Components of a complete FastOx system

Sierra Energy’s FastOx gasification technology is the central component of a waste-to-value system. A full system includes the gasifier vessel, injection lances, oxygen generation, and gas cleaning down to end product requirements.

A system using FastOx gasification can produce a variety of end products including electricity, diesel, hydrogen gas, ammonia, and combined heat and power (CHP).

A complete system using FastOx gasification involves several processes, including feedstock preparation, gasification, syngas cooling & conditioning, and product conversion. The process flow is depicted in the diagram below.

These components can be summarized as component isles: preprocessing, gasification, additional gas cleaning (if needed for back end system), utilities sub-system, and back end system.

Continue reading for a more detailed description of the complete gasification system.

Gasification Process Overview

Preprocessing

After waste materials are delivered to the site, they are sent to a shredder and sized appropriately for the diameter of the gasifier (for more information, please see Feedstock). The FastOx gasifier requires minimal pre-treatment of feedstock because it can handle high-moisture contents (up to 50% by weight.) Competing technologies, such as fluidized bed gasifiers and other downdraft gasifiers, require extensive feedstock handling processes to dry the waste streams to a maximum of 10% to 20% by weight.

Gasification

After the waste is shredded, it is fed into the top of the FastOx gasifier via conveyor belt. At the top of the gasifier, the waste is held in an airlock chamber prior to discharging into the vessel. The residence time of feedstock is controlled by the supply of injected oxygen and steam at the bottom of the gasifier. By injecting oxygen, FastOx gasification increases efficiencies and avoids many of the environmental impacts associated with iron-making blast furnaces, which commonly inject nitrogen-rich ambient air.

Next, the waste descends through the gasifier by gravity through four reaction zones: drying, devolatilization, gasification, and melting. The process produces high-quality syngas, liquid metal, and inert stone. The syngas rises up to the top of the vessel and moves on to the syngas conditioning process, while the molten liquid metal and vitrified stone descend to the bottom of the gasifier.

Gas Cleaning and Conditioning

Once it has left the gasifier, the syngas enters into a polisher that converts any trace organic compounds into additional syngas. Next, a waste-heat boiler (heat exchanger) removes excess heat from the shifted syngas and creates high-pressure steam. The steam is recycled back into the steam isle and re-injected into the gasifier. The steam is continuously controlled to provide the right ratio of hydrogen to carbon monoxide in the outlet syngas. Syngas then goes into the gas-cleaning isle that removes all particulates and gaseous contaminant matter to meet the requirements of subsequent syngas-to-product conversion processes.

Back End Processing

At the back end of a FastOx gasification system, the clean syngas is further processed to produce high-value end products, such as electricity, biodiesel, hydrogen, and RNG. To learn more about the possibilities of our back end syngas processing, visit our valuable end-products page.

Preprocessing for Gasification Plants

The purpose of the preprocessing subsystem is to adequately condition waste in preparation for gasification. FastOx gasifiers accept nearly any kind of waste (view acceptable waste types here). Our systems can also handle upwards of 50% (by weight) moisture content in the incoming feedstock.

Sierra Energy generally aims for its gasifier feedstock to contain 20% moisture by weight and size according to the dimensions of the gasifier. For example, feedstock for a 10 metric tons per day unit should be less than 2 inches in size.
Given that waste streams are as diverse as the regions they come from, preprocessing requirements will vary widely. However, most preprocessing systems will operate in three stages – separation/sorting, processing, and storage and metering.

Separation & Sorting

The separating/sorting stage adequately sorts waste that is either recyclable, not ideal for the gasification process, or needs to be treated prior to gasification. The isolation of undesirable waste will make the gasification process more efficient.

Processing

The processing stage will condition the sorted feedstock to adequately prepare it for gasification. It is during this stage that waste will be physically dried to an appropriate moisture content and shredded to an appropriate size. As such, examples of equipment present in this stage of preprocessing typically include, at minimum:

• Pre-Drying – Rotary-Kiln dryers provide a low-maintenance and consistent method of obtaining an optimum feed material content using waste heat from the plant to pre-dry the high-moisture waste materials.
• Shredder – Shredders mechanically shear the material to less than a system-specified nominal dimension (typically no less than 3”).

