GB2484530A

GB2484530A - Waste treatment and electricity generation

Info

Publication number: GB2484530A
Application number: GB1017439.9A
Authority: GB
Inventors: Anthony Thomas Smith; Richard Adrian John Bennett; Malcolm Richard Bailey; Janet Milson
Original assignee: CARBOGEN Ltd
Current assignee: CARBOGEN Ltd
Priority date: 2010-10-15
Filing date: 2010-10-15
Publication date: 2012-04-18
Also published as: GB201017439D0

Abstract

An installation for the treatment of organic waste material comprises an anaerobic digester 1 for processing the waste material to produce a combustible fuel gas and a digestate slurry, a separator 4 for separating solids from the digestate slurry and an algal biomass production unit 3 receiving the liquid part of the digestate from the separator to promote growth of algae. The biogas can be used as fuel in an engine, such as a combined heat and power (CHP) unit 2, the engine driving an electricity generator. The engine also produces heat and exhaust gas containing carbon dioxide which can be supplied to the algal biomass production unit to further promote growth of the algae. Preferably, the algal production unit comprises a photobioreactor. Algae produced by the installation may be formed into pellets or briquettes for use as fuel. A method of treatment of organic waste material is also claimed. In another aspect, a method of algal biomass production comprises the burning of the biomass as fuel and subsequently extracting nutrients from the resultant ash. The nutrients are then used to promote growth of further algal biomass.

Description

INSTALLATION AND METHOD FOR WASTE TREATMENT AND

ELECTRICITY GENERATION

Field of the Invention

This invention relates to an installation for and method of treatment of waste and generation of electricity.

Background to the Invention

The use of anaerobic digestion to reduce biomass to combustible meth-ane gas, carbon dioxide and water is being economically encouraged as a means of producing environmentally sustainable energy. A by-product of this process is a waste stream of liquid digestate containing fibre that is ascribed a "fertiliser" value based on nitrogen and phosphate content. Frequently this value is not fully realised as the fibre fraction is removed, leaving a stream of water containing very dilute nitrogen and phosphorus. In many cases, particularly in sewage treatment, additional energy and chemicals are used to eliminate this nitrogen via nitrification and denitrification processes yielding nitrogen gas and effluent water of no value.

Concurrently the cultivation of crops for energy is also being promoted to help reduce the use of fossil fuels. This will inevitably lead to pressure on land availability for food production and increase upward price pressures on inor-ganic fertilisers.

The cultivation of algae has long been considered for the production of biomass (for example see "A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae", NREL/TP-580-24190, July 1998) but has only very recently started to enter mainstream thinking. Under appropriate conditions algae have the potential to fix more carbon dioxide per unit of surface area than the higher plants. To establish high rate growth algae require:-* Water * A pH optimally between 6.5 and 7.5 * Depending on species a temperature between 15 and 25 degrees centigrade * Carbon dioxide, possibly in the form of soluble carbonate or bicar-bonate * Sunlight * A source of nitrogen * Asourceofphosphate * A source of potassium * Trace elements such as iron, copper and zinc Clearly an associated cost can be placed against these items that will amount to a baseline cost of producing algae. Current concepts to minimise these costs have included locating the algal unit next to a fossil fuel power plant and capturing carbon dioxide, using a waste water stream to provide nutrients or placing the unit in the desert to get low cost land and sunlight.

Summary of the Invention

Accordingly, the invention provides an installation for the treatment of or-ganic waste material, comprising an anaerobic digester for processing the waste material to produce a combustible fuel gas and a digestate slurry, a sepa- rator for separating solids from the digestate slurry, and an algal biomass pro-duction means receiving the liquid part of the digestate from the separator to promote growth of algae.

The installation preferably also comprises an engine fuelled by the fuel gas, the engine driving an electricity generator and outputting heat and exhaust gas containing carbon dioxide, the heat and exhaust gas being supplied to the algal biomass production means to further promote growth of algae.

