capture with CaCO3, MgCO3
Reaction of nitrogen contained i n air and feedstock
Corrosion, environmental pollution
Corrosion, agglomerations and fouling. Health hazard
[40], Bhattacharya et al. [31], Uma et al. [41], Ramadhas et al. [42], Ramadhas et al. [43] and by Banapurmath and Tewari [44]. It was reported in [44] that the power de-rating of engines observed in such an operational mode was between 20 and 30%. A case of fuelling modern spark ignition engines with producer gas was described by Muñoz et al. [37]. Due to the relatively low compression ratio of this type of engine, namely between 8 and 12 [48], the power de-rating observed was considerably greater than that in diesel engines. According to Sridhar et al. [38], the reduction in power in an engine run on producer gas is mainly attributed to the lower net calorific value of the air/fuel mixture. Lapuerta et al. [45] suggested that the engine’s power was limited by the volume of the gas/air mixture which enters the engine cylinders. The amount of the combustible mixture supplied to a cylinder is determined by the cylinder’s displaced volume, pressure and temperature conditions inside the cylinder and by the pressure and temperature of the gas/air mixture. The stoichiometric mass ratio in the air/producer gas mixture is between 1.0 and 1.2, compared to 17 for methane and thus an adequate mixing and dosage device is necessary for an engine to operate with high performance. The conventional carburetors are suitable for high calorific gaseous fuels, such as natural gas, and form a mixture with a high stoichiometric ratio. Low calorific gases require modified carburetors and Sridhar et al. [49] described such a gas carburetor for low energy density fuels. This device with its improved control and minimal pressure losses very accurately maintained the required air/fuel ratio over a wide load range and provided the smooth running of the engine at a comparatively high efficiency. 6. Parameters affecting the performance of RICEs fuelled with producer gas The parameters which mainly affect the performance of RICEs are the energy density or the heating value of the producer gas/air mixture, the displaced volume of the engine, the methane or octane number of the fuel, the flame speed of the fuel/air mixture, the auto-ignition delay period, the compression ratio of the engine (which is related to the knock tendency) and the spark timing. Some results reported in the open literature on RICEs fuelled with producer gas are systematised in Table 3.
Removal, cracking
biomass gasification in a downdraft reactor using air as an oxidizing agent is around 5000 kJ/Nm3 with the following average composition of the combustible components: H2 - 20%, CO - 20% and CH4 - 1%. With such concentrations of combustible substances the energy density of the producer gas/air mixture is about 2400 kJ/Nm3. This value is lower than the energy density of a natural gas/air mixture which is 3400 kJ/Nm3 (natural gas is assumed to be 100% CH4). The theoretical value of the power de-rating when a natural gas engine is switched to operate on producer gas is about 30%. This value is consistent with the estimation made by Tinaut et al. [16]. In this study the authors used the EFQ parameter to analyze the performance of an engine operating on a particular fuel. They predicted that the power of the engine fuelled with producer gas in the full load regime of operation will be approximately two-thirds of the maximum power obtained with conventional liquid fuel [16]. However, a thermodynamic analysis demonstrated that a lesser value of power de-rating at the level of 15e20% could be achieved if the producer gas is used in engines with a higher compression ratio [49]. The current engine technology intensively exploits advantages of lean combustion operation. For producer gas/air mixtures the lean combustion condition is achieved when the actual air/fuel ratio is greater than 2 and in such conditions the relative energy density of the producer gas/air mixture may be higher than some fossil fuels, such as gasoline [45]. This results in the power derating being lower when the producer gas/air mixture is used. Producer gas is adequate for a lean burn and combustion of a corresponding fuel/air mixture results in low NOx emissions due to the lower combustion temperature and in the low specific fuel consumption. 6.2. Cylinder volume The amount of a combustible mixture which can be delivered to a combustion chamber in a cylinder is determined by the engine’s displaced volume and by the fuel’s initial pressure and temperature. Thus, to maintain the power level in a conventional natural gas engine switched to operate on low heating value fuel, such as producer gas, the fuel amount should be increased significantly which would exceed the engine’s capacity. This can be achieved using a turbocharger for increasing the pressure of the air-fuel mixture in the beginning of the compression process in a cylinder.
