Renewable Energy Technology 3(2+1)

Lesson 10. Operational maintenance of gasifier

Reactor

Generally the reactor is made up of MS of suitable thickness to withstand the higher temperature attained during gasification.  The insulation of reactor gives significant reduction of heat from the gasification zone to atmosphere.  The perfectness of insulation has to be assessed periodically to avoid heat transfer.  The loss of heat through improper insulation not only led to heat loss but also will lead to reduced thermal efficiency of gasifier. 

The reactor is made up of mild steel in most of the cases.  It is always subjected to very high temperature of around 1200°C during gasification.  Hence, heat injury or damage in the reactor surface has to be inspected periodically for better performance of the gasifiers. 

Feeding system

Manual feeding system does not need much of maintenance.  However the automated systems like mechanical feeding like screw systems, elevator or bucket systems have to be maintained.  As these systems have more moving parts and also dealing with higher temperature of the reactor zone and also the dusty biomaterials.  Proper greasing and lubrication of the system has to be carried out.  The joints, hinges, ropes, chains, belts etc have to be checked.  They not only under perform during loading but also will cause severe injury to the persons working with the gasifier.    

Air flow system

Air flow has significant contribution in the generation of quantity of producer gas and also the quality in term of energy content of producer gas.  Hence, it has to be assessed whether the air supply is proper during gasification.  Hence the performance of blower can be assessed to determine the quantity of air supply to the gasifier.  The speed has major role in deciding the air flow rate of the blowers.  The air flow rate of the blower has to be measured against the specified blower speeds and has to be checked with the manufacturer’s specification.  The pressure drop against various air flow rate of blowers has to be checked and compared with standards.  Id the deviation is more the inner surface of air supply system (pipes) have to be checked for blockage due to condensed tar, particulate matters etc. and may be cleaned for smoother air supply.         

The damage in the air supply system or tuyers are predominant as these systems are in contact with oxidation zone of gasifier whereas the temperature will be about 1200°C.  If the damage is beyond repair immediate replacement has to be made to maintain proper air supply.  The blockage of the air supply system also has to be considered as the biomaterials will release more tar during gasification process.  After closing of gasification process the residual tar will be condensed in the surface of the reactor as well as the opening of the air supply system.  Hence, the blockage has to be removed periodically for proper air supply to the gasification system. 

Grate

Grate separates the ash, producer gas and feed material.  This is the part exposed always in the higher temperature at about 1500°C.  Though it is made up of cast iron, they will be damaged easily due to not only the higher temperature but also the chemical constituents of the gas and ash due to the varying chemical composition of the feed materials.  Hence,

Gas out let

The gas outlet system should consider the direction of flow of producer gas.  Feed materials with higher moisture content or tar release are used for gasification and if the producer gas passes though the materials, the quality of the gas will be affected.  Hence, the gas produced after reduction zone has to be immediately taken out for further applications.      

Gas cleaning system

The outlet producer gas consists of particulate matters, tar particles, moisture as impurities.  For thermal applications the impurities may not have much problem.  However, the tar can be removed easily after condensation in the outlet pipe during its transit.  For engine applications, the gas should be free from all the foreign materials, otherwise it will affect the performance of the engine and further that will damage the engine. 

Hence, the gas cleaning systems are employed at different stages to remove the impurities.  They are:  

Coarse particles are separated from the gas stream in a high efficiency cyclone separator. Entrained particles in the producer gas are collected in a separate bin at the bottom of the cyclone assembly and can be cleaned periodically.

Intermediate size particles and most of the fines are removed from the gasifier in an impingement separator. This separator is insulated so as to avoid condensation of vapours and to allow passage of hot gas (above 150°C) to the glass fibre cloth filter.

To guard the engine against any solid material, the product gas is passed through a glass fibre cloth filter, which removes any solids that have slipped through the previous filter sections.

The cooling section consists of a tubular heat exchanger. Condensate can be tapped from the cooler by drain vessels, equipped with safety taps. Cooler outlet temperatures vary between 30° and 50°C, depending on environmental conditions (temperature, wind) and engine load.

Starting up of gasifier

Initially add charcoal over the grate for about 10 cm depth.  The feeding system may be filled with the fuel materials.  Start the blower by adjusting the flow rate valves to allow the calculated amount of air supply in to the system.  Open the ignition port of the gasifier and ignite using gas flame.  After a few minutes of smoky exhaust, test the gas for consistent burning.  If the gasifier is used for running the engine, the blower may be stopped and the engine can be started.  This will facilitate the sucking action from the engine for required air supply in to the gasifier.  If the gasifier is used for thermal applications, the burner can be ignited with continuous supply of air using blowers.

Operational aspects

Refill the feeding system before it becomes completely empty.  Periodically shake down the ash from the grate.  Periodically drain water from the cooler. 

Shutting down of gasifier

For shutting down the gasifier, completely close the air control valve.  The lid of the fuel supply system or gasifier has to be closed fully to avoid any further air entry. 

Routine maintenance

Regular checking of the welding and fitting portions for air tightness or leakages is an essential maintenance. 

Daily maintenance

Complete removal of ash by shaking or any other method from the gasifier for easier starting and smoother operation during the next run. 

Residual unburnt fuel material has to be removed immediately after completion of the gasifier operation to avoid further combustion of the material and also wastage of fuel materials.

Drain out eh tar condensate in the gas pipe line and also in the filter or cleaning units.

Weekly maintenance

            Clean the reactor drum if necessary

            Clean the feeding system

            Filter system

                        General cleaning

                        Replace the filter materials inside the filter

                        Ensure air tightness on filter pipes and filter lids

In addition to the above maintenance, the following occasional maintenance such as once in a fortnight or every 30 hours of operation may be carried out.

            Checking up of all the pipe lines of the gasifier and to the accessories for air tightness

            Check the connections with the producer gas appliances with burners or engines

            Check for rusting or damage in the reactor and piping

            Check the thermal insulation in the system