Another aspect of the invention provides a method of treatment of or-ganic waste material, comprising anaerobically digesting the waste material to produce a combustible fuel gas and a digestate slurry, separating solids from the liquid part of the digestate slurry, and using the liquid part of the digestate to promote growth of algae in algal biomass production.

Preferably, the method also comprises fuelling an engine with the fuel gas, the engine driving an electricity generator and outputting heat and exhaust gas containing carbon dioxide, and using the heat and exhaust gas to further promote growth of the algae.

The algal biomass is suitably dewatered to provide a fuel, which can then be burnt to generate steam to drive, through a suitable steam engine, an elec-tricity generator. Waste heat from this process, as well as carbon dioxide in the combustion gases, can also be used to promote algal growth in the biomass production.

It is further within the method of the invention to recover the ashes from the incineration stage and recover the phosphate and potassium therein to en-able it to be re-used as fertiliser to grow crops.

Other possible uses for the algal biomass are also envisaged such as the production of fuel pellets or briquettes, extraction of oils for the production of bio-diesel or other uses and the extraction of other high value compounds such as alginates.

The method of the invention suitably links the algal unit to an anaerobic digestion unit operating in tandem with a combined heat and power (CHP) unit.

This will provide the algae with most of its requirements using what are currently waste streams:-* Water * Low grade heat from the CHP to maintain temperatures in winter * Carbon dioxide from the CHP * Nitrogen, phosphate, potassium and trace elements from the di-gestate It will not provide the neutral pH and the alkalinity of most digestates must be adjusted to enable algal growth.

Sunlight can be quantified in terms of surface area of land required to produce a given number of tonnes of biomass per annum. By employing shal- low open ponds it has been calculated from theoretical, laboratory, and field-generated data that algae grown for oil production can out-produce corn by over 250 times and canola by over 30 times. The process is based on the concept of outperforming other biomass crops that could be grown by a factor of 10 to 1 thereby minimising the land requirements, whilst having as a starting point the other requirements covered by streams that are not delivering value.

Whilst some processes for the production of algae are based around the use of selected species or strains of algae that may have high growth rates or lipid contents, the process of the invention does not rely on the use of such cul-ture methods as they require sterile techniques and axenic cultures to limit the invasion of more robust native species. Productive strains of algae are well- known in the art and the person skilled in the art will select suitable strains ac- cording to the circumstances. Various strains are indentified in the NREL publi-cation referred to herein.

The invention further provides a method of algal biomass production, wherein the biomass is burnt as fuel, and nutrients extracted from the ash re- sulting from the burning of the biomass is used to promote growth of further al-gal biomass.

Brief Description of the Drawing

The Drawing illustrates diagrammatically an installation according to an exemplary embodiment of the invention.

Detailed Description of the Illustrated Embodiment

The installation comprises an anaerobic digester I of conventional form receiving one or more of sewage sludge, farm waste, biomass or process waste (for example from food processing), together with water, and producing biogas (essentially methane), which is supplied as fuel to a combined heat and power unit 2. This will typically include a prime mover, such as an internal combustion engine, driving an alternator to generate electricity for use locally or for sale to the Grid. Heat from the unit 2 can also be employed locally, but can also be supplied to an algal biomass production unit 3 to warm the water passing there-through, thereby promoting growth of the algae. The exhaust gases from the engine forming part of the unit 2 can serve as a source of carbon dioxide for the biomass production unit 3.

The digestate from the anaerobic digester 1 is essentially a slurry which passes to a separator 4, where solids are removed for use as a land fertiliser, while the remaining water, containing nitrogen, phosphate and potash is fed to the biomass production unit 3 to form a nutrient-rich feed for the algal growth.

The algal biomass produced in the production unit 3 is passed to a de-watering unit 5, the water being low in nutrients and therefore disposable. The de-watered biomass passes to a preparation stage 6, where it is formed into pellets, briquettes or the like, either for sale as fuel, or for use in a further power unit 7, the combustion producing heat to generate steam to drive a steam en-gine, which in turn drives an alternator, carbon dioxide, which can be supplied to the biomass production unit 3, and ash, which can be passed to an extraction stage 8 for extraction of phosphate and potash for fertiliser use, and silicates and other residue for block manufacture.

