Among the possible technologies of small scale hydrogen production, pyrolysis is the cleanest hydrogen production process without purification, but suffered from low efficiency due to the carbon by-product. By the integration of a fuel cell to the pyrolysis reactor, the present invention provide a system to produce both hydrogen and electricity, and to utilize the would-be wasted by-products of the pyrolysis and fuel cell processes to improve the efficiency. The pyrolysis unit generates hydrogen from hydrocarbon fuel input, and produce solid carbon, which can be gasified to provide fuel source for the fuel cell to generate electricity. The fuel cell further functions as a steam and heat source for the gasification of solid carbon and possibly for the pyrolysis reaction. This results in an integrated system that not only generates both electricity and hydrogen, but does so at much higher efficiency as well as with greater simplicity and lower cost.

The improved process is achieved by introducing depuration of green liquor and lime before causticization and two special causticization steps in causticization procedure of conventional chemical recovery processes. It not only avoids the generation of waste residue (lime mud) from the beginning and saves the re-burning and filling land of lime mud, but also can directly produce a series of precipitated calcium carbonate products with various particle sizes by regulating process conditions while alkali is recovered. The CaCO₃ products include CaCO₃ wherein ≧50% of CaCO₃ has a particle size below 2 μm, CaCO₃ wherein ≧90% of CaCO₃ have a particle size below 2 μm, and a superfine CaCO₃ having an average particle size <1 μm. The CaCO₃ obtained by this process can be directly used for papermaking and other industrial applications.