6.1. Energy density 6.3. Flame speed and spark timing The energy density of any fuel/air stoichiometric mixture (HVm) can be determined in terms of the volumetric heating value (kJ/Nm3). The energy density of the producer gas/air mixture mainly depends on the concentration of the combustible components in producer gas. The low heating value of the producer gas obtained from
The speed of the flame depends on the chemical composition of fuel, the amount of air used in the combustion process, which is characterized by the parameter ER, and the pressure and temperature of the fuel/air mixture. Additionally, the speed of the flame
J.D. Martínez et al. / Renewable Energy 38 (2012) 1e9
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Table 3 Performance of some RICEs fuelled with producer gas. Biomass
Engine
RPM
CR
Producer gas fuelled (%)
Modifications in the engine
Power (kW)
Spark timing (o BTDC
Power de-ratinga (%)
Exhaust gas temperature ( C)
Engine’s thermal efficiency (%)
Combustion equivalence ratio
Overall Efficiencyb (%)
Ref
Simulated gas Wood
Spark ignition Diesel
2500
8.2:1
100
None
2.3c
n.a
n.a
n.a
n.a
n.a
[15]
1500
11.5:1
100
Ignition systemd Ignition systemg Ignition system None
12e16e
35
n.a
634 @ 2000 rpm 360- 430 C
28e32
n.a
21e24f
[16]
Wood
Diesel
1500
17:1
100
Wood chips
Diesel
n.a
n.a
100
Coconut shell
Diesel
1500
18.5:1
81
Wood
Diesel
1500 50
17:1
100
Wood
a b c d e f g h i j k l m
Diesel
1500
17.5:1
65% 60%
Ignition system None Ignition system
2.3
c
15e20 11.44c 13.22e 17.5e 4
c
10 e
20
h
310 e 370 C
19.05
i
n.a
n.a
[17]
i
n.a
25
f
[20]
n.a
20
n.a
28
n.a
21
488.2
14.7j
n.a
11.69f
[18]
6 (MBT)
16.7k
n.a
n.a
1.05l
16.6m
[14]
n.a n.a
n.a n.a
[23]
27
20
410 450
i
22 24i
Calculated as a fraction of a nominal engine power. Gasification and engine system. Brake power. Combustion chamber was re-designed for the combustion with the turbulent flame propagation, CR was changed from 17 to 11.5. Electric power. From biomass to net electricity. The engine can operate in diesel and dual fuel mode. Assuming the alternator and transmission efficiency of 80% and 95% respectively. On the shaft. Engineegenerator system. In brake power. Fuel/air equivalence ratio. From biomass input to shaft output.
depends on the turbulence intensity, which in its turn, is pre-defined by the engine speed [48]. Rich mixtures ignite more readily and are characterised with a higher flame speed and therefore provide a more reliable start of the overall combustion process [48]. The flame speed in its turn has a significant effect on the performance of the engine and on the level of pollutant emissions [50]. Kanitkar et al. [51] reported experimental results on the combustion of the producer gas/air mixture at ambient conditions (0.95 atm and 300 K). The flame temperature was 1546 25 K with the flame speed of 0.5 0.05 m/s and the limits of the flame propagation varied from the mixture being lean (26% of fuel with an ER value of 0.47) to rich (56% of fuel with an ER value of 1.65). The peak flame speed was registered to be 0.55 0.05 m/s in an air/fuel ratio varied between 1.2 and 1.4 and this value is greater than that for methane and carbon monoxide, but much less compared to that for hydrogen [51]. In a more recent study Hernández et al. [50] reported that an experimental value of the laminar flame velocity was 0.5 m/s at 300K and 1 bar conditions in the burning process of the stoichiometric air/producer gas mixture in a combustion bomb. The authors also used a software tool, namely CHEMKIN, together with GRI-Mech for computing the laminar flame velocity for different producer gas compositions at various pressure and temperature conditions for a range of the producer gas/air equivalence ratios. For the stoichiometric air/fuel ratio at 300 K and 1 bar conditions the laminar flame velocity of the producer gas was found to be 0.42 m/s. At typical engine operating conditions, which are characterized by high pressures and temperatures, the producer gas laminar flame velocity values were calculated to be lower than that of isooctane but higher than that of methane. 6.4. Spark timing According to Sridhar [49] the considerable hydrogen concentration in producer gas makes it necessary to use the smaller spark