The production unit 3 may consist of a high intensity algal culture unit that may be a photobioreactor, that feeds to the start point of a channelled la- goon, pond or raceway into which the raw digestate liquor is fed after pH correc- tion. This concept will provide a high density inoculum of the desired algal spe-cies that has been selected for optimum biomass yield from locally available cultures. The high intensity unit may use digestate as a culture medium or may use water with the required amounts of inorganic fertiliser added to maintain the stock.

The amount of algal biomass that can be grown is limited by the avail-able nitrogen or phosphate in the digestate. The ratio of N to P will in most cases be close to the required ratio, however if either N or P is particularly low they can be supplemented using inorganic fertiliser based on an economic model. The time requirement T90 for the yield of biomass to take up between 50 and 90% (or 100%) of the available N or P, whichever is limiting, dictates the effective volume of the lagoon, pond or raceway. The lagoon, pond or raceway can be composed of one single linear system giving T9° or several parallel units that add up to T90.

Heat recovered from the CHP unit and carbon dioxide are introduced such that sunlight becomes the limiting factor for biomass production until such time as the available nutrients are depleted to the point where they limit growth rates. Figures of 50 g of algae m2d1 have been achieved by others provided temperatures can be kept within the optimum range for the algae in question.

Cellular doubling times have been reported greater than once every 24 hours.

The harvesting of the algae is done on the basis of maintaining a near optimum cell density in the system for the uptake of available light. This requires a continuous off take controlled by an algal cell density monitoring system as the water flows down the system. Water recovered still rich in nutrients is re- turned to the lagoon, pond or raceway. At the end of the process water sepa-rated from the algae is purified to the point where it can be returned to the plant that feeds the digester and possibly used as make up water, blended with final effluent and put to a recipient or used for irrigation. The algae is harvested, de-watered and dried using known technology. Once dewatered the algae may take one of two routes: 1. If it is to be used as a fuel off site it is either pelletised, for industrial use, or converted to briquettes for more domestic fuel usage. In the former as-suming the algae is not co-fired with fossil fuels or the combustion process is of such high temperature that the phosphate and potassium content of the ash be-comes vitrified the ash is to be recovered and processed for nutrient recovery.

2. If it is to be used as fuel on site it is sufficient to dry sufficiently and in- cinerate. Again assuming the combustion process is suitable the ash to be re-covered for nutrient recovery.

The nutrient recovery phase permits the process user to get two uses from the phosphate and potassium elements of the material available in the ini-tial digestate liquor whilst generating energy from the fixed carbon. The nitrogen element is lost as NOx in the combustion process.

Combustion ash has to be tested for suitability when co-fired with other biomass fuels. Certain heavy metals can be concentrated up in wood and other crops to the point where the recovery of the nutrients is compromised without their removal. However wood ash contains significantly greater concentrations of potassium than algal biomass so provides an incentive to include some.

Given low temperature combustion processes the recovery is a two stage proc-ess: 1. Potassium is recovered in the form of lye of potash using a water ex- traction process followed by a standard solids/liquid separation. The lye of pot- ash to be concentrated up using waste heat from the combustion plant to com-mercially viable levels.

2. The solid fraction is then treated with sulphuric acid to produce a dilute phosphoric acid solution, calcium sulphate, other sulphates and other acid in- soluble fractions. The solids are separated from the liquid and recycled to con- struction block manufacture. The dilute phosphoric acid is concentrated up us-ing waste heat from the combustion plant to commercially viable levels.

Given an ash created from burning algal biomass alone there is an op- tion for omitting the water extraction phase and recovering the potassium as po-tassium sulphate along with the phosphoric acid.

