RISI

Managing the overlap - Part II

By Mitch Morgan Mon, Jan 14, 2013
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BRUSSELS, Jan. 14, 2013 (RISI) -Energy has historically been a major component of an integrated pulp and paper mill's overall operating cost structure. As energy costs have rapidly escalated over the last five years, how well a mill positions itself to manage this spend can be a major determinant in its long-term viability.Click here to read Part I.

Process benefits: Burnt toast

Once the Nalco team has developed a comprehensive understanding of the utilities area through thermodynamic modeling and the benchmarking exercise has been completed, attention then shifts towards the mill's pulping and papermaking processes.

A team comprised of Nalco process experts and mill staff utilize this information to develop hypotheses regarding the utilities area and how any changes within the utilities might impact the process area. Often the process benefit of a change initiated in the utility area can be much greater than the energy benefit.

Conversely, the Nalco team is able to review and assess the process area against their internal benchmarks and field experience from other mill operations and suggest changes to improve the process area. The energy impact is then calculated using the TM. An example of this approach is discussed later in this paper.

In one EO project an analysis of the liquor cycle indicated significant opportunities. For a karft pulp mill, the concentrated black liquor represents a large and inexpensive energy source. The organic portion of black liquor is combustible and provides energy to produce steam in the recovery boiler, which in turn helps power the mill. The inorganic portion is noncombustible and represents the pulping chemicals that the mill recovers to recycle as pulping liquor. The higher the organic proportion, the higher the heating value of the black liquor fuel. The data analysis found that the higher heating value (HHV) averaged 5,852 BTU/lb. When compared with other similar mill operations it was noted that an HHV of 6,000 BTU/lb should be a conservative target. There are two main reasons for this reduction in thermal capacity of the returning black liquor. The first is that the recovery furnace is not reducing the sulfur as efficiently as it should be. The result is that a higher proportion of inert sodium sulfate is recirculating with the liquor, Fig. 4.

Figure 4 - Liquor cycle: Lower recovery furnace reduction efficiency leads to increased Na2SO4 or deadload in the returning liquor

This slight depression and liquor heating value means that other purchased fuels must be used to provide the mill's thermal demands for production requirements. This has the modest financial impact of reducing the amount of wood fuel purchased in the summer months but has a much more dramatic impact on natural gas consumption in the winter months, Table 1.

Table 1 - The purchased fuel impact of a depressed liquor heating value

Further liquor cycle process analysis indicated that the causticizing efficiency was 4- 6% below industry benchmarks, Fig. 5. This is expressed as sodium carbonate which is part of the inorganic smelt of the recovery furnace not being converted to working sodium hydroxide needed for pulping. As with the sulfur situation described earlier, inert sodium carbonate remains in the white liquor supply in the pulping area and the recovery furnace and is not helpful to either.

The presence of the sodium carbonate deadload not only depresses the HHV of the black liquor but also combines with sodium sulfate to form burkeite scale in the evaporators.

Scale formation in the evaporators hampers their ability to produce high solids black liquor for combustion in the recovery boiler. Liquor moisture content is the single largest factor of the many factors influencing recovery boiler efficiency. In this particular case, it was determined that existing liquor solids averaged 67.5% solids as compared with an industry benchmark of 72%.

The projected financial impact of these identified process improvements was in excess of $3,500,000 per year.

Figure 5 - Liquor cycle: Lower causticizing efficiency leads to increased Na2CO3 or deadload in the returning liquor which depresses the black liquor HHV

Conclusions

Optimizing an integrated pulp and paper mill's energy costs is a complex endeavor requiring a multitude of technical skills from a variety of disciplines. Multiple projects have illustrated how the Nalco EO approach serves to complement both internal mill investigations and external conventional energy firm's management efforts. It is also clear that although a very similar protocol to determining energy cost-reduction is used, the strategies and the solutions that emerge are vastly different for each mill. PPI

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