Energy Utilisation in Kraft Pulp Mills

Abstract

In recent years, the operating environment of the pulp and paper industry has become more challenging because of uncertainties regarding the price and availability of energy. Energy provides a considerable potential to reduce operational costs. There are basically three ways to improve the energy efficiency of a pulp mill; by decreasing the energy consumption of pulping processes, increasing the energy production of the recovery boiler and power boiler or improving the energy procurement. This thesis concentrates on reviewing pulp mills' energy consumption per ton of pulp produced.

The aim of thesis was to define the energy consumption of a modem pulp mill and estimate the limits within the consumption vary in different departments. In addition, the variables affecting the consumption for each department are discussed. Energy consumption data from books, reports, Pöyry Forest Industry's internal database and data obtained by interviewing experts are combined into figures for illustrating the consumption in selected mill departments. The actual mill data consists of 26 mills, which are located in Europe, South-America and North-America.

In general, the energy consumption of pulp mills has decreased significantly since the 1970s. In some departments, the consumption has, however, increased. Modern recovery boilers consume more steam and electricity because of increased preheating and higher feedwater pressure. The excess energy generated is manifold compared to the increase in consumption. Many environmental technologies such as electrostatic precipitators and aerated waste water treatment plants, also increases the demand for electricity.

A departmental-level comparison between mills can be misleading, because the boundaries between departments are vague and not well defined; departmental consumptions may be combined or they may include additional equipment. This applies in particular to electricity consumption, because electrical switchgear centres are not department-specific. Hence, it is important to know the reasons behind the energy consumption and understand how changes in energy consumption in one process area affect other areas. Departmental-level improvements may result in drawbacks at mill level.

The median heat consumption for the selected mills is 11.9 GJ/ADt and the median electricity consumption 667 kWh/ADt. In a new pulp mill, the heat consumption is lower (under 10 GJ/ADt), but the electricity consumption is about the same (670 kWh/ADt).