Theoretical model A medium size sewage works taking in sludges from the surrounding area produces 700 cubic metres per day of digestate liquor after removal of the fibre fraction. This liquor contains between 500 and 1,500 mg/I of ammonia and to 200 mg/I of P in the form of soluble phosphate. Using average figures this would be 1,000 mg/I of ammonia and 125 mg/I of P. On an annual basis this is 210 tonnes of nitrogen and 32 tonnes of phosphorus.

The nitrogen content of algal biomass on a dry weight basis is normally between 4 and 9 %. At one hundred percent uptake and efficiency this stream allows for the development of 5,250 tonnes of low protein algae or 2,333 tonnes of high protein algae per year.

Algae typically contains between 50 and 60 % carbon on a dry weight basis, so carbon capture from carbon dioxide recovery would be between 1,166 tonnes and 3,150 tonnes per year.

Typical figures for energy potential of biomass are about 15 MJ/kg on a dry weight basis. (3.6 MJ = 1 kWh). Total energy potential is therefore between 9,720 and 25,500 mWh.

Phosphate recovery post incineration would amount to 20 to 25 tonnes as P or as is more commonly used 46 to 57 tonnes per year of P205. Potassium recovery would be dependent on the blend of biofuels burnt.

Claims

CLAIMS1. An installation for the treatment of organic waste material, com-prising an anaerobic digester for processing the waste material to produce a combustible fuel gas and a digestate slurry, a separator for separating solids from the digestate slurry, and an algal biomass production means receiving the liquid part of the digestate from the separator to promote growth of algae.
2. An installation according to Claim 1, further comprising an engine fuelled by the fuel gas, the engine driving an electricity generator and outputting heat and exhaust gas containing carbon dioxide, the heat and exhaust gas be-ing supplied to the algal biomass production means to further promote growth of algae.
3. An installation according to Claim 1 or 2, comprising separation means for receiving algal biomass from the algal biomass production means and for separating water therefrom.
4. An installation according to Claim 3, comprising a furnace for combustion of the separated biomass, and means for conversion of energy from said combustion.
5. An installation according to Claim 4, wherein the means for con-version of energy include a heat exchanger for heating water.
6. An installation according to Claim 5, wherein the heat exchanger is a boiler for generating steam for supply to a steam engine driving an electric-ity generator.
7. An installation according to any preceding claim, wherein the algal biomass production means comprises a photobioreactor.
8. An installation according to any of Claims I to 6, wherein the algal biomass production means comprises a channelled lagoon, pond or raceway into which the liquid part of the digestate flows.
9. An installation according to Claim 3, comprising means for forming the algal biomass into pellets or briquettes for use as fuel.
10. A method of treatment of organic waste material, comprising an-aerobically digesting the waste material to produce a combustible fuel gas and a digestate slurry, separating solids from the liquid part of the digestate slurry, and using the liquid part of the digestate to promote growth of algae in algal biomass production.
11. A method according to Claim 10, fuelling an engine with the fuel gas, the engine driving an electricity generator and outputting heat and exhaust gas containing carbon dioxide, and using the heat and exhaust gas to further promote growth of the algae.
12. A method according to Claim 10 or 11, comprising dewatering the algal biomass to form a fuel.
13. A method according to Claim 12, comprising a further energy re-covery step of burning the fuel to generate steam, and supplying the steam to a steam engine driving a generator to generate electricity.
14. A method according to Claim 13, comprising using waste heat and combustion gases from the further energy recovery step to further promote the growth of algae in the algal biomass production.
15. A method according to Claim 12, further comprising forming the dewatered biomass into pellets or briquettes.
16. A method according to Claim 13 or 14, comprising processing ash resulting from the combustion of the algal biomass to extract plant nutrients.
17. An installation for the treatment of organic waste material, sub-stantially as described with reference to the Drawing.
18. A method of treatment of organic waste material, substantially as described with reference to the Drawing.
19. A method of algal biomass production, wherein the biomass is burnt as fuel, and nutrients extracted from the ash resulting from the burning of the biomass is used to promote growth of further algal biomass.