Gasification carbon conversion

The Shell syngas contains about 80-83% of the energy in the coal feed (the cold gas efficiency). This high efficiency is due to the high carbon conversion (>96%) during gasification (Tables 3.4 through 3.6).5... 

These simple and nearly ideal equations reveal that the conversion of sugars into alkanes or methanol proceeds at best with carbon and energy efficiencies of 50% C or 75% LHV. Any yield loss in the gasification of conversion step results in further drop in efficiency. Reaction (6) also reveals that the oxygen is removed from the biomass, 2/3 as C02 and 1/3 as water. 

The char yield in a gasification process can be optimized to maximize carbon conversion or the char can be thermally oxidized to provide heat for the process. Char is partially oxidized or gasified according to the following reactions ... 

By the process of carbonisation of biomass and subsequent gasification of the volatile products from carbonisation, a portion of carbon element in biomass is stabilised as solid carbon, and the remaining portion of carbon is converted by gasification and conversion process to synthetic fuels, which can replace fossil fuels for energy supply. 

The specific char surface area, Sc, the char porosity, e, and the effective diffusivities, De vary with char conversion and thus have to be determined from a model of the pore structure evolution Various models can be used for that purpose [10-12]. We chose to use Gavalas s random capillary model [12,13] to describe the widening of the pores and the eventual collapse of the char structure This model provides exact expressions for Sc, e, and De in terms of a local carbon conversion, q(r,t), which represents the length the pore surface has retreated at time t due to char gasification, i.e. the pore radius at the radial coordinate r and time t the initial pore radius + q(r,t). The conservation equation for this local carbon conversion takes the form ... 

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

Gasification. The only study on gasification kinetics of Texas lignite has been performed by Bass (23). Using a differential reactor, he obtained rate data at 700°C and for pressures ranging from 61.6 to 225.9 kPa. Rate equations as a function of steam partial pressure and carbon conversion were developed. 

Kurkela E. Stahlberg P, (1992) Air Gasification of Peat, Wood and Brown Coal in Pressurised Fluidized Bed Reactor. I. Carbon Conversion, Gas Yield, and Tar Formation. Fuel Process. Technol, 31, 1-21. 

The energetic performance of a CASST/combined cycle (=CASST/CC) systan has been compared with an air-blown gastfication/cmnbined cycle (=ABG/CC) system. At a carbon conversion of 80% in foe diarcoal gasification step, the electrical efficiency of the CASST/CC system is lower than for an ABG/CC syst n, 39,0 vs. 41.7%... 

LHV). Increasing foe carbon conversion in the charcoal gasification step to 95% almost eliminates the difforenoe in the performance of both systems. 

Not only the tar cracking will be affected by the flow pattern and mixing, but also the carbon conversion is expected to be influenced by the local gas concentration differences. At CFB gasification conditions (T=850 C - k.=0.30- t=1 [s ), combustion is the dominant mechanism for carbon conversion, because carbon gasification reactions are too slow at the prevailing conditions Hence, good contact of oxygen and caibon is required. 

Of course, also other aspects of pressurised fluidised bed gasification were studied, like tar composition in the generated low calorific value (LCV) gas and carbon conversion. The ignition of the LCV gas during pressurised combustion of the LCV gas was also of interest. Besides the fuel, the main process parameters were the applied air stoichiometry and operating pressure. 

Further research work is necessary to extensively determine the influence of the biomass fuel type used and synergistic effects of brown coal/coal as additive to biomass with respect to Carbon conversion, tar formation and NO, precursor formation. Also, gasification modeling is further optimised and validated with these and more experimental data to describe the basic processes of drying, devolatilisation, partial combustion and gasification. 

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