However, more complicated waste streams, such as those that contain hazardous materials or medical waste, may require additional measures to ensure proper handling of these materials, which may require additional equipment. These additional measures are primarily to ensure the safety of the personnel handling the waste and are not necessarily required for the actual gasification process.

Storage & Metering

The storage and metering stage physically transports waste from storage containers into the gasifier and accurately monitors the feed rate.  Waste will be stored appropriately in a manner that is safe, adequately sized given the waste acceptance and processing capacities, and does not subject waste to conditions that would require additional pretreatment. The exact capacity of the storage will be dependent on the size of the system as well as the nature and frequency of waste being received onsite. Examples of waste storage equipment include:

• Refuse Storage Bunkers – a containing structure for the storage of waste
• Walking floors – hydraulically-driven moving floor conveyance system that facilitates and automates the loading and unloading of material and/or palletized product
• Feedstock storage hoppers – multiple storage hoppers used to store the material streams and optimized-size feedstock before entry to the gasification isle

Typically, a good rule of thumb for continual operation is to size feedstock storage for at least two days of inventory. This is to ensure continual operation in case the feedstock supplier cannot deliver the waste in two consecutive days, during atypical business hours, or during the weekend.
Waste will be moved from the storage section into the gasifier by use of conveyor belts and the feed rate of waste will be closely monitored to ensure proper and optimized operation of the gasifier and back-end processes.

Gasification Isle

The FastOx gasification isle consists of the following:

• Charging system
• Gasifier
• Oxygen and steam injectants
• Polisher
• Recuperator

Charging System

The purpose of the charging system is to feed waste into the gasifier. As the feedstock ascends the conveyor belt from the preprocessing section to the gasifier, it first enters a weigh hopper that collects waste up to a certain quantity. Once that quantity is reached, the weigh hopper releases waste into the airlock on top of the gasifier vessel. This airlock has three valves to monitor and govern the addition of waste into the gasifier, as well as the escape of syngas out of the gasifier vessel. The “flap” valve, which is the closest to the gasifier, is the ultimate gateway for feedstock into the actual vessel. The top-most valve, known as the upper slide gate, seals off the waste from the ambient environment. In-between these is the lower slide gate valve, which acts as a fail-safe precaution.
The flap valve is closed while the weigh hopper fills. Once the weigh hopper is full of feedstock, the conveyor stops and the lower slide gate opens. Once the lower slide gate is confirmed to be open, the upper slide gate opens, and the weigh hopper releases waste into the airlock. The upper and lower slide gates then close and seal the feedstock off from the ambient environment.
Once the gates are confirmed to be closed, the airlock is purged with nitrogen gas. Next, the flap valve opens and the feedstock is released into the gasifier. As the feed leaves the airlock, syngas flows into it before the flap valve is closed. In order to prevent the syngas from escaping to the atmosphere upon the next opening of the upper slide gate, the airlock is purged with nitrogen gas. During this process, the flap valve is closed so that all syngas present flows to the onsite flare. Once the airlock is free of syngas and full of nitrogen, the upper slide gate opens to receive the next load of feedstock.

Gasifier

The gasifier vessel’s structural components are predominately composed of the metal shell, refractory, cooling plates, and taphole. The exterior shell is comprised of carbon steel. The shell’s interior is lined with refractory. The refractory is split into three sections to accommodate the various temperatures and operating conditions seen throughout the vessel. Refractory type of each zone is chosen based on that zone’s requirements.

The cooling plates are present to help regulate the gasifier vessel temperature, providing one of two temperature control methods.  The second cooling method employed is steam injection, which can moderate temperature gains caused by the oxygen injection.

Each gasifier is equipped with a taphole for the purpose of removing molten vitrified stone from the gasifier base. Simply opening the taphole allows for the inert stone to flow out due to the low viscosity of the molten vitrified stone as well as the slightly higher pressure inside the gasifier. The metals and inorganic materials separate in the collector due the comparatively higher density of the metal.

In some cases, it may be strategic to employ the use of a continuous tapper, which allows for a constant, steady state flow of inert stone from the system. The continuous tapper would attach to the taphole, and the molten inert stone products and metals would flow into a water bath for cooling. Alternatively, they could be air cooled. The best method to use will depend upon the exact nature of the project in question.

