Combined cycle gasification of the black liquor waste stream generated in pulp and paper mills offers the potential for more efficient recovery of the energy content of the stream as well as a reduction in emissions and operation of a system without the potential for molten smelt - water explosions. Many companies and organizations have studied, proposed and/or developed black liquor gasification systems. Two of these have been taken to the stage of implementation in operating North American mills. The lower temperature process is being employed at two semi-chem mills; one in Big Island, Virginia, and one in Trenton, Ontario. The higher temperature process is being used at a kraft mill in New Bern, North Carolina. For both processes, the performance of containment materials has been a serious issue. With the higher temperature process, degradation of both refractory and metallic components are a cause for concern. This paper describes the refractories and alloys used in this gasifier, laboratory studies to identify improved refractories, and real-time measurements to monitor the expansion of the refractory. The work has led to the selection of materials that should give considerably longer lifetime than the previously used refractories.
The predominant paper-making process in North American mills is the kraft process in which an aqueous solution of sodium hydroxide and sodium sulfide is used to effect the fiber separation. The Tomlinson boiler is an essential component of a kraft mill because it generates the steam necessary for operating the mill, and it allows the mill to recycle the pulping chemicals thus avoiding the expense of replacing the pulping chemicals and preventing damage to the environment if the spent chemicals were discarded. This boiler, generally referred to as a black liquor recovery boiler, processes the concentrated waste stream containing the spent pulping chemicals and the unused organic components of the wood. Burning the organic components produces enough steam to satisfy the mill?s requirements as well as to generate more than half the electrical power required for operation of the mill. However, the boilers are relatively inefficient with respect to steam and power production, and power, and they emit relatively high levels of pollutants. In addition, the recovery boiler is the most expensive component of a kraft mill, and the boilers have serious safety issues if the cooling water in the tubes that form the vessel walls comes in contact with the mixture of molten salts, generally referred to as smelt, that accumulates on the floor of the boiler.
Many paper companies, most recovery boiler manufacturers and a few other organizations have investigated alternatives to recovery boilers. Black liquor gasification is one of the most widely studied alternatives, and a paper by Whitty and Baxter provides a good summary of the many approaches that have been pursued. Thorough economic studies have 1 been conducted; two reports by teams led by Larson have shown that combined cycle black liquor gasification (CCBLG) has the potential for more efficient recovery of the energy content of the black liquor. In addition to more efficient energy recovery, black liquor gasification has 2,3 a potential for reduction of emissions, lessening or elimination of the molten smelt-water explosion issue and, potentially, having a favorable rate of return on the incremental investment.
As described by Whitty and Baxter, two of the gasification processes have been 1 developed to a greater extent, and these processes are being utilized in three North American mills. Two mills, one in Virginia and one in Ontario, that operate on the semi-chem pulping process have