Equipment and testing procedures
Results and discussion
ZheJiang University, HangZhou 310027, China
This article is mainly concerned with combustion of biomass gas in lean pattern produced from the vaporous circulating fluidized bed (VCFB) gasification system. A 2135D diesel engine is modified into a spark ignition (SI) gas engine. The gas fueled (GF) engine characteristics were studied experimentally. Results indicated that the specific energy consumption Q and thermal efficiency were satisfactory at high torque. The influence of air/gas (AG) ratio on the combustion rate and minimum ignition energy is discussed. The way for using higher LHV biomass gas reasonably and effectively is found.
Along with the rapid development of the national economy in China and other developing countries, the emergence of a universal need for energy resources has re-emphasized throughout those regions where biomass energy resources are abundant. Attention was focused on the conversion of biomass into clean energy, which is an opportunity to expand the energy resource while reducing the pollution now associated with waste disposal. The proposed energy resources (4.5 X 109 T for crop straw, 50 X106 T for rice husks and 10 X 106 m3 for forestry residues per year) in China represents recoverable energy resources. VCFB gasification is one of the preferable technologies for producing combustible gas with much higher LHV, and less tar from low quality biomass . The latter can be burned in an engine as an alternative to fossil fuel.
The VCFB process involves burning the biowaste that has been fluidized by jets of hot vapor. The VCFB (Fig.1) consists of two fluidized beds - gasification and combustion chamber. Part of the semicoke reacts with the gasification agent to produce gas which in turn is passed through the cleaner to remove tar, water and particles. The clean gas is fed into the engine for power generation. The operation temperature of the gasifier is 750-800 °C. The semicoke produced in the gasifier is sent into the combustor for burning to heat the cold circulated solid into a high temperature one that provides the heat required for the gasifier. The operation temperature of the combustor is 900-950 °C and it is fluidized by air. The high temperature gas from the gasifier and flue gas from the combustor are cooled in the steam generator in which the saturated steam is produced to feed the gasifier or other users. A two-cylinder, naturally aspirated, water cooled, four-stroke, open combustion chamber 2135D diesel and gas engine were used for testing. Both have the same engine speed (1500 rpm), piston stroke (140 mm) and cylinder diameter (135 mm), but have a different compression ratio (16.5 and 11) for CI and SI running. The value could be decreased to the value of 11 by reducing the height of the diesel piston to 7.2 mm. Thus, the height of the "Omega" combustion chamber inside the piston will have a value of 12.8 mm (as compared to 20 mm for diesel engine piston). The ignition timing of the gas engine was 17 CA before top dead center (BTDC). The engine was connected to the hydrodynamometer for loading and torque measurement. The engine speed, gas and fuel consumption, exhaust and inlet temperature were determined by using the corresponding instruments. An external blower forced gas into the combustion chamber through the engine manifold at a fixed flow rate of 25N/h. Gas composition was detected by a GC/MS (Gas chromatograph/mass spectrometer) system.
Fig. 1 Flow Diagram of VCFB System
The analysis of biomass (rice husks) gas from a VCFB system shows that the gas components by volume percent are: 40% CO, 29% CO2, 13 % H2, 14 % CH4, 3 % N2, 1 % CH.4.. The rich active reagents, less tar and higher LHV 10-11.7MJ/N m³(as compared to 4.98 MJ/N m from fixed bed producer with the same biomass) is best for combustion in SI gas engine 
The range of biomass AG ratio of 2.50-2.81 was capable of achieving minimum ignition energy and maximum flame propagation velocity. A lean pattern of air-gas combustion can be realized in SI gas engine.
Fig. 2 Improved Combustion Chamber
The combustion process at open chamber is very sensitive to homogeneity of the mixture which depends entirely on air motion. Attempts were made by this author to consider both by swirl (preliminary) induced by inlet mixture and squish (secondary). The latter is a compression induced air motion caused by a small clearance space at top part of the piston. Three kinds of improved combustion chamber were designed to strengthen the squish effect (Fig 2). The depth of No.1, No.2, No.3 combustion chamber designs was 30 mm, 27 mm, and 28 mm respectively. Engine performance was conducted upon two cylinders but running on a single cylinder to save gas. Tests which were carried out under the same compression ratio 11 indicated that all the improved designs were successful. An example of No.1 design is given below. Three spiral form inclines from the top to the bottom part of the combustion chamber inside the piston were uniformly arranged around the cycle of the No.1 combustion chamber. Evidently, rotary direction of spiral inclines should be coincide with those of the induced air motion. Smooth running at the speed of 1500 rpm of the gas engine with No.1 combustion chamber is obtained. Nevertheless, in the gas engine with the original design sharp detonation appears at a speed of 1000 rpm. Improvement in energy consumption Q and thermal efficiency Ç e when using gaseous fuel at heavy load was shown in (Fig 3). The Optimal Value of Q of No.1 design was 4.81 J/w-s, comparing to 5.2 J/w-s of the original one, but conversely at light load. The change of thermal efficiency Ç e has the same trend as that of the optimal value Q.
Fig.3 Performance Curves for the Original and Improved Combustion Chamber of the gas engine
The following concepts are drawn from tests with favorable modification on combustion chamber:
1. The high LHV and lower tar of the gas made by using steam with less N2 instead of air, as the gasification media in a VCFB system is best for combustion in internal combustion engines.
2. Gas engine with original combustion chamber exposed a drop in brake parameters as worse quality of mixture and detonation at heavy load. Mixture formation induced by a new design combustion chamber is accelerated effectively by squish velocity. Optimal Q and thermal efficiency Ç e are well demonstrated at heavy load.
3. It seems advisable to further explore any possibility of rising Î of the present design and to make the favorable operation over a broad range of engine torque levels. Similar tests in gas engines with higher Î have been made by some researchers. The following are two examples with different fuels. A test result of a S-195 biogas engine with a modified swirl chamber is of Î value in 12-13. G.A.Karim et al report that rational value of Î is in the range of 11.5-15.5 in a CFR gas engine using lean combustion of CH4, C3H8 fuel at equivalent ratio 0.50-1.30 .
This research was conducted in the internal combustion engine laboratory of ZheJiang university. Prof. Xia LaiQing of ZheJiang University gave his continuous support and guidance throughout my graduate studies and research. I deeply appreciate Prof. Zhang BaoZhao of the China National Rice Research Institute for his valuable advice and constructive comments which aided me in completing this paper.
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