Oxygen and Steam Injectants

FastOx gasifiers inject oxygen and steam to fuel the process of waste conversion. Pure oxygen, as opposed to nitrogen-rich air, is generated on-site using a cryogenic air shift unit (ASU).

Unlike conventional gasification, which typically injects atmospheric air (composed of 78% nitrogen) that dilutes and impedes the system, oxygen and steam are the only injectants of the FastOx gasifier. This has several favorable effects on the gasification process:

• With the exclusion of nitrogen, the total gas volume inside the gasifier is significantly reduced.  This increases the partial pressure of reacting species inside the vessel and shifts the reaction equilibrium to more products.
• Lower gas volumes allow for smaller solid material sizing, which increases the total surface area and the number of reaction sites of the material. This causes an increase in reaction rates.
• Oxygen increases local temperatures. Temperatures are then controlled by the addition of steam, which also provides additional hydrogen to the system. Hydrogen–a higher-diffusing molecule–increases internal reaction rates.
• The presence of nitrogen interferes with the mass-transfer mechanisms of reactants and products. Therefore, by excluding nitrogen, a significant increase in reaction rates is observed.
• Heat energy that is generally absorbed by inert nitrogen during conventional gasification may instead be used for other endothermic reactions in the upper zones. This allows for more solid materials to be converted with FastOx gasification and increases the productivity.

Oxygen

Oxygen can be purchased or generated from atmospheric air using a cryogenic air separation unit (ASU). Unlike the plasma arc gasification technology, FastOx gasifiers release energy from the reaction between the oxygen and residual char. Thus, the only electricity required is that to power the ASU.

The purity of the oxygen depends on specific project goals such as the desired end product for the syngas produced. If the project is focused on the production of electricity, the purity of the oxygen only needs to be above 60% by volume. If renewable fuel is chosen as the end-product, the purity of the oxygen should be above 90% by volume because FT catalysts work better with minimal nitrogen or diluents in the syngas.

Steam

Steam can be self-sufficiently generated by the FastOx system by utilizing both recovered water and recovered sensible heat from hot syngas. The recovered water (moisture) is removed from the incoming waste streams in the “drying” zone. Water is separated from the syngas in the gas cleaning isle, and recycled back to the FastOx gasifier’s steam feed.

The extent of the impact of these steam-recycling methods will be highly dependent on the initial moisture content of the feedstock in question. It is most likely that the steam recycling will help reduce the amount of water being pulled from the grid, rather than replacing it all together.

Polisher

In a FastOx system syngas flows directly from the gasifier into the polisher unit.

The purpose of the polisher is to further react condensable hydrocarbons present in the syngas into additional syngas. This not only removes the possibility of the compounds condensing out in low-velocity and/or low-temperature zones (which could lead to a build up) but also increases the volume of usable syngas available for conversion to end products.

Like the gasifier, the polisher is lined with refractory materials and encased in a metal shell made of steel. The polisher is also equipped with a tapping system to remove the inert stone that will collect at its base.

Gas Cleaning & Conditioning

The gas cleaning and conditioning stage contains all of the equipment and subsystems to adequately clean and prepare the syngas to meet the specifications for the desired end-product.

Additional gas conditioning and preparation is required only for the production of hydrogen, Fischer-Tropsch (FT) liquids, and certain chemicals. This equipment is not required for the production of electricity via gas turbines or reciprocating internal combustion engines, which are already included in the system. Each end product requires specific conditions of syngas in order to process it. For example, the Fischer-Tropsch process does not permit even trace amounts of hydrogen sulfide present in the syngas stream because it will cause rapid degradation of the catalyst.

For most end products, a specific amount of hydrogen is required. To meet this requirement, syngas is moved to the gas conditioning and preparation unit after cleanup where it is compressed and mixed with high-pressure saturated steam and fed into the Water-Gas Shift (WGS) reactor where the following reaction takes place:

The amount of steam injected into the WGS reactor is continuously controlled to provide optimal hydrogen to carbon monoxide ratios (H₂/CO) in the outlet syngas. The WGS reaction is a mildly exothermic reaction and excess heat is removed from the shifted syngas with a waste-heat boiler (heat exchanger) that makes high-pressure steam.

