Several studies have been published on steam gasification using CaO as sorbent. Acharya et al. (2009) worked on the hydrogen production from sawdust using steam gasification and CaO as CO2 sorbent in bubbling fluidized bed reactor. Furthermore, they proposed the regenerator along the gasifier for the regeneration of calcium carbonate in the system. They predict through experimental setup around 71 vol.% of H2 with 0 vol.% of CO2 at 853 K, steam/biomass ratio of 1.0, and Ca/C ratio of 1. They also proved that using CaO as sorbent the purity of hydrogen increased more than 30 vol.% compared to the process without CaO. Moreover, CaO not only cap­tures CO2 from the system, but also increases the efficiency of the system due to the exothermic nature of carbonation reaction as follows.

Carbonation reaction

CaO + CO2 ® CaCO3 — 178.3kJ / mol

They also reported that in steam gasification with in situ CO2 capture, the water gas shift reaction moves in the forward direction due to the low partial pressure of CO2 in the system, as CaO absorbs the CO2.

Pfeifer et al. (2009) used duel fluidized bed gasifier to study the effect of CaO on the product gas composition from biomass steam gasification. The hydrogen content in the product gas achieved 40 vol.% without CaO, but with the CaO the hydrogen content increased to 75 vol.%. They named this concept as “absorption enhanced reforming—AER concept.”

Furthermore, they presented a simplified flow sheet for power generation using AER process for 100 kW at Vienna University of Technology, Austria.

Guoxin and Hao (2009) studied hydrogen production using pine tree sawdust as wet biomass in quartz reactor. They investigated the effect of temperature, Ca/C ratio, and the moisture content of the biomass on hydrogen production. They pre­dicted that the CaO not only acts as sorbent but also acts as catalyst. Furthermore, CaO has strong impact on watergas shift reaction rather than steam reforming of methane. Moreover, the high temperature is not in favor of carbonation reaction. They reported that the optimum temperature for biomass steam gasification with CaO as sorbent is 923-973 K. Their results showed more than 55 vol.% of hydrogen in the product gas at 923 K with Ca/C ratio of 0.5.

Acharya et al. (2010) have reported biomass steam gasification using sawdust as biomass and CaO as sorbent. They investigated the effect of variables (temperature, steam/biomass ratio, and CaO/biomass ratio) on the hydrogen purity and hydrogen yield. They predicted 54.43 vol.% of hydrogen at 943 K, steam/biomass ratio of 0.83, and CaO/biomass ratio of 2. Furthermore, they have reported that hydrogen yield increased by increasing temperature.

Han et al. (2011) studied on biomass steam gasification in the presence of CaO. They investigated the effect of temperature (762-1,013 K), steam/C ratio (1.2-2.18), and CaO/C ratio (0-2) on the hydrogen purity and yield. Taking sawdust as biomass they performed experiments in the fluidized bed gasifier.

They reported that all three factors, i. e., temperature, steam/C ratio, and CaO/C ratio, are in favor of hydrogen production. The addition of steam along with CaO is in favor of more hydrogen as it shifts the thermodynamic equilibrium of char gasification and water gas shift reaction to product side. They have predicted the maximum hydrogen concentration 62 vol.% with yield of 72 g/kg of biomass at 1,013 K, steam/C ratio of 2.18, and CaO/C ratio of 1. In addition, they observed that carbonation reaction temperature range is 753-1,043 K best for the gasifica­tion process in order to get more pure hydrogen by absorbing CO2 from the system. They reported that within these temperature ranges not only the carbonation reac­tion moves in forward direction but also water gas shift reaction moves to product side due to the lower partial pressure of CO2 in the system. In addition the results showed that by increasing temperature H2 and CO2 increase while CO and CH4 decrease.

A detailed comparison of the literature based on the operating conditions, optimized parameters, and results based on optimum conditions is given in Table 19.1.