advancement (retarding) in the spark timing to achieve a better engine performance. In this case the spark is fired at the instance of the cycle when the piston is very close to its top dead center (TDC). This is because of the high hydrogen concentration (around 20%) in producer gas. Hydrogen has a higher flame speed which is about 2.7 m/s at ambient conditions [51]. However, it is necessary to take into account that the spark timing also depends on other variables, such as the level of load and the engine’s speed. Theoretically the ideal instance of time for the spark to be fired is when the piston is at the permissible high position in the cylinder and when the fuel/ air mixture is fully compressed so the piston’s power stroke occurs with the extraction of the maximum power. Setting the correct spark timing results in a high torque exerted at the fixed speed which corresponds to a higher power output from the engine and to a lower specific fuel consumption. This point is usually known as a maximum brake torque (MBT) point and can be observed in the pressure vs. crank angle (p-q) curve as a peak in the instance in the cycle corresponding to 16-17 degrees of crank angle after the piston’s TDC [48]. Experiments in a single cylinder, four stroke, water cooled diesel engine with a compression ratio of 17 which was adapted to run as a spark ignition engine were carried out by Ramachandra [40] and it was observed that a spark timing set to 10 BTDC provides a smooth running of the engine. The above spark timings in engines fuelled by producer gas are retarded compared to the spark timing for conventional spark ignition engines fuelled with gasoline which vary between 10 and 40 BTDC [48]. Retarding in the spark timing is necessary for engines fuelled with producer gas to achieve a higher efficiency. According to Sridhar et al. [49] the ignition timing has to be retarded with an increase in the compression ratio in order to achieve the MBT point. This is because the pressure and the temperatures are greater at the higher compression ratios and therefore the combustion process occurs faster, requiring the instance of the spark firing to be located close to the piston’s TDC.
8
J.D. Martínez et al. / Renewable Energy 38 (2012) 1e9
6.5. Knock tendency For gaseous fuels, the methane number is used to compare knock properties and this is analogous to the octane number used to quantify knock properties of liquid gasoline fuels. Engines with a high compression ratio require fuels with the high octane/ methane number in order to avoid an uncontrolled self-ignition of the fuel and the formation of sharp pressure peaks in the engine’s cylinder after the start of such a combustion process. Malenshek [52] developed a model and an experimental apparatus which was used to blend simulated alternative gaseous fuels and measure their methane number. Numerous engines were run on natural gas and the methane number was varied between 75 and 95. Producer gas has a higher methane number than natural gas and therefore it is not prone to detonation during the compression stroke. The knock is caused by a combination of factors, such as the combustion chamber design, the equivalence ratio, the intake air temperature and the pressure, the spark timing and fuel properties. The experiments conducted by Sridhar et al. [38] demonstrated a smooth engine operation at the compression ratio of 17 without any traces of knocking when the engine was fuelled with producer gas with the following composition: 19 1% of H2; 19 1% of CO; 2% of CH4; 12 1% of CO2; 2 0:5% of H2O with the remaining made up of N2. Also in [38] the tendency of producer gas to uncontrolled self-ignition in high compression ratio RICEs due to the high concentration of the hydrogen in the gas was addressed by the authors. The high concentration of inert gases in producer gas, such as CO2 and N2, namely 12e15% and 48e50%, respectively, acts as a knock suppressor and explains the high methane number compared to that of natural gas. Shrestha and Rodrigues [53] demonstrated the increased knock resistance within the extended engine’s operational range due to the presence of carbon dioxide and nitrogen in gaseous fuels. The carbon dioxide’s presence in the fuel is an efficient knock suppressor, providing the controlled combustion of the fuel composition with the low methane number, which otherwise would be prone to a high intensity knock. Nitrogen also demonstrates a similar knock suppressing quality, but not to the such extent as the carbon dioxide does [53]. Additionally, the maximum flame temperature attainable with producer gas is lower compared to conventional fuel such as methane and therefore a better knock resistivity could be expected when an engine runs on producer gas [49]. All the above results substantiate the possibility of running RICEs on producer gas with a higher compression ratio. Gaseous fuels with high hydrogen concentrations usually are less resistant to detonation. However, the high flame speed of the corresponding fuel/air mixture reduces the probability of knocking. According to [48], the greater the flame speed in an air/fuel mixture the higher is the octane number of the fuel. With a high flame speed, the portion of the air/fuel mixture that is heated above the self-ignition temperature will be burnt off during the ignition delay period and therefore the occurrence of knocking will be avoided. 