Excess hydrogen from the post-WGS is removed via a hydrogen membrane module. The hydrogen is then transferred to a pressure swing adsorption (PSA) module for purification.

Back End Systems

After cleanup, syngas can be converted into numerous renewable energy products through the following syngas utilization technologies:

Electricity – Combined Cycle Power Plant (CCPP)

A combined-cycle power plant utilizes a gas turbine to produce electricity from a generator attached to the gas turbine shaft. This first ‘Brayton Cycle’ gas turbine recovers approximately 25–30% of the chemical energy in the syngas as electricity with a large volume of high temperature exhaust.

Installing an additional ‘Rankine Cycle’ Heat Recovery Steam Generator (HRSG) can recover and additional 15–20% of the chemical energy in the syngas to produce high-pressure steam from the gas turbine exhaust. A steam turbine attached to another generator produces a second electrical product stream. The combination of both the gas turbine and HRSG gives the combined cycle close to 45% efficiency. This is significantly more electricity per tonne of waste compared to a conventional IC engine generation set.

Fischer-Tropsch Diesel

The Fischer-Tropsch or FT system converts syngas through a series of chemical reactions over a catalyst (typically cobalt and iron) to produce sulfur-free and aromatic-free renewable diesel. The diesel we produce is twenty times cleaner than the ultra-low sulfur diesel standards in California.

When aiming for long-chain products such as renewable diesel, FT synthesis requires typical operating conditions of temperatures of 200–250°C (400-480°F) and pressures of 25–60 bar. The polymerization-like chain growth reaction results in a range of products comprising light hydrocarbons, liquefied petroleum gas (LPG), naphtha, diesel, and wax fractions. The distribution of the products depends on the catalyst composition and the process operation conditions.
In general, the discharge of the FT reactor contains byproduct water (pressurized to stay in the liquid form), low molecular weight hydrocarbons (gases), and a wide range of liquid hydrocarbons that include naphtha, diesel, and wax cuts. A three-phase separator on the discharge end of the FT reactor separates the gases, the liquid hydrocarbons, and the water. The tail-gas can be recycled to the FastOx gasifier or used to generate additional electricity on site. The almost-pure water is cooled and used within the plant and the liquid hydrocarbons continue on to the fractionator. The fractionator then separates appropriate fractions of the liquid hydrocarbons leaving the final desired product in the proper quantities.To learn more about the production and economics of diesel through FastOx gasification, click here.

Hydrogen

The production of hydrogen from syngas is one of the easiest renewable products to be formed with a FastOx gasifier as our syngas typically contains over 25% by volume of hydrogen even before the WGS reactions. To learn more about the production and economics of hydrogen through FastOx gasification, click here.

Utilities Sub-Systems

This subsystem reflects the ancillary utilities that support FastOx gasifiers. A complete FastOx gasification system may require ancillary utilities to facilitate the conversions and reactions in each isle/module. FastOx systems require the ability to place heat into specific components of the system and remove heat from others.

The following are examples of ancillary utilities that would be required at a waste processing facility, note that some of these may already exist onsite if the FastOx system were to be placed at an existing industrial site:

• Oxygen Supply – used for the FastOx gasification and polishing processes. Can obtain oxygen from either liquid oxygen, or an Air Separation Unit (ASU) which separates oxygen from atmospheric air and compresses it to the pressures required by the FastOx gasifier
• Plant Air – small amounts of compressed air are used for instrumentation of valves and for start-up procedures
• Steam system – steam is produced using recovered water from wet waste and heat recovered from the plant (for example, using water to remove heat from the syngas during the Gas Cleanup stage). All produced steam is collected and metered from a central steam isle
• Cooling System – necessary for regulating the temperatures of specific pieces of equipment (such as the Fischer-Tropsch reactor in diesel production) by use of either water or air
• Heat Supply – fuel gases are needed to supply necessary heat to specific system components such as heating vessels, drying materials, start-up procedure, etc. All FastOx systems will be designed to recycle clean syngas to completely offset the need for any fossil fuels (LPG, natural gas etc.)
• Fire Suppression System – nitrogen purging to act as an emergency shut-off
• General Safety Equipment – analyzers strategically placed throughout the system to assist with the monitoring of performance, including temperature sensors, flame and heat detectors, and gas analyzers