Installation of Belt Conveyors in Place of Pneumatic Conveying System for Wood Chips Transfer

Background

The hard wood and soft wood logs (basic raw materials) are cut into small size by band saw. After wetting these logs with water spray to remove sand and dirt particles, they are fed into chippers. These small size logs are chipped to get chips of smaller size (1/2 " to 1").

These chips are then transported from the chipper house, located at the ground floor, to the top of the digester house (at a height of about 12 to 15 m), for cooking operations.

Conventionally, the chips were being transported pneumatically. The pneumatic conveying, though simple and easy to install, consumes more energy. Mechanical conveying is more energy efficient and consumes only 25 - 30% of the energy consumed by pneumatic conveying. Hence, many plants have converted their conveying systems, from pneumatic to mechanical and reduced their energy consumption. The existing layout of the chipper house and the digester house, governs the alternatives of mechanical conveying systems chosen. The installation of a belt conveyor can be taken up in those plants, where the horizontal distance between the digester and the chipper is sufficiently large. In case the horizontal distance is less and the inclination of conveying required is more, then a belt conveyor will not be suitable. In such cases, modified systems, such as, the cleated belt conveyors have been installed with good benefits.

This case study describes both the mechanical conveying alternatives, which have been implemented in two different paper plants in the country.

Previous status

Plant - A In an integrated paper plant, manufacturing paper & paperboards, the wood chips from the chipper house to the digester house, were transported using pneumatic conveying system. The horizontal distance between the chipper house and the digester house, was more and the inclination permitted the installation of a belt conveyor.

Plant – B In an integrated paper and paperboard plant, the wood chips from the chipper house to the silos and from the silos to the digester house, were transported using pneumatic conveying system.The horizontal distance between the chipper house and the digester was less. Also, the inclination of conveying was more, than what a conventional belt conveyor could accommodate.

Energy saving project

Plant – A The plant team replaced the pneumatic conveying system with mechanical belt conveying system.

Plant – B The plant team replaced the pneumatic conveying system with mechanical conveying system. As the inclination of conveying was more, the belt conveyor was not suitable. A modified belt conveyor called the 'cleated belt conveyor' was used for the conversion. This modified conveyor has 'cleats' on the surface of the belt and these cleats physically hold the chips from sliding down.

Concept of the project Pneumatic conveying is highly energy intensive and is at-least 3 - 4 times costlier than mechanical conveying. The latest trend among all the paper plants is to opt for mechanical belt conveying systems.

Implementation status, problems faced and time frame

Plant – A The pneumatic conveying system was replaced with mechanical conveying system. Belt conveyors were installed for the transportation of wood chips. In this particular plant, designing the layout of the belt conveying system was difficult, due to the lack of adequate space. The gradient however was favourable for installing a belt conveyor. The belt conveyor was installed after compromising certain minor factors, such as reduced passages and corridors. The project was implemented in 8 months time, once the layout was decided.

Plant – B In this particular plant, designing the layout of the belt conveying system was difficult, due to the lack of adequate space and steep gradient. Hence, cleated belt conveyors were installed for the transportation of wood chips. In the initial days of operation, problems were faced on account of the failure of pulley and belt. The supplier studied these problems and the design was improved, to avoid recurrence of these problems.

The project was implemented in two phases as below:

The cleated belt was installed, for chips transfer from the chipper house to silo, in the first phase. After successful commissioning and operation of the system, the second phase was taken up for implementation. During the second phase, the cleated belt was introduced for chip transfer, from silos to digester house. The implementation of each phase of the project took 6 months.

Benefits achieved

Plant – A There was a reduction in power consumption of 3868 units per day (equivalent to 193.4 kW per day), on installation of belt conveyors. Plant -B The summary of the benefits achieved is as follows:

Parameter	Units	Chipper to House of Silo	Silo to House Digester
Annual operating hours	hrs/yr	5600	4900

Average power consumption with pneumatic conveying	kW	77.00	63.25
Power consumption of cleated belt conveyor	kW	11.72	4.60
Power savings with the conversion	kW	65.28	58.65

Financial analysis

Plant - A (Belt conveyor) The annual energy saving achieved was Rs.19.59 lakhs. This required an investment of Rs.37.50 lakhs, and was paid back in 23 months.

Plant -B (Cleated belt conveyor) The annual energy saving achieved was Rs.38.30 lakhs. The investment made was Rs.75.00 lakhs, which was paid back in 24 months.

Benefits of installing belt conveyers

Lower power consumption -25% of pneumatic conveying power Belt conveyor for chips conveying Cleated belt conveyor for chips conveying.

Cost Benefit Analysis

Annual Savings - Rs. 38.30 lakhs with cleated belt conveyor

Investment - Rs. 75.00 lakhs

Simple payback - 25 months