We frequently are asked about the fuel value of waste materials from pulp and paper mills. This question arises because, when screen rejects and knots & shives are pressed, the dry press cake that is formed can be burned instead of landfilled. The combustion can be either in a coal fired boiler or in a bark burner. Tipping fees are an important part of the analysis. A high rate, like $30/ton, would make press cake disposal more important.

The heat value of the press cake is a function of the percent moisture as well as the ash and clay content. In general, the solids will have 2,900 BTU/# “as fired”. Even when you take into account that there can be up to 60% water in the press cake, there is still some economic advantage to burning the sludge, in terms of energy contribution. That is, if you start with two pounds of press cake at 50% moisture, there will be one pound of solids burned, contributing 2,900 BTU, but there will also be one pound of water to evaporate, requiring 1,200 BTU. Thus there would be a net contribution of 850 BTU per pound of press cake.

Comparable figures are 10,000 BTU/# for coal and 5,000 BTU/# for wood. The fuel value of primary sludge, 4,000 BTU/#, is good. Coal is an ideal fuel for burning along with pressed screen rejects because it burns hot and has low moisture.

Most commonly the coal fired boiler will use one of two types of grates for burning press cake (rejects, sludge): Rotograte and Vibratory. With the Rotograte there is a limit of 400º F inlet air temperature, so the amount of sludge that can be dried is limited. Blending 10% press cake with coal would be recommended.

With the Vibratory grate the inlet air temperature can go up to 600º to 650º F because the grate is water cooled. This permits mixing the press cake with the fuel up to a higher percentage. General guidelines for the mixture are 30% press cake with 70% bark or 40% press cake with 60% coal.

It should be noted that, with the low inlet air condition, the system counts on radiant heat to evaporate the water in the press cake. This cools down the furnace, causing combustion problems. Also, running up the ratio of press cake to coal leads to poor air distribution across the grate, which also leads to uneven drying and burning.

It should be remembered that if a ton of press cake is burned, there will still be 400 pounds to haul to landfill: that is 1,000 pounds of water evaporated, 600 pounds burned up, and 400 pounds of ash.
Issue 20