6.6. Auto-ignition period The auto-ignition delay period of a fuel/air mixture is an important parameter in the RICEs operation and also can be used to characterise the knock tendency. This parameter is defined as the time required for the mixture to spontaneously ignite at certain temperature and pressure conditions. The length of the ignition delay depends on the producer gas composition and on the producer gas/air ratio in the engine [54]. The auto-ignition delay time of a fuel/air mixture was theoretically determined by Lapuerta et al. [45] using CHEMKIN III software. The results obtained demonstrated that at a constant cylinder pressure of 20 bar the
auto-ignition delay period for producer gas is much longer than that for gasoline in the low temperature range, namely below 950 K. However, the ignition delay period significantly shortens at the high temperature range. With the pressure increased to the 50 bar the auto-ignition delay period for producer gas remains just slightly shorter than that for gasoline in the high temperature range [45] and therefore the producer gas/air mixture has a low knock tendency. According to the authors in [45] the expected lower combustion temperatures, together with the longer auto-ignition delay period, for the producer gas/air mixture would make it possible to increase the compression ratio of the engine without increasing the knock tendency. Another problem associated with using producer gas in RICEs is the possibility of backfiring which is ignition of the gas/air mixture in the intake manifold and its burning in an explosive manner causing the engine to stop. This is attributed to the relatively weak ionization of the hydrogen/oxygen flame. An electric potential present in the ignition cables could cause abnormal ignitions in the RICEs. The authors recommended shielding the ignition cables to avoid these kinds of difficulties. 7. Conclusions The main parameters governing the gasification process of biomass in downdraft reactors using air as an oxidizing agent were discussed. The effects of the equivalence ratio (which should be kept between 0.2 and 0.4), the biomass particle size (which usually should be less than 5 cm), the moisture content (which should be less than 25%) and the influence of gasifier design features were analyzed. The literature review carried out in this subject area indicates that the low heating value and the process cold efficiency for a downdraft type reactor are around 4e6 MJ/Nm3 and 50e70%, respectively. The average temperature in the combustion zone is about 1000 C. The specifics of the use of producer gas in RICEs (diesel and spark ignition ones) were discussed. The low energy density of the producer gas/air mixture and the engine’s volumetric efficiency are the main factors causing the power de-rating of engines. Due to the relatively high flame speed of the producer gas/ air mixture caused by the presence of hydrogen in the mixture it is necessary to retard the spark ignition time in order to achieve a greater efficiency in the operation of the engine. Also the possibility of using engines with a higher compression ratio when fuelled with producer gas without any increase in the knock tendency is highlighted. The rise of the engine’s compression ratio results in the reduction of the power de-rating. The use of air as an oxidizing agent in the biomass gasification process leads to high concentrations of nitrogen (between 40 and 50%) in the fuel/air mixture and the nitrogen acts as a knock suppressor which is beneficial in cases when engines with the high compression ratio are employed. Acknowledgment The authors would like to express their gratitude to Companhia Paulista de Força e Luz (CPFL) for the financial support received through project PD153. Also we would like to thank the Committee on Coordination of Improvements in Higher Education (CAPES) for the allocation of a scholarship and the National Research Council of Brazil (CNPq) for their financial support. Furthermore, we would like to acknowledge the Foundation of Science support from the Minas Gerais State (FAPEMIG). Finally, we would like to thank the Royal Society (UK) for the financial support to perform this collaborative research between teams at Northumbria University (UK) and the Federal University of Itajuba (Brazil).
J.D. Martínez et al. / Renewable Energy 38 (2012) 1e9
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