In nature we often speak of symbiosis. In its most virtuous form, this occurs when an organism or an entire species associates with another to benefit from the resources made available by nature in a mutualistic way.
The two symbiotic organisms, in addition to drawing directly from nature a part of the resources that each one needs, reciprocally exchange others in such a way that the overall effort made by the two to supply themselves is less than the sum of the efforts they would have had to make if they had acted individually. Mutualistic symbiosis is the way in which nature spontaneously makes "efficiency".
This concept is obviously applied actively and consciously by man in the many realities that make up the civil society, in the many production sectors and, in particular, in the industrial sector. The paper industry also presents various opportunities for mutual symbiosis or virtuous integration with other industrial sectors or services. Consider, for example, the fact that, since paper is a "recyclable" asset in many of its forms, the paper mill can receive raw material from the community through the separate collection service and provide the community itself with the paper for its multiple uses.
Industrial integration types
In the case of paper mills, integrations with other types of plants can be classified into two main types: those for energy purposes and those for the supply of raw materials.
As for the first case, we will begin by saying that the paper mill is a large user of electricity and heat. The paper industry in fact absorbs around 10% of the energy produced in the world, and is second only to that of steel. Much of the thermal energy is transferred to the machine in the form of steam, mainly for multi-cylinder continuous machines, or high temperature gas, in the case of Tissue paper plants.
The fundamental property of steam is that of being able to transport large amounts of energy in the form of latent heat and to receive or transfer this energy quickly and efficiently through changes of state, respectively evaporation and condensation. In order to take full advantage of these properties, paper machines are designed to operate with saturated steam which, by condensing in the drying cylinder batteries, releases the thermal energy needed to dry the paper. The paper machine is therefore a condenser par excellence.
Electricity, steam and high-temperature gas are the same fluids that we find in thermoelectric power plants. As is known, these plants have a physical need, dictated by the unavoidable principles of thermodynamics, to dispose of a quantity of thermal energy equal to at least the electricity produced. This energy is actually a process "waste", typically in the form of saturated steam, which must be dissipated in some way in order to condense the steam itself and ensure that the resulting condensate can resume the path of the next thermal cycle. The thermoelectric power plant therefore needs a condenser. Using the environment (the atmosphere, the river or the sea) as a condenser is in effect a waste of energy and damage from the point of view of environmental sustainability.
Here, by combining the conclusions of the two previous paragraphs, one senses the opportunity of integrating the paper mill with the thermoelectric power plant.
Methods of integration and mutual benefits
This integration can take place in two ways:
- By building a paper mill near a thermoelectric power plant
- By equipping the paper mill with its own thermoelectric power plant that can, on the one hand, supply it with electricity and, on the other, receive back the condensate resulting from the cooling of the steam.
The latter concept, called cogeneration, takes the form of an auxiliary plant capable of simultaneously and virtuously generating the two forms of energy necessary for the paper mill, eliminating the need on the one hand to draw electricity from the grid and on the other hand to burn gas to produce steam in a boiler.
A typical cogeneration plant used in paper mills is the turbogas one. In these systems, the exhaust gases of the turbine are used in two ways:
- Directly, to produce the steam necessary to feed the multi-cylinder dryers
- In the case of Tissue plants, they are first passed through the high temperature hood associated with the Yankee Dryer and only subsequently used for the generation of the steam, which in turn is used for heating the Yankee Dryer itself from the inside.
When the paper mill is instead connected to an independent thermoelectric power plant, it receives steam from the latter with which it can directly heat the drying cylinders or, in the case of the Tissue machine, heated air to be sent into the Yankee hood.
For its part, the paper mill can supply the thermoelectric power plant with raw materials. A thermoelectric power plant of the "fluidized bed" type, for example, can use the organic residues of a paper plant as fuel, in particular those resulting from the cleaning cycles of the paper pulp and the purification of waste water. These residues can be relatively small for plants that use virgin cellulose, but they can represent double-digit percentages for plants that use recycled paper as raw material and where the waste fraction can be of the same order of magnitude as the net production of the plant. With increasingly strict environmental standards and a constant increase in costs associated with disposal, the virtuous use of these residues is in many cases an indispensable condition for the sustainability of the plant itself.
The same residues mentioned above can have more noble uses, constituting a genuine raw material for paper mills that produce medium and heavy cardboard such as coreboard, in which the organic residues dispersed in the fibrous matrix constitute an excellent low cost filler. In this way, a significant disposal cost that the transferring plant would have to bear is eliminated.
In the absence of viable alternatives, typically for historical reasons of location, the paper mill can choose to equip itself with a biomass plant for the biological digestion of its organic residues. The process associated with this type of plant produces biogas with a high energy content which can typically be burned to generate steam or to heat other process fluids.
Further examples of integration: plastic disposal and solar energy
Plastics are particular organic residues. In the paper plant, they derive from the cleaning of waste paper, especially those deriving from packaging. The plastic residues that are obtained downstream of pulping of the waste bales are not normally recyclable directly in the plastic supply chain due to their heterogeneity. A possible alternative to disposal as waste is combustion in pelletized or direct form in the boiler in order to generate noble thermal energy for use in the papermaking process. The combustion of plastic is a complex matter due to potentially harmful emissions such as dioxins; however, today there are high temperature combustion and filtration technologies that allow plastics to be burned without such environmental risks.
As for solar energy, this certainly represents the arrival point of the future energy supply on a planetary level. The collection of solar energy through photovoltaic systems involves the availability of large areas exposed to the sun and not usable for other purposes.
The paper mill, with its huge raw material and finished product storage spaces, is characterized by large surfaces, often covered, which can be ideal for the installation of large photovoltaic systems. These can also be installed above the raw material storage areas in the open, if the support points of the essential support frames have been provided. Consider, for example, that the coverage of a 10 thousand square meter raw material depot can generate 1 Megawatt of power in a sunny day, enough to run all the motors of a medium-sized Tissue paper machine.
It is therefore virtuous nowadays, when a new paper plant is designed, to provide for the insertion of a photovoltaic system on all higher level surfaces not used for other purposes from a structural, ergonomic and safety point of view.